Synthesis of Kaempferol 3-O-(3′′,6′′-Di-O-E-p-coumaroyl)-β-d-glucopyranoside, Efficient Glycosylation of Flavonol 3-OH with Glycosyl o-Alkynylbenzoates as Donors

Abstract

Kaempferol 3-O-(3'',6''-di-O-E-p-coumaroyl)-β-D-glucopyranoside (1), an optimal metabolite of Scots pine seedlings for protection of deep-lying tissue against damaging UV-B, represents a typical acylated flavonol 3-O-glycoside. This compound was synthesized for the first time via two approaches. The first approach, starting with kaempferol, featured formation of the flavonol 3-O-glycosidic linkage with a glycosyl bromide under conventional PTC conditions. In the second approach, 5,7,4'-tri-O-benzyl-kaempferol was readily prepared from 2',4',6'-trihydroxyacetophenone and p-hydroxybenzoic acid, which was coupled with a glucopyranosyl o-hexynylbenzoate under the catalysis of a gold(I) complex to provide the desired 3-O-glycoside in excellent yield. A variety of the glycosyl o-hexanylbenzoates equipped with the 2-O-benzoyl group were also proven to be highly efficient donors for construction of the flavonol 3-O-glycosidic linkages.