Organocatalytic asymmetric

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Cite this: DOI: 10.1039/c6cc06367e Received 1st August 2016, Accepted 2nd September 2016

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Organocatalytic asymmetric [3+2] cycloaddition of N-2,2,2-trifluoroethylisatin ketimines with 3-alkenyl-5-arylfuran-2(3H)-ones† Zhen-Hua Wang,ab Zhi-Jun Wu,c Deng-Feng Yue,ab Wen-Fei Hu,ab Xiao-Mei Zhang,a Xiao-Ying Xu*a and Wei-Cheng Yuan*a

DOI: 10.1039/c6cc06367e www.rsc.org/chemcomm

A highly diastereo- and enantioselective [3+2] cycloaddition reaction of N-2,2,2-trifluoroethylisatin ketimines and 3-alkenyl5-arylfuran-2(3H)-ones was developed with 1 mol% thiourea–tertiary amine as the catalyst. A series of spiro[pyrrolidin-3,2 0 -oxindoles] bearing four consecutive stereocenters, including two vicinal spiroquaternary chiral centers, were efficiently obtained with excellent results (up to 499% yield, 420 : 1 dr, and 499% ee).

Spirooxindole scaffolds represent privileged structural motifs that feature in a number of natural products and clinical pharmaceuticals, which exhibit important biological activities.1 In particular, spiro[pyrrolidin-3,20 -oxindole] ring systems constitute the central skeleton of numerous alkaloids and pharmacologically important compounds.2 Consequently, developing efficient methodologies for the construction of the spiro[pyrrolidin-3,20 oxindole] skeletons with structural complexity and diversity has been the focus of intensive efforts from organic chemists, and a variety of efficient strategies for the synthesis of them have been reported.3 However, among these well-developed methods, the vast majority of them are confined to the synthesis of spiro[pyrrolidin-3,2 0 -oxindole] compounds containing only one spiro-quaternary chiral center. In contrast, there are only a handful of reported examples regarding the synthesis of spiro[pyrrolidin-3,2 0 -oxindoles] containing two vicinal spiroquaternary stereocenters fused in one ring structure,4 despite their prevalence and importance as the key structural core for a number of bioactive compounds (Fig. 1).5 Actually, the asymmetric construction of two vicinal spiro-quaternary chiral

centers at one ring represents a great synthetic challenge in chemical synthesis due to the great steric hindrance. Meanwhile, the structural complexity and richness in stereogenic centers of a molecule are generally important to its potentially useful biological properties.6 In this context, the development of an efficient asymmetric method to access enantioenriched spiro[pyrrolidin-3,20 -oxindole] derivatives, particularly containing two vicinal spiro-quaternary chiral centers, is highly desirable. Organocatalytic asymmetric cycloaddition reactions have turned out to be one of the most efficient methods for the construction of various spirocyclic compounds.7 Among them, the enantioselective [3+2] cycloaddition of azomethine ylides with activated alkenes should be the most straightforward way to access optically active pyrrolidine derivatives.8 Recently, N-2,2,2-trifluoroethylisatin ketimines serving as efficient azomethine ylide precursors have been used to generate structurally diverse CF3-containing spiro[pyrrolidin-3,2 0 -oxindole] derivatives.9 Meanwhile, based on our continuing interest in the construction of diverse complex and novel spirocyclic oxindole skeletons with organocatalysis,10 we speculated that a catalytic asymmetric [3+2] cycloaddition reaction between

a

National Engineering Research Center of Chiral Drugs, Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, China. E-mail: [email protected], [email protected] b University of Chinese Academy of Sciences, Beijing 100049, China c Chengdu Institute of Biology, Chinese Academy of Sciences, Chengdu 610041, China † Electronic supplementary information (ESI) available: Experimental procedures, spectral data of new compounds, and crystallographic data. CCDC 1474514. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6cc06367e

This journal is © The Royal Society of Chemistry 2016

Fig. 1 Some biologically active compounds with a spiro[pyrrolidin-3,2 0 oxindole] core bearing two vicinal spiro-quaternary chiral centers, and our products in this work.

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Table 1

Scheme 1

Optimization of the conditionsa

Strategy for spiro[pyrrolidin-3,2 0 -oxindole] scaffold synthesis.

