Phosphine-Catalyzed [3+2] Annulations of γ-Functionalized Butynoates and 1C,3O-Bisnucleophiles: Highly Selective Reagent-Controlled Pathways to Polysubstituted Furans

In this paper, a reagent-controlled [3 + 2] annulation of gamma-functionalized butynoates 1 and 1C,3O-bisnucleophiles 2 is reported, which leads to three distinct furan skeletons 3-5. A PPh3 catalyst preferentially attached the beta-position of 1a, facilitating a-addition to furnish Type I annulations. With the assistance of Ag2O, Type II annulations were achieved via selective gamma-substitution. In the absence of the PPh3 catalyst, the reagent Cs2CO3 promoted beta-addition to realize Type III annulations.