Convergent synthesis of vitamin D-3 metabolites. Control of the stereoselectivity in samarium-induced cyclopropanations of cyclopentenes

The 25-hydroxy and 1 alpha,25-dihydroxy vitamin D-3 metabolites are obtained by solvolytic rearrangements of the 1-desoxy and 1 alpha-hydroxy cyclopropyl vinylogous alcohols 31 and 29, respectively, with simultaneous formation of the vitamin D structural triene and the 3-hydroxy function. Two complementary methods have been employed to direct the stereoselectivity ofthe samarium induced olefin cyclopropanations which ultimately lead to key chiral ring A precursors. One protocol uses the two stereogenic centers of the (R,R)-B,S-butanediol ketal moiety of 8, while the other method uses the allylic hydroxyl group of(R)-16.