K. Ajay Kumar*, N. Renuka, G. Vasanth Kumar and D. M. Lokeshwari ..... A. Venkatesham; N.K. Bejjanki; N. Kommu; S. Pombala; C. Ganesh Kumar; T.P. Rao; ...
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Journal of Chemical and Pharmaceutical Research, 2015, 7(11):693-700
Review Article
ISSN : 0975-7384 CODEN(USA) : JCPRC5
Pyrans: Heterocycles of chemical and biological interest K. Ajay Kumar*, N. Renuka, G. Vasanth Kumar and D. M. Lokeshwari Post Graduate Department of Chemistry, Yuvaraja College, University of Mysore, India _____________________________________________________________________________________________ ABSTRACT Six-membered heterocyclic compounds containing oxygen such as 2H-pyran and 4H-pyrans constitute an important class of biologically active natural and synthetic products, playing a fundamental role in bioorganic chemistry and continuing to attract interest. Pyrans and their analogues occupy prime position in Bioorganic chemistry due to their diverse applications. In this review, up to date information about the developments, exploration of new methodologies and varied biological activities of pyran analogues was discussed. Key words: Antimicrobial, anticancer, microwave, multicomponent, sonochemistry. _____________________________________________________________________________________________ INTRODUCTION Pyran derivatives constitute a useful class of heterocyclic compounds, which are widely distributed in nature [1]. The fused pyran ring skeleton is a well-known heterocycle and important core unit in a number of natural products. Pyran and fused pyran derivatives have attarcted a great deal of interest due to their association with various kinds of biological properties. Substituted benzo(b)pyran derivatives synthesized were reported to exhibit anticancer activities against three human cell lines even at very low concentrations [2]. A number of 2-amino-4H-pyrans are used as photoactive materials [3] pigments [4] and potentially biodegradable agrochemicals [5]. Naphthopyrans are class of photochromic compounds and the molecules have the ability to generate a yellow colour on being irradiated with UV light (van). Pyranochalcones have been reported to exhibit antimutagenic, antimicrobial, antiulcer and antitumor activities [6]. Pyrans and fused pyran derivatives have attracted a great deal of interest due to their association with various kinds of biological properties. The pyran heterocycles embedded with other heterocyclic moiety either in the form of a substituent or as a fused component changes its properties and converts it into a novel heterocyclic derivatives. Pyranopyrazoles were first obtained in 1973 by reaction between 3-methyl-1-phenylpyrazolin-5-one and tetracyanoethylene. After this Otto had proposed the synthesis of the dihydropyrano[2,3-c]pyrazoles in 1974 via the [7] base catalysed cycloaddition of 4-aryliden-5-pyrazolone [8]. Pyrano[2,3-c]pyrazoles were evaluated for their bovine brain adenosine A1 A2A receptor binding affinity and also they are of much interest because of their structural similarity with the flavones and flavanones that exhibit interesting biological activity [9]. SYNTHESIS OF PYRANS Tetrahydropyrans were synthesized by the readily available α-ketoglutaric acid as a precursor. Firstly α-ketoglutaric acid is converted into ketal ester using trimethylorthoformate in methanol in the presence of sulphuric acid. Ketal ester was then reduced to diol using LiAlH4 in THF. Diol undergo mesylation with sodium hydride in absolute THF obtained the tetrahydropyrans [10]. An organocatalytic construction of optically enriched substituted pyran derivatives via amine-catalyzed Michael addition and subsequent enolization/cyclisation was achieved starting from electronically poor alkenes. The report reveals that the functionalized pyrans were obtained in high enantioselectivities (96%) and good yields (90%) having three contiguous chiral centers (Scheme-1) [11].
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Ethyl 3-aryl-4-oxo-3,3a,4,6-tetrahydro-1H-furo[3,4-c]pyran-3a-carboxylates were prepared through the metalcatalyzed domino reaction of alkylidene malonates and 1,4-butynediol under a one pot reaction condition at room temperature. Their in vitro anti-proliferative activity results showed that the most of the compounds possess potent anti-tumor activity against HeLa cells [12]. A series of novel 2-amino-4H-pyran derivatives were synthesized in excellent yields via a three-component reaction of α,β-unsaturated ketones, malononitrile and aldehydes using DBU as a catalyst in ethanol as a cheap, safe, and environmentally benign solvent under mild conditions. Their antitumor activity was evaluated in three human tumor cell lines sucha as human colon cancer (HCT116), human cervical cancer (Hela), and non-small cell lung cancer (H1975) (Scheme-2) [13].
