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

Abstract

The selective formation of an annulated 4 H ‐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 (PhCO 2 H), and annulated 3,4‐dihydro‐2 H ‐pyran was obtained as the major product.