Abstract: Vitamin D.sub.3 analogues of the formula (I): ##STR1## wherein R.sub.1 is hydrogen or hydroxy, R.sub.2 is hydrogen, (C.sub.1 -C.sub.3) alkyl, phenyl, or trifluoromethyl, R.sub.3 is branched or unbranched (C.sub.1 -C.sub.4) alkyl, cyclopropyl or CF.sub.3, A and B are individually hydrogen or methyl, or A and B together form methylene, X is CH.sub.2 or O, and n is 2 or 3. These compounds are useful for treating various skin and bone disorders and cancers. These compounds are also useful in cosmetic applications and for diagnostic purposes. Compounds of formula (I) are prepared by reacting a C--25 ester compound with an organometallic compound or by coupling a CD ring ester moiety with a Wittig reagent or an enyne compound to form the desired diene or triene system.