Synthesis of 3,4,6-Tri-O-acetyl-2-oximino-α-D-hexopyranosides

Abstract

The reaction of the dimeric nltrosyl chloride adducts of tri-O-acetyl-D-glucal and tri-O-acetyl-D-galactal with alcohols and phenols in dimethylformamide at room temperature provides the corresponding trl-O-acetyl-2-oximino-[alpha]-D-hexopyranosides In good to excellent yields. The condensation can also be carried out in tetrahydrofuran in the presence of pyridine or simply in refluxing methylene chloride for the simple alkyl alcohols. The stereospecificity of the reaction is extreme; only products of one configuration for both glycosidlc linkage and the oxlmlno group were detected. This result is rationalized on the basis of a cis-configuration for a tri-O-acetyl-2-nitroso-D-glycal intermediate in the H1 half-chair conformation. The introduction of methyl groups on the C of the methoxy group of methyl [alpha] -D-glucopyranosides causes a progressive deshielding of the anomeric proton. The data indicate that isopropyl [alpha]-D-glycopyranosides avoid the orientation for the aglycon which has the anomeric H projecting between the 2 methyl groups of the aglycon. Methyl tri-O-acetyl-6-O-(tri-O-acetyl-2-oximino-[alpha]-D-arabino-hexopyranosyl)-[beta]-D-gluco-pyranoside was prepared in 70% yield to illustrate the use of the method for the preparation of an a-linked disaccharide.