Addition of nitrile oxides to benzocyclobutene. Generation of 3-(p-methoxyphenyl)benzo[3,4]cyclobuta[1,2-d]isoxazole, a heterocyclic analogue of biphenylene

Abstract

1,3-Dipolar cycloaddition to cyclobutadienes has been observed for the first time in the reaction of benzocyclobutene with benzonitrile oxides; the products, 3-aryl-3a, 7b-dihydrobenzo[3,4]cyclobuta[1,2-d]isoxazoles (1), were isolated in good yields. Adducts were not obtained from cyclobutadiene or benzocyclobutene with several other 1,3-dipolar reagents; possible reasons for this are discussed. Monobromination of the 3-p-methoxyphenyl adduct (1c) followed by dehydrobromination with potassium t-butoxide gave 3-(p-methoxyphenyl)benzo[3,4]-cyclobuta[1,2-d]isoxazole (2) as a transient intermediate which rapidly formed the t-butyl alcohol 3a, 7b-addition product (6).