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DOI: 10.1039/C7QO00768J (Research Article) Org. Chem. Front., 2018, 5, 32-35

Asymmetric synthesis of 3-aminodihydrocoumarins via the chiral guanidine catalyzed cascade reaction of azlactones†

Sai Ruan , Xiaobin Lin , Lihua Xie , Lili Lin , Xiaoming Feng and Xiaohua Liu *
Key Laboratory of Green Chemistry & Technology, Ministry of Education, College of Chemistry, Sichuan University, Chengdu 610064, People's Republic of China. E-mail: liuxh@scu.edu.cn

Received 29th August 2017 , Accepted 17th September 2017

First published on 18th September 2017

Abstract

A highly efficient bifunctional guanidine catalyst was developed for the asymmetric cascade reaction between 2-nitrovinylphenols and azlactones. A wide variety of 3-aminodihydrocoumarin derivatives could be obtained in high yields (up to 99% yield) with excellent enantioselectivities (up to 99% ee) and diastereoselectivities (>19[thin space (1/6-em)]:[thin space (1/6-em)]1 dr).

Introduction

The asymmetric catalytic synthesis of substituted dihydrocoumarins has particular significance in modern organic chemistry.1 This is attributed to the fact that this functional structural motif frequently occurs in biologically and pharmacologically active compounds as well as natural products.2 During the past few years, many methods have been reported to generate these valuable molecules.3 For example, 3-aminodihydrocoumarin derivatives are used as antihypertensive agents and platelet aggregation inhibitors.4 New methods based on the reactions of alkylideneoxazolones5 and azlactones5e,6,7 are the representative routes for the construction of 3-aminodihydrocoumarins (Scheme 1). A chiral thiourea catalyzed cascade reaction between o-hydroxy aromatic aldimine A and alkylidene azlactones yielded chromeno[4,3-b]pyrrolidines (eqn (1)).5b A tandem Friedel–Crafts/transesterification process with phenol B gave access to aryl(benzoylamino)chromanones albeit in racemic version (eqn (2)).5c The formal asymmetric [4 + 2] cycloaddition of azlactones with o-quinone methides C generated from various precursors enabled the formation of 4-aryl/alkyl substituted aminochromanones, which has been realized separately by several groups (eqn (3)).7a–d In addition, enantioselective organocatalyzed nucleophilic addition/transesterification cascade reactions8 of azlactones with 2-hydroxyphenyl substituted electrophiles D benefited the formation of various 4-functionalized 3-aminochromanones (eqn (4)).7e,f Albrecht7g and Wang7h developed the diastereo- and enantioselective cascade reaction between o-hydroxychalcones and azlactones using cinchona-alkaloid-based squaramide or thiourea as the catalyst, respectively. Zhou and coworkers used L-tert-leucine derived squaramide for the related reaction of 2-(2-nitrovinyl)phenols7e to obtain 4-nitromethyl substituted chroman-2-ones. Excellent yield and stereoselectivity were achieved but the reaction is limited in the scope of azlactones.
image file: c7qo00768j-s1.tif
Scheme 1 Representative synthesis routes for 3-aminodihydrocoumarin derivatives.

In our efforts to develop new chiral bifunctional guanidine catalysts,9,10 we succeeded in several asymmetric transformations of azlactones, including a cascade reaction with o-hydroxy aldimines to obtain 3,4-diamino chroman-2-ones.7f Thus we sought to exploit chiral guanidine in the cascade reaction for the generation of 4-nitromethyl 3-aminochromanones. Herein, we report the highly diastereo- and enantioselective conjugate addition/lactonization between various 2-nitrovinylphenols11 and azlactones.

