alexa Synthesis of Four New Brassinosteroids Analogues 11-Oxo-Functionalized on C Ring, with 24-Nor Side Chain and Containing 5β-Cholanic Acid Skeleton | Open Access Journals
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Organic Chemistry: Current Research
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Synthesis of Four New Brassinosteroids Analogues 11-Oxo-Functionalized on C Ring, with 24-Nor Side Chain and Containing 5β-Cholanic Acid Skeleton

Luis Espinoza*
Departamento de Química, Universidad Técnica Federico Santa María, Av. España 1680, Valparaíso, Chile
Corresponding Author : Luis Espinoza
Departamento de Química, Universidad Técnica Federico Santa María
Av. España 1680, Valparaíso, Chile
Tel: +560322654225
Fax: +560322654782
E-mail: luis.espinozac@usm.cl
Received: December 17, 2015; Accepted: December 23, 2015; Published: December 30, 2015
Citation: Espinoza L (2015) Synthesis of Four New Brassinosteroids Analogues 11-Oxo-Functionalized on C Ring, with 24-Nor Side Chain and Containing 5β-Cholanic Acid Skeleton. Organic Chem Curr Res 4:156. doi:10.4172/2161-0401.1000156
Copyright: © 2015 Espinoza L. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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Abstract

In this work, I report the synthesis of four new brassinosteroids analogues with 24-nor side chain and 11-oxo functionalized on C ring, containing 5β-cholanic acid skeleton: 3α, 12β-diacetoxy-22(S), 23-dihydroxy-24-nor-5β-cholan- 11-one (20); 3α, 12β, 22(S), 23-tetrahydroxy-24-nor-5β-cholan-11-one (21); 3α, 12β, 22(S), 23-tetraacetoxy-24-nor-5β- cholan-11-one (22) and 3α, 12β-diacetoxy-[2,2-dimethyl-22(S), 23-dioxolane]-24-nor-5β-cholan-11-one (23) derivatives from commercial deoxycholic acid.

Keywords
Brassinosteroids; C-functionalized; 24-nor side chain
Introduction
Brassinosteroids (Brs) are a naturally occurring steroidal plant hormones group that regulates plant growth and development by producing an array of physiological changes. Brs occur at low concentrations throughout the plant kingdom. They have been detected in all plant organs (pollen, anthers, seeds, leaves, stems, roots, flowers, and grains) and also in the insect and crown galls [1,2]. Further work has demonstrated that Brs not only induce stem elongation but also increase biomass and total crop yield. Moreover, Brs are recognized to have an ameliorative role in plants subjected to various biotic and abiotic stresses, such as high temperature [3], heavy metals excess [4,5], salinity [6], water stress [7,8] and extreme temperatures [9]. Several authors and mainly Hayat et al. have reported structure-activity relationships (SAR) of brassinosteroid [10]. These SAR are based on the functions contained in the A, B ring, A/B ring fusion and in the side chain. However, in recent decades, efforts have been focused on the synthesis of new brassinosteroid analogues, keeping common patterns of organic functions in the A/B rings and cis-trans fusion between these, as in some natural brassinosteroids, but with dramatic structural changes in the side chain (shorter side chains, different oxygenated functions, spirostanic, aromatic and cyclic substituents, esters, carboxylic acids, etc.) and oxygenated functions in C ring. Surprisingly these analogs have presented very important biological activities. On the other hand, the isolation of natural brassinosteroids with oxygenated function in ring C has not been reported. However, the synthesis of this type of analogs is very important for SAR studies of this kind of phytohormone. In this direction, hecogenin (1) is an abundantly available steroidal sapogenin, used as raw material in the production of a large number of Brs spirostanic analogs with oxygenated function in ring C [11-21]. Examples of brassinosteroid analogues oxo-functionalized in C ring are shown in Figure 1. Others active Brs C- oxo and oxa functionalized analogs (6-8) bearing a cholanic acid skeleton were derived from cholic acids [22]. Nevertheless, C-functionalized analogs 9-16 (Figure 2) were obtained from deoxycholic acid [23-25]. The plant growth-promoting activity of compounds 9 and 13 was tested in the hypocotiles elongation and cothyledons expansion of radish bioassay, where the compound 9 showed growth promoting activity at 10-5 mg/mL concentration in both bioassays, whereas compound 13 showed inhibiter effect in the cothyledons expansion test at the same concentration [24]. Compound 14 showed an increase of 38.6% by weight at 10-5 mg/mL concentration in cotyledons expansion of radish bioassay [25]. In this work is reported the synthesis and structural determination of four new Brs analogues, obtained from deoxycholic acid, with a cis-A/B ring junction, 24- nor side chain and 11-oxo-12β-hydroxyl/acetate function on C ring containing with 5β-cholanic acid skeleton.
