First [2+2]-cycloaddition of a 3,4-didehydropyridine and a ketene dialkyl acetal

The 3- or 4-halopyridines and a ketene dialkyl acetal were shown to permit the synthesis of pyrido[b]cyclobutene derivatives. This finding constitutes the first published report of cycloadditions involving an olefin and a 3,4-pyridyne partner. The reaction is totally regioselective and the structure of the cycloadduct is determined unambiguously by NMR experiments.