A-ring Phosphine Oxide Research Articles

Synthesis of new 26,27-difluoro- and 26,26,27,27-tetrafluoro-25-hydroxyvitamin D3: Effects of terminal fluorine atoms on biological activity and half-life.

As an extension of our research on providing a chemical library of side-chain fluorinated vitamin D3 analogues, we newly designed and synthesized 26,27-difluoro-25-hydroxyvitamin D3 (1) and 26,26,27,27-tetrafluoro-25-hydroxyvitamin D3 (2) using a convergent method applying the Wittig-Horner coupling reaction between CD-ring ketones (13,14) and A-ring phosphine oxide (5).The basic biological activities of analogues, 1, 2, and 26,26,26,27,27,27-hexafluoro-25-hydroxyvitamin D3 [HF-25(OH)D3] were examined. Although the tetrafluorinated new compound 2 exhibited higher binding affinity for vitamin D receptor (VDR) and resistance to CYP24A1-dependent metabolism compared with the difluorinated 1 and its non-fluorinated counterpart 25-hydroxyvitamin D3 [25(OH)D3], HF-25(OH)D3 showed the highest activity among these compounds. Osteocalcin promoter transactivation activity of these fluorinated analogues was tested, and it decreased in the order of HF-25(OH)D3, 2, 1, and 25(OH)D3 in which HF-25(OH)D3 showed 19-times greater activity than the natural 25(OH)D3.

Synthesis of (22R)-, (22S)-22-Fluoro-, and 22,22-Difluoro-25-hydroxyvitamin D3 and Effects of Side-Chain Fluorination on Biological Activity and CYP24A1-Dependent Metabolism.

Three novel analogues of C22-fluoro-25-hydroxyvitamin D3 (5-7) were synthesized and evaluated to investigate the effects of side-chain fluorination on biological activity and metabolism of vitamin D. These novel analogues were constructed by convergent synthesis applying the Wittig-Horner coupling reaction between CD-ring ketones (41,42,44) and A-ring phosphine oxide (11). The introduction of C22-fluoro units was achieved by stereoselective deoxy-fluorination for synthesizing 5 and 6 or two-step cationic fluorination for 7. The absolute configuration of the C22-fluoro-8-oxo-CD-ring (39) was confirmed by X-ray crystallographic structure determination. The basic biological activity of the side-chain fluorinated analogues, including compounds (5-7), was evaluated. Generally, osteocalcin promoter transactivation activity decreased in the order of C24-fluoro, C23-fluoro, and C22-fluoro analogues. In addition, the metabolic stability of C22-fluoro-25-hydroxyvitamin D3 (5-7) against hCYP24A1 metabolism was also evaluated. 22,22-Difluoro-25(OH)D3 (7) was more stable against hCYP24A1 metabolism compared with its non-fluorinated counterpart 25-hydroxyvitamin D3 (1), but fluorination at the C22 position had little effect on the metabolic stability compared with C24- and C23-fluoro analogues. Our research clarified that side-chain fluorination in vitamin D markedly changes CYP24A1 metabolic stability depending on the fluorinating position.

Design, synthesis and biological evaluation of novel 2-alkylidene 19-norcalcitriol analogs

Continuing our studies aimed at A-ring modified vitamin D compounds, we designed novel 19-norcalcitriol derivatives bearing at C-2 pegylated chains of different lengths. The terminal fragments of these substituents contain hydroxyls or moieties possessing nitrogen and/or sulfur atoms capable of transition metal ions complexation. Also, two conjugate–type platinum(II) complexes of 19-norcalcitriol were obtained in which l-methionine served as chelating moiety. The convergent synthesis of the target 19-norcalcitriol analogs involved several steps with the crucial one being condensation of A-ring phosphine oxide and the known Grundmann ketone by Wittig-Horner reaction. Further elaboration of the 2-alkylidene substituent provided all final compounds which were then tested to determine their affinity for the vitamin D receptor and cytotoxic activity.

Synthesis and Biological Activity of 2,22-Dimethylene Analogues of 19-Norcalcitriol and Related Compounds.