Materials and Methods
General
All reagents were purchased from commercial suppliers and used without further purification (Merck, Darmstadt, Germany or Aldrich, St. Louis, MO, USA). Melting points were measured on a Stuart-Scientific SMP3 apparatus (Staffordshire, ST15 OSA, UK) and are uncorrected. Optical rotations were obtained for CHCl3 or CH3OH, solutions on a Perkin-Elmer 241 polarimeter (Wellesley, Massachusetts, USA) and their concentrations are expressed in g/100 mL. NMR spectra were recorded on a Bruker AM-200 (Bruker, Rheinstetten, Germany) spectrometer operating at 200.1 MHz for 1H and 50.3 MHz for 13C. Chemical shifts are expressed in ppm downfield relative to TMS (δ scale) in CDC13 solutions and coupling constants (J) are given in hertz. Carbon multiplicity were established by a DEPT pulse sequence. IR spectra were recorder as KBr disks in a Bruker FTIR Vector-22 (Bruker, Germany) and frequencies are in cm-1. Elemental analyses were obtained on a Fisons-Carlo-Erba EA-1108 Automost microanalyzer (Fisons Instruments/Carlo-Erba Instruments, Milano, Italy) For analytical TLC, Merck silica gel 60 in 0.25 mm layer was used and TLC spots were detected by heating after spraying with 25% H2SO4 in H2O. Chromatographic separations were carried out by conventional column on Merck silica gel 60 (230-400 Mesh) using hexane-EtOAc gradients of increasing polarity. All organic extracts were dried over anhydrous magnesium sulfate and evaporated under reduced pressure, below 40°C.
Synthesis
24-oic 3α, 12β-diacetoxy-11-oxo-5β-cholan acid (17)
A solution of 16 (3.2 g, 7.61 mmol) and K2CO3 (0.8 g, 7.55 mmol) in MeOH (150 mL) was stirred at room temperature for 1.5 h. The end of the reaction was verified by TLC. Then the solvent was removed (until a 40 mL approximate volume) and the residue acidified with 2% HCl (15 mL) and extracted with EtOAc (3 × 20 mL). The organic layer was washed with 5% NaHCO3 (30 mL) and water (2 × 15 mL), dried over Na2SO4, and filtered. The solvent was evaporated under reduced pressure. The crude was re-dissolved in CH2Cl2 (5 mL) and chromatographed on silica gel with petroleum ether/EtOAc mixtures of increasing polarity (19.8:0.2→10.2:9.8). Compound 17 (3.17 g, 85% yield) was as a colorless solid: m.p. 85.5-91.7°C (MeOH/Et2O); [α] D 25+46.2° (c=0.405, CHCl3); IR: 3432-2516; 1735; 1243; 1028. 1H NMR: 0.69 (s, 3H, H-18); 0.93 (d, J=6.3 Hz, 3H, H-21); 1.16 (s, 3H, H-19); 2.02 (s, 3H, CH3CO); 2.15 (s, 3H, CH3CO); 2.35 (m, 2H, H-23); 2.49 (d, J=10.6 Hz, 1H, H-9); 4.70 (m, 1H, H-3); 4.90 (s, 1H, H-12). Elemental analysis: found C, 68.22%; H, 8.57%; C28H42O7 requires C, 68.54%; H, 8.63%.
