Studies of O-methylation. Some factors affecting the ratio of glycosides produced in the methylation of L-rhamnose derivatives

Abstract

The ratio of α- to β-glycosides obtained on methylating 2,3-O-isopropylidene-4-O-methyl-α-L-rhamnopyranose (1) and 2,3-O-isopropylidene-5-O-tosyl-α-L-rhamnofuranose (5) depends markedly on the method used. The Kuhn procedure yields over 90% of the α-isomer in each case, and similar results were obtained with a substantially smaller proportion of dimethylformamide present. Treatment with sodium hydride–methyl iodide yields predominantly β-glycosides. Methylation of compound (1) by the latter method in the presence of dicyclohexyl-18-crown-6 (a cyclic polyether capable of complexing sodium ions) reverses completely the glycoside ratio of the product. The factors governing these ratios are discussed.