Chemoenzymatic resolution of β-azidophenylethanols by candida antarctica and their application for the synthesis of chiral benzotriazoles

ARTICLE

Chemoenzymatic resolution of β-azidophenylethanols by candida antarctica and their application for the synthesis of chiral benzotriazoles

Lenilson C. Rocha^I; Isac G. Rosset^I; Gliseida Z. Melgar^I; Cristiano Raminelli^II; André L. M. Porto^I; Alex H. Jeller^I,III,*

^IInstituto de Química de São Carlos, Universidade de São Paulo, Av. Trabalhador São-carlense, 400, 13560-970 São Carlos-SP, Brazil

^IIDepartamento de Ciências Exatas e da Terra, Universidade Federal de São Paulo, Rua Prof. Artur Riedel, 275, 09972-27 Diadema-SP, Brazil

^IIICoordenação de Química, Universidade Estadual de Mato Grosso do Sul, Rod. Itahum, km 12, 79804-97 Dourados-MS, Brazil

ABSTRACT

The kinetic resolutions of (±)-β-azidophenylethanols were carried out using lipase from Candida antarctica, and enantiomerically enriched (R)-β-azidophenylethanols and their corresponding (S)-β-azidophenylethyl acetates were obtained in good enantiomeric excesses (up to > 99%). The enantiomerically enriched (R)-β-azidophenylethanols were subjected to cyclization reaction with 2-(trimethylsilyl)phenyl triflate and CsF producing chiral 1,2,3-benzotriazole compounds in good yields (75-86%) by a [3 + 2] cycloaddition, which involves the benzyne formation.

Keywords: CALB, lipase, biocatalysis, click chemistry, [3 + 2] cycloaddition

RESUMO

As resoluções cinéticas de (±)-β-azidofeniletanóis foram realizadas usando a lipase de Candida antarctica fornecendo os compostos enantiomericamente enriquecidos, (R)-β-azidofeniletanóis e acetato de (S)-β-azidofeniletila em bons excessos enantioméricos (até > 99%). Os (R)-β-azidofeniletanóis enantiomericamente enriquecidos foram submetidos à reação de ciclização com o triflato de 2-(trimetilsilil)fenila e CsF resultando em 1,2,3-benzotriazóis em bons rendimentos (75-86%) pela reação de cicloadição [3 + 2], a qual envolve a formação in situ de benzino.

Introduction

The chemistry of azides has been reviewed several times, some of these overviews focus only on one subclass of azides.¹ The synthesis of α-azidoketones can be commonly achieved by azide-halogen exchange reactions of α-haloketones.² These compounds are precursors of β-azidoalcohols known as synthons of molecules such as agrochemicals, fragrances, drugs, among others.^3,4

Lipases have been used for the optical resolution of several highly functionalized chiral molecules, such as β-azidoalcohols, amino acids and hydroxy acids.^5-7 The use of these enzymes is advantageous in several respects, particularly to simplify the separation of the product, and avoid undesired product formation. Chiral β-azidoalcohols are also immediate precursors of chiral aziridines and amino alcohols.⁴ There has been growing interest in chiral aziridines due to the increasing importance of functionalized aziridines in organic synthesis and their presence in bioactive molecules, e.g., radiation sensitizers and enzyme inhibitors.⁸ Another example, chiral 1,2-aminoalcohols are very important substances, as illustrated by the biologically active natural product ephedrine and the pharmacologically active bronchodilators, such as salmeterol and albuterol.⁵

Chiral β-azidoalcohols have been reported for the synthesis of different triazoles.⁹ With the advent of click chemistry, specifically the use of Cu(I)-catalyzed or benzyne precursor by a 1,3-dipolar cycloaddition reaction with derivative azides, yielding the 1,2,3-triazoles subunit that has become a significant component of various small molecules and macromolecular systems, ranging from therapeutics, self-assembling systems and polymers to proton exchange membranes.^10,11 In addition, triazole compounds constitute promising therapeutic agents for the treatment of fungal infections, for which there is not effective therapy. It has been reported that triazole derivatives exhibit higher antifungal and antibacterial properties.¹² Triazole compounds containing three nitrogen atoms in the five-membered aromatic azole ring are readily able to bind with a variety of enzymes and receptors in biological system via diverse non-covalent interactions, and thus display versatile biological activities.¹³

