Phosphine‐Catalyzed [4+2] Annulations of 2‐Alkylallenoates and Olefins: Synthesis of Multisubstituted Cyclohexenes

From our investigations on phosphine‐catalyzed [4+2] annulations between α‐alkyl allenoates and activated olefins for the synthesis of cyclohexenes, we discovered a hexamethylphosphorous triamide (HMPT)‐catalyzed [4+2] reaction between α‐alkyl allenoates 1 and arylidene malonates or arylidene cyanoacetates 2 that provides highly functionalized cyclohexenes 3 and 4 in synthetically useful yields (30–89 %), with moderate to exclusive regioselectivity, and reasonable diastereoselectivity. Interestingly, the [4+2] annulations between the α‐alkyl allenoates 1 and the olefins 2 manifested a polarity inversion of the 1,4‐dipole synthon 1 , depending on the structure of the olefin, thus providing cyclohexenes 3 exclusively when using arylidene cyanoacetates. The polarity inversion of α‐alkyl allenoates from a 1,4‐dipole A to B under phosphine catalysis can be explained by an equilibrium between the phosphonium dienolate C and the phosphorous ylide D .