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