Abstract
Reactions of 5-cyano-1,4-diphenylpyridazino[4,5-a]indolizines with dimethyl acetylenedicarboxylate afforded the 1:2 adducts regioselectively and, unexpectedly, in a Michael fashion rather than in a 1,3-dipolar manner. The structure of the products was established by X-ray crystallography. Regiospecific formation of the observed product 3 can be explained by the higher nucleophilicity of the nitrogen at the 2 position than at the 3 position of the 5-cyano-1,4-diphenylpyridazino[4,5-a]indolizine 1 owing to the contribution of a resonance structure 5 to the hybrid. This favors two successive Michael additions to two equivalents of dimethyl acetylenedicarboxylate affording the observed 1:2 adducts 3.
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