Synthesis of Benzofuran-2-One Derivatives and Evaluation of Their

molecules Article

Synthesis of Benzofuran-2-One Derivatives and Evaluation of Their Antioxidant Capacity by Comparing DPPH Assay and Cyclic Voltammetry Martina Miceli 1, * ID , Elia Roma 1 ID , Paolo Rosa 2 Daniela Tofani 1 ID and Tecla Gasperi 1, * ID 1 2 3 4

*

ID

, Marta Feroci 3

ID

, M. Antonietta Loreto 4 ,

Dipartimento di Scienze, Sezione di Nanoscienze e Nanotecnologie, Università degli Studi di Roma Tre, via della Vasca Navale 79, I-00146 Roma, Italy; [email protected] (E.R.); [email protected] (D.T.) Dipartimento di Scienze e Biotecnologie Medico-Chirurgiche, Sapienza Università di Roma—Polo Pontino, Corso della Repubblica 79, 04100 Latina, Italy; [email protected] Dipartimento di Scienze di Base e Applicate per l’Ingegneria, Sapienza Università di Roma, via Castro Laurenziano 7, I-00161 Roma, Italy; [email protected] Dipartimento di Chimica, Sapienza Università di Roma, p.le A. Moro 5, I-00185 Roma, Italy; [email protected] Correspondence: [email protected] (M.M.); [email protected] (T.G.); Tel.: +39-338-671-1045 (T.G.)

Received: 3 March 2018; Accepted: 19 March 2018; Published: 21 March 2018







Abstract: The present work aimed to synthesise promising antioxidant compounds as a valuable alternative to the currently expensive and easily degradable molecules that are employed as stabilizers in industrial preparation. Taking into account our experience concerning domino Friedel-Crafts/lactonization reactions, we successfully improved and extended the previously reported methodology toward the synthesis of 3,3-disubstituted-3H-benzofuran-2-one derivatives 9–20 starting from polyphenols 1–6 as substrates and either diethylketomalonate (7) or 3,3,3-trifluoromethyl pyruvate (8) as electrophilic counterpart. The antioxidant capacity of the most stable compounds (9–11 and 15–20) was evaluated by both DPPH assay and Cyclic Voltammetry analyses performed in alcoholic media (methanol) as well as in aprotic solvent (acetonitrile). By comparing the recorded experimental data, a remarkable activity can be attributed to few of the tested lactones. Keywords: antioxidant activity; cyclic voltammetry; DPPH; domino reaction; benzofuran-2-ones

1. Introduction The interplay between free radicals and antioxidants represents a crucial point in the clinical and nutritional research field [1–5]. Free radicals are responsible for oxidative stress, which is balanced by endogenous antioxidant defense mechanisms as well as by the ingestion of exogenous antioxidants [6,7]. In the human body, the overproduction of radical species can cause oxidative damage to fundamental biomolecules (i.e., DNA, lipids, proteins, carbohydrates, etc.), favoring cell apoptosis [8] and becoming a primary cause of several both chronic and degenerative diseases (i.e., aging related pathogenesis, neurological and cardiovascular diseases, skin disorders, cancer, etc.) [9–11]. In order to prevent the undesirable outbreaks of such issues, various natural or synthetic antioxidants are nowadays used as dietary supplements (Smart Food), and are also considered candidate drugs for the reduction of the oxidative damage [12,13]. Among others, tocopherols and tocotrienols, different forms of Vitamin E [14], flavonoids (quercetin) [15,16], hydroxytyrosol (HT) [17], gallic acid [18,19], and their derivatives are noteworthy phenolic antioxidants (Figure 1) which have even shown fascinating antithrombotic activities (i.e., inhibition of LDL oxidation, Molecules 2018, 23, 710; doi:10.3390/molecules23040710

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Molecules FOR PEER REVIEW Molecules 2018, 2018, 23, 23, x710

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have even shown fascinating antithrombotic activities (i.e., inhibition of LDL oxidation, platelet platelet aggregation, and endothelial cell activation) [20]. outstanding Such outstanding actions probably on aggregation, and endothelial cell activation) [20]. Such actions probably rely rely on the the catechol system, which is a common feature in the above bioactive compounds [21]. Unfortunately, catechol system, which is a common feature in the above bioactive compounds [21]. Unfortunately, the currently currently employed employed products products are are seldomly seldomly used used not not only as dietary supplements supplements but but also as the stabilizers in in foods, foods, cosmetics, cosmetics, or or industrial industrial preparations, preparations, mainly mainly due due to to their their frequent frequent degradation. degradation. stabilizers Consequently, the research of more stable antioxidants with a broad application application scope scope remains remains a challenging endeavor of great interest.

Figure 1. Examples of natural phenolic antioxidants. Figure 1. Examples of natural phenolic antioxidants.

Prompted by these considerations, we planned the synthesis of new generation antioxidant Prompted these considerations, we plannedactivity the synthesis of anew generation antioxidant compounds andbythe evaluation of their antioxidant through deep comparison between compounds and the evaluation of their antioxidant activity through a deep comparison between DPPH assay and Cyclic Voltammetry analyses. Specifically, we design potential antioxidant DPPH assay and Cyclic Voltammetry analyses. Specifically, we design potential antioxidant molecules molecules characterized by the 3,3-disubstituted-3H-benzofuran-2-one framework decorated with characterized by the 3,3-disubstituted-3H-benzofuran-2-one framework decorated with one more one or more hydroxyl groups on the aromatic ring. Such a distinguishing scaffold not only isorfound hydroxyl groups ontowards the aromatic ring. Such a distinguishing scaffold not onlybut is occurs found also in key in key intermediates the synthesis of biologically active molecules [22,23], as towards the natural synthesis of biologically active molecules [22,23], but occurs also as aa aintermediates common feature of many medicinal products, among which the antioxidant species play common feature of many natural medicinal products, among which the antioxidant speciesclass playofa critical role [24–27]. Indeed, the 3H-benzofuran-2-ones, or 2-coumaranones, are a significant critical role [24–27]. Indeed, 3H-benzofuran-2-ones, or 2-coumaranones, are a with significant class of heterocyclic molecules highlythe widespread in nature, consisting of a benzene fused a furan-2-one heterocyclic molecules highly widespread in nature, consisting of a benzene fused with a furan-2-one ring [27–30]. For example, the 3,3-disubstitued 2-coumaronone scaffold is a prominent structural ring [27–30]. example, the 3,3-disubstitued 2-coumaronone is aSchidigera, prominentwhich structural motif in many For natural compounds, such as in yuccaol A–E, isolatedscaffold from Yucca have motif in many natural compounds, such as in yuccaol A–E, isolated from Yucca Schidigera, which exhibited antioxidant, radical scavenging, and inflammatory properties, as well as inducible NO have exhibited scavenging, and inflammatory properties, as well as inducible NO synthase (iNOS)antioxidant, expressionradical and platelet aggregation inhibition [31–35]. synthase (iNOS) expression and platelet aggregation inhibition [31–35]. 2. Results and Discussion 2. Results and Discussion 2.1. 2.1. Synthesis Synthesis Within strategies for for the the construction of the Within this this context, context, although althoughseveral severalelegant elegant strategies construction of 3Hthe benzofuran-2-one scaffold [23,36–38] have been reported, the number of approaches 3H-benzofuran-2-one scaffold [23,36–38] have been reported, the number of approaches to to the the corresponding 3-hydroxy derivatives is much more limited. Nevertheless, following our recent corresponding 3-hydroxy derivatives is much more limited. Nevertheless, following our recent development practical route to 3-hydroxy-3H-benzofuran-2-one [39,40], we development of ofalternative, alternative,short, short,and and practical route to 3-hydroxy-3H-benzofuran-2-one [39,40], have figured out the possibility of obtaining various phenolic derivatives by domino reaction we have figured out the possibility of obtaining various phenolic derivatives by domino reaction involving involving aa first first Friedel–Crafts Friedel–Crafts alkylation alkylation and andaasubsequent subsequentintramolecular intramolecularlactonization. lactonization.Specifically, Specifically, the reaction was performed employing the polyphenols 1–6 as substrates and either ketomalonate 7 the reaction was performed employing the polyphenols 1–6 as substrates and either ketomalonate 7 or or 3,3,3-trifluoromethyl pyruvate 8 as the electrophilic counterpart in the presence of TiCl 4 (10 mol%) 3,3,3-trifluoromethyl pyruvate 8 as the electrophilic counterpart in the presence of TiCl4 (10 mol%) as as catalyst, which should activate alkylating agent well postulated intermediate (Table catalyst, which should activate thethe alkylating agent as as well as as thethe postulated intermediate AA (Table 1). 1).

Molecules 2018, 2018, 23, 23, x 710 of 17 17 Molecules FOR PEER REVIEW 33 of Molecules 2018, 23, xxFOR PEER REVIEW 33ofof17 Molecules 2018, 23, FOR PEER REVIEW 17 Molecules Molecules 2018, 2018, 23, 23, x x FOR FOR PEER PEER REVIEW REVIEW 3 3 ofof17 17 Molecules 2018, 23, FOR PEER REVIEW 17 Molecules 2018, 23, xx FOR PEER REVIEW 33 ofof 17 Molecules 2018, 23, x FOR PEER REVIEW 3 of 17 Molecules 2018, 23, x FOR PEER REVIEW 3 of 17 Molecules 2018, 23, x FOR PEER REVIEW 3 of 17 Molecules 2018, 23, x FOR PEER REVIEW 3 of 17 Molecules 2018, 3333of Molecules 2018, 23,xxxxFOR FORPEER PEERREVIEW REVIEW of17 17 Table 1. 1.23, Friedel–Crafts alkylation/lactonization of ofpolyphenols 1–61–6performed with TiCl 4 as Molecules 2018, 23, FOR PEER REVIEW of 17 Molecules 2018, 23, FOR PEER REVIEW of 17 Molecules 2018, 23, FOR PEER REVIEW of 17 Molecules 2018, 23,Friedel–Crafts FORPEER PEER REVIEW of17 17 Table alkylation/lactonization polyphenols performed with TiCl 4a4. as Table 1. alkylation/lactonization of polyphenols 1–6 performed with TiCl as Table 1. Friedel–Crafts alkylation/lactonization of polyphenols 1–6 performed with TiCl4 as catalyst Molecules 2018, 23, xxFriedel–Crafts FOR REVIEW 3333of Molecules 2018, 23, xx FOR PEER REVIEW of 17 Table Table 1. 1. Friedel–Crafts Friedel–Crafts alkylation/lactonization alkylation/lactonization of of polyphenols polyphenols 1–6 1–6 performed performed with with TiCl TiCl 4 4 as as a Molecules 2018, 23, x FOR PEER REVIEW 3 of 17 Molecules 2018, 23, x FOR PEER REVIEW 3 of 17 Table 1. Friedel–Crafts alkylation/lactonization of polyphenols 1–6 performed with TiCl 4 as Table 1. Friedel–Crafts alkylation/lactonization of polyphenols 1–6 performed with TiCl 4 as catalyst . a Molecules 2018, 23, x FOR PEER REVIEW 3 of 17 Molecules 2018, 23, x FOR PEER REVIEW 3 of 17 a Table 1. Friedel–Crafts alkylation/lactonization of polyphenols 1–6 performed with TiCl 4 as Table 1. Friedel–Crafts alkylation/lactonization of polyphenols 1–6 performed with TiCl 4 as catalyst . . Friedel–Crafts catalyst Molecules 2018, 33ofof1717 Molecules 2018, xFOR FORPEER PEERREVIEW REVIEW Table 1.a23, alkylation/lactonization Table 1. alkylation/lactonization ofof polyphenols polyphenols 1–6 1–6 performed performed with with TiCl TiCl4 4 as as catalyst catalyst .a23, . xFriedel–Crafts

Table a Friedel–Crafts Table 1. 1. Friedel–Crafts alkylation/lactonization alkylation/lactonization of of polyphenols polyphenols 1–6 1–6 performed performed with with TiCl TiCl4444 as as Table 1.a1. Friedel–Crafts alkylation/lactonization of polyphenols 1–6 performed with TiCl as Table Friedel–Crafts alkylation/lactonization of polyphenols 1–6 performed with TiCl as catalyst catalyst a.a... Friedel–Crafts Table 1. alkylation/lactonization of polyphenols 1–6 performed with TiCl as Table Friedel–Crafts alkylation/lactonization alkylation/lactonization of of polyphenols polyphenols 1–6 1–6 performed performed with with TiCl TiCl44 44 as as catalyst catalyst a1. Table 1. Table Friedel–Crafts alkylation/lactonization of polyphenols 1–6 performed with TiCl as catalyst .aaa.. Friedel–Crafts catalyst a1. Table 1. Friedel–Crafts alkylation/lactonization of polyphenols 1–6 performed with TiCl 4 4 as catalyst . a Table 1. Friedel–Crafts alkylation/lactonization of polyphenols 1–6 performed with TiCl as catalyst catalyst . catalyst . a1. a Table 1. Friedel–Crafts alkylation/lactonization of polyphenols 1–6 performed with TiCl 4 as Table Friedel–Crafts alkylation/lactonization of polyphenols 1–6 performed with TiCl 4 as catalyst . catalyst Table Table 1.aa1. Friedel–Crafts alkylation/lactonization alkylation/lactonization ofof polyphenols polyphenols 1–6 1–6 performed performed with with TiCl TiCl4 4 asas catalyst ..aa.. Friedel–Crafts catalyst catalyst catalyst . aa catalyst catalyst catalyst catalysta..a..

