Preparation of benzocyclobutenones via the photochemical cyclization of 1-(ortho-alkylaryl)-2,2,4-trimethylpentane-1,3-diones

Irradiation of 1-(ortho-methylaryl)- and 1-(ortho-ethylaryl)-2,2,4-trimethylpentane-1,3-diones 4a–g in hexane gave benzocyclobutenols 10a–g. However, irradiation of 1-(ortho-isopropylaryl)-2,2,4-trimethylpentane-1,3-diones 4i and 4j resulted in no reaction. Irradiation of 1-mesityl-2,2,4-trimethylpentane-1,3-dione 4h gave a complex mixture of products. Pyrolysis of benzocyclobutenols 10a, b, d–f gave benzocyclobutenones 11a,b, d–f and 2,4-dimethylpentan-3-one 2, whereas that of the benzocyclobutenols10c,g gave the starting 1,3-diketones4c, g predominantly along with small amounts of benzocyclobutenonas11c, g and the dimethylpentanone 2.