Reaction of Aromatic N-Oxides with Dipolarophiles. XVI. Cycloaddition Behavior of Aromatic N-Oxides toward Electron-Deficient Allenes and X-Ray Structure of the 1, 4-Dipolar Cycloadduct

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In connection with the pericyclic reaction of pyridine N-oxides with dipolarophiles, the cycloaddition behavior of some aromatic N-oxides toward electron-deficient allenes was investigated. In the reaction of 2-phenylpyridine N-oxide with dimethyl 2, 3-pentadienedioate, the 2, 3-dihydropyridine type 1 : 1 cycloadducts, which resulted from 1, 5-sigmatropic rearrangement of the primary cycloadduct, were isolated. The reactio of 3, 5-dihalogenopyridine N-oxides with the allene gave the dehydrohalogenated cycloadducts of the 1, 5-sigmatropic rearrangement products. The reaction of 3, 5-dimethylpyridine N-oxide with the allene caused deoxygenation to give 3, 5-dimethylpyridine. which in turn reacted with two molecules of the allene to give the 1 : 2 cycloadduct (1, 4-dipolar cycloaddition product). The structure of the cycloadduct was determined by single crystal X-ray analysis.The observed reaction behaviors are discussed in terms of frontier molecular orbital considerations.