Continuing our search for vitamin D analogues, we explored the modification of the steroidal side chain and inserted a methylene moiety in position C-22 together with either lengthening the side chain or introducing a ring at the terminal end. Our conformational studies confirmed that the presence of a methylene group attached to C-22 restricts the conformational flexibility of the side chain, which can result in changes in biological characteristics of a molecule. All synthesized 1α,25-dihydroxy-2,22-dimethylene-19-norvitamin D3 analogues proved equal to calcitriol in their ability to bind to the vitamin D receptor, and most of them exert significantly higher differentiation and transcriptional activity than calcitriol. The most active compounds were characterized by the presence of an elongated side chain or 26,27-dimethylene bridge. The synthetic strategy was based on the Wittig-Horner coupling of the known A-ring phosphine oxide with the corresponding Grundmann ketones prepared from a 20-epi-Inhoffen-Lythgoe diol derived from vitamin D2.

Synthesis of Gemini analogs of 19-norcalcitriol and their platinum(II) complexes

Hormonally active vitamin D3 metabolite, calcitriol, plays an important role in calcium-phosphate homeostasis, immune system actions and cell differentiation. Although anticancer activity of calcitriol is well documented and thousands of its analogs have been synthesized, none has been approved as a potential drug against cancer. Therefore, we attempted to introduce the cytotoxic effect to the calcitriol molecule by its linking to cisplatin. Herein, we present the synthesis of vitamin D compounds, designed on the basis of molecular modeling and docking experiments to the vitamin D receptor, and characterized by the presence of significantly different two side chains attached to C-20. In this study, a new synthetic approach to Gemini analogs was developed. Preparation of the target 19-norcalcitriol compounds involved separate syntheses of several building blocks (the A-ring, C/D-rings and side-chain fragments). The convergent synthetic strategy was used to combine these components by the different coupling processes, the crucial one being Wittig-Horner reaction of the Grundmann ketone analog with the known 2-methylene A-ring phosphine oxide. Due to the nature of the constructed steroidal side chains (bidentate ligands), which allowed coordination of metal ions, the first conjugate-type platinum(II) complexes of the vitamin D analogs were also successfully prepared and characterized. The target vitamin D compounds, displaying significant affinity for a vitamin D receptor, were assessed in vitro for their anti-proliferative activities towards several cell lines.

Concise synthesis of 23-hydroxylated vitamin D3 metabolites

Three 23-hydroxylated vitamin D3 derivatives, which are metabolites of 25-hydroxyvitamin D3 produced by CYP24A1 and a related diastereomer, were efficiently synthesized. Each C23 hydroxy unit was constructed by the Claisen condensation reaction with ethyl acetate or the Grignard reaction with 2-methylallymagnesium chloride. Stereochemistry at the C23 position was determined by a modified Mosher’s method. The triene structures were constructed by the Wittig-Horner reaction utilizing the A-ring phosphine oxide moiety.

Synthesis of 19-norcalcitriol analogs with pegylated alkylidene chains at C-2

The results presented in this paper constitute an extension of our synthetic efforts focused on 19-norvitamin D compounds possessing elongated 2-alkylidene substituents. Based on our previous results, molecular modeling studies, and docking experiments, we selected a novel 19-norcalcitriol analog with long chain at C-2 containing several ether moieties and terminated by 2-(pyridin-2′-yl)ethylamino fragment. It was expected that such structural modification might allow binding of transition metal by the ligand, increase solubility of the formed complexes as well as improve their affinity to the VDR. For comparison, a 19-norcalcitriol analog was also obtained with the terminal hydroxyl group at its pegylated 2-alkylidene substituent. The synthesis of the target vitamin D compounds described in this work was performed using the Wittig-Horner approach. The respective A-ring phosphine oxide was obtained starting from the D-(-)-quinic acid and then coupled with the known Grundmann ketone.

