Studies on the Synthesis of Vitamin D Analogs with Aromatic D-Ring

ABSTRACT

Herein, we describe our studies on the synthesis of 1α,25-dihydroxyvitamin D₃ analogs possessing a benzene ring replacing the natural 5-membered D-ring by the Wittig-Horner and dienyne approaches. A key feature is the synthesis of a Cr(CO)₃-complexed previtamin D derivative that enables the construction of vitamin D analogs with aromatic D-ring through a thermal [1,7]-H sigmatropic shift. This study establishes the basis for the design of new vitamin D analogs containing aromatic D-ring, complexed or uncomplexed to Cr(CO)₃ type moieties for specific molecular recognition and drug research and development.

Key words:
D-ring modified vitamin D analogs; aryl-Cr(CO)₃ complexes; cancer; sigmatropic rearrangements; synthesis

INTRODUCTION

1α,25-Dihydroxyvitamin D₃ [1, 1,25D, calcitriol, Figure 1], the most potent steroid hormone known and the active metabolite of the seco-steroid vitamin D₃, interacts with the vitamin D nuclear receptor (VDR), a member of the nuclear receptor superfamily, to dimerize with the retinoid X receptor (RXR). The heterodimer binds to the vitamin D response elements (VDREs) in target gene promoters and recruit coactivator proteins to induce a cascade of events including control of mineral homeostasis and various important cellular processes such as differentiation, anti-proliferation, growth, angiogenesis, apoptosis, and immunomodulation (Norman et al. 1979, Evans et al. 1988, Kliewer et al. 1992, Bouillon et al. 1995, Feldman et al. 2011). Recently, it was discovered that 1,25D activates more than 229 genes associated with several diseases, including arthritis, diabetes and cancer, suggesting that this hormone might have an even broader range of biological functions than originally thought (Lincoln et al. 2010) 1,25D itself has clinical applications, but the pharmacological doses required for treatment of cancer induce strong, undesired hypercalcemia (Plum et al. 2010, Feldman et al. 1997). Structure-function studies of numerous 1,25D analogs have shown that the undesired calcemic effects can be reduced during cell-differentiating activities, but the mechanism of this selectivity has not yet been unraveled (Feldman et al. 2011, Glebocka et al. 2012). Increasing synthetic efforts have been directed at the development of non-calcemic analogs of the natural hormone 1,25D for treatment of specific disorders, but only a few have found clinical applications. Among these, calcipotriol (2, Figure 1) and OCT (3), two analogs with structural modifications at the side chain, are being successfully used for treatment of psoriasis (Feldman et al. 1997). A few structural features that reduce the calcemic activity include: a) unsaturations at the side chain or D-ring (Verlinden et al. 2000), b) the lack of the 19-methylene group (Kensler et al. 2000), c) 14-epi-configuration (Verlinden et al. 2000, Ma et al. 2013), d) 3-epi-configuration (Molnar et al. 2011), e) short non-hydroxylated side chains (Plum et al. 2004), f) CD-carboranic mimics (Yamada et al. 2014) and g) o-carboranic side chains (Otero et al. 2016). Nonsecosteroidal vitamin D receptor modulators that reduce calcemic effects have also been reported (Eduardo-Canosa et al. 2010).

Figure 1
Structures of 1,25D (1), calcipotriol (2), OCT (3) and target D-ring-aromatic compound 4.

Our continued interest in the synthesis of 1,25D analogs with selective properties as potential drugs for treatment of cancer and/or psoriasis led us to study the synthesis of aromatic compounds 4a and 4b (Figure 1) as models, which bear a benzene ring replacing the natural 5-membered D-ring. Preliminary studies to build the triene system of 4 utilizing the Wittig-Horner coupling approach (route A ) (Lythgoe 1980) between phosphine-oxide-lithium anion 5 and ketone 6 resulted in the recovery of starting ketone presumably through the corresponding enolate (Figure 2). Attempts to synthesize 4b through the dienyne approach (route B ) (Castedo et al. 1986) were hampered by difficulties to accomplish the sigmatropic [1,7]-H shift on the previtamin 7, presumably due to cross-conjugation of the triene system with the aromatic ring. Here we describe our efforts to synthesize 4a through route A and 4b through route B , from ketone-Cr(CO)₃ complex 9. Complexation with Cr(CO)₆ was envisioned as a way to reduce conjugation of the aromatic ring with either the carbonyl group in tetralone 6 or the triene system in 7.

