Phosphine-catalyzed [3 + 2] and [4 + 2] annulation reactions of ynones with barbiturate-derived alkenes

The phosphine-catalyzed [3 + 2] annulation reaction of ynones and barbiturate-derived alkenes has been developed with the assistance of a weak acid, giving functionalized spirobarbiturate-cyclopentanones in moderate to excellent yields with excellent E/Z stereoselectivity. An unprecedented [4 + 2] annulation of ynones with barbiturate-derived alkenes was also achieved in the presence of a phosphine catalyst and an inorganic base, affording biologically interesting 1,5-dihydro-2H-pyrano[2,3-d]pyrimidine-2,4(3H)-dione derivatives. An asymmetric variant of the [3 + 2] annulation reaction has been explored and a moderate enantioselectivity was obtained when a bifunctional chiral phosphine was used as a chiral catalyst. A plausible mechanism was proposed to illuminate two different reaction pathways.