Methylation of Quercetin by Diazomethane and Hypoglycemic Activity of its Tetra-O-Methyl Ether.

The tetra-O-methyl ether of quercetin (QU) 3 (54%), 3,7,4′-tri-O-methyl ether 4 (30%), and a previously unreported 3,7,3-tri-O-methyl ether of QU 5 (7%) were obtained via methylation of QU by an excess of diazomethane in dioxane. Their structures were established using 2D NMR (1H–1H COSY, 1H–1H NOESY, 1H–13C HSQC, 1H–13C HMBC). Tetra-O-methyl ether of QU 3 exhibited pronounced hypoglycemic activity, reduced alloxan-induced hyperglycemia in rats by 44.5% compared to a control, and was 2.7 times more active than QU. [ABSTRACT FROM AUTHOR]