Figure 2
Retrosynthesis of target 4 by the Wittig-Horner and Dienyne approaches.

MATERIALS AND METHODS

GENERAL MATERIALS AND METHODS

Reagents were purchased from Aldrich Chemical (www.sigma-aldrich.com) or Acros Organics (www.acros.com) and used without further purification. All reactions involving oxygen or moisture sensitive compounds were carried out under dry argon atmosphere using oven-dried or flame-dried glassware and standard syring/septa tecniques. All dry solvents were distilled under argon immediately prior to use: Tetrahydrofuran (THF), Et₂O and n-Bu₂O were distilled from Na/benzophenone. CH₂Cl₂ and Ac₂O were distilled from P₂O₅. Hexanes, n-heptane, diisopropilamine (i-PrNH₂) and triethylamine (Et₃N) were distilled from CaH₂. MeOH was distilled from Mg/I₂. DMSO was distilled from CaH₂ and stored over activated 4 Å molecular sieves. Solutions of n-butyllithium in hexanes were titrated with N-benzylbenzamide before use. Hexanes refer to a distilled mixture of hexane isomers. Reaction temperatures refer to external bath temperatures. Acetone-dry ice baths were used for reactions at low temperature. Alternatively, acetone baths were cooled with a CRYOCOOL immersion cooler, provided with a temperature regulator. Organic extracts were dried over anhydrous Na₂SO₄, filtered and concentrated using rotary evaporator at aspirator pressure (20−30 mmHg). Sat refers to aqueous saturated solution. Reactions were monitored by thin-layer chromatography (TLC) using aluminum-backed Merck 60 silica gel plates (0.2 mm thickness). After visualization under ultraviolet light at 254 nm, the plates were developed by immersion in a solution containing either a mixture of p-anisaldehyde (2.5%), acetic acid (1%), and sulfuric acid (3.4%) in 95% ethanol or a solution of ceric ammonium nitrate (0.5 g) and ammonium molybdate (4.8 g) in H₂O (100 mL) and H₂SO₄ (5.6 mL) followed by heating with a heater gun. Flash column chromatography was performed with Merck silica gel (230−400 mesh). HPLC purifications were performed on a Shimadzu preparative liquid chromatograph, model LC- 8A, equipped with a TSP 1100 UV absorbance detector using a HPLC Phenomenex-Luna silica column (Ø 25 x 250 mm 5Å~ 10 mm). NMR spectra were recorded in CDCl₃ solutions on a Bruker AMX 500 MHz, Varian Inova 400 MHz or Bruker DPX 250 MHz. Chemical shifts are reported on the δ scale (ppm) downfield from tetramethylsilane (δ = 0.0 ppm) using the residual solvent signal at δ = 7.26 ppm (¹H, CDCl₃) or δ = 77.0 ppm (¹³C, t, CDCl₃) as internal standard; coupling constants (J) are reported in Hz. Distortionless enhancement by polarization transfer (DEPT-135) was used to assign carbon types. Low (MS) and high resolution mass spectra (HRMS) were performed in a Micromas Instruments Autospec spectrometer.

SYNTHESIS

1,2,3,4-Tetrahydronaphthalen-1-ol (10). NaBH₄ (0.850 g, 22.45 mmol, 1.16 equiv) was added in one portion to a -78 ºC cooled solution of ketone 6 (2.83 g, 19.35 mmol, 1 equiv) in MeOH (85 mL). The mixture was allowed to reach 23 ºC and then concentrated in vacuo. The residue was dissolved in CH₂Cl₂ (40 mL) and washed with sat NaCl (30 mL). The organic phase was dried, filtered and concentrated in vacuo. The residue was purified by flash chromatography (10% EtOAc/hexanes) to give alcohols 10 (Zhao et al. 2012) [2.810 g, 18.96 mmol, 98%, colorless oil, R_f = 0.56 (40% EtOAc/hexanes)]. ¹H-NMR (250 MHz, CDCl₃): δ 7.46 (1H, m), 7.25 (2H, m), 7.16 (1H, m), 4.78 (1H, m), 2.81 (2H, m), 2.33 (1H, s), 2.11-1.73 (4H, m). ¹³C-NMR (63 MHz, CDCl₃): δ 138.7 (C), 137.0 (C), 128.8 (CH), 128.6 (CH), 127.4 (CH), 126.0 (CH), 67.9 (CH, C-1), 32.1 (CH₂), 29.1 (CH₂), 18.7 (CH₂).

