Synthesis, biological activity, and conformational analysis of four seco-D-15,19-bisnor-1 alpha,25-dihydroxyvitamin D analogues, diastereomeric at C17 and C20

The synthesis of four CD-ring-modified 19-nor-1 alpha,25-dihydroxyvitamin D g derivatives lacking C15, referred to as 6C analogues, and diastereomeric at C17 and C20 is described. The synthesis involves an Ireland-Claisen rear rangement of a 3-methyl-substituted ester of(1R)-3-methyl-2-cyclohexen-1-ol as the key step, followed by elaboration of the side chain, transformation into a C8 cyclohexanone derivative, and final Wittig-Horner coupling with the 19-nor A-ring phosphine oxide. Despite possessing a more flexible side chain than the parent hormone, biological evaluation showed an unexpected s uperagonistic antiproliferative and prodifferentiating activity (10-50 time s higher as compared to that of 1 alpha,25(OH)(2)D-3) for the diastereomer with the "natural" configuration at C17 and C20. The other diastereomers ex hibit a 25-90% decrease in activity. All four analogues show a decreased bi nding affinity (45% or less), and their calcemic activity is 4-400 times le ss than that of 1 alpha,25(OH)(2)D-3. The conformational behavior of their side chain was studied using molecular mechanics calculations, and the resu lt is presented as volume maps. A relative activity volume was determined b y subtraction of the volume map of the least active analogue from the volum e map of the most active one. This shows three regions corresponding to pre ferred orientations in space of the side chain of the active analogue. One of these regions was found to overlap with the region that is preferentiall y occupied by the most active of the four diastereomeric 22-methyl-substitu ted 1 alpha,25(OH)(2)D-3 analogues.