Cob(I)alamin als Katalysator, 5. Mitteilung [1]. Enantioselektive Reduktion α,β‐ungesättigter Carbonylderivate

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/CH 3 COOH/H 2 O, 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 .