Entry Entry Entry Entry

b Substrate RR22R22 Product Yield (%) b bb Substrate Product Yield (%) Substrate Product Yield Substrate R Product Yield (%) Entry Entry Substrate Substrate RR R22222 Product Product Yield Yield(%) (%) (%)bbbbb Entry Substrate Product Yield (%) Entry Substrate R Product Yield (%) 22 b Entry Substrate R Product Yield (%) Entry Substrate R Product Yield (%) 2 Entry Substrate R Product Yield (%) Entry 1 Substrate R2222 Product Yield (%)bbbbbb 9 9 1Entry CO 2R Et 7070(%) Substrate Product Yield Entry Substrate R Product Yield (%) Entry Substrate REt Product Yield (%) Substrate R Product Yield (%) bb 1 1 CO 2Et 1 Entry 1 CO 9 70 1 9 1 CO 2 Et 70 Entry Substrate R Product Yield (%) Entry Substrate R Product Yield (%) 2 2 b 2 Et b CO CO 2Et 70 70 Entry Substrate R Product Yield (%) Entry Substrate Product Yield (%) CO 22Et 70 111111 Substrate 999999 111111 CO 70 Entry Substrate RR Product Yield (%) Entry Substrate R22222Et Product Yield (%)bbbbb CO 22Et Et 70 CO 70 Entry R Product Yield (%) Entry Substrate R Product Yield (%) 999 CO Et 70 b CO 22Et 70 Entry R Product Yield (%) Entry Substrate R Product Yield (%) 111111 Substrate 9 111111 CO 2 Et 70 CO 2 Et 70 CO Et 70 CO Et 70 99999 CO 222Et 70 CO 2Et 70 10 2 1112111 2Et 62 2 1112111 CO 9 CO 2Et 70 CO 2Et Et 70 10 2 62 CO 9 70 CO 2Et 70 10 62 2 11221212 9910 CO 22Et 70 10 10 62 62 999 CO 22Et 70 2 2 2 CO 10 62 2 Et CO 2Et 70 22Et 62 CO 10 CO Et 70 62 2112222112 10 2Et Et 62 CO 10 222 62 CO 10 2 22Et 62 2 CO 10 2 Et 62 2 CO 10 2222 2 Et 62 2222 CO 10 2 Et 62 CO 10 10 Et 62 CO 2Et Et 62 CO 10 22 62 22 CO 10 2Et 62 CO222Et 10 62 CO 10 Et 62 CO 1110 3 2223222 CO 2Et222Et 8162 3 2223222 10 62 CO 10 Et 62 CO 11 CO 2 Et 81 2 Et CO 10 2 Et 62 CO 11 33 2 81 33 10 2 2Et 62 2 CO 10 2 2Et 62 2 CO 11 11 3 81 81 3 11 CO 2 Et 81 3 3 333 CO Et 11 81 11 CO 2 Et 81 3333 2 22Et 11 33 CO Et 81 11 CO 81 11 3 CO 2Et 81 3 11 CO 2 Et 81 3 11 3333 CO 81 3333 11 CO222Et 2Et Et 81 11 11 CO 81 CO 2Et Et 81 11 CO 81 11 CO2Et 2Et 81 11 333333 CO 81 333333 11 CO 2Et 81 11 CO 2 Et 81 c) 11 CO 2 Et 81 4 3343 12 CO 4 3343 2Et 4 (22 11 CO 22Et 81 11 CO 2 Et 81 12 444(22 (22 11 CO 2Et 81 12 (22 11 344 CO 2Et 81ccc)c)cc)) 344 4 12 12 CO CO 4 2Et 2Et 4 (22 12 CO 2Et (22 12 CO 4444 2Et 44444(22 (22 c c)c) 4 12 4 4 4444 CO 12 CO 2Et Et (22 12 CO (22 2 Et 12 CO 22Et (22 12 CO 2Et (22ccc)))ccc)))) 44444 12 CO 444444 22Et 444444(22 12 CO 2Et Et (22 12 12 CO CO (22 2 Et (22 12 CO Et (22 12 CO22Et 2Et (22ccc)))ccc))) 444 12 CO 444 444444(22 12 CO 2Et (22 c) )) 12 CO (22 12 CO Et (22 5 445444 13 5 445444 CO 2Et22Et 7 (18 cc)cc 12 CO 22Et 447474(22 12 CO 2Et (22 13 CO (18 cc))cc) 13 22Et (18 45455 12 CO 45455 22Et (22 12 CO (22 13 13 CO Et 2Et 7 7 (18 (18 c)) c)) 13 CO 2Et 7 (18 5555 13 5555 CO 2Et 7 (18 c 13 CO 2Et Et 7 (18 c 13 CO 2 7 (18 c)c)c))) 5 13 5 CO 2Et (18 5 13 5 5 CO Et 13 7 (18 5 CO 2 Et 7 (18 c 2 5555 13 5555 CO 22Et 7777(18 c)) cc) 13 CO 2Et Et (18 13 13 CO (18 CO 2 Et (18 c 13 CO Et 777(18 (18 13 CO22Et 2Et (18cc))ccc))) 555555 13 555555 CO 7 13 CO 2Et (18 13 CO 22Et (18 c) c ) 13 CO Et (18 ) c) 6 5565 1413 CO 6 5565 2Et 3 (19 CO 22Et 77737773(18 13 CO 2Et (18 cc) c 14 (19 13 CO 2Et (18 14 CO (19 566 13 566 2Et (18 c))c) 6 14 14 CO CO 6 2Et 2Et 3 3 (19 (19 ) c c 14 CO 2Et (19 6666 14 CO 6666 2Et 3333(19 c)c) 14 CO 2Et Et (19 14 CO (19 14 CO 22Et 333(19 (19 cccc)))ccc))) 6 14 CO 2Et (19 14 6 6 66 CO Et 14 CO 666666 2 Et 3 (19 6 14 CO 2 Et (19 ) 2 222Et 66 14 14 CO CO Et 33(19 (19 Et (19 14 CO 14 CO 2Et (19ccc)))ccc))) 6666 14 CO 6666 2Et 333333(19 14 CO 2Et (19 14 CO 22Et (19 14 CO Et (19cc))c)) 1 66166 15 7 66766 CF 3 35 14 CO 2 Et 3 (19 14 CO 23Et (19 c 15 CF 35 61161 14 CO 67767 2Et 333(19 15 CF 3 35 14 CO (19 15 15 CF CF 32Et 3 35 35 ) ) 15 CF 35 1111 15 7777 CF 33 35 15 CF 3 35 15 CF 3 35 11 15 CF 35 15 CF3333 35 15 777777 CF 35 15 CF 35 CF 15 35 7 1 1 15 15 CF 35 CF 35 3 3333 111111 15 777 CF 35 15 CF 35 1 15 7 CF 33 35 15 CF 35 2 1121 1615 8 7787 CFCF 3 33 2835 15 CF 35 15 CF 35 16 CF 3 28 12 15 78 CF 35 16 28 15 CF 35 16 16 28 28 15 CF3333333 35 16 CF 28 21222212 16 87888878 CF 28 16 CF 3 28 16 CF 3 28 16 CF 33 28 16 CF 28 222222 16 888888 CF 3 28 16 CF3333 28 16 16 CF 28 CF 28 222 16 CF 28 16 CF 28 8 2 2 CF 16 28 3 3 33 16 888888 CF 28 16 CF 28 2 16 CF 33 28 2 16 CF 28 16 28 3 22322 17 9 88988 CFCF 3 33 61 16 CF 28 16 CF 33 33 28 16 17 CF 28 61 33 17 99 61 3 17 17 9 CF CF 33 61 61 17 CF 61 3333 17 9999 CF 33 61 17 CF 3 61 17 CF 3 61 17 CF 33 61 17 CF 61 333333 17 999999 CF 3 61 17 CF3333 61 17 17 CF 61 CF 61 333 17 CF 61 17 CF333 61 17 999999 CF 61 9 3 3 CF 17 61 17 CF 61 3 33 3 17 CF 61 3 17 CF 61 17 9999 33 61 17 CF 61 18 4 33433 CFCF 3 5 1010 17 CF 33 3 61 17 CF 18 CF 561 18 44 CF 3 5 10 18 18 4 CF CF 33 5 10 10 18 CF 10 18 4444 CF 33 55555 10 18 CF 3 10 18 CF 3 10 18 CF 10 18 CF3333 10 18 444444 CF 555555 10 18 CF 10 18 18 CF CF 10 10 18 444 CF 10 18 CF333333 10 18 4 CF 555555 10 18 CF 10 1918 18 CF 33 33 1111 CF 10 10 1110 4 5 44 CF 55 18 45 CF 10 5 19 11 CF 18 CF 3333 18 4 CF 55 10 19 11 11 18 4 CF 511 10 18 CF33333 10 19 19 11 11 11 11 19 11 CF 5555545 19 11 CF 11 19 11 CF 3 11 19 11 CF 3 11 19 11 CF 11 19 11 CF3333 11 555555 19 11 CF 11 19 11 CF 11 19 19 11 CF 11 11 CF 11 555 19 11 CF 11 19 11 CF333333 11 5 19 11 CF 11 19 11 CF 11 2019 6 55 CFCF 3 33 7 11 1211 19 11 CF 11 19 11 CF 11 20 CF 12 19 11 CF 11 CF 19 11 20 7 12 3 333333 19 11 CF 11 19 11 CF 11 20 20 7711 12 12 5 5656665566 20 CF 777 12 20 CF 33 7 12 20 6 CF 3 12 20 6 CF 3 7 12 20 CF 12 20 CF3333 12 20 666666 CF 777777 and 20 CF 12 a Unless 12 20 20 CF CF 12 12 otherwise stated, thethe reactions were performed with indicated phenols (2.0(2.0 mmol) 20 666stated, CF 333 12 a a Unless 20 CF 12otherwise reactions were with indicated phenols mmol) and 20 6 CF 3 33performed 777777 and 12 Unless otherwise stated, the reactions were performed with indicated phenols (2.0 mmol) and 20 CF 12 a aUnless 20 666stated, CF 12 Unless otherwise otherwise stated, stated, the thereactions reactions reactions were were performed performed with with indicated indicated phenols phenols (2.0 (2.03mmol) mmol) mmol) 20 CF3333performed 12 band a a Unless otherwise stated, the reactions were performed with indicated phenols (2.0 mmol) and 20 6 alkylating agents (7 or 8; 2.2 mmol) in the presence of TiCl 420 (10 mol%) in anhydrous CHCl (9 mL); CF 7 12 otherwise the were with indicated phenols (2.0 and CF 7 12 a Unless bb a Unless otherwise stated, the reactions were performed with indicated phenols (2.0 mmol) and 20 6 CF 3 3performed 7 12 agents (7 or 8; 2.2 mmol) in the presence of TiCl 4with (10 mol%) in anhydrous CHCl 37 (9 mL); Unless otherwise stated, the reactions were with indicated phenols (2.0 mmol) and aalkylating 20 6 12 6 CF 20 CF 7 12 aalkylating agents (7 or 8; 2.2 mmol) in the presence of TiCl 4 (10 mol%) in anhydrous CHCl 3 (9 mL); 3 Unless otherwise stated, the reactions were performed indicated phenols (2.0 mmol) and Unless otherwise stated, the reactions were performed with indicated phenols (2.0 mmol) and aalkylating aalkylatingagents agents(7(7or or8;8;2.2 2.2mmol) mmol)ininthe thepresence presenceofofTiCl TiCl4 4(10 (10mol%) mol%)ininanhydrous anhydrousCHCl CHCl3 3(9(9mL); mL);b b

