Some reactions of 5-hydroxybenzo[b]thiophen and its 3-methyl derivative

Abstract

Formylation of 5-hydroxybenzo[b]thiophen by a modified Gattermann procedure gave the 4-formyl derivative in high yield, and condensation of this with ethyl acetoacetate in the presence of methanolic hydrogen chloride gave an almost theoretical yield of the angular 9-methylthieno[3,2-f][1]benzopyran-7-one. Similarly, nitration, bromination, or formylation of 5-hydroxy-3-methylbenzo[b]thiophen gave the corresponding 4-substituted compound as did the Claisen rearrangement of the O-allyl ether. However, the Fries rearrangement of the O-acetyl derivative gave 2-acetyl-5-hydroxy-3-methylbenzo[b]thiophen, and condensation of this with ethyl acetoacetate as above gave the linear cyclisation product, 3,8-dimethylthieno[2,3-g][1]benzopyran-6-one. Dibromination of 5-hydroxy-3-methylbenzo[b]thiophen gave the 4,6-dibromo-derivative and dinitration the 4,6-dinitro-derivative. Nitration of 4-bromo-5-hydroxy-3-methylbenzo[b]thiophen also occurred in the 6-position.