METHANOLYSIS OF CARBOHYDRATE ORTHOACETATES

Abstract

Acid methanolysis of D-mannose 1,2-(methyl orthoacetate) 3,4,6-triacetate (I) yields mainly 3,4,6-tri-O-acetyl-D-mannose (II). D-glucose 1,2-(ethyl orthoacetate) 3,4,6-triacetate (VIII) yields 3,4,6-tri-O-acetyl-D-glucose and methyl 3,4,6-tri-O-acetyl-β-D-glucopyranoside. The rate of methanolysis of II differs little from that of VIII, in marked contrast to the relative rates of hydrolysis of these compounds in aqueous acid. Several concurrent reactions appear to take place in methanol initiated by protonation at different positions on the orthoester ring.During isolation, II is converted partially to 2,4,6-tri-O-acetyl-D-mannose (VI), a novel instance of acyl migration in the sugar series. Structural assignments for these two triacetates are based on chemical and proton magnetic resonance spectral data. The 2-acetate group of VI migrates during methylation, both II and VI yielding the same 2-O-methyl ether.