STUDIES OF THE SELECTIVE O-ALKYLATION AND DEALKYLATION OF FLAVONOIDS .17. SYNTHESIS OF 5,8-DIHYDROXY-6,7-DIMETHOXYFLAVONES AND REVISED STRUCTURES FOR SOME NATURAL FLAVONES

摘要

Six 5,8-dihydroxy-6,7-dimethoxyflavones and three 8-hydroxy-5,6,7-trimethoxyflavones were synthesized from 2',5'-dihydroxy-3',4',6'-trimethoxyacetophenone by adapting the selective O-alkylation and dealkylation, and the differentiation between the flavones and their isomeric 6-hydroxyflavones was clarified by H-1 NMR and UV spectra. Four natural flavones proposed as 5,8-dihydroxy-6,7-dimethoxyflavones, must have other structures and three are shown to be the isomeric 5,7-dihydroxy-6, 8-dimethoxyflavones. A flavone, is elated from Ageratum conyzoides, is correctly identified as 8-hydroxy- 5,6,7,3',4', 5'-hexamethoxyflavone, but the structure of a flavone, isolated from Helichrysum, is revised to the isomeric 7-hydroxy-5,6,8-trimethoxyflavone.