Synthesis and binding-analysis of 5E-[19-(2-bromoacetoxy)methyl]25-hydroxyvitamin D3 and 5E-25-hydroxyvitamin D3-19-methyl[(4-azido-2-nitro)phenyl]glycinate: novel C19-modified affinity and photoaffinity analogs of 25-hydroxyvitamin D3.

Synthesis of novel C19-modified affinity and photoaffinity analogs of vitamin D3 and 25-hydroxyvitamin D3(25-OH-D3) is described. A key step in the synthesis is a Horner-Emmons reaction between C19-nor-cyclovitamin D3-C19-ketone or C19-nor-25-hydroxy-cyclovitamin D3-C19-ketone and diethyl cyanomethylphosphonate. Competitive radioligand binding assays with human serum vitamin D-binding protein (DBP) and 5E-[19-(2-bromoacetoxy)methyl]25-hydroxyvitamin D3 and 5E-25-hydroxyvitamin D3-19-methyl[(4-azido-2-nitro)phenyl]-glycinate, 25-OH-D3-analogs containing affinity and photoaffinity probes at C19-position, demonstrated that these compounds displaced radiolabeled 25-OH-D3 from the binding pocket of DBP in a dose-dependent manner. Thus, these affinity and photoaffinity analogs are potentially useful in determining the ligand binding site topographies of DBP and possibly the vitamin D receptor.