Conformationally restricted analogs of 1 alpha, 25-dihydroxyvitamin D3 and its 20-epimer: compounds for study of the three-dimensional structure of vitamin D responsible for binding to the receptor.

Two proteins play important roles in the expression of vitamin D function: the specific nuclear receptor protein (vitamin D receptor, VDR) and the transport protein (vitamin D binding protein, DBP). This study was conducted to clarify the conformation of vitamin D responsible for binding to those proteins. For the purpose, the side chain mobility of 1,25(OH)2D3 (1) and its 20-epimer, 20-epi-1,25(OH)2D3 (2), was analyzed by a systematic conformational search. The results were depicted as a three-dimensional dot map, which indicates that the side chains of the two vitamins (1 and 2) occupy different spatial regions that are separated in two areas. We denoted these areas as A and G for 1 and EA and EG for 2. Four analogs, the diastereomers at C(20) and C(22) (3-6) of 22-methylated 1,25(OH)2D3 whose side chains were confined to occupy G, A, EA, and EG, respectively, were designed. These analogs (3-6) were synthesized efficiently by a stereoselective conjugate addition of organocuprate to steroidal E- and Z-22-en-24-ones as the key step. In binding to the VDR the affinities of the analogs (3-6) relative to 1,25-(OH)2D3 (1) were 1/60, 1/3, 20, and 1/100, respectively. These results indicate that the A region is responsible for binding of 1 to VDR and the EA region for binding of 2. Only isomer 4 showed significant affinity for DBP, indicating only the A region is responsible for binding to DBP. Thus, 5 showed clear separation of binding affinities for two proteins, VDR and DBP. Having the highest known VDR affinity, (22R)-22-methyl-20-epi-1,25(OH)2D3 (5) has potential both as a therapeutic agent and as a tool to study the molecular mechanism of vitamin D-mediated gene transcription.