Photochromic heteroaromatic thiofulgides and dimethoxybutanoic acid lactones

Thiofulgides 1, (X = S) cyclise on irradiation with UV light to form therma lly stable photochromes 2, (X = S) that absorb at much longer wavelengths t han photochromes 2, (X = O), formed from corresponding fulgides 1, (X = O), reverse reactions occur on exposure to white light; photochromic thiofulgi des are prepared by reaction of fulgides with sodium hydrosulfide in hot to luene and photochromic lactones 9 and 12 are prepared by reaction of fulgid es with sodium hydrosulfide in cold methanol followed by cyclisation with a carbodiimide.