Synthesis and characterization of 20-hydroxyvitamin D3 with the A-ring modification.

Two novel 20-hydroxyvitamin D3 analogues (4a,b) with the A-ring modification have been synthesized by a convergent manner. An alternative pathway of vitamin D3 metabolism by cytochrome P450scc CYP11A1 was reported to afford 20-hydroxyvitamin D3 (3), functions of which remain to be explored. Based on the structure of the 20-hydroxy metabolite, novel analogues (4a,b) with the modifications, including the 1α-hydroxy, 25-hydroxy and 2α-methyl groups, have been designed. The side chain of the requisite CD-ring portions (9a,b) was introduced by Grignard reaction as a key step, and the stereochemistry at the C20 position was confirmed by the X-ray crystal structure analysis of the synthetic intermediate (8b). Preliminary biological characterization using the bovine thymus vitamin D receptor suggested that the introduction of the active motifs into the 20-hydroxyvitamin D3 scaffold elevated the receptor affinity.

Synthetic strategy and biological activity of A-ring stereoisomers of 1,25- dihydroxyvitamin D3 and C2-modified analogues.

The hormonally active form of vitamin D3, 1α,25-dihydroxyvitamin D3 (1a), has a wide variety of biological activities and its major molecular target is considered to be the vitamin D receptor (VDR). The A-ring stereoisomers of 1a as well as its C2-modified analogues, which have different stereochemistry at the C1 and/or C3 hydroxy groups, are of interest since recent metabolic studies have shown that catabolism could occur through A-ring modification. In this review, a practical and versatile synthesis of the A-ring enyne precursors by the convergent method of Trost and coworkers, which is needed to construct all possible A-ring stereoisomers of 1,25-dihydroxyvitamin D3 (1a-d), and the C2-modified analogues (4a-d, 5a-d, 6a-d and 7a-d) is described. A strategy for the synthesis and evaluation of all possible A-ring stereoisomers of 1a and their A-ring modified analogues is important, and this will stimulate synthesis and biological studies into vitamin D.

Synthesis and biological evaluation of 1α,25-dihydroxyvitamin D3 analogues with aromatic side chains attached at C-17.

Two new analogues of the steroid hormone 1α,25-dihydroxyvitamin D3 with aromatic side chains attached at C-17 were designed to investigate their effects on VDR, HL-60 cell differentiation and tumor cell proliferation. These analogues were prepared by the classical photochemical ring opening approach. After the protection of both the 1α- and 3ß-hydroxyl in 1α-hydroxydehydroepiandrosterone with TBS groups, followed by bromination with NBS and debromination in the presence of γ-collidine, the diene intermediate was obtained. Hydrazone formation followed by iodine oxidation gave a vinyl iodide. The aromatic side chain at C-17 was introduced via the Negishi coupling of the resulting intermediate with an in situ generated zinc reagent with the substituted aryl bromide (CD-side chain) in the presence of catalytic amount of Pd(PPh3)4. After the removal of the TBDMS and MOM protective groups, followed by UV irradiation and the subsequent thermal reaction, the 1α,25-(OH)2-D3 analogues with a substituted phenyl ring attached at C-17 to replace the C-20 and C-21 were prepared. In the VDR competitive binding assay, compounds 2 and 3 almost lost their binding ability, and were only 0.01% and 0.015% as potent as the 1α,25-dihydroxyvitamin D3. However, compounds 2 and 3 were as potent as 1α,25-(OH)2-D3 in inducing HL-60 cell differentiation at concentrations of 30, 100, 300, 1000 nM, respectively. Moreover, compounds 2 and 3 exhibited similar or better antiproliferative potency against MCF-7 human breast cancer cells, the IC50 values for analogues 2, 3 and the natural hormone were 7.08, 7.56, and 12.5 µM, respectively.

Design and synthesis of novel 1,25-dihydroxyvitamin D3 analogues having a spiro-oxetane fused at the C2 position in the A-ring.

Four structurally novel stereoisomeric analogues of 1,25-dihydroxyvitamin D3 (3a-d) bearing a spiro-oxetane fused at the C2 position of the A-ring have been designed and synthesised in a convergent manner. The requisite A-ring enyne precursors (13a,b) for the vitamin D analogues (3a,b) and (3c,d), respectively, were synthesised from pentaerythritol according to an eleven-step procedure. Preliminary biological evaluation of the analogues using the bovine thymus vitamin D receptor (VDR) suggested that the incorporation of the spiro-oxetane moiety instead of a gem-dimethyl group at the C2 position had a beneficial effect on the VDR affinity.