濃オルト燐酸によるカルコンのフラバノン閉環 (第3報)

フェノール系並びにレゾルシン系カルコン類の閉環

抄録

Although the flavanone cyclization was not effected by heating only with conc. phosphoric acid, 2′-hydroxy-4-methoxychalcone (II), and 4′-methoxyl (V:R=H) and 6′-methoxyl (IV) derivatives of (II) underwent cyclization to the corresponding flavanones in a good yield on heating with a mixture of glacial acetic acid and conc. phosphoric acid for 5-10 minutes. Chalcones with methyl group introduced into the ring, i.e. 6′-methyl (VIII), 3′-methyl (VI), and 5′-methyl (V:R=CH₃) derivatives of (V:R=H), and 3′-methyl compound (VII) of (IV), also underwent similar cyclization. The reaction was especially facile in the case of (VIII), in which methyl group is present in the position meta to the hydroxyl, the heating of 1 minute being sufficient. The 5-nitro (R=NO₂), 5-bromo (R=Br), and 5-(4-methoxy-cinnamoyl) (R=CH₃O-C₆H₄-CH=CH-CO) compounds of (V) also underwent cyclization to the corresponding flavanones, although the heating required 30-60 minutes and the respective yield was poor.