Unsaturated carbohydrates. Part X. Epoxidations and hydroxylations of 2,3-dideoxy-α-D-hex-2-enopyranosides. The four methyl 4,6-di-O-acetyl-2,3-anhydro-α-D-hexopyranosides

Abstract

Preliminary experiments have been carried out on the oxidation of the double bonds of 2,3-dideoxy-α-D-hex-2-enopyranosides with a view to developing glycoside syntheses. Hydroxylation of 4,6-di-O-acetyl-2,3-dideoxy-α-erythro-hex-2-enoside compounds with neutral permanganate affords predominantly manno-adducts as was exemplified by the synthesis of 6-O-α-D-mannopyranosyl-D-galactose from an unsaturated disaccharide derivative.

Epoxidation of methyl 4,6-di-O-acetyl-2,3-dideoxy-α-D-erythro-hex-2-enopyranoside gave the manno- and allo-adducts in the ratio 3:2. This ratio could be controlled to some extent by varying the allylic substituents; the analogous t-butyl glycoside gave a ratio 4:1, and for the deacetylated methyl compound the ratio was 1:3. Similar epoxidations of methyl 4,6-di-O-acetyl-2,3-dideoxy-α-D-threo-hex-2-enopyranoside and its deacetylated derivative were studied. The four methyl 4,6-di-O-acetyl-2,3-anhydro-α-D-hexopyranosides prepared by these means were isolated by preparative g.l.c. and characterised by n.m.r. spectroscopy.

Epoxidation of 6-O-(4,6-di-O-acetyl-2,3-dideoxy-α-D-erythro-hex-2-enopyranosyl)-1,2:3,4-di-O-isopropylidene-α-D-galactopyranose gave a crystalline 1:1 mixture of the isomeric epoxides in high yield. Controlled hydrolysis of the product afforded 6-O-α-D-altropyranosyl-D-galactose and 6-O-(3,6-anhydro-α-D-glucopyranosyl)-D-galactose which were isolated in pure form. Hydrolyses of the monosaccharide epoxides were examined briefly, crystalline α-D-altopyranosyl derivatives were isolated during the study, and the problem of synthesising glucosides by this means was considered.