Tricarbonyl [η⁶-(1,2,3,4-tetrahydronaftalen-1-ol)] chromium(0) (11). Alcohol 10 (0.100 g, 0.67 mmol, 1.5 equiv) and Cr(CO)₆ (0.222 g, 1.01 mmol, 1 equiv) were dissolved in a mixture of n-Bu₂O/n-heptane (8 mL, 1:1). The solution was deoxygenated. A stream of argon was passed through the solution. Dry THF (0.4 mL) was added. The reaction mixture was heated at reflux in the dark for 54 h. The mixture was allowed to reach 23 ºC and filtered through a pad of celite. The solids were washed with CH₂Cl₂ and the combined solution was concentrated in vacuo. The residue was purified by flash chromatography (50% CH₂Cl₂/hexanes) to give 11 (Schmalz et al. 1992) [74 mg, 0.26 mmol, 39%, yellow solid, R_f = 0.20 (EtOAc/CH₂Cl₂/hexanes, 1:10:10)] and starting material 10 [40 mg, 0.27 mmol, 40%]. ¹H-NMR (250 MHz, CDCl₃): δ 5.81 (1H, d, J = 6.5 Hz), 5.49 (1H, t, J = 6.2 Hz), 5.09 (2H, m), 4.49 (1H, m), 2.84-2.50 (2H, m), 2.23-1.85 (2H, m), 1.79-1.61 (2H, m). ¹³C-NMR (63 MHz, CDCl₃): δ 233.3 (3xC, Cr(CO)₃), 113.8 (C), 112.7 (C), 95.1 (CH), 93.4 (CH), 90.1 (CH), 88.9 (CH), 66.6 (CH, C-1), 32.2 (CH₂), 27.5 (CH₂), 19.2 (CH₂).

Tricarbonyl-[η⁶-(3,4-dihydronaftalen-1(2H)-one)] chromium(0) (9). A solution of alcohol 11 (0.600 g, 2.11 mmol) in Ac₂O (9 mL) and DMSO (13.5 mL) was stirred at 23 ºC for 3.5 h. The reaction mixture was poured into a 0 ºC cooled solution of NaOH (250 mL, 10%). The resulting mixture was extracted with Et₂O (3x30 mL). The combined organic layers were dried, filtered and concentrated in vacuo. The residue was purified by flash chromatography [EtOAc/CH₂Cl₂/hexanes (1:10:10)] to give ketone 9 [0.500 g, 1.77 mmol, 84%, orange solid, R_f = 0.44 (EtOAc/CH₂Cl₂/hexanes, 1:10:10)]. ¹H-NMR (250 MHz, CDCl₃): δ 6.15 (1H, d, J = 6.6 Hz), 5.62 (1H, t, J = 6.1 Hz), 5.28 (1H, t, J = 6.3 Hz), 5.14 (1H, d, J = 5.4 Hz), 2.95 (1H, m), 2.71 (2H, dt, J ₁= 16.8 Hz, J ₂= 4.1 Hz), 2.43 (1H, m), 2.12 (2H, m). ¹³C-NMR (63 MHz, CDCl₃): δ 230.7 (3xC, Cr(CO)₃), 196.0 (CO, C-1), 115.4 (C), 94.8 (CH), 92.6 (C), 91.2 (CH), 89.8 (CH), 89.2 (CH), 37.7 (CH₂), 28.3 (CH₂), 21.6 (CH₂).

