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

The 1,4-addition of magnesium divinylcuprate prepared from vinylmagnesium bromide and cuprous bromide to some 4-O-crotonyl derivatives of methyl α-D-glucopyranoside proceeds with a high level of diastereochemical induction, providing the adduct in good-to-excellent yields. Other organocuprates also serve as effective carbon nucleophiles for the 1,4-addition. Removal of the carbohydrate moiety from each adduct afforded a variety of β-C-substituted butanoic esters in remarkable enantiomeric excess. The 1,4-addition of the same cuprate to some methyl α-D-manno- or α-D-galactopyranosidic substrates in which a crotonyl group was incorporated, each at 3-OH, was also investigated. The reverse π-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. Furthermore, some D-galacto-type substrates provided 1,4-adducts with higher diastereoselectivities.