Epimerization, transacylation and brominationof dihydroquercetin acetates; synthesisof 8-bromodihydroquercetin

摘要

Dihydroquercetin (dhq) and its 3-acetate react with acetic anhydride in the absence of a base catalyst to yield mixtures of par tiallyacetylated 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 par tially conver ted to dhq 3,3',4'-triacetate and dhq pentaacetate byintermolecular 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 par tial epimerization: 3 new acetates with 2,3-cis stereochemistry(dhq 3-, 3,7,3',4'-tetra- and penta-) were identifed. Dhq and its 3,7,3',4'-tetraacetate undergo regiospecifc dibromination at C-6 andC-8 with excess N-bromosuccinimide in polar solvents, and 6,8-dibromo-dhq can be regioselectively debrominated to 8-bromo-dhqwith sodium sulfte.