Nilesh and co-workers [14] reported the synthesis of 6-amino-4-(3-aryl-1-phenyl-1H-pyrazol-4-yl)-3-methyl-2,4dihydropyrano[2,3-c]pyrazole-5-carbonitrile from 3-aryl-1-phenylpyrazole-4-carbaldehyde with malononitrile and 3-methyl-2-pyrazolin-5-one in the presence of ethanol and piperidine under water bath reflux conditions (Scheme3). 3-aryl-1-phenylpyrazole-4-carbaldehyde was in turn prepared by the reaction of substituted hydrazones with Vilsmeier Haack reagent.
The one pot reaction of estrone with the aromatic aldehydes and either of malononitrile or ethyl cyanoacetate afforded the fused pyran derivatives. The cytotoxicity of the these synthesized products was evaluated against six human cancer and normal cell lines where the results showed that two of the synthesized compounds exhibited optimal cytotoxic effect against the cancer cell lines, with IC50’s in the nM range [15]. Pyrano[2,3-c]pyrazoles derivatives were synthesized by the condensation reaction of aryl aldehydes , ethylacetoacetate, malononitrile and phenyl hydrazines in the presence of sodium benzoate under aqueous condition. It is safe, mild, green and environmental friendly reaction with shorter time and high yield (Scheme-4)[16].
A novel sugar fused diaryl tetrahydropyrans are synthesized via Prins Friedel-Crafts cyclization reaction by coupling m-chlorobenzaldehyde with a homoallylic alcohol derived from D-glucose in benzene using a catalytic amount of BF3.OEt2, the reaction preceded smoothly at 25 °C to yield product in 72% yield [17]. An expedient, eco-friendly and efficient procedure for the preparation of novel pyran derivatives was developed through a solvent-free, one-pot reaction of various aldehydes, malononitrile and either methylacetoacetate or ethyl benzoylacetate in the presence of dibutylamine (2.5 mol %) at room temperature. This procedure is advantageous because the product did not necessitate separation via extraction and column chromatography (Scheme-5)[18].
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The DABCO-catalyzed reaction of propargyl alcohols with dialkyl acetylene dicarboxylates and N-bromo-/Niodosuccinimides under mild conditions has been developed. The reactions give 3-bromo-/3-iodo-2H-pyrans up to 98% yield [19]. Changsheng Yao and co-workers synthesized a series of 4-aryl-cyclopenta[b]pyran derivatives via multi-component reaction under solvent-free and catalyst-free conditions (Scheme-6) [20].
The environmentally benign ZnFe2O4 nanopowder catalyst provides both acidic (Fe3+) and basic functionalities (O2−) as the reaction requires was applied in the one-pot, three-component synthesis of 4H-pyrans (1) in water from 2-napthol, 4-nitrobenzaldehyde and 1H-indene-1,3(2H)-dione [21]. The method exploited water as both as a reaction medium as well as a medium for synthesis of the catalyst and puts forward an application of 4H-pyrans. Diethyl 2,6dimethyl-4-aryl-4H-pyran-3,5-dicarboxylates (2) were synthesized by the reaction of aryl aldehyde and 1,3-diketone catalyzed by ZnCl2 under ultrasound irradiation [22]. Compared with the conventional thermal methods, the method offered remarkable advantages such as simple experimental procedure, shorter reaction time and high yield of product.
An efficient, convenient method for the synthesis of 3,4-dihydropyrano[3,2-c]chromene derivatives by one-pot, three-componentreaction of aldehydes, malononitrile/cyanoacetate, and 4-hydroxycoumarin in the presence of a catalytic amount of thiourea dioxide as efficient, reusable organic catalyst. The salient features of the protocol are mild reaction conditions, high yields, short reaction time, safety, high atom-economy, eco-friendly standards, easy isolation of products and reusability of the catalyst (Scheme-7) [23].