(1E)-Tricarbonyl-{η⁶-1,2,3,4-tetrahydro-1-[(Z)-2-((3S,5S)-3,5-bis-[(triisopropilsilyl)oxy]-2-methylenecyclohexyliden)ethyliden]-naftalen} chromium(0) (12). A solution of n-BuLi in hexanes (0.69 mL, 1.46 mmol, 2.1M, 2.7 equiv) was slowly added to a -78 ºC cooled solution of 5 (1.10 g, 1.65 mmol, 3 equiv) in dry THF (20 mL). After 1 h, a solution of 9 (0.155 g, 0.55 mmol, 1 equiv) in dry THF (8 mL) was added via cannula. The reaction mixture was allowed to reach 15 ºC. H₂O (0.5 mL) were added. The mixture was concentrated in vacuo. The residue was dissolved in Et₂O (30 mL) and successively washed with sat NaHCO₃ (30 mL) and sat NaCl (30 mL). The combined aqueous layers were re-extracted with Et₂O (3x20 mL). The combined organic extracts were dried, filtered and concentrated in vacuo. The residue was purified by flash chromatography (10% EtOAc/hexanes) to give a mixture of compounds 12 and 13 (40 mg) and the starting material 9 (99 mg, 0.35 mmol, 64%). The mixture of 12 and 13 was purified by preparative HPLC (hexanes) to afford four diasteroisomers: 12a (3 mg, 4 μmol, 0.7%), 12b (24 mg, 0.03 mmol, 5.5%), 13a (2 mg, 2.7 μmol, 0.5%) and 13b (7 mg, 0.01 mmol, 1.7%). Compound 12b: ¹H-NMR (500 MHz, CDCl₃): δ 6.96 (1H, d, J = 11.1 Hz), 6.24 (1H, d, J = 11.1 Hz), 5.60 (1H, m), 5.45 (1H, s), 5.30-5.24 (3H, m), 5.00 (1H, s), 4.60 (1H, m), 4.37 (1H, m), 2.78 (1H, dt, J ₁= 9.2 Hz, J ₂= 4.4 Hz), 2.68-2.53 (3H, m), 2.43-2.30 (2H, m), 2.08 (1H, m), 1.93-1.82 (3H, m), 1.15-1.03 (42H, m). ¹³C-NMR (75 MHz, CDCl₃): δ 233.5 (3xC, Cr(CO)₃), 149.2 (C), 142.3 (C), 128.8 (C), 122.5 (CH), 122.4 (CH), 111.8 (CH₂), 109.5 (C), 106.6 (C), 93.8 (CH), 91.5 (CH), 91.2 (CH), 87.1 (CH), 71.1 (CH), 67.8 (CH), 46.1 (CH₂), 45.1 (CH₂), 29.4 (CH₂), 25.7 (CH₂), 22.0 (CH₂), 18.2 (6xCH₃), 18.1 (6xCH₃), 12.4 (3xCH), 12.3 (3xCH). MS ([CI]⁺, m/z, %): 730 ([M]⁺, 10), 687 ([M-ⁱ Pr]⁺, 32), 594 ([M-Cr(CO)₃]⁺, 70), 557 ([M-OTIPS]⁺, 84), 247 [M-H-2xOTIPS-Cr(CO)₃]⁺, 100). HMRS ([CI]⁺): calcd for: [C₄₀H₆₂CrO₅Si₂]⁺ ([M]⁺): 730.3541; found: 730.3550. Compound 13b: ¹H-NMR (500 MHz, CDCl₃): δ 6.45 (1H, d, J = 11.2 Hz), 6.40 (1H, d, J = 11.1 Hz), 5.79 (1H, d, J = 6.3 Hz), 5.40 (1H, s), 5.35 (1H, m), 5.28 (2H, m), 4.99 (1H, s), 4.59 (1H, m), 4.37 (1H, m), 2.69 (2H), 2.58 (1H, d, J = 13.0 Hz), 2.45 (1H, m), 2.39 (2H, m), 2.01 (2H, m), 1.89 (1H, m), 1.81 (1H, m), 1.16-0.99 (42H, m). ¹³C-NMR (75.47 MHz, CDCl₃): δ 233.3 (3xC, Cr(CO)₃), 148.6 (C), 141.9 (C), 130.9 (C), 126.2 (CH), 123.5 (CH), 112.1 (CH₂), 110.3 (C), 107.0 (C), 94.1 (CH), 92.6 (CH), 92.5 (CH), 90.1 (CH), 71.0 (CH), 67.8 (CH), 45.8 (CH₂), 45.0 (CH₂), 34.0 (CH₂), 28.2 (CH₂), 23.2 (CH₂), 18.2 (6xCH₃), 18.1 (6xCH₃), 12.4 (3xCH), 12.3 (3xCH).

Tricarbonyl-{η ⁶ -[3,4-dihydro-1-((trifluoromethanesulfonyl)oxy)naftalen]} chromium(0) (14).