N-2,2,2-trifluoroethylisatin ketimines 2 and 3-alkenyl-5arylfuran-2(3H)-ones 1, a class of promising reagents for the synthesis of important molecules which are butenolidecontaining,11 should be realized under certain enantioselective conditions. If so, this strategy would provide a straightforward route to complex spiro[pyrrolidin-3,2 0 -oxindole] compounds (Scheme 1). More importantly, four consecutive stereocenters including sterically congested two vicinal spiro-quaternary chiral centers would be generated in a single stereoselective process (Scheme 1). Herein, we wish to describe the successful development of this reaction. The initial investigation commenced with the reaction of 3-benzylidene-5-phenylfuran-2(3H)-one 1a12 and N-2,2,2-trifluoroethylisatin ketimine 2a in chloroform at room temperature. Using cinchonidine-derived thiourea A as the catalyst, the reaction could give the spirocyclic oxindole 3a in 97% yield with 93% ee and 420 : 1 dr after 10 h (Table 1, entry 1). Then, bifunctional organocatalysts B–D were evaluated with the model reaction. It was found that a cinchona alkaloid skeleton was essential for achieving high asymmetric induction (Table 1, entries 1–3 vs. entry 4). Particularly, quinine derived thiourea catalyst C afforded 3a with the best results (499% yield, 420 : 1 dr, 95% ee) (Table 1, entry 3). Afterwards, a survey of various solvents, including CH2Cl2, THF, methyl tert-butyl ether (MTBE), toluene, 1,2-dichloroethane (DCE) and acetonitrile revealed that DCE was the most suitable reaction media for the reaction (Table 1, entry 9 vs. entries 5–8, 10). When lowering the catalyst loading from 10 mol% to 5 mol%, there was no effect on the reactivity and stereoselectivity (Table 1, entry 11). On further decreasing the catalyst loading to 1 mol%, an excellent yield and diastereo- and enantiocontrol could still be achieved but with a prolonged reaction time (Table 1, entry 12). As the conditions of choice, we utilized 3-alkenyl-5-arylfuran-2(3H)-ones and N-2,2,2trifluoroethylisatin ketimines at a molar ratio of 2 : 1 in DCE with 1 mol% catalyst C at room temperature. Having established the optimized conditions, the scope of N-2,2,2-trifluoroethylisatin ketimines for the catalytic asymmetric [3+2] cycloaddition reaction was firstly investigated. As shown in Table 2, a series of ketamines 2b–k tolerated the reaction and furnished the corresponding products 3b–k in excellent yields (86–99%) with good to excellent stereoselectivity (12 : 1 Z 20 : 1 dr and 83 Z 99% ee). It was found that those ketamines bearing N-Et, N-Bn, N-2-allyl, N-Ac and N-Ts could smoothly react with 3-benzylidene-5-phenylfuran2(3H)-one 1a under the standard conditions, affording products 3b–f with good to excellent results (Table 2, entries 1–5). Additionally, the substrate with no substituent on the N-1

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Entry

Catalyst

Solvent

Time (h)

Yieldb (%)

drc

eec (%)

1 2 3 4 5 6 7 8 9 10 11d 12e

A B C D C C C C C C C C

CHCl3 CHCl3 CHCl3 CHCl3 CH2Cl2 THF MTBE Toluene DCE CH3CN DCE DCE

10 10 5 48 5 20 5 3 3 5 4 11

97 99 499 93 99 96 99 98 96 93 97 98

420 : 1 420 : 1 420 : 1 4:1 420 : 1 12 : 1 420 : 1 420 : 1 420 : 1 4:1 420 : 1 420 : 1

93 92 95 49 97 97 93 91 97 96 97 97

a

Unless otherwise noted, the reactions were conducted with 1a (0.20 mmol), 2a (0.10 mmol) and 10 mol% catalyst in 1.0 mL of solvent at room temperature for the indicated reaction time. b Isolated yields. c Determined by chiral HPLC analysis. d 5 mol% catalyst was used. e 1 mol% catalyst was used.

