Concise synthesis and biological activities of 2α-Alkyl- and 2α-(ω-Hydroxyalkyl)-20- epi -1α,25-dihydroxyvitamin D 3

The Centennial Collection of VDR Ligands: Metabolites, Analogs, Hybrids and Non-Secosteroidal Ligands

Since the discovery of vitamin D a century ago, a great number of metabolites, analogs, hybrids and nonsteroidal VDR ligands have been developed. An enormous effort has been made to synthesize compounds which present beneficial properties while attaining lower calcium serum levels than calcitriol. This structural review covers VDR ligands published to date.

Novel approach to A-ring synthon for Pd-catalyzed synthesis of 1α-hydroxylated vitamin D metabolites

A novel approach to an A-ring synthon for Pd-catalyzed synthesis of 1α-hydroxylated vitamin D metabolites is described. Key step is an asymmetric glyoxylate ene reaction to access a highly diastereomerically pure α-hydroxy ester. Subsequent stereospecific transformation to an anti-1,3-diol and appropriate chemical modifications at both ends of the acyclic precursor leads to a diastereomerically and enantiomerically pure silylated anti-1,3-diol enyne, serving as a versatile A-ring synthon for its use in vitamin D synthesis.

Pd-catalyzed steroid reactions

We review the most important achievements of the last decade in the field of steroid synthesis in the presence of palladium catalysts. Various palladium-catalyzed cross-coupling reactions, including Heck, Suzuki, Stille, Sonogashira, Negishi and others, are exemplified with steroid transformations.

In Pursuit of an Ideal C-C Bond-Forming Reaction: Development and Applications of the Hydrovinylation of Olefins

Attempts to introduce the highly versatile vinyl group into other organic molecules in a chemo-, regio- and stereoselective fashion via catalytic activation of ethylene provided challenging opportunities to explore new ligand and salt effects in homogeneous catalysis. This review provides a personal account of the development of enantioselective reactions involving ethylene.

Structural refinement of seco-steroidal skeleton and the biological activity through nuclear receptors

1alpha,25-Dihydroxyvitamin D(3) (1) regulates a variety of biological actions through vitamin D receptor (VDR), including calcium and phosphorus homeostasis, bone remodeling, cellular proliferation and differentiation and many other functions. To enhance its potency and to study the structure/function relationship, we synthesized a series of analogs of 1 with a modification at the C-2alpha position. Introducing 2alpha-methyl, 2alpha-(3-hydroxypropyl), or 2alpha-(3-hydroxypropoxy) group increased its binding affinity for the VDR 2- to 4-fold compared to 1. The crystal structures of the VDR bound to these analogs provide a molecular explanation for the interaction between the 2alpha-substituents and water molecules exist in the VDR-ligand binding domain. Based on the accumulated knowledge in VDR agonists, we synthesized 2-substituted analogs of 'double side chain' (gemini), 19-norvitamin D(3) (MART-10), TEI-9647 (VDR antagonist), 1-alkylated vitamin D(3), 14-epi-previtamin D(3) etc. Gemini analogs showed potent HL-60 cell differentiation activity (13-38 times compared to 1), and MART-10 exhibited remarkable antiproliferative activity on PZ-HPV-7 cells even at 10(-10) M. (24S)-2alpha-(3-Hydroxypropoxy)-24-propyl-TEI-9647 showed potent VDR antagonism, and its IC(50) value was 7.4 pM against 10 nM of 1. 1alpha-Methyl-2alpha-(3-hydroxypropyl)-25-hydroxyvitamin D(3) improved the binding affinity for the mutant VDR(Arg274Leu), which causes hereditary vitamin D resistant rickets. 1alpha,25-Dihydroxy-2alpha-methyl-14-epi-previtamin D(3) showed moderate osteocalcin transcriptional activity on HOS cells. We theorize that modification at A-ring alone and in combination with functionalization of the other parts of the vitamin D molecule would provide important new information on the mechanism of vitamin D actions that could lead to the development of new therapeutic regimes for the treatment of various diseases.

