COMMUNICATION DOI: 10.1002/asia.201301011

Bicyclic-Guanidine-Catalyzed Asymmetric Michael Addition of 3Substituted Oxindoles to 2-Cyclopentenone Caiyun Yang,[a] Wenchao Chen,[a] Wenguo Yang,[b] Bo Zhu,[a] Lin Yan,[a] Choon-Hong Tan,[b] and Zhiyong Jiang*[a, b]

3,3-Disubstituted oxindoles, containing a quaternary stereocenter at the C3-position, are privileged heterocyclic motifs found in many biologically active natural and nonnatural products as well as therapeutic agents.[1] In this context, the asymmetric construction of 3,3-disubstituted oxindoles has attracted significant attention of organic chemists.[2] Especially, the addition reactions of prochiral 3-substituted oxindoles to various electrophiles have been recognized as a straightforward approach to access the synthetically important 3,3-disubstituted oxindoles.[2] Over the past decades, numerous catalytic asymmetric transformations of 3-substituted oxindoles have been developed, including the Michael reaction,[3] the Mannich reaction,[4] amination,[5] hydroxylation,[6] aminooxygenation,[7] fluorination,[8] acyl migration,[9] allylic alkylation,[10] the aldol reaction,[11] and sulfenylation.[12] Despite the success to date, the asymmetric reaction of 3substituted oxindoles still remains challenging. For example, in 2010, Melchiorre and co-workers introduced a Cinchona alkaloid-derived primary-amine-catalyzed highly enantioselective Michael addition of 3-substituted oxindoles to cyclic enones, thereby affording a series of oxindole derivatives with vicinal quaternary and tertiary carbon centers.[13a] Subsequently, in 2011, Tsogoeva and Freund reported a l-proline-catalyzed Michael addition of 3-substititued oxindoles to enones.[13b] However, despite these results, the highly stereoselective reaction between 3-substituted oxindoles and 2cyclopentenone has never been successfully achieved [Scheme 1, Eq. (1)]. It is well known that 2-cyclopentenone is an important type of cyclic enone, which has been used in asymmetric reactions to construct a variety of bioactive

Scheme 1. The preliminary work for investigating the Michael addition of 3-benzyl-substituted oxindoles to 2-cyclopentenone catalyzed by chiral bicyclic guanidine. Pg = protective group.

compounds.[14] As part of our ongoing investigation on the synthesis of 3,3-disubstituted oxindoles and other important compounds bearing a quaternary chiral center,[3j, 6c, 12d, 15] we are keen to develop an efficient organocatalyic asymmetric Michael addition of 3-substituted oxindoles to 2-cyclopentenone. Chiral guanidines have been developed as an important type of organocatalyst in numerous asymmetric reactions, particularly the axially chiral guanidine and the C2-symmetric bicylic guanidines.[16] In the past few years, we have developed a series of C2-symmetric bicyclic-guanidine-catalyzed reactions with excellent results, such as the Diels– Alder reaction,[17a] protonation,[17b–d] the Mannich reaction,[17e] decarboxylative reaction,[17f] the Michael reaction,[3j, 14d, e, 17g–i] vinylogous amination,[17j] and amination.[17k]

[a] C. Yang, W. Chen, B. Zhu, Dr. L. Yan, Prof. Z. Jiang Key Laboratory of Natural Medicine and Immuno-Engineering of Henan Province Henan University Kaifeng, Henan, 475004 (P. R. China) Fax: (+ 86) 378-2864-665 E-mail: [email protected] [email protected] [b] W. Yang, Prof. C.-H. Tan, Prof. Z. Jiang Division of Chemistry and Biological Chemistry Nanyang Technological University 21 Nanyang Link, 637371 (Singapore) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201301011.