3α, 12β-diacetoxy-24-nor-5β-cholan-22-en-11-one (18)
To a solution of 17 (3.42 g, 7.69 mmol) in anhydrous benzene (300 mL) were added Cu(OAc)2 (0.25 g, 1.38 mmol) and pyridine (1.0 mL). Then refluxed and Pb(OAc)4 (8.34 g, 18.81 mmol) was added in four portions at hourly intervals. After the addition was completed, the reaction was continued for 1 h. The end of reaction was verified by TLC, and then the mixture was filtered and the solvent was evaporated under reduced pressure. The crude was re-dissolved in CH2Cl2 (5 mL) and chromatographed on silica gel with petroleum ether/EtOAc mixtures of increasing polarity (19.8:0.2→16.4:3.6). Compound 18 (2.24 g, 72.3% yield) was obtained as a colorless solid: m.p. 170.9-172.8°C (hexane/ Et2O); [α]D25+64.8° (c=0.466, CHCl3); IR: 3069; 1732; 1723; 1637; 1450; 1364; 1237; 1029; 910. 1H NMR: 0.66 (s, 3H, H-18); 0.99 (d, J=6.9 Hz, 3H, H-21); 1.16 (s, 3H, H-19); 2.02 (s, 3H, CH3CO); 2.18 (s, 3H, CH3CO); 2.46 (m, 2H, H-1 and H-20); 2.48 (d, J=10.2 Hz, 1H, H-9); 4.70 (m, 1H, H-3); 4.90 (dd, J=10.1 and 2.1 Hz, 1H, H-23); 4.92 (s, 1H, H-12); 4.95 (ddd, J=17.1, 2.1 and 0.8 Hz, 1H, H-23); 5.77 (ddd, J=17.1, 10.1 and 8.8 Hz, 1H, H-22). Elemental analysis: found C, 72.87%; H, 9.13%; C27H40O5 requires C, 72.94%; H, 9.07%.
3α, 12β-dihydroxy-24-nor-5β-cholan-22-en-11-one (19)
To a solution of 18 (1.58 g, 3.55 mmol) in MeOH (60 mL) was added K2CO3 (0.79 g, 7.54 mmol), then the suspension was stirred at room temperature for 6 h. The end of the reaction was verified by TLC. Then the solvent was removed (until a 10 mL approximate volume) and the residue acidified with 2% HCl (10 mL) and extracted with EtOAc (2 × 20 mL). The organic layer was washed with 5% NaHCO3 (20 mL) and water (2 × 10 mL), dried over Na2SO4, and filtered. The solvent was evaporated under reduced pressure. The crude was re-dissolved in CH2Cl2 (5 mL) and chromatographed on silica gel with petroleum ether/EtOAc mixtures of increasing polarity (19.8:0.2→13.8:6.2). Compound 19 (1.26 g, 98% yield) was as a colorless solid: m.p. 120.3- 121.2°C (MeOH/Et2O); [α]D 25+61.4° (c=0.44, CHCl3); IR: 3490; 3258; 3066; 1709; 1638; 1453; 1020; 922; 906. 1H NMR: 0.43 (s, 3H, H-18); 1.06 (d, J=6.9 Hz, 3H, H-21); 1.13 (s, 3H, H-19); 2.43 (m, 1H, H-1α); 2.55 (m, 1H, H-20); 3.57 (m, 1H, H-3); 3.84 (s, 1H, H-12); 4.82 (dd, J=10.2 and 2.2 Hz, 1H, H-23); 4.91 (ddd, J=17.1, 2.2 and 0.8 Hz, 1H, H-23); 5.73 (ddd, J=17.1, 10.2 and 8.7 Hz, 1H, H-22). Elemental analysis: found C, 76.49%; H, 10.15%; C23H36O3 requires C, 76.62%; H, 10.06%.
