The Acylation of 2,3-Dimethyl-bz-hydroxybenzofurans and the Syntheses of Di- and Tri-methylfuroisoflavones

The acylation of 2,3-dimethyl-4-hydroxybenzofuran (1) with acetic acid, phenylacetic acid, 2-methoxyphenylacetic acid or 2,4,5-trimethoxyphenylacetic acid, and polyphosphoric acid gave a mixture of the 5- and 7-acyl compounds (2a, b, c, d, and 3a, c, d respectively). Analogously, the isomeric 2,3-dimethyl-6-hydroxybenzofuran (6) gave the 5-acyl compounds (7a, b, c, d), two (7a, b) of which were also obtained by the Friedel-Crafts acylation of the methyl ether of 6 (9), followed by demethylation. On the other hand, the formation of a dimethylfuran ring on 2,4-dihydroxyphenyl ketones by the action of polyphosphoric acid on their 4-(α-acetylethoxy) derivatives (11a, b, c) also afforded mixtures of the ketones (7a, b, c) and isomeric ketones (2a, b, c). The benzofuranyl ketones (2 and 7) thus obtained were converted to the corresponding 4″,5″-dimethyl-furo[2″,3″:7,8]isoflavones (12b, c, d) and isomeric -furo[3″,2″:6,7]isoflavones (14b, c), and also their 2-methyl derivatives (13b, c, d and isomeric 15b, c), b...