Lewis Acid Catalyzed Inter- and Intramolecular [2 + 2] Cycloadditions of Conjugated Allenic Esters to Alkenes

Abstract

The [2 + 2] cycloadditions of the substituted allenic esters methyl 2,3-pentadienoate (2), methyl 4-methyl-2,3-pentadienoate (3), and methyl 2-methyl-2,3-butadienoate (4) with alkenes occur at carbons 3 and 4 as with methyl 2,3-butadienoate to give similar mixtures of products. The stereo- and regioselectivities of these Lewis acid catalyzed reactions are remarkably similar to the thermal cycloaddition reactions of the corresponding ketenes, which indicates that the mechanistic models used to rationalize ketene cycloaddition reactions are probably also applicable to the allenic ester cycloadditions. In particular, the cycloaddition of 2 with cis-3-hexene is selective for the cycloadduct 12 expected from a concerted reaction proceeding through transition state. Cycloadditions of the intramolecular allenic esters 41, 44, and 48 proceed in excellent yield to give primarily the [2 + 2] cycloadducts. These reactions complement intramolecular ketene cycloadditions which often proceed in reasonable yield only when activated ketenes are used. © 1986, American Chemical Society. All rights reserved.