| Abstract: | 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. |
