Synthesis of photochromic thieno-2H-chromene derivatives

Syntheses of photochromic thieno-2H-chromenes from hydroxybenzo[b]thiophenes and 1,1diphenyl-2-propyn-1-ol using an acid catalyst are described. The hydroxybenzo[b]thiophenes were prepared by demethylation of methoxybenzo[b]thiophenes with pyridinium chloride or boron tribromide. The precursors were synthesized in two steps, which involved the reaction of methoxythiophenols with 3-bromobutan-2-one in an alkaline medium, to give ketoarylsulfides, followed by cyclization using phosphorus pentoxide or polyphosphoric acid. The photochromic behaviour of the target compounds was evaluated with the aid of the usual spectrokinetic parameters, and the results were compared to data from reference compounds that are benzoannellated in the 5,6 and 7,8 positions of the 2H-chromene system.