Protonation of the highly reactive 1:1 intermediates produced in the reaction between triphenylphosphine and dialkyl acetylenedicarboxylates by triformylmethane leads to vinyltriphenylphosphonium salts which undergo an intramolecular Wittig reaction to produce dialkyl 5-formyl-2H-pyran-2,3-dicarboxylates in fairly high yields.