3α, 12β-diacetoxy-22(S), 23-dihydroxy-24-nor-5β-cholan-11-one (20)
To a solution of alkene 18 (2.05 g, 4.28 mmol) in acetone (150 mL) was added NMO (0.45 g, 3.84 mmol). Then the mixture was homogenized by magnetic stirring and 1.5 mL of 4% OsO4 (0.157 mmol) were added dropwise with stirring for 12 h at room temperature. The end of the reaction was verified by TLC. Then the solvent was removed (until a 25 mL approximate volume) and water (25 mL) and Na2S2O3.5H2O (25 mL saturated solution) were added. The organic layer was extracted with EtOAc (2 × 30 mL), washed with water (2 × 50 mL), dried over Na2SO4, and filtered. The solvent was evaporated under reduced pressure. The crude was re-dissolved in CH2Cl2 (5 mL) and chromatographed on silica gel with petroleum ether/EtOAc mixtures of increasing polarity (19.8:0.2→15.6:4.4), by subsequent recrystallization (MeOH/Et2O) compound 20 was obtained as a colorless solid (1.50 g, 68% yield): m.p. 183.7-186.6°C (MeOH/Et2O); [α]D 25+27.6° (c=0.156, CHCl3); IR: 3423; 1747; 1723; 1248; 1026. 1H NMR: 0.67 (s, 3H, H-18); 0.91 (d, J=6.3 Hz, 3H, H-21); 1.15 (s, 3H, H-19); 2.00 (s, 3H, CH3CO); 2.14 (s, 3H, CH3CO); 2.47 (d, J=11.1 Hz, 1H, H-9); 3.47 (m, 1H, H-23); 3.63 (m, 1H, H-23); 3.72 (m, 1H, H-22); 4.68 (m, 1H, H-3); 4.88 (s, 1H, H-12). Elemental analysis: found C, 67.60%; H, 8.87%; C27H42O7 requires C, 67.75%; H, 8.85%.
3α, 12β, 22(S), 23-tetrahydroxy-24-nor-5β-cholan-11-one (21)
To a solution of 20 (0.07 g, 0.146 mmol) in MeOH (50 mL) was added K2CO3 (0.035 g, 0.215 mmol), then the suspension was stirred at room temperature for 6 h. The end of the reaction was verified by TLC. Then the solvent was removed (until a 10 mL approximate volume) and the residue acidified with 2% HCl (2 mL) and extracted with EtOAc (2 × 10 mL). The organic layer was washed with 5% NaHCO3 (10 mL) and water (2 × 10 mL), dried over Na2SO4, and filtered. The solvent was evaporated under reduced pressure. Subsequent recrystallization (MeOH/Et2O) gave compound 21 (0.0485 g, 84.1% yield) which it was identified as a colorless solid: m.p. 152.8-154°C (MeOH/Et2O); [α]D 25+99.5° (c=0.20, MeOH); IR: 3428; 1700; 1066; 1022. Elemental analysis: found C, 69.71%; H, 9.61%; C23H38O5 requires C, 70.02%; H, 9.71%.
