J . Org. Chem. 1990,55, 929-935
929
Benzyne-Oxazole Cycloadducts: Isolation and Retro-Diels-Alder Reactions' Scott E. Whitney, Michael Winters, and Bruce Rickborn* Department of Chemistry, University of California, Santa Barbara, California 93106 Received July 10, 1989
A literature procedure for generating benzyne from 1-aminobenzotriazole and lead tetraacetate was modified by the use of two syringe pumps to effect simultaneous addition through opposing ports of a three-neck flask. Applied to the reaction of benzyne with 4-phenyloxazole at 0 "C, this modification resulted in essentially quantitative formation of cycloadduct. Retro-Dieh-Alder expulsion of benzonitrile with concurrent formation of isobenzofuran occurred when the adduct was heated. The sequence constitutes a useful method of preparation of isobenzofuran, which may be utilized in situ or isolated as a solution for subsequent application. Benzyne cycloadducts were also prepared from 4-(4-nitrophenyl)- and 4-(4-methoxyphenyl)oxazoles, in order to assess the effect of remote substituents on the retro-Diels-Alder reaction. First-order rate constants were determined at three temperatures in the range of 40-70 "C. The order of reactivity was p-N02 > p-H > p-OMe; the substituent effects were small, with a factor
