ACID-CATALYZED AND PHOTOCHEMICAL ISOMERIZATION OF STEROIDAL CYCLOPROPENES.

Article abstract of DOI:10.1021/ja00351a047

The acid-catalyzed and photochemical isomerization of two naturally occurring steroidal cyclopropenes, (28R)-calysterol and (23R)-23H-isocalysterol is described. Cyclopropene on treatment with methanolic sulfuric acid afforded 23-ethylated cholesterol derivatives containing conjugated diene systems or methoxylated side chains. The sole production of C-23 ethylated sterols demonstrates that this cyclopropene undergoes acid-promoted bond cleavage only between C-24 and C-28. By contrast, methanolic sulfuric acid treatment of the isomeric cyclopropene generated conjugated dienes with 24-methyl-24-homocholestane and 24-ethylcholestane side chains, together with their methoxylated derivatives. This implies that bond fission between C-23 and C-24, as well as C-23 and C-28, is operating in this instance, which is of mechanistic significance.