Synthesis of 19-norcalcitriol analogs with elongated side chain

Pronounced biological potency of 19-norvitamin D compounds as well as interesting biological action of the vitamin D analogs possessing elongated side chains encouraged us to expand the scope of our structure-activity studies to encompass such modifications of the 1α,25-(OH)2D3 (calcitriol) molecule. The aim of our studies was the synthesis of calcitriol analog, designed on the basis of results of molecular modeling and docking experiments, and characterized by a presence of a long, nitrogen-containing substituent attached to carbon 26, and an exomethylene moiety transferred from C-10 to C-2. The convergent synthesis of such 19-norcalcitriol compound, described in this communication, consisted of the preparation and combining four building blocks. The crucial point of the synthesis, coupling of the known A-ring phosphine oxide and the synthesized Grundmann ketone analog, was achieved using Wittig-Horner protocol. It provided the protected analog of 1α,25-dihydroxy-2-methylene-19-norvitamin D3 which was further transformed into the target compound.

Design, synthesis and biological properties of seco-d-ring modified 1α,25-dihydroxyvitamin D3 analogues

As a continuation of our efforts directed to the structure-activity relationship studies of vitamin D compounds, we present in this paper the synthesis of new analogues of 1α,25-(OH)2D3 characterized by numerous structural modifications, especially a cleaved D ring. Total synthesis of the CD fragment required for the construction of the target vitamins was based on the Stork approach. The structure of the key intermediate – bicyclic hydroxy lactone – was established by crystallographic and electronic circular dichroism (ECD) spectral analysis. Following the attachment of the hydroxyalkyl side chain, the formed D-seco Grundmann ketone was subjected to Wittig-Horner coupling with the corresponding A-ring phosphine oxides providing two desired D-seco analogues of 19-nor-1α,25-(OH)2D3, one without a substituent at C-2 and the other possessing a 2-exomethylene group. Both compounds were biologically tested and the latter was found to be more active in in vitro tests. Despite so many structural changes introduced in its structure, the biological activity of the 2-methylene analogue approached that of the natural hormone. The synthesized D-seco vitamins, however, proved to be inactive on bone and intestine in vivo.

Structural revisions of the reported A-ring phosphine oxide synthon for ED-71 (Eldecalcitol) and a new synthesis

In a reported procedure for the synthesis of the ED-71 A-ring phosphine oxide, we discovered that unusual TBS transfer occurred from the 1α-position to the sterically more constrained 2β-position, and the subsequent Michael addition of ethyl acrylate predominated at the 1α-position. The X-ray analysis of a key intermediate confirmed our observations. We made a structural revision of the reported A-ring phosphine oxide synthon and ED-71. In addition, we provided the first stereoselective synthetic approach to ED-71 A-ring phosphine oxide applying the stereochemistry of d-mannitol.

Stereoselective reduction of C-20 ketone of vitamin D CD-ring and a new synthetic approach to maxacalcitol

Stereoselective reduction of C-20 ketone of vitamin D CD-ring precursor using Corey’s CBS reagents, (R)-(+)-2-methyl-CBS-oxazaborolidine and (S)-(−)-2-methyl-CBS-oxazaborolidine, led to the (20S)-alcohol 5 and (20R)-epimer 4 in approximately 17:1 selectivity. A new synthetic approach to the 22-oxa-25-hydroxy Grundmann’s ketone 11 was developed through the Williamson etherification of (20S)-alcohol 5 with 1-bromomethyl-2,2-dimethyloxirane, followed by regioselective reductive epoxide ring opening with LiAlH4, the removal of the silyl protecting group by TBAF, and the environmentally benign TEMPO-mediated oxidation using inexpensive Oxone as a co-oxidizing agent. The preparation of drug maxacalcitol was achieved on gram scale by the convergent Lythgoe coupling via Wittig–Horner reaction of the A-ring phosphine oxide synthon with the CD-ring fragment.

