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

Article abstract of DOI:10.1002/hlca.19820650144

It is shown that the acid-catalyzed ring-opening of the two diastereoisomeric 23,24-methylenecholosterols 3 and 5 on treatment with gaseous HCl in acetic acid leads sterospecifically to the naturally occuring 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 chain for tracer experiments.The mechanistic features of the reaction with respect to its particular stereospecificity are discussed.