The chemoenzymatic kinetic resolution of a set of aromatic and aliphatic alcohols by immobilized Candida antarctica lipase (CALB) in organic media was recently investigated. These resolutions have shown the dependence between the structure of the substrate and the enantioselectivity of enzymatic transesterification according to the empirical Kazlauskas rule.^14-17

In this work, we studied the kinetic resolutions of (±)-β-azidophenylethanols to the production of enantiomerically enriched (R)-β-azidophenylethanols, and their corresponding (S)-β-azidophenylethyl acetates using lipase from CALB in organic medium. In addition, the enantiomerically enriched (R)-β-azidophenylethanol products were subjected to the cyclization reaction with 2-(trimethylsilyl)phenyl triflate and cesium fluoride in order to produce chiral 1,2,3-benzotriazole compounds by a [3 + 2] cycloaddition via benzyne formation and organic azides.

Results and Discussion

α-Haloacetophenones 1-5 were transformed into their α-azidophenylketones 6-10 by treatment with NaN₃ in acetone. In addition, the compounds 6-10 were reduced using sodium borohydride in methanol to yield (±)-β-azidophenylethanols 11-15. All carried out reactions led to the desired products in good yields (Scheme 1). Products 11-15 were characterized by spectroscopic data which are in agreement with those reported in the literature.^6,7,18-20

The (±)-β-azidophenylethanols 11-15 were previously acetylated using acetic anhydride and pyridine, yielding the (±)-β-azidophenyl acetates 16-20, which were used as standards by chromatographic analysis on chiral column.

The enzymatic resolutions of the (±)-β-azidophenylethanols 11-15 were performed using immobilized CALB, vinyl acetate as the acyl donor and hexane as solvent in an orbital shaker. The results are shown in Table 1. The development of the reactions was followed by TLC (thin layer chromatography) analysis and aliquots were collected between 5 and 10 days of incubation to conclusion of the reaction. Next, aliquots were analyzed by HPLC (high performance liquid chromatography) using chiral phase column to determine conversion of β-azidophenyl acetates and enantiomeric excess.

In these conditions, the R-alcohols 11-15 and S-acetates 16-20 were obtained with excellent selectivity (E > 200). The β-azidoalcohols 11, 12 and 14, and their respective acetates 16, 17 and 19, were obtained in excellent yields and high enantiomeric excesses (> 99% ee). While for the β-azidoalcohol 13 remaining in reaction for a period of 10 days, no complete resolution was observed (Table 1). No complete resolution was observed for the nitroazidoalcohol 15, possibly owing to the nitro electron-withdrawing group hindering the interaction of the substrate with the active site of CALB.²¹ The attribution of the absolute configuration of the alcohols 11-14 was assigned as R configuration by comparison with optical rotation value described in the literature.^6,7,19 Consequently, it was possible to suggest that the immobilized CALB has stereochemical preference for S-alcohol esterification. Kazlauskas rule predicts this esterification preference.^22,23

The (R)-β-azidophenylethanols 11-15 were subjected to cyclization reaction with 2-(trimethylsilyl)phenyl triflate and CsF in acetonitrile at room temperature, producing chiral 1,2,3-benzotriazole 21-25 in good yields (75-86%) by a [3 + 2] cycloaddition, which involves the benzyne formation (Scheme 2).^24-27

The [3 + 2] cycloaddition involving chiral azides and arynes, as shown in Scheme 2, may be considered a versatile extension of the click chemistry, providing rapid access to chiral benzotriazoles, which are known to possess important biological properties, such as: antimicrobial activity, anti-inflammatory activity, analgesic activity and anticancer activity.²⁸

Conclusions

In summary, a simple and efficient synthesis of 1,2,3-triazoles 21-25, known as benzotriazoles, was developed. Chiral β-azidoalcohols 11-15 were obtained by immobilized Candida antarctica lipase, producing S-azidoalcohols 11-15 and R-azido acetates 16-20 in high selectivities (E > 200). The enantiomerically enriched S-azidoalcohols 11-15 yielded chiral benzotriazoles by [3 + 2] cycloaddition reaction in good yields (75-86%).