Results and discussion

In our initial research, we chose (E)-2-(2-nitrovinyl)phenol 1a and azlactone 2a as the model substrates, and the reaction was investigated at −30 °C in ethyl acetate with various chiral guanidine organocatalysts (Table 1). Firstly, bisguanidine BG-1 and its hemi-salt BG-1·HBArF47f,10c,d which were proved to be efficient for several asymmetric transformations of azlactones were used for the current cascade reaction. It was found that these two catalysts could promote the reaction to give the desired 3-aminodihydrocoumarin 3aa but with poor diastereo- and enantioselectivities (entries 1 and 2). Nevertheless, other bifunctional guanidines G-1 to G-5 which were derived from chiral amino acids and 1,2-diphenylethane-1,2-diamine but embellishing a sulfonamide unit showed excellent reactivity (94–99% yield) and diastereoselectivity (>19[thin space (1/6-em)]:[thin space (1/6-em)]1) (entries 3–7). The amino acid backbone had an obvious influence on the enantioselectivity of the reaction, and tetrahydroisoquinoline-based G-4 and G-5 gave higher results than guanidines G-1–G-3 derived from L-proline, L-pipecolic acid, and L-ramipril subsequently.12 Further adjustment of the sulfonamide substituent indicated that the installation of the 2,6-difluorobenzenesulfonamide group onto G-5 enhanced the enantioselection to 85% ee in comparison with the TsNH substituted G-4 (entry 7 vs. entry 6). When the solvent was changed from ethyl acetate to THF, the enantioselectivity of the reaction became 88% ee with the yield and dr maintained (entry 8). When the reaction was performed at −60 °C, the enantioselectivity greatly improved to 95% ee (entry 9). If the catalyst loading of G-5 was reduced to 5 mol%, the ee value of 3aa slightly declined but with maintained yield and diastereoselectivity (entry 10).
Table 1 Optimization of the reaction conditionsa

image file: c7qo00768j-u1.tif
Entrya Cat Yieldb (%) drc eed (%)
a Unless otherwise noted, the reactions were carried out with guanidine (10 mol%), 1a (0.10 mmol) and 2a (1.2 equiv.) in AcOEt (1.0 mL) at −30 °C for 24 h. b Isolated yield. c Determined by NMR analysis. d Determined by UPC2 analysis. e THF (1.0 mL) instead of AcOEt. f At −60 °C for 72 h. g G-5 (5 mol%).
1 BG-1 86 2.5[thin space (1/6-em)]:[thin space (1/6-em)]1 24/14
2 BG-1·HBArF4 75 2[thin space (1/6-em)]:[thin space (1/6-em)]1 23/11
3 G-1 98 >19[thin space (1/6-em)]:[thin space (1/6-em)]1 47
4 G-2 94 >19[thin space (1/6-em)]:[thin space (1/6-em)]1 52
5e G-3 97 >19[thin space (1/6-em)]:[thin space (1/6-em)]1 41
6 G-4 95 >19[thin space (1/6-em)]:[thin space (1/6-em)]1 74
7 G-5 99 >19[thin space (1/6-em)]:[thin space (1/6-em)]1 85
8e G-5 99 >19[thin space (1/6-em)]:[thin space (1/6-em)]1 88
9f G-5 99 >19[thin space (1/6-em)]:[thin space (1/6-em)]1 95
10f,g G-5 99 >19[thin space (1/6-em)]:[thin space (1/6-em)]1 93

Having identified the optimized reaction conditions (Table 1, entry 9), we next investigated the scope of (E)-2-(2-nitrovinyl) phenols 1 which were employed to react with azlactone 2a. As shown in Table 2, a variety of substrates bearing electron-withdrawing and electron-donating substituted phenolic groups were experimented on. Both the position and electronic nature of the substituent affected the yield obviously. For halo-substituted nitroolefins 1b–1d, the yield decreased along with the increase in atomic volume but without affecting the enantioselectivity (entries 2–4). 5-Substituted nitroalkenes 1e–1g underwent the reaction in relatively lower yield compared with the others (entries 5–7). It was noteworthy that the hydroxyl group tolerated well in the reaction, and the transformation of the substrate 1g afforded the corresponding product 3ga in 80% yield and 96% ee (entry 7). 6-Substituted nitroolefins 1h–1n performed the reaction well to give the related dihydrocoumarins 3ha–3na in 80–99% yield and 91–95% ee (entries 8–14). It was obvious that C6-electron-withdrawing groups resulted in lower yield than the electron-donating ones. Particularly, substrate 1n bearing the sterically hindered tert-butyl group adjacent to the hydroxyl group participated in the cascade reaction with 99% yield and 95% ee (entry 14). In all cases, the diastereoselectivity of the reaction was as high as 19[thin space (1/6-em)]:[thin space (1/6-em)]1.

