Highly diastereoselective 1,4-addition of an organocuprate to methyl alpha-D-gluco-, alpha-D-manno-, or alpha-D-galactopyranosides tethering an alpha,beta-unsaturated ester. Novel asymmetric access to beta-C-substituted butanoic acids

The 1,4-addition of magnesium divinylcuprate prepared from vinylmagnesium b romide and cuprous bromide to some 4-O-crotonyl derivatives of methyl alpha -D-glucopyranoside proceeds with a high level of diastereochemical inducti on, providing the adduct in good-to-excellent yields. Other organocuprates also serve as effective carbon nucleophiles for the 1,4-addition. Removal o f the carbohydrate moiety from each adduct afforded a variety of beta -C-su bstituted butanoic esters in remarkable enantiomeric excess. The 1,4-additi on of the same cuprate to some methyl alpha -D-manno- or alpha -D-galactopy ranosidic substrates in which a crotonyl group was incorporated, each at 3- OH, was also investigated. The reverse pi -facial attack of the cuprate was observed when some D-manno-type substrates were subjected to 1,4-addition conditions similar to those used for the D-gluco-type substrates. Furthermo re, some D-galacto-type substrates provided 1,4-adducts with higher diaster eoselectivities.