Experimental

General methods

Reagents (vinyl acetate, α-halobenzophenones 1 -5, anhydride acetic, pyridine, cesium fluoride and 2-(trimethylsilyl)phenyl trifluoromethanesulfonate) and solvents (ethyl acetate (EtOAc), hexane, methanol, isopropanol and acetonitrile) were purchased from Sigma-Aldrich (USA) and Synth (Brazil), respectively. Acetonitrile was distilled from calcium hydride under a nitrogen atmosphere prior to use. Purification of the reaction products was carried out by column chromatography (CC) over silica gel (230-400 mesh) eluted with mixtures of n-hexane and EtOAc (9:1, 8:2, 7:3). The reactions were monitored by TLC using aluminum plates precoated with silica gel 60 F254, eluted with hexane and EtOAc, and visualized by spraying with phosphomolybdic acid or p-anisaldehyde staining solutions followed by heating. Novozyme 435^®, immobilized lipase from Candida antarctica, was provided by NovoNordisk (Araucária, Paraná, Brazil). ¹H and ¹³C nuclear magnetic resonance (NMR) spectra were obtained on a Bruker AC-200 III spectrometer (¹H and ¹³C at 200 and 50 MHz, respectively) and on a Bruker/AVANCE spectrometer (¹H and ¹³C at 400 and 100 MHz, respectively). The spectra were taken in deuterated chloroform (CDCl₃) or dimethylsulfoxide (DMSO-d₆) and the chemical shifts were given in ppm using tetramethylsilane (TMS) as internal standard. Infrared (IR) spectra were recorded on a Bomem MB-102 spectrometer. High resolution mass spectrometry (HRMS) analyses were carried out on a Brucker Daltonics MicroTOF focus mass spectrometer equipped with an electrospray ionization (ESI) source. External calibration was achieved with 0.1 mol L^-1 sodium formate solution. The samples were prepared in a 1:1 water/ethanol mixture and measured in the positive mode. Enzymatic kinetic resolutions were carried out using a Tecnal TE-421 orbital shaker. Optical rotation values were measured with a Perkin-Elmer 241 polarimeter. The reported data were determined using the sodium D line (589 nm) and a 1 dm cuvette. The absolute configurations of the compounds were determined comparing their specific optical rotation values with the corresponding values reported in the literature.^6,7,19 Enzymatic reactions were analyzed using Shimadzu LC-10AD or a Shimadzu 20AT equipments with UV detector (190-254 nm) and a Chiralcel^® OD-H chiral column (0.46 cm × 25 cm; 5 µm); hexane/2-propanol (95:5), 0.5 mL min^-1 flow rate. The Shimadzu LC-10AD^® model, equipped with detector photodiode array SPD-M10A, degasser DGU-14A, control center SCL-10A and injector manual Rheodyne^® and Shimadzu LC-20AT^® model, equipped with detector diode array SPD- M20A, degasser DGU-20A5, control center CBM-20A, automatic injector Sil-20A and oven CTO-20A. Acquisition and data analysis were performed using the application LCSolution/CLASS-VP software. The enantiomeric excesses of azidophenylethanols 11-15 and azidophenyl acetates 16-20 were determined by HPLC analysis employing chiral column with hexane/2-propanol mobile phase. Enantioseparations of azidophenylethanols 11-15 and azidophenyl acetates 16-20 obtained by HPLC analysis are as follow: (R-11, 35.0 min; S-11, 33.0 min), (R-12, 27.0 min; S-12, 23.0 min), (R-13, 24.0 min; S-13,

