BrOnsted Acid Tuned, Lewis Base Promoted [4+2] Annulation Reactions of Allenoates with Electron-Deficient Olefins

The selective formation of an annulated 4H-pyran from an allenoate and an olefin demonstrated the dual roles of triethylenediamine (DABCO) in both Lewis base promoted [4 + 2] annulation and BrOnsted base mediated C=C isomerization. Interestingly, the role of the BrOnsted base could be quenched selectively with the presence of a BrOnsted acid (PhCO2H), and annulated 3,4-dihydro-2H-pyran was obtained as the major product.