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The reaction of N-vinyliminotrimethoxyphosphorane reacts with electron deficient acetylenes results in the formation of formal [4+2] cycloadducts, 1,2-λ5-azaphosphorines 4a-c. However, N-vinyliminotriphenylphosphorane reacts with dimethyl acetylenedicarboxylate (DMAD) in a formal [2+2] manner. The product 4a or 4b also reacts with DMAD to give λ5-phosphorine derivatives.
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Kobayashi et al. (1985) studied this question.
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