General procedures

Preparation of (±)-β-azidophenylethanols 11-15 and (±)-β-azidophenyl acetates 16-20

The azidoacetophenones 6-10 were obtained from the commercial α-bromoacetophenones 1-5, respectively, using NaN₃ in acetone.²⁹ These compounds were subsequently reduced with NaBH₄ providing the (±)-azidophenylethanols 11-15.³⁰ The products were purified by CC over silica gel eluted with n-hexane/EtOAc (8:2) and obtained in good yields (Scheme 1). The (±)-β-azidophenylethyl acetates 16-20 were obtained by acetylation of their corresponding (±)-β-azidophenylethanols 11-15 using acetic anhydride in pyridine.¹⁵ Products 11-15 were characterized by spectroscopic data which are in agreement with those reported in the literature.^6,7,18-20

Kinetic resolution of (±)-azidophenylethanols 11-15 by immobilized CALB

HPLC grade hexane (10 mL), vinyl acetate (1 mL), immobilized CALB (100 mg, 10,000 propyl laurate units per g) and the appropriate (±)-azidophenylethanols 11-15 (1.20, 1.0, 0.8, 1.0, 0.9 mmol, respectively) were added to 50 mL Erlenmeyer flasks. These flasks were sealed using a rubber stopper, and the reaction mixture was stirred in orbital shaker at 32 ºC and 130 rpm. The reaction progress was monitored by collecting samples (1.0 mL) according to the time indicated in Table 1, which were analyzed by liquid chromatography with chiral stationary phase. After the reaction proceeds to completion, the immobilized lipase was filtered off. The filtrate was evaporated under reduced pressure. The residue was purified by column chromatography on silica gel using hexane/EtOAc (8:2) as eluent yielding the alcohols 11-15 and acetates 16-20. The attributions of the absolute configuration of the alcohols 11-14 and acetates 16-19 were assigned and compared with the literature values (Table 2).^6,7,19

Preparation of chiral triazole compounds 21-25 by click chemistry reaction

The (R)-β-azidophenylethanols 11-15 (1.0 mmol), 2-(trimethylsilyl)phenyl triflate (1.5 mmol, 0.45 g), acetonitrile (5 mL) and CsF (3.0 mmol, 0.46 g) were added to a vial (10 mL). The vial was sealed using a cap, and the reaction mixture was stirred for 18-24 h at room temperature. Afterward, a solution of 10% NaHCO₃ (5 mL) was added to the mixture, which was extracted with EtOAc (3 × 10 mL). The organic phase was dried over MgSO₄. After filtration, the solvent was evaporated under reduced pressure. The residue was purified by CC on silica gel, using a mixture of hexane/EtOAc (7:3) as eluent, yielding the desired products 21-25. The structures of compounds 21-25 were assigned on the basis of a variety of spectroscopic techniques, namely, according to their IR, and ¹H and ¹³C NMR spectra, one-bond (HSQC) and long-range (HMBC) ¹H-¹³C NMR correlation experiments. All compounds 21-25 provided HRMS data that agree with the proposed structures.