In general, it was observed that ultrasound irradiations sometimes change the reaction path in comparing with silent condition. An improved method for the synthesis of novel fused pyrans from the reaction of the tetrahydropyran-4one with arylidine malononitriles carried out under silent and ultrasonic conditions in good yields was reported [24]. Banitaba et al [25] reported a green and simple approach to assembling of 2-amino-4,8-dihydropyrano[3,2-b]pyran3-carbonitrile scaffolds via three-component reaction of kojic acid, malononitrile, and aromatic aldehydes in aqueous media under ultrasound irradiation (Scheme-8). In comparison to conventional methods, experimental simplicity, functional group tolerance, excellent yields, short routine, and selectivity without the need for a transition metal or base catalyst.
It was demonstrated that, an inexpensive and commercially available nano-powder magnetite or iron(III) oxide used as catalyst in a new, straightforward, and fast protocol for the construction of 4-substituted-4H-pyrans. The reaction
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K. Ajay Kumar et al J. Chem. Pharm. Res., 2015, 7(11):693-700 ______________________________________________________________________________ implies a tandem process, involving an aldol condensation, a Michael-type addition, and dehydrating annulations [26]. Multi-component reaction of an isocyanide, a dialkyl acetylenedicarboxylate, and tetronic acid in dichloromethane at room temperature afforded 4H-furo[3,4-b]pyrans that showed a broad spectrum of biological activities [27]. Polyfunctionalized 4H-pyrans bearing fluorochloro pyridyl moiety were readily prepared in high yields via one-pot multicomponent reaction catalyzed by piperidine. The protocol provides an efficient synthetic route to the target compounds with the characteristics of short reaction time, high yield, and easy separation of the products [28]. Fluorinated vinylcopper reagents undergo stereospecific syn addition to hexafluoro-2-butyne, to provide in situ the dienylcopper reagent. Subsequent acylation of the dienylcopper reagent gave a dienylketone, which spontaneously cyclized to the 2H-pyrans [29]. DABCO-catalyzed reactions of α-halo carbonyl compounds with dimethyl acetylenedicarboxylate (DMAD) at room temperature produced polysubstituted furans and highly functionalized 2H-pyrans in good yields [30]. Khalilzadeh et al [31] reported multicomponent reaction of dithiocarbamates, dialkyl acetylenedicarboxylates, and isocyanides in solvent-free conditions to obtain 2H-pyran-3,4-dicarboxylates (3). A one-pot practical, efficient, and environmentally benign multicomponent synthesis of 4H-pyrans (4) by a three component reaction of an aldehyde, malononitrile, and 5,5-dimethyl-1,3-cyclohexanedione or ethyl acetoacetate at room temperature or refluxing in ethanol has been developed [32].
An efficient synthesis of polyfunctionalized 4H-pyrans is carried out in one pot through condensation of an aldehyde, malononitrile, and an active methylenic diketo compound using a heterogeneous strong basic Mg/La mixed oxide catalyst (Scheme-9) [33]. The protocol offered high yields, short reaction times, and mild reaction conditions, with reusability of the catalyst.
A concise approach to the synthesis of 5-arylamino-4H-pyran-4-ones via palladium-catalyzed amination reaction was reported. The method involves protection/deprotection protocols and on manipulation of the 5-hydroxy group of readily available kojic acid [34]. The mildly basic ionic liquid N,N,N,N-tetramethylguanidinium triflate (TMGTf) was found to be a very effective solvent for the reaction between 4-hydroxy-6-methyl-2H-pyran-2-one, Meldrum’s acid, and aldehydes to afford some novel pyrano[4,3-b]pyran-2,5-diones in high yields at room temperature (Scheme-10) [35].
One pot synthesis of 2,2-dialkyl-3-dialkylamino-2,3-dihydro-1H-naphtho[2,1-b]pyrans from 2-naphthol, a secondary amine, and 3-hydroxy-2,2-dialkylpropanal in the presence of a catalytic amount of p-toluenesulfonic acid. The reaction involves retro-aldol disintegration of 3-hydroxy-2,2-dialkylpropanal followed by formation of a Mannich base intermediate from 2-naphthol, a secondary amine, and formaldehyde. The Mannich base then disproportionates into a quinone methide intermediate which undergoes electrocyclic ring closure with enamines to
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K. Ajay Kumar et al J. Chem. Pharm. Res., 2015, 7(11):693-700 ______________________________________________________________________________ produce the products [36]. Rhodium(II)-catalyzed reactions of diazo compound with a variety of ethynyl compounds provide a rapid route for preparing a variety of furo[2,3-b]pyran-6-ones in one-pot via cascade reactions of metal carbenoid reaction/ketene formation/[2+2]cycloaddition/ring expansion [37]. A series of 2-amino-4-aryl-4H,8H-6-methyl-8-oxo-pyrano[3,2-b]pyrans [38] were synthesized efficiently from aromatic aldehyde, malononitrile or cyanoacetate, and 5-hydroxy-2-methyl-4H-pyran-4-one via an one-pot threecomponent reaction catalyzed by Et3N in ionic liquid [bmim]BF4. Kumar and co-workers [39, 40] reported the synthesis of a series of coumarin appended formyl-pyrazoles by a simple and accessible approach. Later they converted these formyl pyrazoles to a series of fused pyran derivatives in excellent yields (Scheme-11) [41].
