Phosphine-catalyzed annulations of 4,4-dicyano-2-methylenebut-3-enoates with maleimides and maleic anhydride.

A novel phosphine-catalyzed [4+1] annulation of maleimides with 4,4-dicyano-2-methylenebut-3-enoates has been developed to afford spirocyclic products, and the maleimides serves as C1  synthons. Moreover, a phosphine-catalyzed formal [3+2] annulation between 4,4-dicyano-2-methylenebut-3-enoates and maleic anhydride has been also achieved, and maleic anhydride behaved as a C3  synthon in the reaction, thus efficiently affording the functionalized cyclopentenones. A stable phosphinium-containing zwitterionic compound is the key reactive intermediate in both annulations and was successfully isolated. Plausible mechanisms have been proposed on the basis of control experiments and deuterium-labeling experiments.