Stereochemical Analysis of the Enzymic Reduction of the Double Bond of alpha - and beta -Substituted Nitrostyrenes and alpha -Ethoxycinnamaldehyde through Deuterium Labelling Experiments

(2)H NMR studies and comparison with authentic labelled reference compounds prepared from the (Z)-acetamidocinnamic acid 10 and from (Z)-2-ethoxy-3-(4-methoxyphenyl) prop-2-en-1-ol (14), respectively, by catalytic syn reduction with deuterium gas show that the baker's yeast-mediated saturation in the presence of deuterated water of the double bond of (E)-alpha-(hydroxymethyl) nitrostyrene (2) and of the (Z)-alpha-ethoxycinnamaldehyde (3) leads to (2R, 3R)-[ 2,3-(2)H(2)]-3-(4-methoxyphenyl)-2-nitropropan-1-ol (5) and to (1R, 2S, 3R)-[1,2,3-(3)H(2)]-2-ethoxy-3-(4-methoxyphenyl) propan-1-ol (6), respectively, thus supporting an anti mode of hydrogen addition across the activated double bond, with hydride delivery in beta-position respect to the activating group from the upper side of the molecule.