Phosphine-Promoted Divergent Annulations of δ-Acetoxy Allenoates with α-Hydroxy-β-carbonyl Ester Derivatives: Synthesis of Tetrasubstituted Cyclopentadienes and Benzenes.

The substrate-dependent annulations of δ-acetoxy allenoates with α-hydroxy-β-carbonyl ester derivatives are reported. 2-[(Methoxycarbonyl)oxy]malonate as a C1 partner is able to undergo (1 + 4) annulations with allenoates in the presence of a catalytic amount of phosphine, affording cyclopentadienes in good yields. However, (2 + 4) annulations of allenoates with 2-(tosyloxy)-3-ketoesters are instead observed with the assistance of stoichiometrical phosphine and base, leading to tetrasubstituted benzene products. In these two reactions, δ-acetoxy allenoate serves as a 1,4-disubstituted diene synthon.