Unless otherwise stated, reactions were performed with indicated phenols (2.0 mmol) and Unless otherwise stated, the reactions were performed with indicated phenols (2.0 mmol) and bb cthe 1of Unless otherwise stated, the reactions were performed with indicated phenols (2.0 mmol) and Unless otherwise stated, the reactions were performed with indicated phenols (2.0 mmol) and aaaalkylating agents (7 or 8; 2.2 mmol) in the presence of TiCl (10 mol%) in anhydrous CHCl (9 mL); alkylating agents (7 or 8; 2.2 mmol) in the presence TiCl 4 4with (10 mol%) in anhydrous CHCl 3 3(9 mL); Yield of of thethe isolated product; Yield calculated at the H-NMR of the crude reaction mixture. a Unless b c cYield 1H-NMR otherwise stated, the reactions were performed indicated phenols (2.0 mmol) and Unless otherwise stated, the reactions were performed with indicated phenols (2.0 mmol) and 1TiCl aYield alkylating agents (7 or 8; 2.2 mmol) in the presence of TiCl 4 (10 (10 mol%) in anhydrous CHCl 3 (9 (9 mL); aYield alkylating agents (7 or 8; 2.2 mmol) in the presence of mol%) in anhydrous CHCl mL); bbb isolated product; calculated at the of the crude reaction mixture. of the isolated product; Yield calculated at the of the crude reaction mixture. Unless otherwise stated, the reactions were performed indicated phenols (2.0 mmol) and Unless otherwise stated, the reactions were performed with indicated phenols (2.0 mmol) and cmmol) cYield 1H-NMR 1H-NMR alkylating agents (7 or 8; 2.2 mmol) in the presence of TiCl 44with (10 mol%) in anhydrous CHCl 333(9 mL); agents (7 or 8; 2.2 in the presence of TiCl 4with (10 mol%) in anhydrous CHCl (9 mL); a aalkylating bbbb Yield Yield of of the the isolated isolated product; product; Yield calculated calculated at at the the H-NMR of of the the crude crude reaction reaction mixture. mixture. Unless otherwise stated, the reactions were performed indicated phenols (2.0 mmol) and alkylating agents (7 or 8; 2.2 mmol) in the presence of TiCl 4 (10 mol%) in anhydrous CHCl 3 (9 mL); Unless otherwise stated, the reactions were performed with indicated phenols (2.0 mmol) and c 1 alkylating agents (7 or 8; 2.2 mmol) in the presence of TiCl 4 (10 mol%) in anhydrous CHCl 3 (9 mL); cmmol) 1H-NMR aYield a Unless alkylating agents (7 or 8; 2.2 mmol) in the presence of TiCl 4 (10 mol%) in anhydrous CHCl 3 (9 mL); alkylating agents (7 or 8; 2.2 in the presence of TiCl 4 (10 mol%) in anhydrous CHCl 3 (9 mL); b Yield of the isolated product; Yield calculated at the H-NMR of the crude reaction mixture. of the isolated product; Yield calculated at the of the crude reaction mixture. otherwise stated, the reactions were performed with indicated phenols (2.0 mmol) and c 1 Unless otherwise stated, the reactions were performed with indicated phenols (2.0 mmol) and c Yield aalkylating agents (7(7 or 8;8; 2.2 mmol) in the presence of mol%) in anhydrous CHCl 3 (9 mL); aUnless alkylating agents (7stated, or 8;2.2 2.2 in thewere presence of TiCl44 with 4(10 (10of mol%) inanhydrous anhydrous CHCl (9mL); mL); b bb Yield of the isolated product; Yield calculated at the H-NMR of the crude reaction mixture. Yield of the isolated product; calculated at the H-NMR of the crude reaction mixture. cmmol) 11H-NMR otherwise the reactions performed indicated phenols (2.0 mmol) and cYield 1TiCl a Unless Unless otherwise stated, the reactions were performed with indicated phenols (2.0 mmol) and alkylating agents (7 or 8; mmol) in the presence of mol%) in CHCl 3 33(9 alkylating agents or 2.2 in the presence of TiCl 4(10 (10 mol%) in anhydrous CHCl (9 mL); Yield of the isolated product; calculated at the the crude reaction mixture. bb Yield ofthe the isolated product; Yield calculated at the H-NMR of the(2.0 crude reaction mixture. cmmol) 11H-NMR otherwise stated, the reactions were performed with indicated phenols mmol) and alkylating agents 1TiCl alkylating agents (7 or 8; 2.2 mmol) in the presence of TiCl 4 4(10 mol%) in anhydrous CHCl 3 3(9 mL); Yield of isolated product; calculated at the of the crude reaction mixture. c ccYield 1H-NMR alkylating agents (7 or 8; 2.2 mmol) in the presence of TiCl (10 mol%) in anhydrous CHCl (9 mL); Yield of the isolated product; Yield calculated at the H-NMR of the crude reaction mixture. b b As depicted in Table 1, employing the previously optimized conditions, only the polyphenols Yield of the isolated product; Yield calculated at the of the crude reaction mixture. Yield of the isolated product; Yield calculated at the H-NMR of the crude reaction mixture. c 1 c 1 agents (7 or 8; 2.2 mmol) in the presence of TiCl 4 4(10 mol%) in anhydrous CHCl 3 3(9 mL); alkylating agents (7 or 2.2 in the presence TiCl (10 mol%) anhydrous CHCl (9 mL); bin bb Yield of the isolated product; Yield calculated atat the H-NMR of the crude reaction mixture. Yield of the isolated product; Yield calculated atthe the H-NMR of the crude reaction mixture. cmmol) 1H-NMR As in Table 1,1, employing the previously optimized conditions, the polyphenols cYield 1TiCl alkylating agents (7 or 8;8; 2.2 mmol) in the presence ofof 4 4(10 mol%) in anhydrous CHCl 3product; mL); (7 alkylating or 8; depicted 2.2 mmol) in the presence of TiCl mol%) in at anhydrous CHCl (9 mL); Yield of theonly isolated As depicted in Table employing the previously optimized conditions, only the polyphenols alkylating agents (7 or 8; 2.2 mmol) in the presence of TiCl (10 mol%) in anhydrous CHCl 3(9 (9 mL); Yield of the isolated product; calculated the crude reaction mixture. 4 (10 3of Yield of the isolated product; Yield calculated the H-NMR of the crude reaction mixture. c 1 c 1 As As depicted depicted in in Table Table 1, 1, employing employing the the previously previously optimized optimized conditions, conditions, only only the the polyphenols polyphenols Yield of the isolated product; Yield calculated at the H-NMR of the crude reaction mixture. Yield of the isolated product; Yield calculated at the H-NMR of the crude reaction mixture. c 1 c 1 c 1 As depicted in Table 1, employing the previously optimized conditions, only the polyphenols As depicted in Table 1, employing the previously optimized conditions, only the polyphenols 1–3 furnished the desired bicyclic compounds 9–11 with good toof high yields (up to 81%, entries 1–3), Yield calculated atin theTable H-NMR of cthe crude reaction mixture. Yield of isolated product; calculated at the crude reaction mixture. Yield the isolated product; Yield calculated the the crude reaction mixture. 1H-NMR As depicted in Table 1, employing the previously optimized conditions, only the polyphenols cYield 1H-NMR As depicted 1, employing the previously optimized conditions, only the polyphenols 1–3 furnished the desired bicyclic compounds 9–11 with good to high yields (up to 81%, entries 1–3), Yield ofof isolated product; Yield calculated atat the H-NMR ofof the crude reaction mixture. 1–3 furnished the desired bicyclic compounds 9–11 with good to high yields (up to 81%, entries 1–3), Yield ofthe the isolated product; Yield calculated atthe the H-NMR of the crude reaction mixture. As depicted in Table 1, employing the previously optimized conditions, only the polyphenols As depicted in Table 1,employing employing thepreviously previously optimized conditions, only thepolyphenols polyphenols 1–3 1–3furnished furnished the the desired desired bicyclic bicyclic compounds compounds 9–11 9–11 with with good good to to high high yields yields (up (up to to 81%, 81%, entries entries1–3), 1–3), As depicted in Table 1, the optimized conditions, only the As depicted in Table 1, employing the previously optimized conditions, only the polyphenols