Table 2 Substrate scope of (E)-2-(2-nitrovinyl) phenols 1a

image file: c7qo00768j-u2.tif
Entrya 1: R1 Yieldb (%) drc eed (%)
a Unless otherwise noted, the reactions were carried out with G-5 (10 mol%), 1 (0.10 mmol) and 2a (1.2 equiv.) in THF (1.0 mL) at −60 °C for 72 h. b Isolated yield. c Determined by NMR analysis. d Determined by UPC2 analysis.
1 1a: H 99 (3aa) >19[thin space (1/6-em)]:[thin space (1/6-em)]1 95
2 1b: 4-F 99 (3ba) >19[thin space (1/6-em)]:[thin space (1/6-em)]1 95
3 1c: 4-Cl 90 (3ca) >19[thin space (1/6-em)]:[thin space (1/6-em)]1 95
4 1d: 4-Br 84 (3da) >19[thin space (1/6-em)]:[thin space (1/6-em)]1 96
5 1e: 5-Me 80 (3ea) >19[thin space (1/6-em)]:[thin space (1/6-em)]1 92
6 1f: 5-Cl 85 (3fa) >19[thin space (1/6-em)]:[thin space (1/6-em)]1 92
7 1g: 5-OH, 6-Me 80 (3ga) >19[thin space (1/6-em)]:[thin space (1/6-em)]1 96
8 1h: 6-F 83 (3ha) >19[thin space (1/6-em)]:[thin space (1/6-em)]1 94
9 1i: 6-Cl 82 (3ia) >19[thin space (1/6-em)]:[thin space (1/6-em)]1 91
10 1j: 6-Br 80 (3ja) >19[thin space (1/6-em)]:[thin space (1/6-em)]1 91
11 1k: 6-Me 92 (3ka) >19[thin space (1/6-em)]:[thin space (1/6-em)]1 95
12 1l: 6-MeO 99 (3la) >19[thin space (1/6-em)]:[thin space (1/6-em)]1 92
13 1m: 6-EtO 95 (3ma) >19[thin space (1/6-em)]:[thin space (1/6-em)]1 93
14 1n: 6-t-Bu 99 (3na) >19[thin space (1/6-em)]:[thin space (1/6-em)]1 95

Encouraged by the above mentioned results, we continued to investigate the scope of azlactones 2 (Table 3). 2-Aromatic substituents of azlactones 2b–2g underwent the reaction resulting in good to excellent yields and satisfactory enantioselectivity (entries 1–6). The electron-deficient substituent at the para-position of the 2-aryl group weakened the reactivity in comparison with electron-donating ones, and cyano-substituted 2d gave the lowest yield of 63% with 93% ee but ethyl-substituted 2g afforded up to 99% yield with 98% ee (entry 6 vs. entry 3). Next, azlactones varied at the C4-position were subjected to the reaction (entries 8–11). We synthesized azlactones 2i–2l from alanine, leucine, 2-amino-4-phenylbutanoic acid, and tryptophan separately, and allowed them to react with 1a. These reactions proceeded smoothly to the desired products in very high yields and enantioselectivities. Particularly, the 3-indolylmethyl substituted one afforded the formation of functional dihydrocoumarin derivative 3al in almost optical purity and equivalent yield.