(-)-(R)-2-(1H-Benzo[d][1,2,3]triazol-1-yl)-1-phenylethanol (21): [α]_D²⁵ -20.5 (c 1.20, CHCl₃, > 99% ee); mp 130-131 ºC; IR (KBr) ν/cm^-1 3217, 2950, 2902, 2849, 1594, 1492, 1451, 1426, 1275, 1233, 1189, 1158, 1124, 1071, 1029, 883, 749, 746, 699, 586, 525, 484; ¹H NMR (400.1 MHz, CDCl₃) δ 4.73 (dd, J 12.0, 8.0, 1H, CH₂), 4,82 (dd, J 12.0, 4.0, 1H, CH₂), 5.36 (dd, J 8.0, 4.0, 1H, CHOH), 7.28-7.30 (m, 1H, Bt-H*), 7.36-7.39 (m, 2H, Ph-H), 7.41-7.45 (m, 3H, 2Ar-H and 1H, Bt-H), 7.50 (dt, J 8.5, 0.9, 1H, Bt-H), 7.91 (dt, 1H, J 8.5, 0.9, 1H, Bt-H); ¹³C NMR (100.6 MHz, CDCl₃): δ 55.3, 73.1, 109.8, 119.5, 123.8, 125.5, 126.0, 127.3, 128.4, 133.8, 140.5, 145.5; HRMS (FTMS + pESI) m/z, calcd. for C₁₄H₁₄N₃O [M]⁺: 240.1131, found: 240.1134; *Bt-H: benzotriazole hydrogens.

(-)-(R)-2-(1H-Benzo[d][1,2,3]triazol-1-yl)-1-(4-methoxyphenyl)ethanol (22): [α]_D²⁵ 43.3 (c 1.50, CHCl₃; > 99% ee); mp 136-137 ºC; IR (KBr) ν/cm^-1 3281, 2926, 2839, 1610, 1581, 1510, 1446, 1311, 1263, 1190, 1132, 1003. 932, 926, 708; ¹H NMR (400.1 MHz, CDCl₃) δ 3.76 (s, 3H, OCH₃), 4.68 (dd, J 14.3, 8.0, 1H, CH₂), 4.73 (dd, J 14.3, 4.2, 1H, CH₂), 5.27 (dd, J 8.0, 4.2, 1H, CHOH), 6.84 (d, J 8.5, 2H, Ar-H), 7.23 (d, J 8.4, 2H, Bt-H*), 7.30 (d, J 8.5, 2H, Ar-H), >7.38 (d, J 8.4, 1H, Bt-H), 7.49 (d, J 8.4, 1H, Bt-H), 7.76 (d, J 8.4, 1H, Bt-H); ¹³C NMR (100.6 MHz, CDCl₃) δ 55.2, 55.5, 72.7, 110.0, 114.0, 119.2, 123.9, 127.0, 127.2, 132.6, 133.6, 145.1, 159.4; HRMS (FTMS + pESI) m/z, calcd. for C₁₅H₁₆N₃O₂ [M]⁺: 270.1237, found: 270.1243; *Bt-H: benzotriazole hydrogens.

(-)-(R)-2-(1H-benzo[d][1,2,3]triazol-1-yl)-1-(4-bromophenyl)ethanol (23): [α]_D²⁵ -6.8 (c 0.90, CHCl₃, 70% ee); mp 165-166 ºC; IR (KBr) ν/cm^-1 3393, 3059, 2941, 2895, 1494, 1493, 1450, 1407, 1245, 1327, 1231, 1158, 1066, 1007, 875, 858, 820, 756, 735, 538, 510; ¹H NMR (400.1 MHz, CDCl₃δ 4.68 (dd, J 16.0, 8.0, 1H, CH₂), 4.82 (dd, 1H, J 16.0, 4.0, 1H, CH₂), 5.41 (dd, J 8.0, 4.0, 1H, CHOH), 7.24 (d, J 8.0, 2H, Ar-H), 7.28 (d, 1H, J 8.0, 1H, Bt-H); 7,43-7,50 (m, 4H, 2Ar-H and 2Bt-H*), 7.87 (d, J 8.0, 1H, Bt-H); ¹³C NMR (100.6 MHz, CDCl₃) δ 55.4, 72.8, 109.5, 119.5, 124.4, 127.6, 127.7, 131.9, 133.8, 134.5, 139.3, 145.0; HRMS (FTMS + pESI) m/z, calcd. for C₁₄H₁₃BrN₃O [M]⁺: 318.0236, found: 318.0246; *Bt-H: benzotriazole hydrogens.

