Epimerization, transacylation and bromination of dihydroquercetin acetates; Synthesis of 8-bromodihydroquercetin

Abstract

Dihydroquercetin (dhq) and its 3-acetate react with acetic anhydride in the absence of a base catalyst to yield mixtures of partially acetylated products. Three new esters were characterized by NMR spectroscopy as dhq 3,7,3'-triacetate, 3,7,4'-triacetate and 5,7,3',4'- tetraacetate. At its melting point neat dhq 3,7,3',4'-tetraacetate is partially converted to dhq 3,3',4'-triacetate and dhq pentaacetate by intermolecular acetyl transfer. Dhq 7,3',4'-triacetate yields exclusively dhq 3',4'-di- and 3,7,3',4'-tetraacetate under these conditions. The acetylation/deacetylation reactions are accompanied by partial epimerization: 3 new acetates with 2,3-cis stereochemistry (dhq 3-, 3,7,3',4'-tetra- and penta-) were identified. Dhq and its 3,7,3',4'-tetraacetate undergo regiospecific dibromination at C-6 and C-8 with excess N-bromosuccinimide in polar solvents, and 6,8-dibromo-dhq can be regioselectively debrominated to 8-bromo-dhq with sodium sulfite. © Versita Sp. z o.o.