Thio-sugars. Part VI. Syntheses of episulphides by reaction of methyl 2,3- and 3,4-anhydroglycopyranosides with potassium thiocyanate or thiourea

Abstract

Episulphides, in which the thiiran ring is of opposite configuration to that of the epoxide, have been prepared by reaction of potassium thiocyanate or thiourea with methyl 2,3-anhydro-4,6-O-benzylidene-α-D-taloside, -β-D-taloside, -α-D-guloside, and -β-D-guloside, methyl 2,3-anhydro-4,6-di-O-methyl-α-D-alloside and -mannoside, methyl 3,4-anhydro-6-deoxy-α-L-taloside, and methyl 3,4-anhydro-6-deoxy-2-O-methyl-α-L-taloside. Some of the episulphides were obtained in high yield, but other products, including unsaturated glycosides (shown to be formed from episulphides), were also identified, and predominated in the reactions of the β-D-glycosides. Prolonged treatment of several of the episulphides with potassium thiocyanate resulted in partial stereomutation of the thiiran ring, whilst prolonged treatment with thiourea led to desulphurisation. Considerable differences in the reactivities of the epoxides, and of the episulphides, are explained by steric and conformational effects.

The ¹H n.m.r. parameters for the epoxides and episulphides are tabulated.