As depicted in Table 1,1, employing the previously optimized conditions, only the polyphenols As depicted in Table employing the previously optimized conditions, only the polyphenols 1–3 furnished the desired bicyclic compounds 9–11 with good to high yields (up to 81%, entries 1–3), 1–3 furnished the desired bicyclic compounds 9–11 with good to high yields (up to 81%, entries 1–3), when the ketomalonate 7 was as alkylating agent; meanwhile substrates 4–5, owing second As depicted in Table 1, employing the previously optimized conditions, only the polyphenols As depicted in Table 1,used employing thepreviously previously optimized conditions, only thethe polyphenols 1–3 furnished the desired bicyclic compounds 9–11 with good to high yields (up to 81%, entries 1–3), 1–3 furnished the desired bicyclic compounds 9–11 with good to high yields (up to 81%, entries 1–3), when the ketomalonate was used as alkylating agent; meanwhile substrates 4–5, owing the second As depicted Table 1, employing the optimized conditions, only the polyphenols when the ketomalonate was used as alkylating agent; meanwhile substrates 4–5, owing the second As depicted in Table 1, employing the previously optimized conditions, only the polyphenols 1–3 furnished the desired bicyclic compounds 9–11 with good to high yields (up to 81%, entries 1–3), 1–3 furnished thein desired bicyclic compounds 9–11 withmeanwhile goodto tohigh high yields(up (up toowing 81%, entries 1–3), As depicted in Table 1, employing the previously optimized conditions, only the polyphenols when when the the ketomalonate ketomalonate 77777was was was used used as asalkylating alkylating alkylating agent; agent; meanwhile substrates substrates 4–5, 4–5, owing the thesecond second second As depicted in Table 1,employing employing thepreviously previously optimized conditions, only thepolyphenols polyphenols 1–3 furnished the desired bicyclic compounds 9–11 with good yields to 81%, entries 1–3), 1–3 furnished the desired bicyclic compounds 9–11 with good to high yields (up to 81%, entries 1–3), 1–3 furnished the desired bicyclic compounds 9–11 with good to high yields (up to 81%, entries 1–3), 1–3 furnished the desired bicyclic compounds 9–11 with good to high yields (up to 81%, entries 1–3), when the ketomalonate was used as alkylating agent; meanwhile substrates 4–5, owing the second As depicted in Table 1, the optimized conditions, only the when the ketomalonate 7 used as agent; meanwhile substrates 4–5, owing the As depicted in Table 1, employing the previously optimized conditions, only the polyphenols hydroxyl group in the ortho position (4) or two OH substituents (5 and 6), led to a complex reaction 1–3 furnished the desired bicyclic compounds 9–11 with good to high yields (up to 81%, entries 1–3), 1–3 furnished the desired bicyclic compounds 9–11 with good to high yields (up to 81%, entries 1–3), As depicted in Table 1, employing the previously optimized conditions, only the polyphenols when the ketomalonate 7 was used as alkylating agent; meanwhile substrates 4–5, owing the second As depicted in Table 1, employing the previously optimized conditions, only the polyphenols 1–3 when the ketomalonate 7 was used as alkylating agent; meanwhile substrates 4–5, owing the second As depicted in Table 1, employing the previously optimized conditions, only the polyphenols hydroxyl group in the ortho position (4) or two OH substituents (5 and 6), led to a complex reaction 1–3 furnished the desired bicyclic compounds 9–11 with good to high yields (up to 81%, entries 1–3), hydroxyl group in the ortho position (4) or two OH substituents (5 and 6), led to a complex reaction 1–3 furnished the desired bicyclic compounds 9–11 with good to high yields (up to 81%, entries 1–3), when the ketomalonate 7 was used as alkylating agent; meanwhile substrates 4–5, owing the second when the ketomalonate 7 was used as alkylating agent; meanwhile substrates 4–5, owing the second 1–3 furnished the desired bicyclic compounds 9–11 with good to high yields (up to 81%, entries 1–3), hydroxyl hydroxyl group group in in the theortho ortho ortho position position (4) (4) or ortwo two two OH OH substituents substituents (5 (5and and and 6), 6),led led led to to aaaowing complex complex reaction reaction 1–3 furnished the desired bicyclic compounds 9–11 with good tohigh high yields (up to 81%, entries 1–3), when the ketomalonate 7777was used as alkylating agent; meanwhile substrates 4–5, the second when thegroup ketomalonate was used as alkylating agent; meanwhile substrates 4–5, owing the second 1led when the ketomalonate was used as alkylating agent; meanwhile substrates 4–5, owing the second when the ketomalonate was used as alkylating agent; meanwhile substrates 4–5, owing the second hydroxyl group in the ortho position (4) or two OH substituents (5 and 6), to complex reaction 1–3 the desired bicyclic compounds 9–11 with good to (up to 81%, entries 1–3), hydroxyl in the position (4) or OH substituents 6), to ato complex reaction 1–3 furnished the desired bicyclic compounds 9–11 with good to high yields (up 81%, entries 1–3), mixture where the corresponding products 12 and 13, initially detected atyields the H-NMR, were isolated when the ketomalonate was used as alkylating agent; meanwhile substrates 4–5, owing the second 1H-NMR, when the ketomalonate was used as alkylating agent; meanwhile substrates 4–5, owing the second 1–3 furnished the desired bicyclic compounds 9–11 with good to high yields (up 81%, entries 1–3), hydroxyl group in the ortho position (4) or two OH substituents (5 and 6), led to complex reaction furnished the desired bicyclic compounds 9–11 with good to high yields (up to 81%, entries 1–3), when 14–5, hydroxyl group in the ortho position (4) or two OH substituents (5 and 6), led to aaaowing complex reaction 1–3furnished furnished the desired bicyclic compounds 9–11 with good to(5 high yields 81%, entries 1–3), mixture where corresponding products 12 and 13, initially detected at the were isolated when the ketomalonate 777777was used as alkylating agent; meanwhile substrates the second mixture where the corresponding products 12 and 13, initially detected at the H-NMR, were isolated when the ketomalonate was used as alkylating agent; meanwhile substrates 4–5, owing the second hydroxyl group in the ortho position (4) or two OH substituents (5 and 6), led to ato complex reaction hydroxyl group in theortho ortho position (4) ortwo two OH substituents (5 and 6), led toto complex reaction 1(up 14–5, when the ketomalonate was used as alkylating agent; meanwhile substrates owing the second mixture mixture where where the the corresponding corresponding products products 12 12 and and 13, 13, initially initially detected detected at at the the H-NMR, H-NMR, were were isolated isolated when the ketomalonate was used as alkylating agent; meanwhile substrates 4–5, owing the second hydroxyl group in the position (4) or OH substituents (5 and 6), led to a complex reaction hydroxyl group in the ortho position (4) or two OH substituents (5 and 6), led to a complex reaction 1 1 hydroxyl group in the ortho position (4) or two OH substituents (5 and 6), led to complex reaction hydroxyl group in the ortho position (4) or two OH substituents (5 and 6), led to aaowing complex reaction mixture where the corresponding products 12 and 13, initially detected at the H-NMR, were isolated the ketomalonate 7777was used as alkylating agent; meanwhile substrates the second mixture where the corresponding products 12 and 13, initially detected at the were isolated when the ketomalonate was used as alkylating agent; meanwhile substrates 4–5, owing the second inwhen very poor yields, mainly due to their instability (entry 4–6). Additionally, the more reactive 3,3,314–5, hydroxyl group in the ortho position (4) or two OH substituents (5 and 6), led to aamore complex reaction 1H-NMR, hydroxyl group in the ortho position (4) or two OH substituents (5 and 6), led to complex reaction when the ketomalonate was used as alkylating agent; meanwhile substrates 4–5, owing the second mixture where the corresponding products 12 and 13, initially detected at the H-NMR, were isolated mixture where the corresponding products 12 and 13, initially detected at the H-NMR, were isolated 1 when the ketomalonate was used as alkylating agent; meanwhile substrates 4–5, owing the second 1 in very poor yields, mainly due to their instability (entry 4–6). Additionally, the more reactive 3,3,3hydroxyl group in the ortho position (4) or two OH substituents (5 and 6), led to a complex reaction hydroxyl group in the ortho position (4) or two OH substituents (5 and 6), led to a complex reaction in very poor yields, mainly due to their instability (entry 4–6). Additionally, the reactive 3,3,3mixture where the corresponding products 12 and 13, initially detected at the H-NMR, were isolated mixture where the corresponding products 12and and 13, initially detected at the H-NMR, werereaction isolated 1the hydroxyl group in the ortho position (4) or two OH substituents (5 and 6), led to amore complex hydroxyl group incorresponding the ortho position (4)instability ortwo two OH substituents (5and and 6), led tomore complex reaction in invery very verypoor poor poor yields, yields, mainly mainly due due to to their their instability instability (entry (entry 4–6). 4–6). Additionally, Additionally, the more reactive reactive 3,3,33,3,3mixture where the products 12 13, initially detected at the H-NMR, were isolated 11H-NMR, mixture where the corresponding products 12 and 13, initially detected at the H-NMR, were isolated mixture where the corresponding products 12 and 13, initially detected at the were isolated mixture where the corresponding products 12 and 13, initially detected at the were isolated 11the hydroxyl group in the ortho position (4) or OH substituents (5 6), led to aamore 1H-NMR, in very poor yields, mainly due to their instability (entry 4–6). Additionally, reactive 3,3,3in yields, mainly due to their (entry 4–6). Additionally, the reactive 3,3,3hydroxyl group in the position (4) or two OH substituents and 6), led to aaacomplex complex reaction trifluoromethyl pyruvate 8ortho provided exclusively the benzofuran-2-one 1717 in acceptable amount (61%, mixture where the corresponding products 12 and 13, initially detected at the H-NMR, were isolated mixture where the corresponding products 12and and 13, initially detected at the H-NMR, werereaction isolated 1the hydroxyl group in the ortho position (4) or two OH substituents (5(5 and 6), led to complex reaction 1H-NMR, in very poor yields, mainly due to their instability (entry 4–6). Additionally, the more reactive 3,3,3hydroxyl group in the ortho position (4) or two OH substituents (5 and 6), led to complex reaction in very poor yields, mainly due to their instability (entry 4–6). Additionally, more reactive 3,3,3trifluoromethyl pyruvate 8 provided exclusively the benzofuran-2-one in acceptable amount (61%, mixture where the corresponding products 12 13, initially detected at the H-NMR, were isolated trifluoromethyl pyruvate 8 provided exclusively the benzofuran-2-one 17 in acceptable amount (61%, mixture where the corresponding products 12 and 13, initially detected at the were isolated 1 in very poor yields, mainly due to their instability (entry 4–6). Additionally, the more reactive 3,3,31 in very poor yields, mainly due to their instability (entry 4–6). Additionally, the more reactive 3,3,3mixture where the corresponding products 12 and 13, initially detected at the were isolated trifluoromethyl trifluoromethyl pyruvate pyruvate provided provided exclusively exclusively the the benzofuran-2-one benzofuran-2-one 17 17 in in acceptable acceptable amount amount (61%, (61%, mixture where the corresponding products 12and and 13, initially detected atin the H-NMR, wereisolated isolated in very yields, mainly due their instability (entry 4–6). Additionally, the more reactive 3,3,3in verypoor poor yields, mainly dueto to their instability (entry 4–6).detected Additionally, the moreamount reactive 3,3,31the 1H-NMR, in very poor yields, mainly due to their instability (entry 4–6). Additionally, more reactive 3,3,3in very poor yields, mainly due to their instability (entry 4–6). Additionally, more reactive 3,3,3trifluoromethyl pyruvate provided exclusively the benzofuran-2-one 17 in acceptable amount (61%, mixture where the corresponding products 12 13, initially at the H-NMR, were trifluoromethyl pyruvate 888888provided exclusively the benzofuran-2-one 17 acceptable (61%, mixture where the corresponding products 12 and 13, initially detected at the were isolated 1the in very poor yields, mainly due to their instability (entry 4–6). Additionally, the more reactive 3,3,31H-NMR, in verypoor poor yields, mainly dueto to their instability (entry 4–6).detected Additionally, the morereactive reactive 3,3,3mixture where the corresponding products 12 13, initially at the H-NMR, were trifluoromethyl pyruvate provided exclusively the benzofuran-2-one 17 in acceptable amount (61%, trifluoromethyl pyruvate provided exclusively the benzofuran-2-one 17 in acceptable amount (61%, mixture where the corresponding products 12and and 13, initially detected atin the H-NMR, wereisolated isolated in very yields, mainly due their instability (entry 4–6). Additionally, the more 3,3,3in very poor yields, mainly due to their instability (entry 4–6). Additionally, the more reactive 3,3,3trifluoromethyl pyruvate 888provided provided exclusively the benzofuran-2-one 17 acceptable amount (61%, trifluoromethyl pyruvate provided exclusively the benzofuran-2-one 17 in acceptable amount (61%, in very poor yields, mainly due to their instability (entry 4–6). Additionally, the more reactive 3,3,3in very poor yields, mainly due to their instability (entry 4–6). Additionally, the more reactive 3,3,3trifluoromethyl pyruvate 8 exclusively the benzofuran-2-one 17 in acceptable amount (61%, trifluoromethyl pyruvate provided exclusively the benzofuran-2-one 17 in acceptable amount (61%, trifluoromethyl pyruvate 8 provided exclusively the benzofuran-2-one 17 in acceptable amount (61%, trifluoromethyl pyruvate 8 provided exclusively the benzofuran-2-one 17 in acceptable amount (61%, in very poor yields, mainly due to their instability (entry 4–6). Additionally, the more reactive 3,3,3in very poor yields, mainly due to their instability (entry 4–6). Additionally, the more reactive 3,3,3trifluoromethyl pyruvate 8 provided exclusively the benzofuran-2-one 17 in acceptable amount (61%, trifluoromethyl pyruvate provided exclusively the benzofuran-2-one 17in inacceptable acceptable amount (61%, in mainly due instability (entry 4–6). the reactive 3,3,3invery verypoor pooryields, yields, mainly duetototheir their instability (entry 4–6).Additionally, Additionally, themore moreamount reactive(61%, 3,3,3trifluoromethyl pyruvate 88888provided exclusively the benzofuran-2-one 17 trifluoromethyl pyruvate provided exclusively the benzofuran-2-one 17 in acceptable amount (61%, trifluoromethyl pyruvate provided exclusively the benzofuran-2-one 17 in acceptable amount (61%, trifluoromethyl pyruvate provided exclusively the benzofuran-2-one 17 in acceptable amount (61%, trifluoromethyl trifluoromethyl pyruvate provided exclusively the benzofuran-2-one 17 in acceptable amount (61%, trifluoromethyl pyruvate provided exclusively the benzofuran-2-one 17 in acceptable amount (61%, trifluoromethylpyruvate pyruvate8888provided providedexclusively exclusivelythe thebenzofuran-2-one benzofuran-2-one17 17in inacceptable acceptableamount amount(61%, (61%,

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entry 9).2018, On the contrary, the same electrophile 8 reacted slowly with hydroxyl phenols 2 and 3, which Molecules 23, 710 4 of 17 furnished the 3-hydroxy lactones 15 (entry 7) and 16 (entry 8) in moderate yields (35% and 28% respectively) and almost no conversion was detected starting from substrates 4–6 (entries 10–12). the ketomalonate 7 was used as alkylating agent; meanwhile substrates owing the second hydroxyl In light of these outcomes, we supposed that the installation of a 4–5, strong interaction between the group in the ortho position (4) or two OH substituents (5 and 6), led to a complex reaction mixture where employed Lewis acid (TiCl4) and the several hydroxyl groups bounded to the substrates should the corresponding products 12 andby 13,prohibiting initially detected at the 1nucleophilic H-NMR, were isolated in alkylating very poor establish various stable complexes any possible attack of the yields, mainly due to their instability (entry 4–6). Additionally, the more reactive 3,3,3-trifluoromethyl agent on aromatic ring. For this reason, we carried out additional investigations in order to optimize pyruvate 8 provided exclusively the benzofuran-2-one 17 inpolyphenols acceptable amount entry 9). On the the domino Friedel–Crafts/lactonization reaction between 1–6 and(61%, 3,3,3-trifluoromethyl contrary, the electrophile reacted slowly with hydroxyl phenols 2 and(hydroquinone) 3, which furnished the pyruvate 8 assame alkylating agent. 8Specifically, we employed p-hydroxyphenol 1 as the 3-hydroxy lactones 15 (entry 7) and 16 (entry 8) in moderate yields (35% and 28% respectively) and model substrate and different reaction conditions. The observed results are summarized in Table 2. almost no conversion was catalytic detected amounts starting from substrates (entries 10–12). Firstly, we examined of two different4–6 Lewis acids, precisely, BF3·Et2O (entries In light of these outcomes, we supposed that the installation of a strong interaction between 1 and 2) and AlCl3 (entry 3), but both of them were neither effective nor efficient, leading to a complex the employed Lewis acid (TiCl ) and the several hydroxyl groups bounded to the substrates 4 reaction mixture without any trace of the target compound. Subsequently, benzoic acid (entriesshould 4 and establish various stable complexes by prohibiting any possible nucleophilic attack of the alkylating 5) and camphorsulfonic acid (CSA) (entries 6 and 7), were tested, varying both their amounts and the agent ontemperature, aromatic ring. For this reason, we additional investigations in order to optimize reaction but unfortunately thecarried startingout materials were recovered almost quantitatively. the domino Friedel–Crafts/lactonization reaction between polyphenols 1–6 and 3,3,3-trifluoromethyl A similar result was achieved using a catalytic amount of either DBU (entry 8) or acetic acid in pyruvate 8 as alkylating Specifically, weany employed 1 as the dichloromethane, where agent. they did not catalyse reactionp-hydroxyphenol and even after 72(hydroquinone) h the starting material model substrate and different reaction conditions. The observed results are summarized in Table 2. was recovered. Table 2. Reaction of 3,3,3-trifluoromethyl pyruvate 8 with polyphenols 1–6 a . Table 2. Reaction of 3,3,3-trifluoromethyl pyruvate 8 with polyphenols 1–6 a.

