Synthesis of 2,3:4,6-di- O-isopropylidene derivatives of alkyl α- and β- d-galactopyranosides, and elucidation of structure by n.m.r. and x-ray analysis

Abstract

Kinetically controlled reaction of 4.5 equiv. of 2-methoxypropene with some alkyl α- and β- d-galactopyranosides gave the 2,3:4,6-di- O-isopropylidene derivatives in high yields (80–85%). With 2 equiv. of 2-methoxypropene, benzyl β- d-galactopyranoside gave the 4,6- and the 3,4-monoacetals in the ratio 30:1 together with ∼20% of the 2,3:4,6-diacetal. The structures of methyl 2,3:4,6-di- O-isopropylidene-α- and -β- d-galactopyranosides were determined by X-ray analysis. The former crystallised in the orthorhombic system, P2 12 12 1, with a = 5.503, b = 16.105, c = 16.822 Å, and Z = 4, the latter in the monoclinic system, P2 1, with a = 10.400, b = 13.344, c = 11.647 Å, β = 111.50, and Z = 4. The α anomer and the two molecules of the β anomer had the d-galactopyranoside and the 1,3-dioxane rings in twist-chair conformations and the dioxolane ring in a half-chair conformation. N.m.r. spectroscopy suggested the occurrence of similar conformations in solution.