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

✍ Scribed by Eberhard Kiehlmann; Monica G. Szczepina

• Book ID: 111488218
• Publisher: SP Versita
• Year: 2011
• Tongue: English
• Weight: 464 KB
• Volume: 9
• Category: Article
• ISSN: 1895-1066
• DOI: 10.2478/s11532-011-0032-8

✦ Synopsis

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.