PPh3-catalyzed (3 + 3) annulations of 5-acetoxypenta-2,3-dienoate with 1C,3O-bisnucleophiles: facile entry to stable monocyclic 2H-pyrans.

Phosphine-catalyzed (3 + 2) and (3 + 3) annulations between 5-acetoxypenta-2,3-dienoate and 1C,3O-bisnucleophiles are presented. The former cases can be achieved with the assistance of base while the latter is dominant without any additive. A series of deuterium-labeling experiments disclosed that the divergence in annulations is likely determined by the involved proton transfer processes.