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 benzocyclobutenols 10c, g gave the starting 1,3-diketones 4c, g predominantly along with small amounts of benzocyclobutenones 11c,g and the dimethylpentanone 2.