A solution of lithium diisopropylamide was prepared by slowly addition of a solution of n-BuLi in hexanes (0.42 mL, 0.92 mmol, 2.2M, 1.3 equiv) to a-78 ºC cooled neat i-Pr₂NH (0.14 mL, 1.01 mmol, 1.4 equiv). The bath was removed and the white slurry was stirred at 23 ºC for 15 min. The suspension was cooled to -78 ºC and dry THF (2.5 mL) was added. After 15 min, a solution of ketone 9 (0.200 g, 0.71 mmol, 1 equiv) in dry THF (2 mL) was added dropwise. The reaction mixture was stirred at -78 ºC for 45 min and a solution of N,N-(5-chloropyridin-2-yl)-bistriflimide (0.444 g, 1.13 mmol, 1.6 equiv) in dry THF (1.5 mL) was added via cannula. The mixture was allowed to reach 23 ºC. The reaction was quenched by the addition of H₂O (10 mL). The mixture was extracted with EtOAc (3x10 mL). The combined organic layers were dried, filtered and concentrated. The residue was purified by flash chromatography (10% EtOAc/hexanes) to give 14 [0.215 g, 0.52 mmol, 73%, orange oil, R_f = 0.44 (20% EtOAc/hexanes)]. ¹H-NMR (250 MHz, CDCl₃): δ 6.02 (1H, m), 5.55 (1H, m), 5.12-5.38 (3H, m), 2.85 (1H, m), 2.40-2.72 (3H, m). ¹³C-NMR (63 MHz, CDCl₃): δ 232 (3xC, Cr(CO)₃), 124.3 (CH), 120.1 (C), 107.5 (C), 95.7 (C), 92.0 (CH), 91.4 (CH), 89.9 (CH), 86.7 (CH), 25.6 (CH₂), 21.5 (CH₂). MS ([CI]⁺, m/z, %): 415 ([M+H]⁺, 97), 414 ([M]⁺, 98), 279 ([M+H-Cr(CO)₃]⁺, 43), 266 ([M+H-OTf]⁺, 96), 147 (100). HMRS ([CI]⁺): calcd for: [C₁₄H₁₀O₆F₃SCr]⁺ ([M+H]⁺): 414.9555; found: 414.9559.

Tricarbonyl-{η⁶-(1,2-dihydro-4-[(3S,5R)-3,5-bis-(tert-butyldimethylsilyloxy)-2-methyl-cyclohex-1-en-1-yl]ethynyl)-naftalen} chromium(0) (16). Compound 15 (0.120 g, 0.31 mmol, 1.3 equiv), PdCl₂(PPh₃)₂ (8 mg, 0.011 mmol, 5 mol%) and CuI (2 mg, 0.011 mmol, 5 mol%) were successively added to a solution of 14 (0.100 g, 0.24 mmol, 1 equiv) in dry Et₃N (3 mL). The reaction mixture was heated at 60 °C for 2 h. The mixture was allowed to reach 23 ºC and then filtered through a pad of celite. The solids were washed with CH₂Cl₂ (3x5 mL) and the combined solution was washed with a sat NH₄Cl (20 mL). The aqueous layer was extracted with CH₂Cl₂ (3x25 mL). The combined organic extracts were dried, filtered and concentrated in vacuo. The residue was purified by flash chromatography (hexanes) to give 16 [0.121 g, 0.187 mmol, 78%, orange oil, R_f = 0.60 (20% EtOAc/hexanes)]. ¹H-NMR (250 MHz, CDCl₃): δ 6.43 (1H, dd, J ₁ = 6.0 Hz, J ₂ = 3.7 Hz), 5.82 (1H, dd, J ₁ = 6.0 Hz, J ₂ = 2.2 Hz), 5.45-5.20 (3H, m), 4.24 (1H, m), 4.14 (1H, m), 2.86-2.35 (5H, m), 2.14 (1H, m), 1.98 (3H, s), 1.87 (1H, m), 1.72 (1H, m), 0.92 (9H, s), 0.90 (9H, s), 0.12 (6H, br s), 0.08 (6H, s). ¹³C-NMR (63 MHz, CDCl₃): δ 233.2 (3xC, Cr(CO)₃), 142.6 (C), 136.7 (CH), 118.9 (C), 114.7 (C), 107.5 (C), 102.2 (C), 92.5 (CH), 92.1 (C), 91.5 (CH), 91.2 (CH), 90.7 (CH), 87.2 (C), 69.8 (CH), 64.1 (CH), 41.1 (CH₂), 39.4 (CH₂), 26.0 (CH₂), 25.9 (3xCH₃), 25.8 (3xCH₃), 23.4 (CH₂), 19.3 (CH₃), 18.1 (C), 18.0 (C), -4.3 (CH₃), -4.6 (CH₃), -4.7 (CH₃), -4.8 (CH₃). MS ([CI]⁺, m/z, %): 644 ([M]⁺, 13), 560 ([M-3xCO]⁺, 8), 377 ([M-OTBS-Cr(CO)₃]⁺, 83), 245 [M-H-2TBSO-Cr(CO)₃]⁺, 100). HMRS ([CI]⁺): calcd for: [C₃₄H₄₈CrO₅Si₂]⁺ ([M]⁺): 644.2445; found: 644.2448.

