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Asymmetric Epoxidation of Chalcones Promoted by Chiral Schiff Bases and Amino Alcohols.

Byline: Tianhua Shen, Junqi Li, Funa Cui, Xiaocong Zhou, Xiaoli Chen and Qingbao Song

Summary: A series of chiral ligands were conveniently prepared from L-phenylalanine and characterized. Their application to the asymmetric epoxidation of chalcone was studied. The results demonstrated that 3a were efficient catalysts for enantioselective epoxidation of chalcone in moderate yield (up to 58.6%) and high enantioselectivities (up to 90% e.e.).

Keywords: Chiral ligands, L-phenylalanine, asymmetric epoxidation, enantioselective.

Introduction

Chiral epoxides are very important building blocks for the synthesis of enantiomerically pure molecules of biologically active compounds and pharmaceuticals [1-3]. Catalytic asymmetric epoxidation of (Alpha) , (Beta)-unsaturated ketone (chalcone) was a useful technique for these purposes and numerous studies were carried out in getting appropriate catalytic systems [4-8]. Among them, more and more efforts have been devoted to the development of metal-free procedures asymmetric epoxidation recently. Since the pioneering work of Julia and Colonna [9-11], a variety of efficient organocatalysts have been explored for this reaction, for instance, polyamino acids [12], amino alcohols [13-19], phase-transfer catalysts (PTCs) [20-22], amines [23], chiral ketones [24] and chiral peroxides [25, 26]. Particularly, chiral amino alcohols were recognized to be superior chiral catalysts due to their high activity, stability, and easy preparation.

Inspired by the successes of Lattanzi and others [13-19], we report herein the preparation of these easy accessible Schiff bases and (Beta)-amino alcohol ligands from natural amino acids and their applications in the reaction of asymmetric epoxidation of chalcones.

Results and Discussion

The synthesis of ligands 3-4 are shown in Scheme 1. We obtained these ligands via short steps. The chiral Schiff-base ligands 3a-c was accomplished via the key intermediate 2a-c in quantitative yield starting from L-phenylalanine. The reduction of the Schiff base was carried out in dry THF with three equivalents of NaBH4 at room temperature for 3-4 h. After work-up, 4a-c were obtained in satisfactory yield (56-73%). Ligands 3-4 as catalysts were initially tested in the enantioselective epoxidation of chalcone with H2O2 as the oxidant at room temperature.

The results were listed in Table-1, which demonstrated that the chiral Schiff-base ligands 3a-c afforded higher e.e. values than the salicylaldehyde-based chiral (Beta)-amino alcohols 4a-c in the presence of 10 mol % of the chiral ligand. It also could be seen that the activities and enantioselectivities of the ligands 3a and 3b were more efficient than other ligands (entries 2-3), whereas the ligands 4a-c have low enantioselectivities and catalytic activities (entries 5- 7).

Table-1: Asymmetric epoxidation of 5a promoted by ligands 3-4a.

Entry###Ligand none###Time (h)###Yieldb (%)###eec (%) (Config.)

1###None###120###20.3###0

2###3a###100###54.4###87.6 (2R, 3S)

3###3b###100###56.8###86.7 (2R, 3S)

4###3c###100###45.6###18.9 (2R, 3S)

5###4a###100###20.7###6.5 (2R, 3S)

6###4b###100###35.6###8.9 (2R, 3S)

7###4c###100###24.8###6.0 (2R, 3S)

a The reaction was carried out at room temperature with 30 equiv of H2O2 (30%, aq) in the presence of 10 mol % of ligands, C = 0.24 M of 5a, 6 equiv of NaOH (1 M). b,c Determination by chiral HPLC. d Absolute configuration was determined by compared the HPLC retention times with those reported in the literature [28-30].

The reaction was performed in 10 mol% of the ligand 3b in different solvents (Table-2, entries 1- 6). This reaction was very slow in toluene or MTBE (entries 1-2), but satisfactory levels of enantioselectivities were observed. The polar and protic solvents such as EtOH and CH3CN were afforded comparable results in terms of conversion, but the epoxide product showed poor e.e. values (entries 3-5).

The EtOH is a suitable solvent for this reaction to give high conversion, but unfortunately, no enantioselectivities was obtained (entry 4), and the temperature had little effect on e.e. values (entries 15- 16). Since non-proton polarity solvents furnished better results in enantioselectivity, the reaction was performed in toluene. We were pleased to purify 6a in 57.6% yield and 87.2% e.e. (entry 1). Increasing the amount of the catalyst could not lead to higher asymmetric induction (entries 12-14). Urea hydrogen peroxide (UHP) as the oxidant gave lower yield (31.2%) and lower e.e. (46.9%, entry 7). The epoxidation with TBHP and NaClO, H2O2/ Na2WO4aC/2H2O gave the enantioselectivity not more than 15% e.e. (entries 8-10).

###Cat.###T Time Yieldb e.e.c (%)

Entry###Solvent###Oxidant

###(mol%)###(degC) (h) (%)###(Config.d)

1###10###toluene###H2O2###r. t. 124 57.6###87.2 (2R, 3S)

2###10###MTBE###H2O2###r.t.###124 54.2###80.7 (2R, 3S)

###MTBE

3e###10###H2O2###r. t.###24 91.0###9.3 (2R, 3S)

###/EtOH

4e###10###EtOH###H2O2###r. t.###24 93.2###0

5e###10###CH3CN###H2O2###r. t.###72 80.1###4.4 (2R, 3S)

6###10###Hexane###H2O2###r. t. 124 36.4###19.5 (2R, 3S)

7f###10###toluene###UHP###r. t###120 31.2###46.9 (2R, 3S)

8g

###10###toluene###H2O2###r. t###120 20.1###13.1 (2R, 3S)

