Kinetic acetonation of d-galactose, d-allose, and d-talose with alkyl isopropenyl ethers as a preparative route to the 4,6- O-isopropylidenealdohexopyranoses

Abstract

The action of 2 molar equiv. of ethyl (or methyl) isopropenyl ether on d-galactose causes acetonation under conditions of kinetic control, to give 4,6- O-isopropylidene-α,β- d-galactopyranose (3), isolated crystalline in 67% yield after separation from a small proportion (14%) of the pyranoid 3,4-acetal and a trace (1-2%) of the furanoid 5,6-acetal. Similar acetonation of d-allose led essentially exclusively (81%) to 4,6- d-isopropylidene- d-allopyranose, which crystallized mainly as the β anomer. Analogous treatment of d-talose gives 4,6- O-isopropylidene-α- d-talopyranose in 93% yield. The pyranoid ring-structures of the acetals were established by the sequence acetylation-deacetonation-acetylation to give the known aldopyranose pentaacetates, and studies by n.m.r. spectroscopy and mass spectrometry established the positions of acetal substitution; compound 3 was also identified by comparison of its properties with data in the literature