Tricarbonyl-{η⁶-[1,2-dihydro-4Z-[(3S,5R)-3,5-bis-(tert-butyldimethylsilyloxy)-2-methyl-ciclohexen-1-yl)vinyl]-naftalen} chromium(0) (17). A suspension of quinoleine in hexanes (50 mL, 2 mmol, 0.04M, 1.25 equiv) and Lindlar catalyst (167 mg, Pd/CaCO₃ poisoned with lead) were successively added to a solution of enyne 16 (0.100 g, 0.16 mmol, 1 equiv) in dry hexanes (27 mL). The system was purged three times with argon and two times with hydrogen. The reaction mixture was stirred under hydrogen at balloon pressure for 1 h and then filtered through a pad of celite. The solids were washed with hexanes and the combined solution was concentrated in vacuo. The residue was purified by flash chromatography (hexanes) to give 16 [0.036 g, 0.06 mmol, 35%, red oil, R_f = 0.70 (20% EtOAc/hexanes)]. ¹H-NMR (250 MHz, CDCl₃): δ 6.24 (1H, d, J = 12.1 Hz), 6.04 (1H, br s), 5.49-5.19 (5H, m), 4.32-3.85 (2H, m), 2.90-1.20 (11H, m), 0.88 (18H, m), 0.05 (12H, m). MS ([CI]⁺, m/z, %): 510 ([M-Cr(CO)₃]⁺, 20), 379 ([M-Cr(CO)₃-OTBS]⁺, 86), 355 (49), 247 ([M-H-2TBSO]⁺, 100). HMRS ([CI]⁺): calcd for: [C₃₄H₅₀CrO₅Si₂]⁺ ([M]⁺): 646.2602; found: 646.2626.

(1R,3S,5Z)-5-[(E)-2-(2,3-Dihydronaphthalen-4(1H)-yliden)ethylidene]-1,3-bis-(tert-butyl dimethylsilyloxi)-4-methylenecyclohexane (4) and (1R,3S)-5-[(Z)-2-(1,2-dihydronaphthalen-4-yl)vinyl]-1,3-bis-(tert-butyldimethylsilyloxi)-4-methyl-4-cyclohexene (7). A solution of 17 (0.030 g, 0.046 mmol) in dry isooctane (5 mL) was heated at reflux for 2 h. The reaction was allowed to reach 23 ºC and then concentrated in vacuo. The residue was re-dissolved in MeOH (3 mL). The resulting solution was cooled at 0 ºC and a solution of CAN [ammonium cerium(IV) nitrate] in MeOH (1.5 mL) was added until green color remained. The mixture was extracted with hexanes (3x5 mL). The combined organic extract was washed with H₂O (10 mL), dried, filtered and concentrated in vacuo. The residue was purified by flash chromatography (5% EtOAc/hexanes) to afford a mixture of compounds 4b and 7 [0.016 g, 0.031 mmol, 68%, ca 1:0.7, colorless oil, R_f = 0.70 (5% EtOAc/hexanes)]. ¹H-NMR (250 MHz, CDCl₃): δ 7.54 (H15-4b, d, J = 7.7 Hz), 7.22-7.04 (Hs-Ar, br m), 6.99 (H7-4b, d, J = 11.3 Hz), 6.34 (H6-4b, d, J = 11.3 Hz), 6.21 (H7 + H6-7, d, J = 12.2 Hz), 5.98 (H9-7, t, J = 4.7 Hz), 5.31 (H19-4b, s), 4.95 (H19-4b, d, J = 2.2 Hz), 4.46 (H1-4b, m), 4.30-3.89 (H3-4b, H1-7, H3-7, m), 2.88-1.50 (Hs-4+7, m), 0.93-0.78 (t-BuSi + Me19-7, m), 0.14-0.03 (MeSi, m).

