Enzyme catalyzed reactions .26. Stereoselective synthesis of (1R)- and (1R,2S)-1-aryl-2-alkylamino alcohols from (R)-cyanohydrins

Hydrogenation of (R)-cyanohydrins (R)-1 with LiAlH4 occurs without racemization to give the (R)-2-amino alcohols (R)-3. (1R,2S)-2-Amino alcohols (1R,2S)4 are obtained with high diastereoselectivity by addition of methyl Grignard to O-silyl protected cyanohydrins (R)-2 and subsequent hydrogenation with NaBH4. The N-alkylated 2-amino alcohols (R)-8 and (1R,2S)-9 can be prepared either by reductive alkylation of the corresponding 2-amino alcohols (R)-3 and (1R,2S)-4, respectively, or by a transimination reaction of the Grignard addition products with primary amines and subsequent hydrogenation with NaBH4. The lower diastereoselectivity of hydrogenation in case of the N-alkylimino compounds in comparison to the N-unsubstituted imines is explained by a weaker chelating effect. (C) 1997 Elsevier Science Ltd. All rights reserved.