Table 2 Scope of the asymmetric [3+2] cycloaddition of 1a with N-2,2,2trifluoroethylisatin ketimines 2b–ka

Entry

R1/R2/R3

Time (h)

3/yieldb (%)

drc

eec (%)

1 2 3 4 5d 6 7 8 9 10

Et/H/H (2b) Bn/H/H (2c) 2-Allyl/H/H (2d) Ac/H/H (2e) Ts/H/H (2f) H/H/H (2g) Me/F/H (2h) Me/Cl/H (2i) Me/Br/H (2j) Me/H/Cl (2k)

18 10 11 11 15 23 14 10 40 11

3b/99 3c/99 3d/99 3e/98 3f/91 3g/91 3h/97 3i/99 3j/86 3k/98

420 : 1 420 : 1 420 : 1 12 : 1 420 : 1e 420 : 1 420 : 1 420 : 1 420 : 1 420 : 1

95 97 96 98 88 83 99 499 499 98

a

Unless otherwise noted, the reactions were conducted with 1a (0.20 mmol), 2 (0.10 mmol) and 1 mol% catalyst C in 1.0 mL of DCE at room temperature for the indicated reaction time. b Isolated yields. c Determined by chiral HPLC analysis. d 5 mol% catalyst was used. e Determined by 1H NMR analysis.

position could also undergo this transformation well and provide 3g with good results (91% yield, 420 : 1 dr and 83% ee; Table 2, entry 6). Moreover, we also evaluated the substituted ketamines


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Table 3 Scope of the asymmetric [3+2] cycloaddition of 3-alkenyl-5arylfuran-2(3H)-ones 1 with 2aa

Entry R4/R5 d

1 2 3 4d 5 6 7 8 9 10 11 12 13d 14 15

4-MeC6H4/Ph (1b) 4-MeOC6H4/Ph (1c) 3-MeC6H4/Ph (1d) 3,4-(MeO)2C6H3/Ph (1e) 4-FC6H4/Ph (1f) 3-BrC6H4/Ph (1g) 2-FC6H4/Ph (1h) 2-ClC6H4/Ph (1i) 2-BrC6H4/Ph (1j) 2-furyl/Ph (1k) 2-thienyl/Ph (1l) 1-naphthyl/Ph (1m) Ph/4-MeC6H4 (1n) Ph/4-FC6H4 (1o) Ph/2-naphthyl (1p)

Time (h) 3/yieldb (%) drc 22 12 24 16 12 12 12 12 12 62 48 24 12 15 12

3l/95 3m/81 3n/99 3o/86 3p/99 3q/96 3r/81 3s/99 3t/98 3u/95 3v/99 3w/94 3x/99 3y/98 3z/96

eec (%) e

420 : 1 98 420 : 1 95 420 : 1e 97 420 : 1 98 420 : 1 98 420 : 1 98 420 : 1e 98 420 : 1 94 420 : 1 98 420 : 1 94 420 : 1e 499 420 : 1 98 420 : 1 98 420 : 1 98 420 : 1 97

a

Unless otherwise noted, the reactions were conducted with 1 (0.20 mmol), 2a (0.10 mmol) and 1 mol% catalyst C in 1.0 mL of DCE at room temperature for the indicated reaction time. b Isolated yields. c Determined by chiral HPLC analysis. d 5 mol% catalyst was used. e Determined by 1H NMR analysis.

bearing 5-F, 5-Cl, 5-Br and 6-Cl substituents on the phenyl ring. Significantly, these substrates also performed very well, delivering the products 3h–k in excellent yields as well as with excellent dr and ee values (Table 2, entries 7–10). Further exploration of the substrate scope was focused on the 3-alkenyl-5-arylfuran-2(3H)-ones 1b–p bearing various substituents (Table 3). The effects of the substituents R4 on 3-alkenyl-5-arylfuran-2(3H)-ones were first investigated. It was found that either electron-donating (1b–e) or electronwithdrawing substituents (1f–j) on the benzene ring were tolerated and delivered the desired spirocyclic oxindoles 3l–t in good to excellent yields (81–99%) with excellent diastereoselectivities (420 : 1 dr) and ee values (94–98% ee) (Table 3, entries 1–9). Moreover, this protocol was also broadened to 2-furyl or 2-thienyl substrates, and the reaction could be completed in high yields and excellent stereoselectivities with prolonging of the reaction time (Table 3, entries 10 and 11). Meanwhile, fused aromatic 1m also reacted efficiently with 2a, giving 3w in 94% yield with 420 : 1 dr and 98% ee (Table 3, entry 12). Additionally, the R5 group on the 3-alkenyl-5arylfuran-2(3H)-ones was tested as shown with substrates 1n–p, the corresponding adducts 3x–z were obtained in 96–99% yield, with 420 : 1 dr and 97–98% ee (Table 3, entries 13–15). For the substrates, 1b, 1e, and 1n bearing electron donating substituents, the reactions proceeded with relatively lower reactivity and a 5 mol% catalyst loading was used to complete the reaction, providing the respective products with satisfactory results (Table 3, entries 1, 4 and 13). Ultimately, the absolute configuration of the adduct 3q was assigned as C5S, C6S, C17R, C23S using single crystal X-ray analysis.