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Notably, in 2007, we reported a highly enantioselective Michael addition of dithiomalonates to 2-cyclopentenone catalyzed by bicyclic guanidine (1) [Scheme 1, Eq. (2)].[14d] Furthermore, we presented a Michael addition of 3-benzyl 2-oxindoles to N-substituted maleimides catalyzed by 1 with excellent enantio- and diastereoselectivities [Scheme 1, Eq. (3)].[3j] The above results indicate the potential of bicyclic guanidine as an enantioselective catalyst owing to hydrogen bonding to both 3-benzyl 2-oxindole and 2-cyclopentenone. Therefore, we were intrigued to investigate the possibility of the highly stereoselective Michael addition of 3benzyl-substituted oxindoles to 2-cyclopentenone by using bicyclic guanidine 1 as a catalyst. Initially, we selected the addition of N-benzyl-3-benzylsubstituted oxindole 2 a to 2-cyclopentenone 3 as the model reaction (Table 1). In the presence of 10 mol % of catalyst 1 at 25 8C in toluene, the desired Michael adduct 4 a was obtained in 32 % yield with 79 % ee and 95:5 d.r. within 36 h (Table 1, entry 1). The good enantioselectivity promoted us to investigate the solvent effect (Table 1, entries 2–8), and the less polar solvents, such as diethyl ether (Table 1, entry 4), mesitylene (Table 1, entry 6), and m-xylene (Table 1, entry 7) gave similar good enantioselectivities. However, toluene was still identified as the ideal solvent. When the loading of catalyst 1 was increased to 20 mol %, the reaction rate was accelerated, thus affording the Michael adduct 4 a in 80 % yield with the same good enantioselectivity after 24 h (Table 1, compare entries 1 and 9). We then sought to improve the reaction by lowering the reaction temperature (Table 1, entries 10–12), and at 10 8C the best

results were achieved (85 % ee and 98:2 d.r., Table 1, entry 11). As we have previously shown, non-chiral amines could accelerate the rate of the Michael reaction whilst retaining the enantioselectivity when chiral bicyclic guanidine 1 was used as the catalyst.[14e] Therefore, four organic amines were examined as additives (Table 1, entries 13–16). It was found that only Et3N accelerated the reaction rate and gave 4 a with similar enantio- and diastereoselectivity (Table 1, entry 13). Considering that Et3N has a low boiling point and subsequent removal from the reaction mixture is not problematic, the amount used was increased to 0.5 equivalents (Table 1, entries 17 and 18). However, we found that the enantio- and diastereoselectivity decreased, although the reaction rate improved. To our surprise, when we continued to increase the amount of Et3N and use it as a solvent, the reaction became slower; this is probably due to the poor solubility of oxindole 2 a in Et3N (Table 1, entry 19). We next investigated the effect of N-benzyl substituents (i.e., 3-benzyl-2-oxindoles 2 b–e) on the stereoselectivity, using 20 mol % of bicyclic guanidine 1, 0.1 equivalents of Et3N as an additive, and toluene as a solvent, at 10 8C. The results are summarized in Table 2. 3-Benzyl-2-oxindole 2 b with a cyano group at the ortho position of the phenyl ring decreased the enantioselectivity significantly (Table 2, entry 1). The fluorine group at the ortho position of 2 c gave good enantioselectivity, thus indicating that the fluorine substituent can achieve better enantioselectivities under the same reaction conditions (Table 2, entry 2). We then employed 2 d that has a fluorine atom at the meta position, and

Table 1. Screening studies of the Michael addition of 3-benzyl-substituted oxindole 2 a to 2-cyclopentenone 3 catalyzed by bicyclic guanidine 1.[a]

Entry

1 [mol %]

Additive

Solvent

Temp [o C]

t [h]

Yield [%][b]

ee [%][c]

d.r.[c]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17[d] 18[e] 19

10 10 10 10 10 10 10 10 20 20 20 20 20 20 20 20 20 20 20

– – – – – – – – – – – – Et3N Et2NH DIPEA Pyridine Et3N Et3N –

Toluene CH2Cl2 THF Et2O CH3CN Mesitylene m-Xylene p-Xylene Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene Et3N

25 25 25 25 25 25 25 25 25 0 10 20 10 10 10 10 10 10 10

36 36 36 36 36 36 36 36 24 72 72 96 72 96 120 120 48 48 48

32 37 40 33 29 34 30 22 80 77 41 30 94 48 45 50 87 93 60

79 75 72 78 46 78 77 77 79 80 85 85 83 81 23 27 77 80 81

95:5 89:11 85:15 91:9 70:30 93:7 93:7 91:9 95:5 96:4 98:2 96:4 95:5 98:2 93:7 94:6 93:7 92:8 92:8

[a] The reaction was carried out with 2 a (0.1 mmol), 3 (0.05 mmol), and guanidine 1 (0.005–0.01 mmol) in solvent (0.5 mL). [b] Yield of isolated product. [c] Determined by HPLC method. [d] Et3N (0.3 equiv) was used. [e] Et3N (0.5 equiv) was used. DIPEA = N,N-Diisopropylethylamine.

