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 d-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 dC preferentially reacted with salicylaldehyde derivative 2, delivering 4H-chromenes 3. On the other hand, allenoates 1 with an alkyl group at delta C readily underwent (4 + 2) annulations with oxo diene 4 to afford 4H-pyrans 5.