Recentrly we have found prominent 1,2- and 1,3-asymmetric induction in the intramolecular Michael addition of y- and δ-carbamoyloxy-α,β-unsaturated esters. High complementary stereoselection was observed by changing the site of carbamoyl group in the reactions of ethyl erythro-4,5-dihydroxy-2 (E)-hexenoate, and it was applied to the highly stereoselective syntheses of N-acyl (±)-acosamine (2b) and (±)-ristosamine (3b). Contrary to the erythro derivative (1,3-syn), relatively low 1,3-anti stereoselection (up to 5:1) was found in the reaction of ethyl threo-5-carbamoyloxy-4-trialkylsilyloxy-2 (E)-hexenoate. The reversal of diastereofacial selectivity in these reactions suggested that the factor governing such stereoselection might be the antiperiplanar effect due to the group at 4-position. Dramatic improvement of the 1,3-anti selectivity in the reaction of threo-diol derivatives, however, has been realized by changing the geometry of the double bond from E to Z, as expected by the mechanistic consideration of the reaction transition state based on the anti-periplanar effect. Thus, the optically active 17 was produced in ratio of >100:1 by the base-catalyzed cyclization of 11, synthesized from t-butyldimethyl-silyl (S)-lactaldehyde (13) and methyl propiolate via the stereoselective reduction of methyl 5-t-butyldimethylsilyloxy-4-oxo-2-hexynoate (15) with L-selectride, culminating in a stereoselective synthesis of N-benzoyl L-daunosamine (1b). New diastereoselective syntheses of optically active 2b and 3b will also be described.