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ic ring of the benzyl groups at the C3-position of 3-(4fluoro-benzyl)-substituted oxindoles 2 did not affect the enantioselectivity, but with the exception of 2 q that contains two trifluoro groups at the 3- and 5-positions (Table 2, entry 8). However, it is worth noting that the electron-neutral and electron-donating substituents (2 s–v; Table 3, entries 10–13) gave higher diastereoselectivities than electronwithdrawing groups (2 k–r, Table 2, entries 2–9). Moreover, the introduction of various substituents at the 5-, 6-, and 7positions of the aromatic ring of oxindoles (2 w–z) appeared to have a very limited effect on the enantioselectivity (Table 2, entries 14–17). Notably, the reaction could be carried out on a gram scale, and gave product 4 f in good yield without compromising the enantioselectivity (Table 2, entry 1). The absolute configurations of the Michael adducts were assigned based on X-ray crystallographic analysis of a single crystal of 4 z (Figure 1).[18] Under the established reaction conditions, the Michael addition of 3-phenyl-substituted oxindole 5 to 2-cyclopentenone 3 was also investigated; the reaction could reach completion within 50 h, thereby affording the adduct 6 in high yield but with moderate enantio- and diastereoselectivity [Scheme 2, Eq. (5)]. When the reaction of 3-benzyl-substituted oxindole 2 f with 2-cyclohexenone 7 was performed under the same conditions, the corresponding adduct 8 was obtained in 65 % yield with 83 % ee and 4:1 d.r. after 96 h

Table 2. Investigation on the effect of the N-substituents of 3-benzyl-substituted oxindoles 2.[a]

Entry

Ar, 2

Yield [%][b]

4

ee [%][c]

d.r.[c]

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

2-CNC6H4, 2 b 2-FC6H4, 2 c 3-FC6H4, 2 d 3,5-(tBu)2C6H3, 2 e 4-FC6H4, 2 f 4-ClC6H4, 2 g 4-BrC6H4, 2 h 4-tBuC6H4, 2 i 4-MeOC6H4, 2 j 4-FC6H4, 2 f 4-FC6H4, 2 f

76 70 77 52 84 67 73 81 47 73 87

4b 4c 4d 4e 4f 4g 4h 4i 4j 4f 4f

37 79 83 79 87 82 86 83 83 88 90

92:8 95:5 95:5 89:11 95:5 91:9 95:5 89:11 95:5 95:5 98:2

[a] Unless otherwise noted, the reaction was carried out with 2 a (0.1 mmol), 3 (0.05 mmol), guanidine 1 (0.01 mmol), and Et3N (0.005 mmol) in toluene (0.5 mL) at 10 8C. [b] Yield of isolated product. [c] Determined by HPLC method. [d] The reaction was carried out with 2 a (0.075 mmol), 3 (0.05 mmol), guanidine 1 (0.01 mmol), and Et3N (0.005 mmol). [e] The reaction was carried out with 2 a (0.075 mmol), 3 (0.05 mmol), guanidine 1 (0.01 mmol), and Et3N (0.01 mmol).

4 d was afforded in 77 % yield with 83 % ee and 95:5 d.r. (Table 2, entry 3). Two bulkier tert-butyl groups were incorporated at the two meta positions of the phenyl ring, but the enantio- and diastereoselectivity were decreased (Table 2, entry 4). When other substituents (F, Cl, Br, tert-butyl, methoxy) were introduced at the para position of the phenyl ring (2 f–j, Table 2, entries 5–9), the enantioselectivities slightly improved with the exception of 3-benzyl-2-oxindole 2 g (Table 2, entry 6). As predicted, 4 f with a fluorine substituent at the para position of the phenyl ring was obtained with the best results (84 % yield, 87 % ee, 95:5 d.r., Table 2, entry 5). To improve the atom economy, we attempted to decrease the number of equivalents of 2 f from 2.0 to 1.5 (Table 2, entry 10). However, we found that the ee and d.r. values of 4 f were maintained but the yield was slightly reduced. To our delight, when the number of equivalents of Et3N was increased from 0.1 to 0.2, 4 f was obtained with 87 % yield, 90 % ee, and 98:2 d.r. (Table 2, entry 11). With the optimized reaction conditions in hand, we evaluated the performance of the asymmetric Michael additions of various 3-(4-fluoro-benzyl)substituted oxindoles 2 to 2-cyclopentenone 3 by using 20 mol % of catalyst 1 (Table 3). The reactions were complete within 59–72 h and gave products in 69–99 % yield with 73–98 % ee and 9:1 to > 20:1 d.r. It was found that the position and electronic properties of the substituents on the aromat-

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Table 3. Michael addition of various 3-(4-fluoro-benzyl)-substituted oxindoles 2 to 2cyclopentenone 3 catalyzed by bicyclic guanidine 1.[a]