13,13-Dimethyl-des-C,D analogues of (20S)-1α,25-dihydroxy-2-methylene-19-norvitamin D3 (2MD): Total synthesis, docking to the VDR, and biological evaluation

As a continuation of our studies focused on the vitamin D compounds lacking the C,D-hydrindane system, 13,13-dimethyl-des-C,D analogues of (20S)-1α,25-dihydroxy-2-methylene-19-norvitamin D3 (2, 2MD) were prepared by total synthesis. The known cyclohexanone 30, a precursor of the desired A-ring phosphine oxide 11, was synthesized starting with the keto acetal 13, whereas the aldehyde 12, constituting an acyclic ‘upper’ building block, was obtained from the isomeric esters 34, prepared previously in our laboratory.The commercial 1,4-cyclohexanedione monoethylene ketal (13) was enantioselectively α-hydroxylated utilizing the α-aminoxylation process catalyzed by l-proline, and the introduced hydroxy group was protected as a TBS, TPDPS, and SEM ether. Then the keto group in the obtained compounds 15–17 was methylenated and the allylic hydroxylation was performed with selenium dioxide and pyridine N-oxide. After separation of the isomers, the newly introduced hydroxy group was protected and the ketal group hydrolyzed to yield the corresponding protected (3R,5R)-3,5-dihydroxycyclohexanones 30–32. The esters 34, starting compounds for the C,D-fragment 12, were first α-methylated, then reduced and the resulted primary alcohols 36 were deoxygenated using the Barton-McCombie protocol. Primary hydroxy group in the obtained diether 38 was deprotected and oxidized to furnish the aldehyde 12. The Wittig–Horner coupling of the latter with the anion of the phosphine oxide 11, followed by hydroxyl deprotection furnished two isomeric 13,13-dimethyl-des-C,D analogues of 2MD (compounds 10 and 42) differing in configuration of their 7,8-double bond. Pure vitamin D analogues were isolated by HPLC and their biological activity was examined. The in vitro tests indicated that, compared to the analogue 7, unsubstituted at C-13, the synthesized vitamin D analogue 10 showed markedly improved VDR binding ability, significantly enhanced HL-60 differentiation activity as well as increased transcriptional potency. Docking simulations provided a rational explanation for the observed binding affinity of these ligands to the VDR. Biological in vivo tests proved that des-C,D compound 10 retained some intestinal activity. Its geometrical isomer 42 was devoid of any biological activity.

ChemInform Abstract: Efficient Fluorination with Tetrabutylammonium Dihydrogen Trifluoride in a Novel Approach Toward 1-α-Fluoro-25-hydroxy-vitamin D3 Analogues.

The known, but hardly accessible, A-ring phosphine oxide 20, a building block for 1-α-fluoro-25-hydroxy-vitamin D3, was prepared by a new route in gram amounts from (S)-(+)-carvone in 20 steps and 0.6% overall yield. Fluorine was introduced at an early stage by the completely regio- and stereoselective trans-diaxial opening of key-epoxide 5 with neat tetrabutylammonium dihydrogen trifluoride at 95 °C. The required 2-carbon chain extension of cyclohexanol 13 was accomplished in moderate yield via SN‘ substitution with cesium phenylselanyl acetate followed by Ireland−Claisen rearrangement of the resulting ester 15. Spontaneous elimination of the derived phenyl selenoxide led stereorandomly to a 1:1 mixture of dienoates E/Z-17, which was transformed into 20 as previously described.

ChemInform Abstract: Asymmetric Intramolecular Diels-Alder Cycloadditions of 2-Pyrone-3-carboxylates and Synthesis of Vitamin D3 A-Ring Phosphine Oxide.

ChemInform Abstract: Asymmetric Intramolecular Diels-Alder Cycloadditions of 2-Pyrone-3-carboxylates and Synthesis of Vitamin D3 A-Ring Phosphine Oxide.

Synthesis of 2β-substituted-14-epi-previtamin D 3 and testing of its genomic activity

2β-Substituted analogs of 14-epi-previtamin D 3 were synthesized for the first time by the thermal isomerization of the corresponding 14-epi-vitamin D 3 that were available using coupling reaction between the A-ring phosphine oxide derived from a chiral epoxide and CD-ring cis-hydrindanone. The VDR binding affinity and transactivation activity of osteocalcin promoter in HOS cells were evaluated, and the new analogs were found to be less active, 0.01–0.18% of VDR binding affinity compared with the natural hormone and EC 50 1.0–9.1 nM for transactivation activity, than 14-epi-previtamin D 3 with 0.5% (VDR) and EC 50 0.46 nM, respectively.