Multicomponent reactions (MCRs) have proved to be one of the most powerful and efficient methods for the preparation of bioactive heterocyclic compounds because of its atom economy, simple procedure, and high yields of the products. One-pot three-component reaction for the synthesis of pyran annulated heterocycles was reported by condensing aromatic aldehydes, ethyl cyanoacetate, or malononitrile and C–H activated acidic compounds in the presence of catalytic amount of 4-(dimethylamino)pyridine (DMAP) in ethanol under reflux conditions [42]. A series of isomeric bridged pyrans tagged to pyrazole and coumarin moiety were synthesized by the condensation reaction of 3,3'-(7-hydroxy-4-methyl-2-oxo-2H-chromene-6,8-diyl)bis(1-aryl-1H-pyrazole-4-carbaldehyde) in ethyl alcohol in the presence of conc. H2SO4 under reflux conditions [43]. Liu and co-workers [44] reported the synthesis of trifluoromethylated cyclopenta[b]pyran derivatives efficiently from 1,3-cyclopentanedione, arylaldehydes, and ethyl 4,4,4-trifluoro-3-oxobutanoate via one-pot multi-component reaction catalyzed by NH4OAc. A green and highly efficient protocol has been developed for the synthesis of 4Hpyran scaffolds installing a one-pot three-component coupling reaction of an aldehyde, malononitrile, and a 1,3diketo compound using nano structured ZnO as the catalyst in aqueous alcoholic medium [45]. Albadi and coworkers [46] proved that CuO–CeO2 is a highly efficient and green recyclable catalyst, for the multicomponent synthesis of 4H-benzo[b]pyran derivatives. The method provides several advantages such as simple work-up procedures, minimal amount of waste generated, short reaction time, and high yields. Pyran structural units are widely occurring in various molecules exhibiting a wide range of biological activities and serve as a specific non-nucleoside reverse transcriptase inhibitor of HIV-1. 4-(Succinimido)-1-butane sulfonic acid as an efficient and reusable Bronsted acid catalyzed the synthesis of pyrano[4,3-b]pyran derivatives under solventfree conditions (Scheme-12) [47]. The catalyst was recycled without significant loss of activity.
A regioselective palladium-catalyzed allylic alkylation cascade forms furo[3,2-c]pyrans from 4-hydroxy-6-methylα-pyrone and 6-((tert-butyldimethylsilyl)oxy)-3,6-dihydro-2H-pyran-3-yl methyl carbonate using toluene as solvent. The combination of allylic carbonate and anomeric siloxy leaving groups in the dihydropyran substrates allows control of the many regiochemical possibilities in this reaction [48]. BIOLOGICAL APPLICATIONS The development of drug resistance to clinically used agents has increased the demand for discovery of new chemical scaffolds with antimicrobial activity. Shamsuzzaman et al [49] reported the green and simple procedure for the synthesis of steroidal 2H-pyrans using chitosan as an eco-friendly heterogeneous catalyst. They have tested the synthesized compounds in vitro against two cancer cell lines [HeLa (cervical) and Jurkat (leukemia)] and one
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K. Ajay Kumar et al J. Chem. Pharm. Res., 2015, 7(11):693-700 ______________________________________________________________________________ normal cell line (PBMC). Their results showed that the have moderate to good activity against the two human cancer cell lines and were less toxic against the non-cancer cell line. New chromeno-annulated cis-fused pyrano[3,4-c]benzopyran and naphtho pyran derivatives were synthesized by domino aldol-type reaction/hetero Diels–Alder reaction of generated from o-quinone methide in situ from 7-Oprenyl derivatives of 8-formyl-2,3-disubstituted chromenones with resorcinols/naphthols in the presence of ethylenediamine diacetate (EDDA), triethylamine as co-catalyst in CH3CN under reflux conditions in good yields. The results showed that some compounds of the series exhibited very potent cytotoxicity against human cervical cancer cell line (HeLa) [50]. Debnath and co-workers [51] synthesized the ethyl 2-amino-[N-[6(4-methylphenyl)-4-(4-chlorophenyl)-4H-pyran-2yl]-N-[(1E)-4-(dimethylamino phenylmethylene]-amine]-3-carboxylate (5) and 4-(4-chlorophenyl)-2-{[E-(2,4dichlorophenyl)-methylidene]-amino}-6-(4-methylphenyl)-4H-pyran-3-carbonitrile (6). These compounds show good antifungal activity against candida albicans when compared with the standard drug fluconazole.