Table 3 Substrate scope of azlactones 2a

image file: c7qo00768j-u3.tif
Entry 2: R2/R3 Yieldb (%) drc eed (%)
a Unless otherwise noted, the reactions were carried out with G-5 (10 mol%), 1a (0.10 mmol) and 2 (1.2 equiv.) in THF (1.0 mL) at −60 °C for 72 h. b Isolated yield. c Determined by NMR analysis. d Determined by UPC2 analysis. e The azlactones 2 (2.0 equiv.).
1 2b: 4-ClC6H4/Bn 87 (3ab) >19[thin space (1/6-em)]:[thin space (1/6-em)]1 95
2 2c: 4-BrC6H4/Bn 87 (3ac) >19[thin space (1/6-em)]:[thin space (1/6-em)]1 96
3 2d: 4-NCC6H4/Bn 63 (3ad) >19[thin space (1/6-em)]:[thin space (1/6-em)]1 93
4 2e: 4-MeC6H4/Bn 99 (3ae) >19[thin space (1/6-em)]:[thin space (1/6-em)]1 95
5 2f: 4-MeOC6H4/Bn 99 (3af) >19[thin space (1/6-em)]:[thin space (1/6-em)]1 95
6 2g: 4-EtC6H4/Bn 99 (3ag) >19[thin space (1/6-em)]:[thin space (1/6-em)]1 98
7 2h: Ph/4-ClC6H4CH2 99 (3ah) >19[thin space (1/6-em)]:[thin space (1/6-em)]1 96
8e 2i: Ph/Me 94 (3ai) >19[thin space (1/6-em)]:[thin space (1/6-em)]1 98
9e 2j: Ph/i-Bu 97 (3aj) >19[thin space (1/6-em)]:[thin space (1/6-em)]1 97
10 2k: Ph/CH2CH2Ph 97 (3ak) >19[thin space (1/6-em)]:[thin space (1/6-em)]1 98
11 2l: Ph/3-Indolylmethyl 99 (3al) >19[thin space (1/6-em)]:[thin space (1/6-em)]1 99

Taking into account the practical application of the catalyst system, the gram-scale synthesis of 3aa was performed. Under the optimal reaction conditions, 3 mmol (0.50 g) 1a and 3.6 mmol (0.91 g) azlactone 2a reacted well to afford the desired adduct 3aa in 84% yield (1.06 g) with 92% ee and >19[thin space (1/6-em)]:[thin space (1/6-em)]1 dr (Scheme 2). The absolute configuration of the product 3aa was determined to be (3R, 4R) by X-ray crystallography analysis, and the others were confirmed in comparison with the Cotton effect in the CD spectra analysis (see the ESI† for details).13 The chiral G-5 acts as a bifunctional catalyst such that the guanidine unit benefits the activation of azlactones and the sulfonamide group activates the nitroolefins via H-bonding. The diastereo- and enantioselective oxazolone–nitroalkene conjugate addition intermediate11f undergoes an intramolecular transesterification to give the final 4-nitromethyl 3-aminochromanone.


image file: c7qo00768j-s2.tif
Scheme 2 The gram-scale synthesis of 3aa.

Conclusions

We have successfully developed an efficient method for the asymmetric synthesis of chiral 3,4-dihydrocoumarins in the presence of a bifunctional guanidine catalyst. A variety of azlactones and ortho-nitrovinylphenols were tolerated well, providing the corresponding 4-functionalized 3-aminochromanones in high yields and excellent diastereo- and enantioselectivities. Further applications of the bifunctional guanidines in asymmetric catalysis are ongoing in our laboratory.

Experimental section

General procedure for the cascade reaction

2-Nitrovinylphenol 1 (0.10 mmol), azlactone 2 (1.2–2.0 equiv.) and the catalyst G-5 (10 mol%) were added into a test tube, followed by the addition of THF (1.0 mL). Then the mixture was stirred at −60 °C. After completion, the reaction mixture was purified by silica gel column chromatography (ethyl acetate/petroleum ether 1/4–1/2) to afford the desired products.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We acknowledge the National Natural Science Foundation of China (No. 21625205 and 21332003) and the National Program for Support of Top-Notch Professionals for financial support.

Notes and references

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  13. CCDC 1560312 (3aa). For more data, see the ESI.†.

Footnote

† Electronic supplementary information (ESI) available. CCDC 1560312. For ESI and crystallographic data in CIF or other electronic format, see DOI: 10.1039/c7qo00768j
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