Entry Sub Catalyst mol% Solvent Temp (°C)◦ Time (h) Product Yield (%) b b Entry Sub Catalyst mol% Solvent Temp ( C) Time (h) Product Yield (%) 15 15 BF·3Et ·Et2OO 30 CH 2Cl2 r.t r.t 48 48 5 5 1 1 11 BF 30 CH 3 2 2 Cl2 1 15 2 BF 3·Et2O 30 THF 60 22 n.d. 2 1 BF3 ·Et2 O 30 THF 60 22 15 n.d. 15 15 30 CH 2ClCl r.t. r.t. 72 72 n.d. n.d. AlCl 3 3 11 AlCl 30 CH 33 22 2 15 15 2H 30 CHCl 3 3 60 60 72 72 n.d. n.d. PhCO 4 4 11 PhCO 30 CHCl 2H 5 5 11 PhCO 100 CHCl 15 15 PhCO 2H 100 CHCl 3 3 60 60 24 24 n.d. n.d. 2H 6 6 11 CSA 30 CHCl 15 15 3 3 60 60 24 24 n.d. n.d. CSA 30 CHCl 7 7 11 CSA 100 Toluene 15 15 CSA 100 Toluene 100100 10 10 n.d. n.d. 8 8 11 DBU 30 CH Cl r.t. 72 15 2 2 15 r.t. 72 n.d. n.d. DBU 30 CH2Cl2 9 1 AcOH 30 CH2 Cl2 r.t. 72 15 n.d. 15 1 r.t. 72 n.d. 9 AcOH 30 CH2Cl2 10 1 AcOH 120 12 15 45 1 15 10 AcOH 120 12 45 11 c 1 AcOH 120 10 15 55 c 15 15 -AcOH 120120 10 4 55 80 11 -AcOH 12 d 11 d 15 16 -AcOH 120120 4 80 72 21 -AcOH 4 13 d 12 d d 2 16 AcOH 120 4 72 86 13 3 AcOH 120 6 17 14 d d 3 17 AcOH 120 6 86 78 14 4 AcOH 120 4 18 15 d d 4 18 AcOH 120 4 78 66 15 5 AcOH 120 5 19 16 d d 5 19 16 AcOH 120 5 66 76 6 AcOH 120 4 20 17 d 6 20 AcOH 120 4 17 a Unless otherwise stated, the reactions were performed under inert atmosphere with the indicated 76 phenols b Yield of the isolated product. a(5.0 mmol) and 3,3,3-trifluoromethyl pyruvate (8; 5.5 mmol) in under 8 mL ofinert solvent; Unless otherwise stated, the reactions were performed atmosphere with the indicated (n.d. = not detected); c 5 mL of AcOH was employed; d 3 mL of AcOH was employed. phenols (5.0 mmol) and 3,3,3-trifluoromethyl pyruvate (8; 5.5 mmol) in 8 mL of solvent; b Yield of the isolated product. (n.d. = not detected); c 5 mL of AcOH was employed; d 3 mL of AcOH was employed.

Firstly, we examined catalytic amounts of two different Lewis acids, precisely, BF3 ·Et2 O (entries 1 and 2) and AlCl 3), observed but both of were neither effectivepreviously nor efficient, leading a complex The best results were bythem exploiting the approach reported bytoDyachenko 3 (entry reaction mixture without any trace of the compound. acid (entries and et al. [41,42], who employed acetic acidtarget as solvent. WithSubsequently, our delight, benzoic benzofuran-2-one 154 was 5) and camphorsulfonic acid (CSA) the (entries 6 and 7),atwere varying both their amountstoand the obtained with a 45% yield keeping temperature 120 tested, °C for 12 h (entry 10). Differently Lewis reactionthe temperature, but of unfortunately starting materials were recovered almost quantitatively. acids, employment acetic acid the does not suffer from the formation of an unreactive A similar result was achieved using athe catalytic amount of either DBUinvolve (entry (see 8) orScheme acetic acid in substrate/catalyst complex. Therefore, mechanistic pathway should SM-1): dichloromethane, where did not catalyse and even after 72 h the starting (i) the initial activation ofthey the alkylating agent 8any byreaction the acidic solvent that promotes (ii) the material Friedel– was recovered.

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The best results were observed by exploiting the approach previously reported by Dyachenko et al. [41,42], who employed acetic acid as solvent. With our delight, benzofuran-2-one 15 was obtained with a 45% yield keeping the temperature at 120 ◦ C for 12 h (entry 10). Differently to Lewis acids, the employment of acetic acid does not suffer from the formation of an unreactive substrate/catalyst complex. Therefore, the mechanistic pathway should involve (see Scheme SM-1): (i) the initial activation of the alkylating agent 8 by the acidic solvent that promotes (ii) the Friedel–Crafts alkylation at the ortho position of substrate 1 to obtain the key intermediate A; (iii) a subsequent acid-assisted intramolecular transesterification to deliver the expected 3-hydroxylactone 15. The subsequent optimization of the applied protocol particularly benefited from a more concentrated reaction mixture and decrease in the reaction time (entries 11 and 12). Finally, the desired lactone 15 was isolated in 80% yield after only 4 h at 120 ◦ C and using 3 mL of acetic acid as solvent. In almost all cases, the desired product was successfully formed in quite good yield (up to 86%). Noteworthy, in phenol 3 containing a hydroxyl group in the meta position, the introduction of an alkyl group in the ortho position considerably enhanced the overall reactivity (86% yield, entry 14), with respect to the corresponding non-methylated phenol 2, which gave the desired lactone 16 with 72% yield (entry 13), as well as the phenol 5, which possesses two additional hydroxyl groups in both the meta positions and reacted even less efficiently furnishing the lactone 19 in 66% yield (entry 16). Moreover, the presence of a catecholic group (entries 15 and 17) did not interfere with the reaction path, as proven by compounds 18 and 20, that were obtained in comparable yields (respectively 78% and 76%) after 4 h. Additionally, hydroxyl phenols 4–6 were tested using AcOH as solvent and diethylketomalonate 7 as alkylating agent at high temperature (120 ◦ C), but unfortunately a complex reaction mixture was obtained and the corresponding products 12–14 were not detected, which confirmed the extreme lability of such compounds. 2.2. Antioxidant Capacity Evaluation In the last decades, there has been an increasing interest in antioxidants, principally in those proposed to prevent the potential injurious effects of free radicals [43]. Therefore, it is very appealing to have convenient and quick methods for estimating the efficacy of a substance as antioxidant. For this purpose, numerous in vitro methods have been developed to evaluate different antioxidant effects [44,45]. Depending upon the reactions involved, these assays can be classified into assays based on hydrogen atom transfer (HAT) reactions, and assays based on electron transfer (ET). The former assays analyse competitive reaction kinetics, quantified through the kinetic curves, while the ET-based assays monitor one redox reaction with the oxidant as an indicator of the reaction endpoint. Both methods are able to measure the radical (or oxidant) scavenging capacity, instead of the preventive antioxidant capacity of a sample [46]. In light of such features, we evaluated the antioxidant capacity of our compounds by DPPH assay and Cyclic Voltammetry analyses. 2.2.1. DPPH Assay The DPPH assaying of compounds 9–11 and 15–20 was performed using Trolox as reference compound. Actually, to achieve more reliable results and to obtain information on the antioxidant activity of these compounds, the assay was carried out both in alcoholic media (methanol, MeOH) as well as in an aprotic solvent (acetonitrile, ACN), i.e., solvents with different abilities in solvating molecules and forming hydrogen bonds. Specifically, to the degassed solutions (2 mL) of antioxidant, with a final concentration from 0 to 60 µM, were added 2 mL of 140 µM DPPH• stock solution (70 µM DPPH final concentration). The solution was stirred at room temperature in the absence of light, and, after 60 min, the absorbance was measured at 517 nm. The working DPPH• concentration was calculated from the absorbance response at 517 nm of the initial solution in the absence of antioxidant and using the calibration curves reported for methanol in Figure 1 and acetonitrile in

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Figure 2, respectively. The DPPH• scavenging percentage or inhibition [I%] was calculated from the measured absorbance using the following expression: Molecules 2018, 23, x FOR PEER REVIEW

I (%) =

( A − A0 ) × 100 A0

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inhibitory in terms of molsantioxidant /molsDPPH•)in was the ŷ-value by where A0 concentration and A represent respectively the absorbance theestimated absence replacing of antioxidant and the 50 in the regression line (see the Supplementary Sometimes it isthe more useful in absorbance at a given antioxidant concentration.Material). The plot of I(%) versus molar ratiotoofdiscuss reagents terms of antiradical power (ARP), that is the inverse of rIC 50 , therefore the higher the ARP value is, (molsantioxidant /molsDPPH• ) presented a linear response (yˆ = ax ± b) until the plateau was reached, the efficient the antioxidant thatmore is, when the inhibition arrivedis. to its maximum [44,47]. The predicted relative IC50 (rIC50 , the half inhibitory concentration in terms of molsantioxidant /molsDPPH• ) was estimated replacing the y-value by ˆ DPPH Assay in Methanol 50 in the regression line (see the Supplementary Material). Sometimes it is more useful to discuss in • for the termsFigure of antiradical that is the inverse versus of rIC50moles , therefore the higher theDPPH ARP value is, 2 shows power the plot(ARP), of inhibition percentage antioxidant/moles the more efficient the antioxidant is. synthesized compounds 9–11 and 15–20.

Figure 2. Regression line of the tested compounds 9–11 and 15–20 in DPPH assay in methanol. Figure 2. Regression line of the tested compounds 9–11 and 15–20 in DPPH assay in methanol.

