Chiral ligands derived from abrine.: Part 6:: Importance of a bulky N-alkyl group in indole-containing chiral β-tertiary amino alcohols for controlling enantioselectivity in addition of diethylzinc toward aldehydes

被引:75
作者
Dai, WM
Zhu, HJ
Hao, XJ
机构
[1] Hong Kong Univ Sci & Technol, Dept Chem, Kowloon, Hong Kong, Peoples R China
[2] Chinese Acad Sci, Kunming Inst Bot, Kunming 650204, Yunnan, Peoples R China
关键词
D O I
10.1016/S0957-4166(00)00189-0
中图分类号
O61 [无机化学];
学科分类号
070301 ; 081704 ;
摘要
A number of chiral beta-amino alcohols possessing a 3-indolylmethyl group have been synthesized from the alkaloid (S)-abrine and elucidated for potency in the catalytic enantioselective ethylation of PhCHO with Et2Zn. In general, the secondary amines 15a-d bearing a dialkylhydroxymethyl group induced (R)-1-phenyl-1-propanol, whereas 15e-g and 18 bearing a diarylhydroxymethyl group favored the (S)-enantiomer. In contrast, the beta-tertiary amino alcohols 20b d and 21 produced (R)-1-phenyl-1-propanol. regardless of the substituents at the carbon bearing the hydroxy group. Enantiomeric excess of 87.5% was obtained for (R)-1-phenyl-1-propanol using ligand 21 as the promoter. Eleven substituted benzaldehydes and naphth-aldehydes were examined for enantioselective ethylation by using 21 and the chiral alcohols were obtained in 93-97% ee, except for o-BrC6H4CHO and p-Me2NC6H4CHO. Excellent enantioselectivity was also observed in the ethylation of cyclohexanecarboxaldehyde (94.8% ee) and 2-thiophenecarboxaldehyde (94.9% ee) by using catalytic 21. The anti 5/4/4-fused tricyclic TS I was proposed to rationalize the asymmetric induction. The diethylhydroxymethyl and N-2-t-butylethyl groups are believed to enforce the preference for the anti-TS(R) I and it results in high enantioselectivity. (C) 2000 Elsevier Science Ltd. All rights reserved.
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页码:2315 / 2337
页数:23
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