A simple and efficient synthetic route has been developed for synthesis of (R)-rugulactone, (6R)-((4R)-hydroxy-6phenyl-hex-2-enyl)-5,6-dihydro-pyran-2-one and its 4S epimer by employing proline-catalyzed α-aminooxylation, Sharpless epoxidation, Mitsunobu reaction as chirality introuducing steps. The antibacterial and antifungal activity of the compounds showed better antibacterial activity against Pseudomonas aeroginosa and Klebsiella pneumonia [52]. 4-Amino-5-(5-chloro-2-phenyl-1H-indol-3-yl)-7-(4-chlorophenyl)-1H-pyrano[2,3-d]pyrimidin-2(5H)-one (7) was synthesized from the 2-amino-4-(5-chloro-2-phenyl-1H-indol-3-yl)-6-(4-chlorophenyl)-4H-pyran-3-carbonitrile using urea and ethanol under reflux condition. This compound shows promising radical scavenging activity, ferric ions reducing antioxidant power and metal chelating activity [53].
2-Amino-4-(4-chlorophenyl)-6-((-6a,8a-dimethyl-4-oxo-dodecahydro-1H-naphtho[2’,1’:4,5]indeno[1,2-d]thiazol10-yl)amino)-4H-pyran-3,5-dicarbonitrile (8) and 2-((-6a,8a-dimethyl-4-oxo-dodecahydro-1Hnaphtho[2’,1’:4,5]indeno[1,2-d]thiazol-10-yl)amino)-6-hydroxy-4-(4-methoxyphenyl)-4H-pyran-3,5-dicarbonitrile (9) were synthesized from 2-Cyano-N-(-6a,8a-dimethyl-4-oxo-dodecahydro-1H-naphtho[2’,1’:4,5]indeno[1,2d]thiazol-10-yl) in absolute ethanol, malanonitrile or ethylacetoacetate and 4-chlorobenzaldehyde or 4methoxybenzaldehyde under reflux condition. These compounds show maximum antiulcer activity and also studied the toxicology against shrimp larvae, showed non-toxicity against the tested organisms [54].