By comparing the recorded absorbances (Figure 2 and Table 3), remarkably compounds 9, 15, DPPH Assay in Methanol 18, and 20 achieve a ratio molar (molsantioxidant/molsDPPH•) between 0.18 and 0.31, which is quite close • for to theFigure value 2obtained with Trolox (0.23), whereas the other benzofuran-2-ones (10, 11, the 16, shows the plot of inhibition percentage versussynthesised moles antioxidant/moles DPPH • 17, and 19) exhibit a poor9–11 to moderate reactivity towards the DPPH . From the regression lines, the synthesized compounds and 15–20. in comparing terms of mols /mols DPPH• was subsequently as well as the corresponding rIC50 By theantioxidant recorded absorbances (Figure 2 andestimated Table 3), remarkably compounds 9, 15, coefficient determination R2, which validated the robustness of our analyses (Table 18, and 20 of achieve a ratio molar (molsundoubtedly /mols ) between 0.18 and 0.31, which is quite DPPH • antioxidant 3). Additionally, antiradical the stoichiometry, and the number of DPPH• reduced close to the valuethe obtained withpower Trolox(ARP), (0.23), whereas the other synthesised benzofuran-2-ones (10, • were asexhibit reported in Table 3. 11, 16,calculated, 17, and 19) a poor to moderate reactivity towards the DPPH . From the regression lines, the rIC50 in terms of molsantioxidant /molsDPPH • was subsequently estimated as well as the a. Table 3.coefficient Antioxidant of compounds 9–11 and 15–20 towards DPPH• the in methanol corresponding ofcapacity determination R2 , which undoubtedly validated robustness of our analyses (Table 3). Additionally, the antiradical power (ARP), the stoichiometry, and the number of Entry Antioxidant rIC50 (molsantiox.t/molsDPPH•) N° DPPH• Reduced b • DPPH reduced were calculated, as reported in Table 3. 1 Trolox 0.23 2.16 2 3 4 5 6 7 8 9 10

9 10 11 15 16 17 18 19 20

0.31 3.62 2.03 0.22 3.52 1.07 0.24 0.62 0.18

1.63 0.14 0.25 2.28 0.14 0.47 2.07 0.80 2.72

All the measures were performed in triplicate and the values were reported as mean ± SD; b The number of DPPH• molecules, reduced by one molecule of antioxidant, is the inverse of the a

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Table 3. Antioxidant capacity of compounds 9–11 and 15–20 towards DPPH• in methanol a . Entry

Antioxidant

rIC50 (molsantiox.t /molsDPPH · )

N◦ DPPH• Reduced b

1 2 3 4 5 6 7 8 9 10

Trolox 9 10 11 15 16 17 18 19 20

0.23 0.31 3.62 2.03 0.22 3.52 1.07 0.24 0.62 0.18

2.16 1.63 0.14 0.25 2.28 0.14 0.47 2.07 0.80 2.72

a All the measures were performed in triplicate and the values were reported as mean ± SD; • molecules, reduced by one molecule of antioxidant, is the inverse of the stoichiometry. DPPH Molecules 2018, 23, x FOR PEER REVIEW

b

The number of 7 of 17

As depicted depicted above, above, compounds compounds 9, 9, 15, 15, 18, 18, and and 20 20 presented presentedrIC rIC50 50 values from 0.18 to 0.31 (entries As 2, 5, 8, and 10 respectively), representing representing the the best best antioxidants antioxidants of of the the analysed analysed series series in in the employed employed 2, conditions. Specifically, Specifically,compounds compoundswith withaasingle singlephenolic phenolicgroup group(i.e., (i.e.,9, 9,15, 15, and and 18) 18) seemed seemed to to reduce reduce conditions. •, whereas the benzolactone 20, decorated with two hydroxyl • approximately two molecules of DPPH approximately two molecules of DPPH , whereas the benzolactone 20, decorated with two hydroxyl These experimental experimental results results groups on on the the aromatic aromatic ring, ring, reduced reduced almost almost three threemolecules molecules of ofDPPH DPPH••.. These groups highlight the possible involvement of the lactone ring in the reduction mechanism, although no highlight the possible involvement of the lactone ring in the reduction mechanism, although no exhaustive mechanistic study was undertaken. Conversely, the remaining compounds (i.e., 10, 11, 16, exhaustive mechanistic study was undertaken. Conversely, the remaining compounds (i.e., 10, 11, 16, 17, and and 19), 19), bearing bearing aa hydroxyl hydroxyl group group in in meta meta position position (with (with respect respect the the oxygen oxygen of of the the lactone lactone ring), ring), 17, show quite high values of rIC 50 (0.62–3.62) and consequently, a limited antioxidant capacity toward show quite high values of rIC50 (0.62–3.62) and consequently, a limited antioxidant capacity toward DPPH• (entries (entries 3, 4, 6, 6, 7, 7, and and 99 respectively). respectively). In In order order to to further further validate validate these these outcomes, outcomes, the the DPPH DPPH DPPH 3, 4, assay was was also also carried carried out out in in an an aprotic aprotic solvent solvent (acetonitrile), (acetonitrile), and and the the results results were were consequently consequently assay compared to to those those obtained obtained in in alcoholic alcoholic media. media. compared DPPH DPPH Assay Assay in in Acetonitrile Acetonitrile The wasperformed performedasasdescribed described above, using acetonitrile as solvent experimental The assay was above, using acetonitrile as solvent (see(see experimental part part for calibration curve of DPPH ACN, SM).The Theabsorbance absorbancemeasurements measurements of of each solution for calibration curve of DPPH in in ACN, SM). solution containing containing a known known concentration concentration of of antioxidant antioxidant and and DPPH DPPH•• were collected. collected. Figure 3 depicted depicted the the plot /mols for synthesized the synthesized compounds plot of of inhibition inhibition percentage versus molsantioxidant /mols DPPH• for• the compounds 9–119–11 and DPPH antioxidant and 15–20. 15–20.

Figure 3. Regression line of the tested compounds 9–11 and 15–20 in DPPH assay in acetonitrile Figure 3. Regression line of the tested compounds 9–11 and 15–20 in DPPH assay in acetonitrile (ACN). (ACN).

By the comparison of achieved results (Figure 3 and Table 4), only compound 20 shows a ratio molar (molsantioxidant/molsDPPH•) of 0.17, which suggests an even better antioxidant capacity than that of Trolox, whereas the other synthesised benzofuran-2-ones (9–11 and 15–19) exhibit a lower reactivity towards the DPPH•. Table 4 shows the corresponding regression lines with the values of R2

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By the comparison of achieved results (Figure 3 and Table 4), only compound 20 shows a ratio molar (molsantioxidant /molsDPPH• ) of 0.17, which suggests an even better antioxidant capacity than that of Trolox, whereas the other synthesised benzofuran-2-ones (9–11 and 15–19) exhibit a lower reactivity towards the DPPH• . Table 4 shows the corresponding regression lines with the values of R2 as well as the rIC50 and all the deducible parameters, i.e., the ARP, the stoichiometric value, and the number of DPPH• molecules that were reduced. Table 4. Antioxidant capacity of compounds 9–11 and 15–20 towards DPPH• in acetonitrile a .

a

Entry

Antiox.

rIC50 (molsantiox. /molsDPPH · )

N◦ DPPH• Reduced b

1 2 3 4 5 6 7 8 9 10

Trolox 9 10 11 15 16 17 18 19 20

0.22 4.26 4.12 3.92 1.69 4.47 3.25 0.54 2.23 0.17

2.24 0.12 0.12 0.13 0.30 0.11 0.15 0.93 0.22 3.02

All the measures were performed in triplicate and the values were reported as mean ± SD; DPPH• molecules, reduced by one molecule of antioxidant, is the inverse of the stoichiometry.

b

The number of

The results depicted above are surprisingly different from the previous ones recorded in methanol. Indeed, almost all the examined compounds showed an inadequate antioxidant capacity toward DPPH• in acetonitrile, since compounds 9–17 and 19 exhibited rIC50 between 1.69 and 4.47, not reducing any molecules of DPPH• . Conversely, compound 18 exhibited a better antioxidant capacity, although not to the level of Trolox, with a rIC50 of 0.54 and a number of reduced molecules of DPPH• amounting approximately to one. Exceptionally, with our delight, the 3-hydroxy benzofuran-2-one derivative 20, bearing two hydroxyl groups in ortho and meta positions, presented a rIC50 of 0.17, which reduced three molecules of DPPH• , as observed before in methanol. These experimental results highlight the different behavior of compounds in the DPPH assay with respect to the employed solvent. In this regard, the rate constants for hydrogen atom abstraction from compounds 9, 15, 18, and 20 by the DPPH• were determined both in methanol and acetonitrile. Such a kind of analyses should help us to verify if the obtained results could have been invalidated by solvent interference. 2.2.2. Measurement of Rate Constants for the Reaction of Compounds 9, 15, 18, and 20 with DPPH• Several studies on the kinetic solvent effects (KSEs) of DPPH• /phenol reactions established that the large KSEs observed for H-atom abstractions from phenols are mainly a consequence of hydrogen bonding to the solvent, when it is a hydrogen bond acceptor (HBA) [48–50]. Precisely, Ingold et al. [51–53] endorsed that the bimolecular rate constant enhancement (ks PhOH/DPPH• , Scheme 1, reaction a) is due to the partial ionization of the phenol in those solvents that supports an ionization process (Scheme 1, reaction b), especially alcohols, and a very fast electron transfer from the phenoxide anion to the DPPH• (Scheme 1, reaction c), leading to profound kinetic consequences. Additionally, enhanced rate constants were a general feature of PhOH/DPPH• reaction also for phenols with low pKa values, in a non-hydroxylic, polar solvent, such as n-butyl ether, acetonitrile, THF, and DMSO.

[51–53] endorsed that the bimolecular rate constant enhancement (ksPhOH/DPPH•, Scheme 1, reaction a) is due to the partial ionization of the phenol in those solvents that supports an ionization process (Scheme 1, reaction b), especially alcohols, and a very fast electron transfer from the phenoxide anion to the DPPH• (Scheme 1, reaction c), leading to profound kinetic consequences. Additionally, enhanced rate constants were a general feature of PhOH/DPPH• reaction also for phenols with9 of low Molecules 2018, 23, 710 17 pKa values, in a non-hydroxylic, polar solvent, such as n-butyl ether, acetonitrile, THF, and DMSO.

Scheme 1. Reaction kinetic solvent solvent effects. effects. Scheme 1. Reaction involved involved in in the the kinetic

In order to better comprehend the possible kinetic solvent effects in the performed DPPH assays In order to better comprehend the possible kinetic solvent effects in the performed DPPH assays of compounds 9, 15, 18, and 20, the rate constant measurement for the reactions of these compounds of compounds 9, 15, 18, and 20, the rate constant measurement for the reactions of these compounds with DPPH• was carried out. Practically, the decay of DPPH• absorbance in the presence of a known with DPPH• was carried out. Practically, the decay of DPPH• absorbance in the presence of a known concentration of phenols was followed at 517 nm and analysed as a pseudo-first-order process to concentration −1of phenols was followed at 517 nm and analysed as a pseudo-first-order process to yield yield the kex/s . Because of this, the antioxidants were always used in large excess (final concentration the kex /s−1 . Because of this, the antioxidants were always used in large excess (final concentration 1–6 mM) over DPPH•• (final concentration 85 μM). Afterwards, the second-order rate constants 1–6 mM) over DPPH (final concentration 85 µM). Afterwards, the second-order rate constants (ksPhOH/DPPH•) of the synthesised compounds were evaluated by plotting the pseudo-first-order (ks PhOH/DPPH • ) of the synthesised compounds were evaluated by plotting the pseudo-first-order constant (kex) versus antioxidant concentration: this plot is linear and its slope gives the bimolecular constant (kex ) versus antioxidant concentration: this plot is linear and its slope gives thes bimolecular constants (see Supporting Material Table SM-5 and Table SM-6). Table 5 shows the k of analysed constants (see Supporting Material Table SM-5 and Table SM-6). Table 5 shows the ks of analysed compounds, as well as the corresponding rIC50 are reported again for a more explicit comparison. compounds, as well as the corresponding rIC50 are reported again for a more explicit comparison. Table 5. Bimolecular rate constants (M−1 ·s−1 ) for H-atom abstraction from compounds 9, 15, 18, and 20 by DPPH• a . Entry

Antiox.

1 2 3 4

9 15 18 20

ks (M−1 ·s−1 ) b

rIC50 (molsantioxidant /molsDPPH• )

ks (M−1 ·s−1 ) b

rIC50 (molsantioxidant /molsDPPH• )

MeOH

MeOH

ACN

ACN

3.26 3.77 1.40 0.77

0.31 0.22 0.24 0.18

8.54 1.26 0.23 2.22

4.26 1.69 0.54 0.17

a

All the measurements were performed in triplicate and processed using a Sigma Plot software, 12.0, Systat software, Inc., San Jose, CA, USA); b The second-order rate constants were the slopes of the plots of kes vs. compound concentration (See Supplementary Material).