Scheme 2 Large-scale experiment and representative transformations of product 3q. Reaction conditions: (a) DMAP, MeOH, 60 1C, 6 h; (b) N2H4 H2O, EtOH, 50 1C, 15 min; (c) AcOH, HCl, reflux, 20 min.

The stereochemistry of the other products in Tables 2 and 3 was assigned by analogy.13 We further performed a preparative scale experiment to evaluate the practicability of this protocol. The reaction of 1g and 2a was conducted on a gram-scale (4 mmol of 2a) with 5 mol% C under the optimized conditions and furnished 3q with excellent results (Scheme 2), which are similar to the results of the original reaction illustrated in Table 3, entry 6. Moreover, only a 1 mol% loading of C is sufficient to afford the corresponding product 3q in high yield with excellent diastereo- and enantiocontrol (93% yield, 420 : 1 dr, 96% ee) (Scheme 2). Then, two synthetic transformations of product 3q into other spirocyclic oxindoles were performed.14 3q could be easily transformed into the intermediate hydrazide 5 with hydrazine hydrate, which was directly treated with a mixture of HCl/AcOH (v/v 4 : 1) to afford spirocyclic compound 6 in 99% yield with 420 : 1 dr and 499% ee. In addition, treatment of 3q with DMAP in methanol could give the spiro[pyrrolidin-3,2 0 oxindole] derivative 4 in 97% yield with excellent stereoselectivity (420 : 1 dr, 98% ee) (Scheme 2). On the basis of the results from our experiment and the absolute configuration of the major isomer 3q, a plausible transition state model is proposed to explicate the observed stereochemical preference. As shown in Scheme 3, the thiourea– tertiary amine catalyst C promotes the [3+2] cycloaddition reaction in a dual activation model: N-2,2,2-trifluoroethyl isatin ketimines are activated by deprotonation by the tertiary amine of catalyst C to enhance the nucleophilicity of the reacting

Scheme 3

Proposed transition state for the [3+2] cycloaddition reaction.

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carbon anion. Concurrently, the H-bonding activation of 3-alkenyl-5-arylfuran-2(3H)-ones by the thiourea moiety of catalyst C facilitates the nucleophilic attack from the Si-face by the carbon anion. Subsequently, the a-carbon center of 3-alkenyl-5-arylfuran2(3H)-ones from the Si-face attacks the Re-face of isatin ketimines, thus leading to the formation of spiro[pyrrolidin-3,20 -oxindole] products bearing four consecutive stereocenters, including two vicinal spiro-quaternary chiral centers. In conclusion, we have developed a highly diastereo- and enantioselective [3+2] cycloaddition reaction of N-2,2,2-trifluoroethylisatin ketimines with 3-alkenyl-5-arylfuran-2(3H)-ones. In the presence of a thiourea–tertiary amine catalyst, a wide range of CF3-containing spiro[pyrrolidin-3,20 -oxindoles] bearing four consecutive stereocenters, including two vicinal spiro-quaternary chiral centers, were efficiently obtained with excellent results (up to 99% yield, 420 : 1 dr, and 499% ee). A large-scale experiment and further conversions of the product were also successfully performed to demonstrate the promising applicability of the methodology. This efficient method features a low catalyst loading (1 mol%) and mild reaction conditions, and also will open a new and straightforward way to access enantioenriched complex spiro[pyrrolidin-3,20 -oxindoles] spiro-fused with butenolides. The development of efficient strategies to synthesize more promising candidates for drug discovery and the biological evaluation of these structurally complex spiro[pyrrolidin-3,20 oxindole] compounds are currently underway. We are grateful for the financial support from the National Natural Science Foundation of China (No. 21372217, 21572223 and 21572224).

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