Ligand tuning in asymmetric hydrovinylation of 1,3-dienes: a stereoselective route to either steroid-C20 (S) or -C20 (R) derivatives

1,3-Dienes derived from steroidal D-ring C 17-ketones undergo Ni(II)-catalyzed hydrovinylation to give 1,2- or 1,4-addition of ethylene. Using finely tuned phosphoramidite ligands, it is possible to synthesize either the C 20 ( R)- or ( S)-derivatives without mutual contamination. The proportion of the 1,4-adduct, which is also formed stereoselectively, can be minimized by optimizing the reaction conditions. Because the two alkenes in the resultant dienes have differing steric demands for many potential reactions, and are ideally juxtaposed for further D-ring functionalization, these intermediates could be useful for the preparation of biologically important compounds such as vitamin D analogs and various antitumor steroidal glycosides.

Biological activities of 2alpha-substituted analogues of 1alpha,25-dihydroxyvitamin D3 in transcriptional regulation and human promyelocytic leukemia (HL-60) cell proliferation and differentiation

Biological activities of 2alpha-substituted 1alpha,25-dihydroxyvitamin D3 analogues were evaluated in vitro. Their binding affinity was examined with calf thymus cytosolic vitamin D receptor (VDR) and rat plasma vitamin D-binding protein (DBP). In addition, the transcriptional activity of the analogues was measured using a rat 25-hydroxyvitamin D3-24-hydroxylase gene promoter, a human osteocalcin gene promoter, and VDR-GAL4 system. This study investigated the biological activities of 2alpha-substituted analogues in comparison with 2beta-substitued analogues at the molecular level, with regard to the structural differences of alkyl, hydroxyalkyl, hydroxyalkoxy substituents at the 2-position of 1alpha,25-dihydroxyvitamin D3.

Further Synthetic and Biological Studies on Vitamin D Hormone Antagonists Based on C24-Alkylation and C2α-Functionalization of 25-Dehydro-1α-hydroxyvitamin D3-26,23-lactones

An efficient synthesis and the biological evaluation of 80 novel analogs of 25-dehydro-1alpha-hydroxyvitamin D3-26,23S-lactone 2 (TEI-9647) and its 23R epimer (3) in which the lactone ring was systematically functionalized by introduction of a C1 to C4 primary alkyl group at the C24 position (5 sets of 4 diastereomers), together with their C2alpha-methyl, 3-hydroxypropyl, and 3-hydroxypropoxy-substituted derivatives were described. The triene structure of the vitamin D3 was constructed using palladium-catalyzed alkenylative cyclization of the A-ring precursor enyne with the CD-ring counterpart bromoolefin having the C24-alkylated lactone moiety on the side chain. The CD-ring precursors having 23,24-cis lactones were prepared by using a chromium-mediated syn-selective allylation-lactonization process, and the 23,24-trans lactone derivatives were derived from these via inversion of the C23 stereochemistry. The biological evaluation revealed that both binding affinity for chick vitamin D hormone receptor and antagonistic activity (inhibition of vitamin D hormone induced HL-60 cell differentiation) were affected by the orientation and chain-length of the primary alkyl group on the lactone ring. Furthermore, the C2alpha-functionalization of the C24-alkylated vitamin D3 lactones dramatically enhanced their biological activities. The most potent compound to emerge, (23S,24S)-2alpha-(3-hydroxypropoxy)-24-propyl exhibited almost 1000-fold stronger antagonistic activity (IC50=7.4 pM) than 2 (IC50=6.3 nM).

Probing a water channel near the A-ring of receptor-bound 1 alpha,25-dihydroxyvitamin D3 with selected 2 alpha-substituted analogues

The crystal structure of the vitamin D receptor (VDR) in complex with 1 alpha,25(OH)2D3 revealed the presence of several water molecules near the A-ring linking the ligand C-2 position to the protein surface. Here, we report the crystal structures of the human VDR ligand binding domain bound to selected C-2 alpha substituted analogues, namely, methyl, propyl, propoxy, hydroxypropyl, and hydroxypropoxy. These specific replacements do not modify the structure of the protein or the ligand, but with the exception of the methyl substituent, all analogues affect the presence and/or the location of the above water molecules. The integrity of the channel interactions and specific C-2 alpha analogue directed additional interactions correlate with the binding affinity of the ligands. In contrast, the resulting loss or gain of H-bonds does not reflect the magnitude of HL60 cell differentiation. Our overall findings highlight a rational approach to the design of more potent ligands by building in features revealed in the crystal structures.

