Pd-catalyzed carbocyclization-Negishi cross-coupling cascade: a novel approach to 1alpha,25-dihydroxyvitamin D3 and analogues

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.

Seco-B-Ring Steroidal Dienynes with Aromatic D Ring: Design, Synthesis and Biological Evaluation

Continuing our structure-activity studies on the vitamin D analogs with the altered intercyclic seco-B-ring fragment, we designed compounds possessing dienyne system conjugated with the benzene D ring. Analysis of the literature data and the docking experiments seemed to indicate that the target compounds could mimic the ligands with a good affinity to the vitamin D receptor (VDR). Multi-step synthesis of the C/D-ring building block of the tetralone structure was achieved and its enol triflate was coupled with the known A-ring fragments, possessing conjugated enyne moiety, using Sonogashira protocol. The structures of the final products were confirmed by NMR, UV and mass spectroscopy. Their binding affinities for the full-length human VDR were determined and it was established that compound substituted at C-2 with exomethylene group showed significant binding to the receptor. This analog was also able to induce monocytic differentiation of HL-60 cells.

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.

Synthesis and biological activity of two C-7 methyl analogues of vitamin D

Two novel vitamin D analogues of the hormone 1α,25-(OH)2D3 modified at C-7, namely, 7-methyl-1α,25-(OH)2D3 (12) and 7-methyl-1α,25-(OH)2-19-nor-D3 (26), were synthesized and biologically evaluated to gain further insights into the structure-function relationships of vitamin D. Key steps in the synthesis of 12 include the functionalization at C-7 by an efficient regioselective hydrostannylation of an allene precursor, and the construction of the triene framework by a palladium-catalyzed intramolecular cyclization-Suzuki-Miyaura coupling cascade. Since the calcitriol analogue 12 was prone to conversion into its previtamin D form by thermal equilibration, the corresponding 19-nor-compound 26 was also synthesized. The diene moiety of compound 26 was constructed by a modified Julia coupling. UV data as well as X-ray analysis indicate that introduction of the methyl group at C-7 results in a significant deviation from planarity of the 5,7-diene moiety. The new vitamin D analogues 12 and 26 retained good VDR binding ability.

Synthesis of novel vitamin D3 analog with an additional ring annulated to A and seco-B rings

A simple method for the synthesis of yet unknown 5E-vitamin D3 analogs with an additional six-membered ring connecting C-6 and C-19 was developed. Ring-closing metathesis (RCM) was used for efficient formation thereof from the corresponding 5E-isomers of 6-alkenyl vitamin D3 compounds which in turn were prepared from the 6-oxo-3,5-cyclovitamin D3. Reinvestigation of the Grignard reactions of this latter compound as well as the following acid-catalyzed cycloreversions showed discrepancies with the literature data describing the course of such processes.

1α,25(OH)2-3-epi-vitamin D3, a natural physiological metabolite of vitamin D3: its synthesis, biological activity and crystal structure with its receptor

Background

The 1α,25-dihydroxy-3-epi-vitamin-D₃ (1α,25(OH)₂-3-epi-D₃), a natural metabolite of the seco-steroid vitamin D₃, exerts its biological activity through binding to its cognate vitamin D nuclear receptor (VDR), a ligand dependent transcription regulator. In vivo action of 1α,25(OH)₂-3-epi-D₃ is tissue-specific and exhibits lowest calcemic effect compared to that induced by 1α,25(OH)₂D₃. To further unveil the structural mechanism and structure-activity relationships of 1α,25(OH)₂-3-epi-D₃ and its receptor complex, we characterized some of its in vitro biological properties and solved its crystal structure complexed with human VDR ligand-binding domain (LBD).

Methodology/Principal Findings

In the present study, we report the more effective synthesis with fewer steps that provides higher yield of the 3-epimer of the 1α,25(OH)₂D₃. We solved the crystal structure of its complex with the human VDR-LBD and found that this natural metabolite displays specific adaptation of the ligand-binding pocket, as the 3-epimer maintains the number of hydrogen bonds by an alternative water-mediated interaction to compensate the abolished interaction with Ser278. In addition, the biological activity of the 1α,25(OH)₂-3-epi-D₃ in primary human keratinocytes and biochemical properties are comparable to 1α,25(OH)₂D₃.

Conclusions/Significance

The physiological role of this pathway as the specific biological action of the 3-epimer remains unclear. However, its high metabolic stability together with its significant biologic activity makes this natural metabolite an interesting ligand for clinical applications. Our new findings contribute to a better understanding at molecular level how natural metabolites of 1α,25(OH)₂D₃ lead to significant activity in biological systems and we conclude that the C3-epimerization pathway produces an active metabolite with similar biochemical and biological properties to those of the 1α,25(OH)₂D₃.

Design and synthesis of active vitamin D analogs

A review of the design and synthesis of structural analogs of the vitamin D hormone recently investigated in our laboratories, and the first report on a new class of vitamin D analogs characterized by an aromatic D-ring, is described.

Synthesis and biological evaluation of 6-methyl analog of 1alpha,25-dihydroxyvitamin D3

Among more than 3000 analogs of 1alpha,25-dihydroxyvitamin D3 synthesized to date, only a few were characterized by structural modifications in the seco-B-ring. The compounds alkylated at C-6 seemed to be interesting targets for synthetic efforts. Such vitamin D analogs easily undergo thermal conversion to their previtamin forms. The results of molecular modeling indicate that significant deviation from planarity must be present in their molecules associated with the interaction of the 6-alkyl substituent and hydrogens from the C-ring. The synthesis of the analog of 1alpha,25-(OH)2D3, being characterized by the presence of the 6-methyl group, is reported here, together with the results of preliminary testing of its biological potency. This 6-alkylated compound was efficiently prepared using a novel stereoconvergent strategy in which the ring A and the triene unit of the vitamin D skeleton are constructed by a one-pot Pd-catalyzed tandem cyclization-Negishi coupling process.

Expedited Approach to the Vitamin D trans-Hydrindane Building Block from the Hajos Dione. Comparative Study on Various Methods for the Selective Deoxygenation of One of the Hydroxy Groups in a Diol

1 alpha,25-Dihydroxyvitamin D3 (calcitriol, 1) is a bioregulator important for the treatment of various human metabolic diseases and biomedical research. Herein, we report an efficient diastereoselective approach to the key trans-hydrindane building block for calcitriol synthesis (2a) starting from the readily accessible optically active tetrahydroindenedione derivative (Hajos dione, 3). It was found that epoxide ring opening in a related hydroxy epoxide (7) with sodium cyanoborohydride-BF3 x Et2O occurs by hydride anion addition at the ring juncture position to produce a vicinal diol with the trans-hydrindane ring system (6a). Four methods for selective deoxygenation of the sterically less shielded hydroxy group in diol 6a were examined with an approach based on a cyclic sulfate of the diol as the most efficient and operationally convenient method.

An expedited approach to the vitamin D trans-hydrindane building block from the Hajos dione

Efficient and operationally simple synthesis of the key trans-hydrindane alcohol building block for the synthesis of calicitriol (1alpha,25-dihydroxyvitamin D(3)) has been developed. Epoxy alcohol prepared almost quantitatively from the Hajos dione was reduced at the quaternary carbon by the Hutchins procedure (NaBH(3)CN-BF(3).Et(2)O). The diol was selectively deoxygenized either using the Barton-McCombie reaction (with Bu(3)SnH-AIBN) or via the respective iodohydrine (with LiAlH(4)). [reaction: see text]