Preparation and crystal and molecular structure of 6-O-[(2S)-2,3-epoxypropyl]-1, 2:3,4-di-O-isopropylidene-α-d-galactopyranose. Pyranoid ring conformation in 1,2:3,4-di-O-isopropylidene galactopyranose and related systems

The title compound was isolated by several recrystallisations from a 1:1 mixture of diastereomers which was obtained in 75% yield from the reaction of 1,2:3,4-di-O- isopropylidene-α-d-galactopyranose with epichlorohydrin. The absolute configuration of this isomer (30% yield) was established by X-ray crystallography. The conformation of the pyranoid ring in this compound is compared to those in related situations. In all these cases the pyranoid ring adopts a heavily distorted conformation which is described on average as between the screw-boat OS5 and the twist-boat (“skew”-boat) OT2 with deviations in the direction of the boat B2,5 in most cases. The puckering parameters for all such reported pyranoid rings were calculated or recalculated, and are given within expanded conventions of carbohydrate nomenclature.