Highly Potent Vitamin D Receptor Antagonists: Design, Synthesis, and Biological Evaluation

Vitamin D receptor (VDR) antagonists have attracted significant levels of interest because of their potential utility in the treatment of Paget's disease, which is known as the most flagrant example of disordered bone remodeling and the second most common bone disease after osteoporosis in Anglo-Saxons. Recent studies on Paget's disease suggested a specific increase in osteoclasts' sensitivity to the differentiation activity of active vitamin D(3) as the principal mechanism for abnormal bone formation. We set out to conduct a structure-activity relationship study on the first VDR antagonists, TEI-9647 and TEI-9648 (25-dehydro-1alpha-hydroxyvitamin D(3)-26,23-lactone), with the goal of improved VDR antagonistic activity. Given that both potent agonists and antagonists must have high affinity for the VDR, we hoped that our accumulated knowledge in the field of VDR agonists would help us identify potent antagonists. First 2alpha-modified TEI-9647 analogues were synthesized, and then 24-substitution to stabilize the lactone structure under physiological conditions was investigated. Finally, 2alpha-modified 24-methyl-, 24,24-dimethyl-, and 24,24-ethano-25-dehydro-1alpha-hydroxyvitamin D(3)-26,23-lactone analogues were synthesized. The synthesis of the 24,24-ethano-TEI lactone was accomplished through Ru-catalyzed intermolecular enyne metathesis of the alkynone CD-ring side chain with ethylene to give a dienone, followed by regioselective cyclopropanation. It was found that 2alpha,24,24-trimethyl-TEI-9647 (39) possessed an antagonistic activity (IC(50)=0.093 nM) approximately 90 times that of the original TEI-9647 (IC(50)=8.3 nM).

Quantitative structure-activity relationship studies of vitamin D receptor affinity for analogues of 1alpha,25-dihydroxyvitamin D3. 1: WHIM descriptors

The weighted holistic invariant molecular (WHIM) approach has been applied to the study of the VDR affinity of 86 vitamin D analogues. A model able to describe more than 71% of the variance in the experimental activity was developed with the use of the mentioned approach. In contrast, none of three different approaches, including the use of BCUT, Galvez topological charge indices, and 2D autocorrelations descriptors, was able to explain more than 38% of the variance in the mentioned property, even with more variables in the equation.

Design and Efficient Synthesis of 2α-(ω-Hydroxyalkoxy)-1α,25-dihydroxyvitamin D3 Analogues, Including 2-epi-ED-71 and Their 20-Epimers with HL-60 Cell Differentiation Activity

A concise and efficient synthetic approach to 2 alpha-(omega-hydroxyalkoxy)-1 alpha,25-dihydroxyvitamin D(3) (4a-c), including 2-epi-ED-71, was developed starting from D-glucose as a chiral template for the construction of the 2 alpha-modified A-ring precursors (11a-c). It was found that the best ligand for the bovine thymus vitamin D receptor (VDR) in this series is 4b, which has 1.8 times greater binding affinity for the bovine thymus VDR than that of the natural hormone 1. Interestingly, potency in the induction of HL-60 cell differentiation for 4a-c was almost the same or weaker than that of 1 despite the strong binding affinity for the VDR. Next, we were interested in the "double modification"of 1 based on 4a-c with C20-epimerization, affording 2 alpha-(omega-hydroxyalkoxy)-20-epi-1 alpha,25-dihydroxyvitamin D(3) (20-epi-4a-c). All three 2 alpha-substituted 20-epi analogues of 1 (20-epi-4a-c) exhibited stronger binding affinities for the VDR, and their conformations in the ligand binding domain of VDR were analyzed by molecular modeling. Double-modified analogues of 20-epi-4a-c showed marked HL-60 cell differentiation activity, and 20-epi-4a possesses an activity 58-fold higher than that of the natural hormone 1.