Stereochemical Aspects of Acid‐Catalyzed Cyclopropane Ring‐Opening Reactions. A Stereospecific Pathway to Crinosterol and Brassicasterol

It is shown that the acid‐catalyzed ring‐opening of the two diastereoisomeric 23:24‐methylenecholesterols 3 and 5 on treatment with gaseous HCl in acetic acid leads stereospecifically to the naturally occurring crinosterol (4) and brassicasterol (6), respectively (Scheme 1). This isomerization can be viewed as a biomimetic model of an in vivo methylation process of the type already known in plant sterol metabolism (cf. cycloeucalenol → obtusifoliol, 1 → 2). The synthetic application of this method provides a convenient labelling of sterol side chains for tracer experiments. The mechanistic features of the reaction with respect to its particular stereospecificity are discussed.