Protonation of the reactive 1:1 intermediates produced in the reaction between triphenylphosphine and dialkyl acetylenedicarboxlates by butane-2,3-dione monoxime leads to vinyltriphenylphosphonium salts, which undergo intramolecular Wittig reaction to produce dialkyl 2,3-dimethyl-N-hydroxypyrrole-4,5-dicarboxylates in fairly high yields.
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Yavari et al. (1996) studied this question.
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