Enzymatic Desymmetrization of 19-nor-Vitamin D3 A-Ring Synthon Precursor: Synthesis, Structure Elucidation, and Biological Activity of 1α,25-Dihydroxy-3-epi-19-nor-vitamin D3 and 1β,25-Dihydroxy-19-nor-vitamin D3

In a search for novel vitamin D derivatives of potential therapeutic value, structurally simple but synthetically challenging A-ring epimers of the 19-nor-Calcitriol [19-nor-1 alpha,25-(OH)(2)-D-3] at C1 and C3 were efficiently synthesized. Both analogues (1-epi- and 3-epi-19-nor-Calcitriol) were obtained through a convergent synthesis starting from cis,cis-1,3,5-cyclohexanetriol and the protected 25-hydroxy Grundmanns ketone. After Julia-Kocienski coupling of the corresponding C,D-ring/side chain sulfone fragment with the A-ring ketone moiety, both vitamin D analogues were isolated. The critical point was how to determine the structural configuration of both diastereoisomers since similar H-1 NMR spectra were observed. For that, a biocatalytic approach was crucial in the synthesis of orthogonally protected derivatives. NMR spectroscopy allows the unambiguous identification of these compounds and as a result the structural elucidation of the desired vitamin D diastereomeric analogues. Affinity studies demonstrated that these 1,25-19-nor analogues have a very low affinity for the vitamin D receptor compared with 1 alpha,25-dihydroxyvitamin D-3 or 1 alpha,25-dihydroxy-19-nor-vitamin D-3. In addition, these analogues have a lower binding affinity for the human vitamin D binding protein than the natural hormone. In vitro cell culture studies revealed that synthesized analogues were less active than 1 alpha,25-dihydroxyvitamin D-3 in inhibiting cell proliferation.