New A-ring analogs of the hormone 1α,25-dihydroxyvitamin D 3: (2′-hydroxymethyl)tetrahydrofuro[1,2- a]-25-hydroxyvitamin D 3

Conceptually new, enantiomerically pure bicyclic tetrahydrofuro[1,2- a]-A-ring phosphine oxides (+)- 4 and (−)- 4 were successfully prepared from methyl 2-pyrone-3-carboxylate and ( S)- or ( R)-2-( tert-butyldimethylsilyloxy)methyl-2,3-dihydrofuran, respectively. In addition, (2′-hydroxymethyl)tetrahydrofuro[1,2- a]-25-hydroxyvitamin D 3 3a and 3b as new A-ring-modified analogs of the natural hormone 1α,25-dihydroxyvitamin D 3 were readily synthesized by using Lythgoe-type coupling of the A-ring phosphine oxides (+)- 4 and (−)- 4 with C,D-ring ketone (+)- 5.

Synthesis and structure–activity relationships of 16-ene-22-thia-1α,25-dihydroxy-26,27-dimethyl-19-norvitamin D 3 analogs having side chains of different sizes

We have synthesized eight novel 16-ene-22-thia-26,27-dimethyl-19-norvitamin D 3 analogs 1– 5 bearing side chains of different sizes, in combination with 20 S- and 20 R-isomers. The target compounds were prepared by Wittig-Horner reaction of A-ring phosphine oxide with 16-ene-22-thia-25-hydroxy Grundmann’s ketones having different sized side chains, which were derived from the S-phenyloxycarbonyl derivative 13 as key intermediates. The binding affinity to the vitamin D receptor (VDR), VDR-mediated transcriptional activity, and osteoclast-inducing activity of synthetic 22-thia-19-norvitamin D analogs 1– 5 were investigated. The (20 S)-22-thia-19,24-dinorvitamin D analog 1a is as active as the natural hormone 1α,25-dihydroxyvitamin D 3 (1α,25-(OH) 2D 3) in terms of biological activities tested in vitro. The analogs 2a and 3a exhibited almost the same potency as 1α,25-(OH) 2D 3 in binding to the VDR, were about 20 times more potent than 1α,25-(OH) 2D 3 in terms of transcriptional activity, and 3a was approximately 100 times as potent as 1α,25-(OH) 2D 3 in eliciting osteoclast formation. The biological activities of (20 S)-22-thia compounds were more potent (by more than 10-fold) than those of the corresponding 20 R-counterparts, but the activity of (20 R)-compounds 1b, 2b, and 3b in stimulating the formation of osteoclasts was similar to that of 1α,25-(OH) 2D 3, and the 24-dihomo- and trihomo-analogs 4a and 5a showed low transcriptional activity. These results suggest that elongation of the side chain in 22-thia analogs by up to one carbon can be stably accommodated in the VDR ligand binding pocket.

Two Convergent Approaches to the Synthesis of 1α,25-Dihydroxy-2β-(3-hydroxypropoxy)vitamin D3 (ED-71) by the Lythgoe and the Trost Coupling Reactions

Two convergent syntheses of 1α,25-dihydroxy-2β-(3-hydroxypropoxy)vitamin D 3 (ED-71) by the Lythgoe coupling reaction between the A-ring phosphine oxide and the C/D-ring ketone and the Trost coupling reaction between the A-ring ene-yne and the C/D-ring bromomethylene are described.

Novel Synthesis of 2‐Substituted 19‐Norvitamin D A‐Ring Phosphine Oxide from D‐Glucose as a Building Block.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Novel synthesis of 2-Substituted 19-norvitamin D A-ring phosphine oxide from d-glucose as a building block

19-Norvitamin D A-ring phosphine oxide 5 was synthesized by a new sequence mode starting from d-glucose as a chiral template. Transformation of the pyranoside ring into the A-ring carbocycle was achieved by the Pd-catalyzed Ferrier rearrangement. The phosphine oxide 5 was obtained in an 18% overall yield by this novel cost-effective method.