濃オルト燐酸によるカルコンのフラバノン閉環 (第5報)

ポンカネチンの構造について

抄録

2′, 5′-Dihydroxy-3′, 4, 4′, 6′-tetramethoxychalcone and 2′-hydroxy-3′, 4, 4′, 5′, 6′-penta-methoxychalcone underwent cyclization by short heating with glacial acetic acid and conc. phosphoric acid to give in a good yield the corresponding 4′, 5, 7, 8-tetrame-thoxy-6-hydroxyflavanone and 4′, 5, 6, 7, 8-pentarnethoxyflavanone, mp 108.5-109.5°. The latter compound was also obtained by methylation of the former with dimethyl sulfate and potassium carbonate and it was found to have a far different melting point from that (mp 152°) of poncanetin. 4′, 5, 6, 7, 8-Pentamethoxyflavone, obtained by the bromination-dehydrobromination of 4′, 5, 6, 7, 8-pentamethoxyflavanone was also obtained by the Baker-Venkataraman method from 2-hydroxy-3, 4, 5, 6-tetrame-thoxyacetophenone and melted at 152.5-153.5°. It follows, therefore, that poncane-tin is not a flavanone but a flavone compound.