Protonation of the reactive intermediates produced in the reaction between dialkyl acetylenedicarboxylates and triphenylphosphine by 2-hydroxy-1-naphthaldehyde leads to vinyltriphenylphosphonium salts, which undergo intramolecular Wittig reaction to produce dialkyl 3H-naphtho[2,1-b]pyran-2,3-dicarboxylates in fairly high yields.
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Yavari et al. (1997) studied this question.
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