Photochemical Methods for the Construction of an Additional Four-membered Carbocycle in Steroids

The photochemical methods for the synthesis of pentacyclic steroids with an additional four-membered carbocycle are considered. A systematic account is given of the available literature data on the [2 + 2] photocycloaddition of steroid enones and dienones to various alkenes and alkynes and also on the dimerisation of dienones and intramolecular cyclisation of steroid ketones.
The mechanism of the photolytic reactions is discussed and the stereo- and regio-specificity of the photocycloaddition are considered as a function of the structure of both the steroid and the alkene. It is shown that the pentacyclic steroids obtained are convenient intermediates for further modifications.
The bibliography includes 106 references.