As shown above, compound 9 (entry 1) gave a high value of ks in acetonitrile, although the corresponding rIC50 revealed a moderate antioxidant capacity. Moreover, even though the kinetic constant is lower in methanol than in acetonitrile, the significant value of ks suggests a partial ionization of the phenol and slight interference by the employed solvent. Instead, compounds 15 and 18 (entries 2 and 3) display values of ks quite in agreement with the corresponding rIC50 : in methanol, a partial ionization of the phenols altered the results obtained with DPPH assay in methanol (rIC50 0.22 and 0.24, respectively), while in acetonitrile, the value of rIC50 was reliable. Finally, the ks values of compound 20 (entry 4), which presented a similar antioxidant capacity towards DPPH• in both solvents, were mildly in disagreement, in fact in acetonitrile the ks was higher than in methanol. Consequently, the obtained result in the DPPH assay, considering the reaction kinetics and possible solvent effects, were reassessed and benzofuran-2-one 20 presented a real quite good antioxidant capacity toward DPPH• . 2.2.3. Cyclic Voltammetry Cyclic voltammetry (CV) is an electrochemical technique frequently used (often along with DPPH assay) for the determination of the antioxidant activity of target compounds [54,55]. It consists of a potential scanning from a starting value to a final one, and then returning to the initial potential, while registering the current flowing between two electrodes. The potential at which an increase of current is registered corresponds to the oxidation (or reduction) of a species in solution. Thus, the oxidation potential measured by cyclic voltammetry can be taken as a measure of the ease of the oxidation

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process. Specifically, low oxidation potentials are associated with a greater facility of a given molecule for electro-donation and thus to act as antioxidant [54,56–58]. Moreover, CV can be carried out in aqueous medium as well as in organic solvents, e.g., in the presence of a supporting electrolytes to ensure the electrical conductivity of the solution [58]. The redox chemistry of the synthesized compounds 9–11 and 15–20 was evaluated using CV, recording the voltammograms in aqueous medium (H2 O) as well as in an organic solvent (acetonitrile) and comparing the first oxidation peak potential to that of Trolox as a reference compound. Table 6 summarizes the observed results. Table 6. First oxidation peaks (Ep ox ) from CV in aqueous medium and acetonitrile relative to compounds 9–11 and 15–20 (Figures 3 and 4). Entry

Compounds

Ep ox (V) 1 (H2 O)

Ep ox (V) 1 (ACN)

1 2 3 4 5 6 7 8 9 10

Trolox 9 10 11 15 16 17 18 19 20

0.52 0.72 1.13 1.11 0.62 1.01 1.05 0.73 1.03 0.85

1.08 1.62 1.44 1.65 1.72 1.92 1.77 0.92 1.88 1.81

1

All the peak potentials are referred to SCE.

Cyclic Voltammetry in Aqueous Medium of Compounds 9–11 and 15–20 The CV in aqueous medium was performed in a cell containing a three-electrode system: (i) the reference electrode (Saturated Calomel Electrode, SCE); (ii) the working electrode (Glassy Carbon, GC); and (iii) a counter electrode (platinum wire). Each antioxidant (2 × 10−3 M final concentration), dissolved in EtOH, was added to 40 mL of a solution of water, containing 0.5 M NaCl, as supporting electrolyte, and then the voltammogram was recorded. As Figure 4 and Table 6 highlight, all the considered compounds showed a similar voltammetric behavior, i.e., an irreversible anodic oxidation whose Ep ox (oxidation peak potential) is between +0.62 and +1.13 V (vs SCE). All Ep ox of the considered products are of higher potential than that of the reference compound, Trolox (Ep ox 0.52 V vs. SCE), which corresponds to a minor easing of the oxidation process. Nevertheless, among these, products 9, 15, and 18 exhibited the lowest values of potential, which, being under 1V (Ep ox 0.72, 0.62, and 0.73 respectively) and quite close to the Trolox one, is indicative of antioxidant capacity. These results suggested that in these electrochemical measurements, the fundamental structural features necessary to increase the antioxidant capacity are the phenolic groups in positions 5 and 7 (in compounds 9, 15, and 18, respectively). Moreover, the presence of a CF3 group seems essential for the ease in the oxidation process, lowering Ep ox by 60–120 mV (compare compound 9 vs. 15, 10 vs. 16 and 11 vs. 17 in Table 6). Cyclic Voltammetry in Acetonitrile of Compounds 9–11 and 15–20 The CV in acetonitrile was performed in a cell containing a three-electrode system involving: (i) the reference electrode (modified SCE electrode, −0.029 V vs. SCE electrode− i.e., containing an organic junction); (ii) the working electrode (Glassy Carbon, GC); and (iii) a counter electrode (platinum wire). Each antioxidant (2 × 10−3 M final concentration), dissolved in ACN, was added to 5 mL of a solution of ACN, containing 0.1 M TEABF4 (tetraethylammonium tetrafluoroborate), as supporting electrolyte. The corresponding results are depicted in Figure 5 and Table 6.

potential, which, being under 1V (Epox 0.72, 0.62, and 0.73 respectively) and quite close to the Trolox one, is indicative of antioxidant capacity. These results suggested that in these electrochemical measurements, the fundamental structural features necessary to increase the antioxidant capacity are the phenolic groups in positions 5 and 7 (in compounds 9, 15, and 18, respectively). Moreover, the presence2018, of a23,CF 3 group seems essential for the ease in the oxidation process, lowering Epox by 60–120 Molecules 710 11 of 17 mV (compare compound 9 vs. 15, 10 vs. 16 and 11 vs. 17 in Table 6).

Figure 4. 4. Voltammetric Voltammetric curves curves of of compounds compounds 9–11 9–11and and15–20 15–20(c(c==2 2××10 10−−33 M) in aqueous aqueous medium medium Figure M) in (H 2 O-0.5 M NaCl). Starting potential: −0.5 V; reversal potential: +1.5 V (GC working electrode, ν = 0.211 of 17 Molecules 2018, 23, x FOR PEER REVIEW (H2 O-0.5 M NaCl). Starting potential: −0.5 V; reversal potential: +1.5 V (GC working electrode, −1, SCE reference − 1 ◦ electrode, 25 °C, N 2 ). V·s ν = 0.2 V·s , SCE reference electrode, 25 C, N2 ).

Cyclic Voltammetry in Acetonitrile of Compounds 9–11 and 15–20 The CV in acetonitrile was performed in a cell containing a three-electrode system involving: (i) the reference electrode (modified SCE electrode, −0.029 V vs. SCE electrode− i.e., containing an organic junction); (ii) the working electrode (Glassy Carbon, GC); and (iii) a counter electrode (platinum wire). Each antioxidant (2 × 10−3 M final concentration), dissolved in ACN, was added to 5 mL of a solution of ACN, containing 0.1 M TEABF4 (tetraethylammonium tetrafluoroborate), as supporting electrolyte. The corresponding results are depicted in Figure 5 and Table 6.

−3−3 M) Figure5.5. Voltammetric Voltammetric curves curvesofofcompounds compounds9–11 9–11and and15–20 15–20ininacetonitrile acetonitrile(c (c= =2 × 2 ×1010 M) in in Figure acetonitrile(MeCN-0.1 (MeCN-0.1MMEtEt 4NBF4). Starting potential: −0.5 V; reversal potential: +1.5 V (GC working acetonitrile 4 NBF4 ). Starting potential: −0.5 V; reversal potential: +1.5 V (GC working −11, SCE reference electrode, 25 °C, − electrode, ν = 0.2 V·s electrode, ν = 0.2 V·s , SCE reference electrode, 25 ◦ C,N N22).).

Asdisplayed displayedin in Table 6, electrochemical the electrochemical obtained CV in acetonitrile were As Table 6, the resultsresults obtained from CVfrom in acetonitrile were relatively relatively in accord with those observedmedium. in aqueous medium.compounds Specifically,9 and compounds 9 and 10 in accord with those observed in aqueous Specifically, 10 showed lower showed lower potential (Epox 1.62 and 1.44, respectively), although they were higher than the value of Trolox (Epox 1.08 V). Remarkably, compound 18 exhibited the first potential peak at 0.92 V, lower than Trolox. This result showed that an essential structural feature which makes the 3-hydroxy3Hbenzofuranone scaffold a possible antioxidant compound is the presence of phenolic group in ortho position and a CF3 group.

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potential (Ep ox 1.62 and 1.44, respectively), although they were higher than the value of Trolox (Ep ox 1.08 V). Remarkably, compound 18 exhibited the first potential peak at 0.92 V, lower than Trolox. This result showed that an essential structural feature which makes the 3-hydroxy-3Hbenzofuranone scaffold a possible antioxidant compound is the presence of phenolic group in ortho position and a CF3 group. 3. Materials and Methods Solvents and common reagents were purchased from a commercial source and used without further purification. All reactions were monitored by thin-layer chromatography (TLC) carried out on Merck F-254 silica glass plates and visualized with UV light or by 5% phosphomolibdic acid/ethanol test. Flash chromatography was performed on Sigma-Aldrich silica gel (60, particle size: 0.040–0.063 mm). 1 H-NMR and 13 C-NMR were recorded in CDCl3 (99.8% in deuterium) using a Varian Gemini 300 spectrometer (300 MHz, Varian Inc., Palo Alto, CA, USA). All chemical shifts are expressed in parts per million (δ scale) and are referenced to the residual protons of the NMR solvent (CDCl3 , δ 7.24 ppm). Coupling constant (J) was expressed in Hz. Infrared spectra (FT-IR) were obtained using a Bruker Vector 22 spectrometer (Bruker, Billerica, MA, USA); data are presented as the frequency of absorption (cm−1 ). High-resolution mass spectrometry (HRMS) spectra were recorded with Micromass Q-TOF micro mass spectrometer (Waters Corporations, Milford, MA, USA) and Micromass LCT (ESI, Waters Corporations, Milford, MA, USA) with Lock-Spray-Injector (Injection Loop-Modus in a HPLC system, Waters, Alliance 2695, Waters Corporations, Milford, MA, USA). UV-Vis measurements were performed with a Shimadzu-UV-2401PC spectrophotometer. Cyclic Voltammetry measurements were acquired on a AMEL552 electrochemical workstation. The standard three-electrode arrangement was employed. In all cases, a Pt wire auxiliary electrode was used, the working electrode was a 3 mm diameter glassy carbon, and the solution was degassed with N2 . Melting points were determined on a Mel-Temp apparatus. 3.1. General Procedure for the Lewis-Acid-Catalysed Friedel–Crafts/Lactonization Reaction The alkylating agent (2.2 mmol) was added in one portion to a stirred solution of the appropriate phenol (2.0 mmol) in anhydrous CHCl3 (9 mL), and then TiCl4 (1 M in anhydrous CH2 Cl2 ; 0.4 mL, 10 mol-%) was added. The system was kept under an argon atmosphere. The clear reddish solution was stirred at the reported temperature until the substrate had been completely consumed (TLC monitoring). Afterwards, the reaction mixture was poured into cold water (18 mL), and the aqueous phase was extracted several times with EtOAc (4 × 20 mL). The combined organic layers were washed with brine, dried with anhydrous Na2 SO4 , and concentrated under vacuum. The residue was purified by flash chromatography on silica gel to give the corresponding products as described below. 3.2. Characterization Data for Benzofuran 9–11 Ethyl 3,5-Dihydroxy-2-oxo-2,3-Dihydrobenzofuran-3-Carboxylate, 9. Following the general procedure, the single product 9 was obtained as a white solid in 70% yield after purification by flash chromatography on silica gel (nHexane/EtOAc = 7/3). m.p. 139–142 ◦ C. IR (CHCl3 ): νe = 3468–3412, 3018, 2979, 2914, 1759, 1725, 1608 cm−1 . 1 H-NMR (CDCl3 , 300 MHz, 25 ◦ C): δ (ppm) = 8.59 (bs, 1H, OHphen ), 7.08 (d, J = 8.6 Hz, 1H, CHarom ), 6.98–6.88 (m, 2H, CHarom ), 6.50 (bs, 1H, OH), 4.30–4.15 (m, 2H, CH2 CH3 ), 1.16 (t, J = 7.1 Hz, 3H, CH2 CH3 ). 13 C-NMR (CDCl3 , 75 MHz, 25 ◦ C): δ (ppm) 173.2, 168.5, 155.4, 147.7, 128.2, 118.6, 112.5, 111.7, 77.9, 63.3, 14.0. HRMS: exact mass calculated for (C11 H10 NaO6 ) requires m/z 261.0370, found m/z 261.0371. Ethyl 3,6-Dihydroxy-2-oxo-2,3-Dihydrobenzofuran-3-Carboxylate, 10. Following the general procedure, the single product 10 was obtained as a white solid in 62% yield after purification by flash chromatography on silica gel (nHexane/EtOAc = 4/6). m.p. 142–144 ◦ C. IR (CHCl3 ): e ν = 3468–3420, 3010, 2972, 2921, 1760, − 1 1 ◦ 1727, 1615 cm . H-NMR (CDCl3 , 300 MHz, 25 C): δ (ppm) = 9.17 (bs, 1H, OHphen ), 7.25 (d, J = 7.9 Hz, 1H, CHarom ), 6.72 (m, 2H, CHarom ), 6.37 (bs, 1H, OH), 4.29–4.08 (m, 2H, CH2 CH3 ), 1.15 (t, J = 7.1 Hz, 3H,