RESULTS AND DISCUSSION

Complex 9 was prepared according to Schmalz’s procedure (Figure 3) (Schmalz et al. 1992). Reduction of ketone 6 with sodium borohydride in methanol provided alcohol 10 in 98% yield. Heating at reflux a mixture of 10 and chromium hexacarbonyl in n-heptane/n-Bu₂O/THF (10:10:1) gave the diastereomeric alcohols 11 in 39% yield, which upon oxidation with Ac₂O and DMSO produced the desired Cr(CO)₃-complex 9 in 84% yield. Wittig-Horner coupling between ketone 9 and phosphine oxide anion 5 in THF furnished a mixture of four diastereomers (12+13), which could be separated by HPLC [12 (two diastereomers 0.7% + 5.5%); 13 (two distereomers 0.5 + 1.7%)] together with recovered starting ketone 9 (64%). Each pair of diastereomers could not be distinguished by ¹H NMR (500 MHz). The H7 and H15 of 12 appear deshielded with respect to those corresponding to 13 (steroid numbering is used for discussion). The structure 12 was established by ¹H NMR nOe analysis.

Figure 3
Effect of Cr(CO)₃-complexation on the Wittig-Horner coupling.

Though the Cr(CO)₃-complexation reduces conjugation of the aromatic unit with the carbonyl group, the yield on Wittig-Horner reaction products is still too low to considerer this approach of preparative value. This result led us to examine the dienyne approach (route B , Figure 2) as an alternative pathway to the target compound 4b (Figure 4). Treatment of ketone 9 with LDA followed trapping of the resulting enolate with 2-[N,N-bis(trifluoromethylsulfonyl)-amino]-5-chloropyridine afforded vinyl triflate 14 (73%), which upon Sonogashira coupling (Sonogashira et al. 1998) with enyne 15 in the presence catalytic amounts of CuI and PdCl₂(PPh₃)₂ provided the dienyne 16 (68%). Semihydrogenation of the triple bond in the presence of Lindlar catalyst and quinoline in hexanes gave the previtamin D 17 in 35% yield. Finally, thermal sigmatropic [1,7]-H shift on 17 in refluxing isooctane followed by Cr(CO)₃-decomplexation with ammonium cerium(IV) nitrate in methanol gave a 1:0.7 mixture of the target protected vitamin 4b and the corresponding previtamin 7 in 68% yield. The formation of the previtamin 7 can be rationalized by vitamin D-previtamin D equilibration during thermal sigmatropic rearragement. Deprotection of 4b (TBAF, THF) gave the corresponding vitamin D compound (95%), which equilibrates with its previtamin D form on standing in CDCl₃ as determined by ¹H-NMR (vitamin D/previtamin D ratio = 1/2.2, equilibration time = 120 h, room temperature).

Figure 4
Effect of Cr(CO)₃-complexation on the sigmatropic [1,7]-H shift.

CONCLUSIONS

In summary, we have demonstrated that the dienyne route is suitable for the synthesis of vitamin D analogs with aromatic D-ring for biological evaluation. The key feature of the synthesis is the complexation of the aromatic D-ring with Cr(CO)₃ to partially release conjugation with the previtamin triene system, thus allowing the thermal sigmatropic [1,7]-H shift to generate the desired vitamin D form. The formation of Cr(CO)₃-complexed vitamin D derivatives opens the way to a novel class of VDR-ligands for biological testing.

(Spectral data (¹H and ¹³C NMR) of all compounds - Supplementary Material)

ACKNOWLEDGMENTS

We thank Xunta de Galicia (project GPC2014/001) and for financial support. Silvina Eduardo thanks the Spanish MEC for a fellowship. Rita Sigüeiro thanks Xunta de Galicia for a post-doctoral fellowship (Axudas posdoutorais, plan I2C, mod B).

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Publication Dates

• Publication in this collection
  2018

History

• Received
  06 Sept 2017
• Accepted
  10 Oct 2017