Phosphine-Catalyzed 4+2 Annulations of 2-Alkylallenoates and Olefins: Synthesis of Multisubstituted Cyclohexenes

摘要

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 a-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.