Enantioselective Construction of 5‐6‐5 Tricyclic Lactone Framework Bearing a Quaternary Bridgehead Carbon via Rh‐Catalyzed Asymmetric [2+2+2] Cycloaddition of Enediynes

Abstract

Herein, we report a Rh-catalyzed asymmetric [2 + 2 + 2] cycloaddition of ene-yne-yne enediynes to generate enantio-enriched tricyclic cyclohexadienes bearing a quaternary bridgehead carbon. We found that the Rh-Phanephos complex is an appropriate catalyst for the cycloaddition of enediynes bearing an unsubstituted propiolate terminus, whereas Rh-biaryl bisphosphine catalysts, which have been widely used for asymmetric cycloadditions of alkynes and alkenes, are not applicable for the reaction of such enediynes. Several control experiments suggest that the reaction using the Rh-Phanephos complex exclusively proceeds via a rhodacyclopentadiene intermediate, unlike when using a Rh-biaryl bisphosphine complex that can form a rhodacyclopentadiene intermediate as well as a rhodacyclopentene intermediate in a substratedependent manner.