A vitamin D compound of the formula ##STR19## wherein R1 is a hydrogen atom or a hydroxy group;R3 is a hydroxy(C1 -C4)alkyl group, a C1 -C4, alkoxymethyl group, a C2 -C5 alkenyl group, a C2 -C5 alkynyl group, a fluorinated C2 -C5 alkyl group or a fluorinated C2 -C5 alkenyl group; R4 is a hydrogen atom or a C1 -C4 alkyl group; R5 is a branched or non-branched, saturated or unsaturated aliphatic hyd