Synthesis of Polycyclic Ring Systems via Intramolecular [2 + 2]‐Cycloaddition Reactions of Cyclopropene Derivatives

Abstract

AbstractStrained bicyclic and polycyclic compounds continue to play an important role in the understanding of many aspects of organic chemistry. For this reason, synthetic efforts in this area have been extensive. Our research group has found that the thermolysis of a number of cyclopropene derivatives containing π‐substituents undergo a novel [2 + 2]‐cycloaddition. This reaction is unique in that the other reported examples of thermal olefin cycloadditions either occur in compounds in which the double bond is subjected to severe torsional strain or else involve reactants that bear substituents capable of stabilizing diradical or dipolar intermediates. In cyclopropene, the torsional angle is close to zero and p‐p overlap should not be significantly different from that of a normal olefin. Thus the propensity of the cyclopropene ring to undergo internal cycloaddition is primarily due to relief of angle bending rather than torsional strain. Details of the results obtained and the probable mechanism of the intramolecular [2 + 2]‐cycloaddition are presented in some detail.