Heterocyclic syntheses from o-halogeno-acids. Part II. Thienopyridinones and thienopyranones from 3-bromothiophen-2- and 4-bromothiophen-3-carboxylic acids

The sodium salt of 3-bromothiophen-2- and 4-bromothiophen-3-carboxylic acids react with carbanions in the presence of copper or copper(II) acetate to give condensation products by displacement of bromide ion (often with simultaneous deacylation). Thus ethyl acetoacetate gives the mono-ethyl esters of the homophthalic acid analogues, 2- and 4-carboxythiophen-3-acetic acids. Cyclisation reactions of the condensation products provide convenient routes to thienopyranones and thienopyridinones. 4-Phenacylthiophen-3-carboxylic acid is reduced to the 4-phenethyl-acid, which is cyclised by polyphosphoric acid at 100 °C to give 9,10-dihydrobenzo[4,5]-cyclohepta[1,2-c]thiophen-4-one. At 160 °C, however, this ketone, or the acid, yields the isomeric 4,5-dihydrobenzo[5,6]cyclohepta[1,2-b]thiophen-10-one, apparently by acidic cleavage to form an acylium ion which recyclises by attack on the α-position of the thiophen ring.