[2 + 2] Photocycloadditions of dichloromaleimide and dichloromaleic anhydride to cyclic olefins

Abstract

The photocycloaddition of dichloromaleimide to cyclohexadiene, cyclopentadiene, and cyclopentene on direct photolysis gave [2 + 2] addition products with the cis,endo- and cis,exo-configurations as the major products. However, a similar photocycloaddition to cyclohexene yielded the trans-fused cycloaddition product (between the four- and six-membered rings) as the major product in addition to the cis,endo- and cis,exo-isomers. The photocycloadditions of dichloromaleic anhydride to cyclopentadiene and cyclohexadiene were reinvestigated; two major products were obtained which were identified as the cis,endo- and cis,exo-addition products, while the reported trans-fused isomers were not isolated. The structures of these photoaddition products were investigated by 400 MHz n.m.r. spectroscopy to ascertain the configuration of the ring fusion. An X-ray crystallographic analysis established the stereochemistry of one product.