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CH2 CH3 ). 13 C-NMR (CDCl3 , 75 MHz, 25 ◦ C): δ (ppm) 173.2, 168.7, 161.1, 155.9, 126.0, 118.0, 112.5, 99.5, 77.2, 63.0, 14.0. HRMS: exact mass calculated for (C11 H10 NaO6 ) requires m/z 261.0370, found m/z 261.0372. Ethyl 3,6-Dihydroxy-7-Methyl-2-oxo-2,3-Dihydrobenzofuran-3-Carboxylate, 11. Following the general procedure, the single product 11 was obtained as a white solid in 81% yield after purification by flash chromatography on silica gel (nHexane/EtOAc = 4/6). m.p. 126–128 ◦ C. IR (CHCl3 ): νe = 3460–3420, 3262, 3005, 2970, 2919, 1801, 1730, 1629 cm−1 . 1 H-NMR (CDCl3 , 300 MHz, 25 ◦ C): δ (ppm) = 8.32 (bs, 1H, OHphen ), 7.02 (d, J = 7,9 Hz, 1H, CHarom ), 6.61 (m, 2H, CHarom ), 5.98 (bs, 1H, OH), 4.41–3.99 (m, 2H, CH2 CH3 ), 2.15 (s, 3H, CCH3 ), 1.15 (t, J = 7.1 Hz, 3H, CH2 CH3 ). 13 C-NMR (CDCl3 , 75 MHz, 25 ◦ C): δ (ppm) 174.3, 169.7, 159.6, 154.9, 122.7, 117.8, 111.6, 109, 78.3, 63.5, 14.1, 9.6. HRMS: exact mass calculated for (C12 H12 NaO6 ) requires m/z 275.0526, found m/z 275.052. 3.3. General Procedure for the Domino Friedel-Crafts/Lactonization Reaction Performed in AcOH The alkylating agent (5.5 mmol) was added in one portion to a stirred solution of the appropriate phenol (5.0 mmol) in acetic acid (3 mL). The system was kept under an argon atmosphere. The clear reddish solution was stirred at reflux temperature until the substrate had been completely consumed (TLC and HPLC monitoring). Afterwards, the reaction mixture was concentrated under vacuum and the residue was purified by flash chromatography on silica gel to give the products as described below. 3.4. Characterization Data for Benzofuran-2(3H)-One 15–20 3,5-Dihydroxy-3-(trifluoromethyl)benzofuran-2(3H)-One, 15. Following the general procedure, the single product 15 was obtained as a white solid in 35% yield after purification by flash chromatography on silica gel (nHexane/Acetone = 8/2). m.p. 136–138 ◦ C. IR (CHCl3 ): νe = 3460–3190, 3005, 2970, 2919, 1801, 1730, 1629, 1498 cm−1 . 1 H-NMR (CDCl3 , 300 MHz, 25 ◦ C): δ (ppm) = 8.72 (bs, 1H, OHphen ), 7.24–6.95 (m, 4H, CHarom + OH). 13 C-NMR (CDCl3 , 75 MHz, 25 ◦ C): δ (ppm) 168.7, 153.8, 145.7, 121.8, 121.6 (q, 1 JCF = 283.3 Hz), 118.1, 111.4, 111.0, 74.0 (q, 2 JCF = 32.5 Hz). HRMS: exact mass calculated for (C9 H5 NaF3 O4 ) requires m/z 257.0032, found m/z 257.0033. 3,6-Dihydroxy-3-(trifluoromethyl)benzofuran-2(3H)-One, 16. Following the general procedure, the single product 16 was obtained as a white solid in 28% yield after purification by flash chromatography on silica gel (nHexane/Et2 O = 1/1). m.p. 138–140 ◦ C. IR (CHCl3 ): νe = 3490–3230, 3010, 2970, 2923, 1806, 1735, 1637 cm−1 . 1 H-NMR (CDCl3 , 300 MHz, 25 ◦ C): δ (ppm) = 9.40 (bs, 1H, OHphen ), 7.45 (d, J = 9 Hz, 2H, CHarom ), 7.02 (bs, 1H, OH), 6.80 (d, J = 9 Hz, 1H, CHarom ), 6.74 (s, 1H, CHarom ). 13 C-NMR (CDCl3 , 75 MHz, 25 ◦ C): δ (ppm) 170.5, 162.1, 155.9, 127.7, 123.6 (q, 1 JCF = 283.9 Hz), 113.2, 113.0, 99.5, 75.2 (q, 2 JCF = 32.8 Hz). HRMS: exact mass calculated for (C9 H5 NaF3 O4 ) requires m/z 257.0032, found m/z 257.0031. 3,6-Dihydroxy-7-methyl-3-(trifluoromethyl)benzofuran-2(3H)-One, 17. Following the general procedure, the single product 17 was obtained as a white solid in 61% yield after purification by flash chromatography on silica gel (nHexane/Et2 O=1/1). m.p. 129–131 ◦ C. IR (CHCl3 ): νe = 3470–3220, 3000, 2975, 2913, 1826, 1724, 1630 cm−1 . 1 H-NMR (CDCl3 , 300 MHz, 25 ◦ C): δ (ppm) = 7.32 (bs, 1H, OHphen ), 7.08 (d, J = 8.4 Hz, 1H, CHarom ), 6.58 (d, J = 8.3 Hz, 1H, CHarom ), 4.78 (bs, 1H, OH), 2.22 (s, 3H, CCH3 ). 13 C-NMR (CDCl3 , 75 MHz, 25 ◦ C): δ (ppm) 170.5, 159.3, 153.4, 123.2, 122.8 (q, 1 JCF = 283.8 Hz), 112.1, 111.6, 108.6, 75.2 (q, 2 JCF = 32.8 Hz), 8.12. HRMS: exact mass calculated for (C10 H7 NaF3 O4 ) requires m/z 271.0189, found m/z 271.0190. 3,7-Dihydroxy-3-(trifluoromethyl)benzofuran-2(3H)-One, 18. Following the general procedure, the single product 18 was obtained as a white solid in 78% yield after purification by flash chromatography on silica gel (nHexane/Et2 O = 1/1). m.p. 142–144 ◦ C. IR (CHCl3 ): νe = 3498–3256, 3012, 2988, 2909, 1831, 1744, 1629 cm−1 . 1 H-NMR (CDCl3 , 300 MHz, 25 ◦ C): δ (ppm) = 9.20 (bs, 1H, OHfen ), 7.23–7.07 (m, 4H, CHarom + OH).13 C-NMR (CDCl3 , 75 MHz, 25 ◦ C): δ (ppm) 169.2, 141.6, 125.9, 123.6, 122.8 (q, 1 JCF = 284.1 Hz), 120.3, 119.8, 116.4, 75.0 (q, 2 JCF = 32.6 Hz). HRMS: exact mass calculated for (C9 H5 NaF3 O4 ) requires m/z 257.0032, found m/z 257.0032.

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3,4,6-Trihydroxy-3-(trifluoromethyl)benzofuran-2(3H)-One, 19. Following the general procedure, the single product 19 was obtained as a white solid in 66% yield after purification by flash chromatography on silica gel (nHexane/Acetone=7/3). m.p. 174–176 ◦ C. IR (CHCl3 ): νe = 3500–3280, 3023, 3000, 2959, 1821, 1754, 1607 cm−1 . 1 H-NMR (CDCl3 , 300 MHz, 25 ◦ C): δ (ppm) = 9.30 (bs, 1H, OHfen ), 9.22 (bs, 1H, OHphen ), 6.71 (bs, 1H, OHfen ), 6.28 (s, 1H, CHarom ), 6.23 (s, 1H, CHarom ).13 C-NMR (CDCl3 , 75 MHz, 25 ◦ C): δ (ppm) 167.3, 162.0, 159.4, 158.0, 123.2 (q, 1 JCF = 285.7 Hz), 122.7, 98.4, 95.6, 75.4 (q, 2 JCF = 33.6 Hz). HRMS: exact mass calculated for (C9 H5 NaF3 O5 ) requires m/z 272.9981, found m/z 272.9983. 3,6,7-Trihydroxy-3-(trifluoromethyl)benzofuran-2(3H)-One, 20. Following the general procedure, the single product 20 was obtained as a white solid in 76% yield after purification by flash chromatography on silica gel (nHexane/Et2 O = 3/7). m.p. 148–150 ◦ C. IR (CHCl3 ): νe = 3517–3239, 3045, 3018, 2960, 1819, 1734, 1615 cm−1 . 1 H-NMR (CDCl3 , 300 MHz, 25 ◦ C): δ (ppm) = 9.03 (bs, 1H, OHphen ), 8.55 (bs, 1H, OHphen ), 6.95 (d, J = 8.4 Hz, 1H, CHarom ), 6.81 (d, J = 8.4 Hz, 1H, CHarom ), 6.65 (bs, 1H, OH).13 C-NMR (CDCl3 , 75 MHz, 25 ◦ C): δ (ppm) 169.7, 149.6, 142.0, 130.0, 122.9 (q, 1 JCF = 284.0 Hz), 116.6, 113.8, 111.9, 75.0 (q, 2 JCF = 35.1 Hz). HRMS: exact mass calculated for (C9 H5 NaF3 O5 ) requires m/z 272.9981, found m/z 272.9980. 4. Conclusions In summary, we have synthesized a series of 3,3-disubstituted-3H-benzofuran-2-one derivatives (9–11 and 15–20) by improving our previous findings concerning the domino Friedel–Crafts/lactonization reaction. With respect the chosen electrophile as well as the various polyphenols (1–6) used as nucleophilic counterpart, two different protocols were followed: (i) the use of TiCl4 as catalyst in CHCl3 at 60 ◦ C; and (ii) the employment of AcOH as solvent at 120 ◦ C without the addition of any further catalyst. Afterwards, the antioxidant capacity of the synthesized compounds (9–11 and 15–20) was evaluated using DPPH assay and Cyclic Voltammetry, performing the experiments in both MeOH and acetonitrile. Specifically, the benzofuran-2-ones 9, 15, 18, and 20 presented the best values of rIC50 . Among them, compound 20 exhibited a remarkable rIC50 of 0.18 (MeOH) and 0.17 (ACN), and therefore possessed the greatest antioxidant activity by reducing three molecules of DPPH• in both solvents, a result that is validated despite the possible interference of the solvent which was evaluated and excluded almost completely by measuring the rate constants for the reaction with DPPH• . Additionally, the electrochemical measurements (CV), recorded in in aqueous medium as well as in acetonitrile, established compounds 18 and 20 as the best reducing agents among the tested molecules. The observed results undeniably suggest that the 3-hydroxy-benzofuran-2-one scaffold should be involved in the antioxidant mechanism: the presence of a hydroxyl group at C-7 position as well as a strong electron withdrawing group at C-3 position are the essential structural features. Detailed additional in vitro tests to further endorse the notable and promising antioxidant activity of the studied compounds are ongoing in our laboratories and will be reported in due course. Supplementary Materials: The following are available online at http://www.mdpi.com/1420-3049/23/4/710/s1. Experimental procedures, characterization data of synthesized compounds, explanation of the reaction mechanism, and the complete set of antioxidant assay are reported in the Supplementary Material. Acknowledgments: The authors gratefully acknowledge Dipartimento di Scienze and Sezione di Nanoscienze e Nanotecnologie (Università di Roma Tre, Roma, Italy) for the financial support. Author Contributions: Tecla Gasperi and Martina Miceli conceived and designed the experiments; Martina Miceli and Elia Roma performed the experiments; Martina Miceli, Paolo Rosa, M. Antonietta Loreto, Daniela Tofani, and Marta Feroci analyzed the data; Tecla Gasperi and Marta Feroci contributed reagents/materials/analysis tools; Tecla Gasperi, Martina Miceli, Marta Feroci, and Paolo Rosa wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

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Sample Availability: Samples of the compounds 9–11 and 15–20 are available from the authors. © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).