Entry [e]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

R H H H H H H H H H H H H H 5-F 5-OMe 6-Br 7-F

Ar

2

C6H5 4-CNC6H4 4-FC6H4 4-BrC6H4 3-FC6H4 2-FC6H4 2-BrC6H4 3,5-(CF3)2C6H4 2,4-Cl2C6H4 4-CH3C6H4 4-CH3OC6H4 3-CH3OC6H4 2-CH3OC6H4 C6H5 C6H5 C6H5 C6H5

2f 2k 2l 2m 2n 2o 2p 2q 2r 2s 2t 2u 2v 2w 2x 2y 2z

t [h] 80 63 72 59 63 72 72 64 64 64 64 64 84 64 64 64 64

Yield [%][b] 82 95 99 98 87 91 99 99 83 85 87 82 98 87 69 97 94

4 4f 4k 4l 4m 4n 4o 4p 4q 4r 4s 4t 4u 4v 4w 4x 4y 4z

ee [%][c] [f]

90 (98) 91 91 94 88 91 95 73 85 96 89 91 90 89 93 95 90

d.r.[d] > 20:1 15:1 12:1 10:1 13:1 13:1 > 20:1 9:1 9:1 > 20:1 > 20:1 > 20:1 > 20:1 > 20:1 > 20:1 10:1 > 20:1

[a] The reaction was carried out with 2 (0.15 mmol), 3 (0.1 mmol), guanidine 1 (0.02 mmol), and Et3N (0.02 mmol) in toluene (1.0 mL) at 10 8C. [b] Yield of isolated product. [c] Determined by HPLC method. [d] Determined by 1H NMR analysis. [e] The reaction was also conducted on a 5.0 mmol scale, thereby affording adduct 4 f in 87 % yield with 90 % ee and > 20:1 d.r. after 96 h. [f] The data in parenthesis were obtained by simple filtration using petroleum ether (b.p. 60–908C) as an eluent.

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Figure 2. Proposed transition-state model.

Figure 1. X-ray crystal structure of adduct 4 z. Thermal ellipsoids set at the 30 % probability level

tension of this protocol to other asymmetric Michael reactions with 3-cyclopentenone as an electrophile.

Experimental Section General procedure for the asymmetric Michael addition of 3-benzylsubstituted oxindoles (2) to 2-cyclopentenone (3) catalyzed by bicyclic guanidine (1) 3-Benzyl-substituted oxindole 2 (0.15 mmol, 1.5 equiv), bicyclic guanidine 1 (0.02 mmol, 0.2 equiv/20 mol %), and Et3N (0.02 mmol) were dissolved in toluene (1.0 mL) and stirred at 10 8C for 10 min, then 2-cyclopentenone 3 (0.1 mmol, 1.0 equiv) was added. The reaction mixture was stirred at 10 8C and monitored by TLC. Upon complete consumption of 3, the reaction mixture was directly loaded onto a short silica-gel column, followed by separation with flash chromatography using gradient elution with petroleum ether/ethyl acetate (50:1 to 5:1). After removal of the solvent under vacuum, the corresponding products 4 were obtained.

Acknowledgements This work was supported by NSFC (nos. 21072044 and 21202034), the Program for New Century Excellent Talents in University of Ministry of Education (NCET-11-0938) and Excellent Youth Foundation of Henan Scientific Committee (114100510003).

Scheme 2. Bicyclic-guanidine-catalyzed Michael addition of 3-phenyl-substituted oxindole 5 to 2-cyclopentenone 3 [Eq. (5)]; Michael addition of 3-benzyl-substituted oxindole 2 f to 2-cyclohexenone 7 [Eq. (6)]; A useful example for the transformation of 4 f [Eq. (7)].

Keywords: 2-cyclopentenone · asymmetric guanidine · Michael addition · organocatalysis

[Scheme 2, Eq. (6)]. Furthermore, to demonstrate the potential of the present catalytic reaction, we attempted to modify the Michael adduct 4. As described in Scheme 2 [Eq. (7)], 4 f could be readily transformed into 9 by deprotection of the N-(4-fluoro-benzyl) group of 4 f with moderate yield and without compromising the enantioselectivity. In our previous work on C2-symmetric bicyclic-guanidinecatalyzed reactions,[3j] we proposed and verified the side-on transition state (TS), which is preferred over the face-on TS. According to the obtained stereochemistry of the Michael adduct, a plausible TS model is proposed in Figure 2. In conclusion, we have developed the first highly enantioand diastereoselective Michael addition of various 3-benzylsubstituted oxindoles to 2-cyclopentenone. A variety of 3,3disubstituted oxindoles were obtained in 69–99 % yield with 73–98 % ee and 9:1 to > 20:1 d.r. Further investigations will focus on the highly stereoselective Michael addition of 3aryl-substituted oxindoles to 3-cyclopentenone, and the ex-

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synthesis

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