COB(I)ALAMIN AS CATALYST .5. ENANTIOSELECTIVE REDUCTION OF ALPHA,BETA-UNSATURATED CARBONYL DERIVATIVES

Cob(I)alamin as Catalyst. 5. Communication [1]. Enantioselective Reduction of α,β‐Unsaturated Carbonyl Derivatives The cob(I)alamin‐catalyzed reduction of an α,β‐unsaturated ethyl ester in aqueous acetic acid produced the (S)‐configurated saturated derivative 2 with an enantiomeric excess of 21%. The starting material 1 is not reduced at pH = 7.0 in the presence of catalytic amounts of cob(I)alamin (see Scheme 2). It is shown that the attack of cob(I)alamin and not of cob(II)alamin, also present in Zn/CH3COOH/H2O, accounts for the enantioselective reduction observed. All the (Z)‐configurated starting materials 1, 3, 5, 7, 9 and 11 have been transformed to the corresponding (S)‐configurated saturated derivatives 2, 4, 6, 8, 10 and 12, respectively. The highest enantiomeric excess revealed to be present in the saturated product 12 (32,7%, S) derived from the (Z)‐configurated methyl ketone 11 (see Scheme 3 and Table 1). The reduction of the (E)‐configurated starting materials led mainly to racemic products. A saturated product having the (R)‐configuration with a rather weak enantiomeric excess (5.9%) has been obtained starting from the (E)‐configurated methyl ketone 23 (see Scheme 5 and Table 2). The allylic alcohols 16 and 24 have been reduced to the saturated racemic derivative 17. Copyright © 1979 Verlag GmbH & Co. KGaA, Weinheim