Synthesis of (23R)- and (23S)-23H-isocalysterols, marine Sterols with a cyclopropene moiety in the Side Chain

A synthesis of (23R)- and (23S)-23H-calysterols 2a and 2b from pregnanoic e ater 10 is reported. Alkylation of 10 with dibromide 19b, followed by reduc tion of the carboethyloxy group to a methyl group, afforded (Z)-vinylic bro mide 22. Dibromocyclopropanation of 22 yielded the diastereomeric tribromoc yclopropane derivatives 15c and 15d. The corresponding 3-hydroxy-5-ene 17c was transformed into 2a via 25 and a cyclopropenyllithium intermediate. An alternative synthetic route involving vinylsilane 13 and (E)-vinylic bromid e 14 has also been examined.