(-)-(R)-2-(1H-Benzo[d][1,2,3]triazol-1-yl)-1-(4-chlorophenyl)ethanol (24): [α]_D²⁵ -34.8 (c 1.00, CHCl₃, > 99% ee); mp 157-158 ºC; IR (KBr) ν/cm^-1 3394, 3058, 2948, 2923, 1492, 1454, 1421, 1307, 1222, 1143, 1062, 744, 541, 516; ¹H NMR (400.1 MHz, CDCl₃) δ 4.68 (dd, J 16.0, 8.0, 1H, CH₂), 4.82 (dd, J 16.0, 4.0, 1H, CH₂), 5.41 (dd, J 8.0, 4.0, 1H, CHOH), 7.30 (td, J 8.3, 1.0, 1H, Bt-H*), 7.36 (d, J 8.0, 2H, Ar-H), 7.41 (d, J 8.0, 2H, Ar-H), 7.47 (td, J 8.3, 1.0, 1H, Bt-H), 7.53 (dt, J 8.3, 1.0, 1H, Bt-H), 7.88 (dt, J 8.3, 1.0, 1H, Bt-H); ¹³C NMR (100.6 MHz, CDCl₃): δ 55.7, 72.7, 109.5, 119.5, 124.1, 127.2, 127.4, 129.2, 134.5, 138.7, 138.9, 145.5; HRMS (FTMS + pESI) m/z, calcd. for C₁₄H₁₃ClN₃O [M]⁺: 274.0741, found: 274.0749; *Bt-H: benzotriazole hydrogens.

(R)-2-(1H-benzo[d][1,2,3]triazol-1-yl)-1-(4-nitrophenyl) ethanol (25): mp 201-203; IR (KBr) ν/cm^-1 3388, 3107, 3062, 2960, 2921, 2850, 1932, 1801, 1606, 1529, 1344, 1261, 1224, 1159, 1114, 1105, 1068, 869, 802, 773, 740, 694, 538, 509, 439; ¹H NMR (200.1 MHz, DMSO-d₆) δ 4.92-4.98 (m, 2H, CH₂), 5.28-5.34 (m, 1H, CH₂), 7.37 (t, J 8.0, 1H, Bt-H*), 7.51 (t, J 8.0, 1H, Bt-H), 7.67 (d, 2H, J 8.5, Ar-H), 7.86 (d, J 8.2, 1H, Bt-H), 8.00 (d, J 8.2, 1H, Bt-H), δ 8.19 (d, J 8.5, 2H, Ar-H); ¹³C NMR (50.3 MHz, DMSO-d₆) δ 59.8, 70.9, 111.4, 118.9, 123.3, 123.8, 127.0, 127.6, 133.7, 138.5, 146.9, 149.8; HRMS (FTMS + pESI) m/z, calcd. for C₁₄H₁₃ClN₄O₃ [M]⁺: 285.0982, found: *Bt-H: benzotriazole hydrogens.

Supplementary Information

Supplementary data are available free of charge at http://jbcs.sbq.org.br as a PDF file.

Acknowledgements

A. L. M. P. gratefully acknowledges CNPq and FAPESP for financial support. L. C. R. acknowledges CAPES and I. G. R. acknowledges CNPq for the grants. A. H. J. acknowledges UEMS is grateful for the Post-Doctoral stage. The authors also thank Novo Nordisk (Curitiba, Paraná, Brazil) for donating the immobilized CALB enzyme (Novozyme 435^®), Professor Timothy Brocksom (UFSCar) by optical rotations analysis and Professor Antonio Gilberto Ferreira (UFSCar) for the NMR analyses. The English language was reviewed by Timothy Roberts (MSc) a native English speaker.

Submitted: April 3, 2013

Published online: August 9, 2013

FAPESP has sponsored the publication of this article.

Supplementary Information

The supplementary material is available in pdf: [Supplementary material]

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Publication Dates

• Publication in this collection
  24 Sept 2013
• Date of issue
  Sept 2013

History

• Received
  03 Apr 2013
• Accepted
  09 Aug 2013