Tertiary Amine-Catalyzed (4 + 2) Annulations of δ‑Acetoxy Allenoates: Synthesis of Multisubstituted 4H‑Pyran and 4H‑Chromene

The DABCO-catalyzed divergent (4 + 2) annulations of δ-acetoxy allenoates 1 are reported. The chemical behavior of 1 under DABCO catalyst was found to be substrate dependent. Allenoate 1 with an aromatic group at δC preferentially reacted with salicylaldehyde derivative 2, delivering 4H-chromenes 3. On the other hand, allenoates 1 with an alkyl group at δC readily underwent (4 + 2) annulations with oxo diene 4 to afford 4H-pyrans 5.