Phosphine-catalyzed [4+2] annulation: Synthesis of cyclohexenes

Phosphine-catalyzed [4 + 2] annulations of alpha-alkylallenoates with activated olefins allow the efficient syntheses of cyclohexenes. Hexamethylphosphorous triamide (HMPT)-catalyzed [4 + 2] annulations of alpha-alkylallenoates with arylidenemalononitriles provided highly functionalized 5,5-dicyano-4,6-disubstituted cyclohex-1-enecarboxylates in excellent yields (77-98%) and moderate to high diastereoselectivities (1:2-12:1). Remarkably, the corresponding triarylphosphine-catalyzed [4 + 2] annulations of alpha-methylallenoate with arylidenemalononitriles manifested a polarity inversion of the 1,4-dipole synthon, providing 4,4-dicyano-5-substituted cyclohex1-enecarboxylates in excellent yields (80-93%). The polarity inversion of alpha-alkylallenoates from one 1,4-dipole to another under phosphine catalysis presumably resulted from a change in the balance of the equilibrium between the phosphonium dienolate and the vinylogous phosphonium ylide intermediate.