3α, 12β, 22(S), 23-tetraacetoxy-24-nor-5β-cholan-11-one (22)
Compound 20 (0.09 g, 0.188 mmol) was dissolved in CH2Cl2 (30 mL) and pyridine (3.0 mL). Later 4-N,N-dimethylaminopyridine (DMAP, 5 mg) and Ac2O (3 mL) were added to the solution. The end of the reaction was verified by TLC (2 h), and then the solvent was reduced to a volume about 5 mL, extracted with EtOAc (2 × 10 mL). The organic layer was washed with 5% KHSO4 (2 × 5 mL) and water (2 × 10 mL), dried over Na2SO4 and filtered. The solvent was evaporated under reduced pressure. The crude was redissolved in CH2Cl2 (3 mL) and chromatographed on silica gel with petroleum ether/EtOAc mixtures of increasing polarity (19.8:0.2 → 14.2:5.8), to yield pure 20 (0.09 g, 85.1% yield) as a colorless solid: m.p. 78.1-79.5°C (hexane/EtOAc); [α] D 25+41.5° (c=0.412, CHCl3); IR: 1740; 1451; 1370; 1243; 1028. 1H NMR: 0.65 (s, 3H, H-18); 0.99 (d, J=6.7 Hz, 3H, H-21); 1.15 (s, 3H, H-19); 2.01 (s, 3H, CH3CO); 2.04 (s, 3H, CH3CO); 2.07 (s, 3H, CH3CO); 2.14 (s, 3H, CH3CO); 2.50 (d, J=11.1 Hz, 1H, H-9); 3.97 (dd, J=12.1 and 9.0 Hz, 1H, H-23); 4.29 (dd, J=12.1 and 2.0 Hz, 1H, H-23); 4.69 (m, 1H, H-3); 4.85 (s, 1H, H-12); 5.14 (m, 1H, H-22). Elemental analysis: found C, 66.10%; H, 8.13%; C31H46O9 requires C, 66.17%; H, 8.24%.
3α, 12β-diacetoxy-[2,2-dimethyl-22(S), 23-dioxolane]-24-nor-5β- cholan-11-one (23)
To a solution of 20 (0.08 g, 0.167 mmol) in dry acetone (50 mL) was added anhydrous CuSO4 (0.250 g, 1.57 mmol), then the suspension was stirred in N2 atmosphere at room temperature for 5 days. The end of the reaction was verified by TLC. Then the mixture was filtered and the solvent was evaporated under reduced pressure. The crude was re-dissolved in CH2Cl2 (3 mL) and chromatographed on silica gel with petroleum ether/EtOAc mixtures of increasing polarity (19.8:0.2→16.8:3.2) to yield pure 23 (0.06 g, 69.2% yield) as a colorless oil; [α]D 25+32.5° (c=0.520, CHCl3); IR: 1734; 1699; 1245; 1066; 1030. 1H NMR: 0.67 (s, 3H, H-18); 0.85 (d, J=6.9 Hz, 3H, H-21); 1.13 (s, 3H, H-19); 1.29 (s, 3H, O2CCH3); 1.36 (s, 3H, O2CCH3); 2.01 (s, 3H, CH3CO); 2.14 (s, 3H, CH3CO); 2.45 (m, 1H, H-9); 3.53 (dd, J=7.8 Hz and 7.8 Hz, 1H, H-23); 3.89 (dd, J=7.8 and 6.3 Hz, 1H, H-23); 4.07 (m, 1 H, H-22); 4.67 (m, 1H, H-3); 4.89 (s, 1H, H-12). Elemental analysis: found C, 69.50%; H, 8.97%; C30H46O7 requires C, 69.47%; H, 8.94%.