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K. Ajay Kumar et al J. Chem. Pharm. Res., 2015, 7(11):693-700 ______________________________________________________________________________ The compounds (E)-2-Amino-4-(3-nitrophenyl)-8-(4-trifluoromethyl)benzylidene)-5,6,7,8-tetrahydro-4H-chromene3-carbonitrile (10) and (E)-2-amino-6-methyl-4-(naphthalene-2-yl)-8-(4-(trifluoromethyl)benzylidene)-5,6,7,8tetrahydro-4H-pyrano[3,2-c]pyridine-3-carbonitrile (11) exhibited noticeable growth inhibitory activity against the tested human tumor cell lines such as human colon cancer (HCT116), human cervical cancer (Hela) and nonsmallcell lung cancer (H1975) [13]. NH2
NH2 NC
NC
O
O
N
10
CF3
CF3 NO2
CH3
11 CONCLUSION
In this review, attempt has been made to present the up to date information about the source, synthetic strategies, reactions and pharmaceutical applications of pyran analogues. Wide range of natural sources and new pyran analogues are being discovered or synthesized on a regular basis. Their physicochemical, physiological, antioxidant, antitumor bactereostatic properties etc. make them as novel class for therapeutic applications. Synthetic procedure and clinical applications of pyrans were critically discussed. Acknowledgements The authors are grateful to the University Grants Commission, New Delhi, for financial assistance through major research project grant (F: 42-230/2013 (SR) Dated 25th March 2013). REFERENCES [1] T. Moriguchi; H. Matsuura; Y. Itakura; H. Katsuki; H. Saito; N. Nishiyama. Life Sci., 1997, 61, 1420. [2] G.H. Abou El-Fotooh; Osama I Abd El-Salam; M.M. Ashraf; A.H. Nagla. Ind. J. Chem., 2005, 44B, 1893. [3] D. Armesto; W.M. Horspool; N. Martin; A. Ramos; C. Seoane. J. Org. Chem., 1989, 54, 3069. [4] J.A. Rideout; I.R. Smith; M.D. Sutherland. Aust. J. Chem., 1976, 29(5), 1087. [5] D. Kumar; V.B. Reddy; S. Sharad; U. Dube; K A. Suman. Eur. J. Med. Chem., 2009, 44, 3805. [6] Yong Rok Lee; Xue Wang; Likai Xia. Molecules, 2007, 12, 1420. [7] H. Junek; H. Aigner. Chem. Ber., 1973, 106, 921. [8] H.H. Otto. Arch. Pharm., 1974, 307, 444. [9] V. Colatta; D. Catarzi; F. Varano; F. Melani; G. Filacchioni; L. Cecci; L. Trincavelli; C. Martini; A. Lucacchini. Il Farmaco.1998, 53, 189. [10] P. M. Andrey; V.D. Aleksadr; O.G. Oleksandr; A.T. Andrey. Arkivoc, 2012, 8, 226. [11] Utpal Das; Chan-Hui Huang; Wenwei Lin. Chem. Commun., 2012, 48, 5590. [12] T. Wang; J. Liu; H. Zhong; H. Chen; Zhiliang Lv; Y. Zhang; M. Zhang; D. Geng; C. Niu; Y. Li; Ke Li. Bioorg. Med. Chem. Lett., 2011, 21, 3381. [13] Dao-Cai Wanga; Yong-Mei Xie; Chen Fan; Shun Yao; Hang Song. Chin. Chem. Lett., 2014, 25, 1011. [14] J.T. Nilesh; P.P. Manish. Arkivoc, 2009, 8, 363. [15] R.M. Mohareb; F. Al-Omran; R.A. Azzam. Steroids. 2014, 84, 46. [16] H. Kiyani; H.A. Samimi; F. Ghorbani; S. Esmaieli. Cur. Chem. Lett., 2013, 2, 197. [17] B.V. Subba Reddy; D.N. Chaya; J.S. Yadav; D. Chatterjee; A.C. Kunwar. Tetrahedron Lett., 2011, 52, 2961. [18] Reddi Mohan Naidu Kalla; Mi Ri Kim; Il Kim. Tetrahedron Lett., 2015, 56, 717. [19] Q. Chong; C. Wang; D. Wang; H. Wang; Fan Wu, X. Xin; B. Wan. Tetrahedron Lett., 2015, 56, 401. [20] C. Yao; B. Jiang; T. Li; B. Qin; X. Feng; H. Zhang; C. Wang; S. Tu. Bioorg. Med. Chem. Lett., 2011, 21, 599. [21] P. Das; A. Dutta; A. Bhaumik; C. Mukhopadhyay. Green Chem., 2014, 16, 1426. [22] C.-L Ni; X.-H. Song; H. Yan; X.-Q. Song; R.-G. Zhong. Ultrason. Sonochem., 2010, 17, 367. [23] S.S. Mansoor; K. Logaiya; K. Aswin; P.N. Sudhan. J. Taibah Univ. Sci., 2015, 9, 213. [24] T.S. Saleh; N.M. Abd El-Rahman; A.A. Elkateb; N.O. Shaker; N.A. Mahmoud; S.A. Gabal. Ultrasonics Sonochem., 2012, 19, 491. [25] S.H. Banitaba; J. Safari; S.D. Khalili. Ultrasonics Sonochem., 2013, 20, 401. [26] R. Cano; D.J. Ramón; M. Yus. Synlett, 2011, 2017. [27] S. Ahmad; S. Ebrahim; S. Afshin; R.A. Hossein. Bioorg. Med. Chem. Lett., 2008, 18, 3968.
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