Synlett 2009(6): 925-928  
DOI: 10.1055/s-0028-1088210
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Mild Michael Addition of Glycine Imines to Aromatic Nitroalkenes Catalyzed by DBU with LiOTf as an Additive

Wei Lia, Han Liua, Da-Ming Du*a,b
a Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Bioorganic Chemistry and Molecular Engineering, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. of China
Fax: +86(10)68914985; e-Mail: dudm@pku.edu.cn;
b School of Chemical Engineering and Environment, Beijing Institute of Technology, Beijing 100081, P. R. of China
Further Information

Publication History

Received 23 September 2008
Publication Date:
16 March 2009 (online)

Abstract

A mild Michael addition of glycine imines to aromaticnitroalkenes catalyzed by 10 mol% DBU with LiOTf as an additive was developed. In most cases, the products could be obtained in good yields (up to 96%) with moderate to good diastereoselectivities (up to 10:1). The selectivity for syn adduct can be reversed to anti when the R group of glycine imines was changed from methyl or ethyl to tert-butyl.

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15

General Procedure for Michael Addition of Glycine Imines to Aromatic Nitroalkenes
To a stirred solution of nitroalkene (1.2 mmol), LiOTf (16 mg, 0.1 mmol), and ethyl diphenylmethyleneiminoacetate (267 mg, 1 mmol) or tert-butyl diphenylmethyleneimino-acetate (295 mg, 1 mmol) in dry THF (1 mL) was added DBU (15 mg, 0.1 mmol) in dry THF (1 mL). The mixture was stirred at r.t. for 24 h. After being quenched by H2O, the mixture was extracted by CH2Cl2. The organic phase was separated and dried with Na2SO4. The diastereoselectivity was determined by NMR analysis of curde product. The sample for analysis was purified on column chromatography (SiO2, 200-300 mesh) using PE-EtOAc (20:1) as eluent and recrystallized from Et2O and PE.
syn -Ethyl 2-Diphenylmethyleneimino-4-nitro-3-phenyl-butanoate (5a)
According to the general procedure, a white solid was obtained; mp 84-85 ˚C. ¹H NMR (300 MHz, CDCl3): δ = 1.20 (t, J = 7.2 Hz, 3 H), 4.11-4.16 (m, 2 H), 4.27-4.38 (m, 2 H), 5.14-5.18 (m, 2 H), 6.60-6.62 (d, J = 6.9 Hz, 2 H), 7.14-7.48 (m, 1 1H), 7.64 (d, J = 6.9 Hz, 2 H). IR: 1735, 1551, 1446, 1368, 1316, 1290, 1190, 1024, 695 cm. MS (70 eV, EI): m/z (%) = 416 (3) [M+], 343 (10), 296 (23), 267 (21), 266 (100), 193 (47), 165 (50). Anal. Calcd (%) for C25H24N2O4: C, 72.10; H, 5.81; N, 6.73. Found: C, 71.74; H, 5.83; N, 6.55.2.
syn -Ethyl 2-Diphenylmethyleneimino-3-(4-methylphenyl)-4-nitrobutanoate (5b)
According to the general procedure, a white solid was obtained; mp 102-103 ˚C. ¹H NMR (300 MHz, CDCl3): δ = 1.19 (t, J = 6.9 Hz, 3 H), 2.29 (s, 3 H), 4.10-4.15 (m, 2 H), 4.27-4.32 (m, 2 H), 5.10-5.12 (m, 2 H), 6.65 (d, J = 6.0 Hz, 2 H), 7.04 (s, 4 H), 7.27-7.45 (m, 6 H), 7.65 (d, J = 7.5 Hz, 2 H). ¹³C NMR (75 MHz, CDCl3): δ = 14.0, 21.0, 46.2, 61.5, 68.7, 76.3, 127.3, 128.0, 128.2, 128.3, 128.6, 128.9, 129.3, 130.9, 134.0, 135.4, 137.4, 138.7, 169.9, 172.6. IR: 1736, 1732, 1619, 1552, 1516, 1446, 1379, 1317, 1288, 1182, 1026, 695 cm. MS (70 eV, EI): m/z (%) = 430 (4) [M+], 413 (3), 357 (7), 310 (17), 267 (27), 266 (100), 238 (22), 193 (69), 165 (61). Anal. Calcd (%) for C26H26N2O4: C, 72.54; H, 6.09; N, 6.51. Found: C, 72.36; H, 6.22; N, 6.35.