Results and Discussion
Previously we have reported the synthesis, and structural determination, of compounds 15 and 16 from deoxycholic acid [25]. Selective saponification of 16 in K2CO3/MeOH at room temperature afforded acid 17 with 85% yield (Scheme 1). The presence of the carboxylic function was confirmed by the signal at δC=179.27 ppm in 13C NMR spectrum (see Table 1). The olefinic intermediate 18 (Scheme 1) was obtained with 72.3% yield by decarboxylation reaction of 17 in Pb(OAc)4/Cu(AcO)2 system, as reported for the preparation of other derivatives [24,26-28]. The presence of terminal exocyclic double bond was confirmed by the signals observed in the 1H NMR spectrum at δH=4.90 ppm (dd, J=10.1 and 2.1 Hz, 1H, H-23); 4.95 ppm (ddd, J=17.1, 2.1 and 0.8 Hz, 1H, H-23) and 5.77 ppm (ddd, J=17.1, 10.1 and 8.8 Hz, 1H, H-22). While two signals at δC=113.36 and 142.79 ppm were observed in the 13C NMR, assigned to at C-22 and C-23 respectively (see Table 1). Saponification of olefin 18 under mild conditions (K2CO3/ MeOH at room temperature for six hours) produces the ketol 19 with 98% yield (Scheme 1) The presence of two signals at δH=3.57 ppm (m, 1H) and δH=3.84 ppm (s, 1H) in the 1H NMR spectrum confirms the removal of acetate groups. These carbinolic hydrogens were assigned as H-C3 and H-C12, respectively, according to their observed hydrogen multiplicities. The next step was the dihydroxylation reaction of olefin 18, however, some authors have reported that the electrophilic reactions at the steroidal C-22 double bond with OsO4, RuCl3/NaIO4, Prevost-Woodward reaction with I2/AgOAc produces predominantly (S) configuration at C-22 [29-31]. Then treatment of the alkene 18, with catalytic OsO4 in NMO and purification of reaction crude by C.C. and subsequent crystallization, afforded compound 20 with 68% yield (Scheme 1). The presence of three carbinolic proton signals at δH=3.47 ppm (m, 1H); 3.63 ppm (m, 1H) and 3.72 ppm (m, 1H) in the 1H NMR spectrum that were assigned to the hydrogens H-23a, H-23b and H-22, respectively by 2D heteronuclear correlations with C-23 and C-22 and 13C DEPT-135 experiment. The configuration at C-22 was established as (S) considering referential aspects for this reaction and comparing NMR spectroscopic data for similar compounds previously reported by us for other derivatives [23,24]. The new derivatives 21, 22 and 23 were prepared from compound 20, according to Scheme 2. Compound 21 was obtained by treatment of 20 under mild saponification conditions (K2CO3/MeOH, r.t.) with 84.1% yield, whereas standard acetylation (Ac2O/DMAP/CH2Cl2) of compound 20 produces the tetra-acetylated derivative 22 with 85.1% yield. 1H NMR spectroscopic evidence indicates the presence of four singlets signals at δH=2.01, 2.04, 2.07 and 2.14 ppm (3H, CH3CO each), while six signals at δC=20.86, 20.90, 21.25, 21.40, 169.94, 2 x 170.46 and 170.94, ppm were observed in 13C NMR (Table 1), so confirming the presence of four acetate groups in the molecule. Finally, by ketalization reaction of 20 with (CH3)2CO/CuSO4 anhydrous system, the ketal derivative 23 was obtained with 69.2% yield, according to the methodology previously described. In the 1H NMR spectrum of compound 23 two singlets signals at δH=1.29 and 1.36 ppm were observed, these were assigned to the methyl of acetonide group [O2C(CH3)2]. While in the 13C NMR spectrum, the signals observed at δC=25.33 (CCH3), 26.39 (CCH3) and 108.30 ppm [O2C(CH3)2], confirming the presence of acetonide group.
Conclusion
We have designed a synthetic sequence, which allows the obtainment of three new synthetic derivatives (compounds 17-19) from commercial deoxycholic acid and four new brassinosteroids analogues (compounds 20-23) with 24-nor side chain, oxygenated function at C-22 (S) and C-23 and 11-oxo functionalized on C ring, containing 5β-cholanic acid skeleton. Bioassays in Rice Lamina Inclination Test (RLIT) and growth in Arabidopsis thaliana, to detect possible biological activity of compounds 20-23, are under being developed and these results will be reported later.
Acknowledgements
The author thank to the Dirección General de Investigación y Postgrado (DGIP-USM Grant No. 27.15.83.) of Universidad Técnica Federico Santa María for financial support of this work.
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