Construction of Complex 1,3-Cyclohexadienes via Phosphine-Catalyzed (4 + 2) Annulations of δ‑Acetoxy Allenoates and Ketones

The phosphine-catalyzed substrate-dependent
(4 + 2) annulations
of δ-acetoxy allenoates with ketones is described. Allenoates 1 with an alkyl substituent at δC are able to react
with cyclic 1,3-diketones 2, wherein the δC is
attacked by the methenyl carbon of 2 while the αC
attacks the ketone of 2. Allenoates 5 with
an aryl group at δC is poised to react with cyclic β-carbonyl
amides 6, in which the αC is attacked by the methenyl
carbon of 6 and the δC undergoes 1,2-addition to
the ketone of 6.