In most potentially applicable vitamin D analogues it is the side chain of 1 that is modified (although only a few of the numerous analogues of this type that have been synthesized in recent decades have in fact proved useful for the treatment of cancers 1. and 2. or psoriasis 3., and ). In particular, analogues with rigid, conformationally restricted structural units in the side chain tend to combine non-negligible antiproliferative activity with low calcaemic activity. 4., and ...