9h###10###toluene###NaClO###r. t###120###5.6###9.0 (2R, 3S)

10###10###toluene###TBHP###r. t###120 20.3###5.6 (2R, 3S)

11###3###toluene###H2O2###r. t###72 23.7###32.1 (2R, 3S)

12###5###toluene###H2O2###r. t###72 45.4###67.4 (2R, 3S)

13###10###toluene###H2O2###r. t###72 52.1###83.5 (2R, 3S)

14###20###toluene###H2O2###r. t###72 55.6###85.2 (2R, 3S)

15###10###EtOH###H2O2###0###10 92.0###Trace

16###10###EtOH###H2O2###-20###10 92.2###Tracee

oxidation, C = 0.24 M of 5a, 6 equiv of NaOH (1M), 30 mol % of tetrabutylammonium bromide. b,c Determination by chiral HPLC. d Absolute configuration of outcome was determined to be (2R, 3S) by comparing the HPLC retention times with those reported in the literature [28-30].

e V(MTBE):V(EtOH) = 1:1; 6 equiv oxidation was used without tetrabutyl- ammonium bromide. f 6 equiv oxidation was used without NaOH and tetrabutylammonium bromide. g The reaction was done using H2O2 and Na2WO4aC/2H2O in a 120:1 molar ratio. h without NaOH Having improved the reaction conditions to practically useful levels of stereocontrol, the optimized protocol, employing catalyst 3a, was applied to a variety of chalcones to study the general scope and limitations of the epoxidation method. The results for the organocatalytic asymmetric epoxidation of the different chalcones are presented in Table-3.

In most of the examples (entries 1-3), diastereoisomerically pure trans-(2R,3S)-epoxides were obtained starting from trans-chalcones. Different types of electronic substitution (electron- donating or electron-withdrawing) on the phenyl ring (entries 2-4) of chalcone led to slow conversion to the epoxide (entries 2 and 4), the p-NO2-substituted chalcone, which failed to afford the corresponding product (entry 3).

Table-3: Asymmetric epoxidation of chalcones 5 promoted by ligand 3aa.

Entry###R1###Yieldb (%)###eec (%) (Config.d)

1###Ph###58.6###89.5 (2R, 3S)

2###p-MeO-C6H5###46.5###75.6 (2R, 3S)

3###o-Cl-C6H5###54.2###87.3 (2R, 3S)

4###p-NO2-C6H5###trace###nd

a The reaction was carried out at room temperature with 30 equiv of H2O2 (30%,aq) in the presence of 10 mol % of ligands, C = 0.24 M of 5a, 6 equiv was determined by compared the HPLC retention times with those reported in the literature [28-30].

Experimental

Chemicals were purified when required according to standard procedures. Reactions were monitored by thin layer chromatography (TLC). Column chromatography purifications were carried out using silica gel (100a"200 mesh). Melting points were recorded on X-4 melting point apparatus and were uncorrected. 1H NMR and 13C NMR spectra were taken in CDCl3 on Bruker AVANCE-500 MHz spectrometers respectively, using TMS as the internal carried out on a HP 5973 spectrometer. IR spectra were recorded on a Perkin Elmer 298 instrument. Optical rotations were recorded on a Perkina"Elmer 341 polarimeter. The e.e. value determination was carried out using chiral HPLC with a Daicel ChiracelA(r)

ADa"H column on ShimadzuA(r)

LC-20 AT

with SPD-20 A detector. All commercially available reagents were purchased from Aldrich. The starting materials 5a, 5b, 5c and 5d were prepared according to literature procedures [27].

Preparation of the hydrogen chloride salt of (S)- phenylalanine methyl ester (1)

A suspension of L-phenylalanine (11.55 g, 0.07 mol) in methanol (150 ml) was cooled to 0AdegC. While vigorously stirring, thionyl chloride (39.4 g, 0.33 mol) was added dropwise for over 30 minutes. The temperature was allowed to rise to room temperature. After stirring overnight, the excess of thionyl chloride and methanol was evaporated. The resultant white solid was washed three times with Et2O and dried under reduced pressure to get a white solid (1), Yield: 14.33 g (95%), mp 157 AdegC. General procedure for the preparation of amino alcohols (2)

The hydrogen chloride salt of L- phenylalanine methyl ester 1 (1.1 g, 5 mmol) was introduced with freshly prepared Grignard reagent RMgBr (60 mmol) in the usual way under 0AdegC and argon atmosphere in diethyl ether. Then the mixture was stirred at ambient temperature overnight, and cold saturated NH4Cl was dropped into it under vigorous stirring. The mixture was extracted with ethyl acetate three times. The combined organic layer was washed with brine and dried with anhydrous Na2SO4, concentrated in vacuum. The pure product was obtained by recrystallization from ethyl acetate and petroleum ether or by column chromatography.

(S)-2-Amino-3-ethyl-3-phenylpentanol (2a)

Yield: 43%; yellow oil. IR (KBr): 3391 (br.), 3059, 3026, 2964, 2926, 2851, 1595, 1494, 1457, 1397, 1376, 1316, 1260, 1153, 1076, 1028, 952, 750, 697 cm-1. (S)-2-Amino-1,1,3-triphenylpropan-1-ol (2b) Yield: 65.3%; a colorless crystal, mp 136 AdegC. IR (KBr): 3366 (br.), 3022, 2863, 1739, 1599, 1575, 1492, 1451, 1243, 1189, 1085, 893, 836, 784, 699 cm-1.

(S)-3-Amino-2-benzyl-1,4-diphenylbutan-2-ol (2c).

Yield: 54%; a white solid, mp 137-138 AdegC. IR (KBr): 3377, 3316, 3026, 2853, 1739, 1600, 1582, 1491, 1452, 1238, 1181, 1077, 1031, 878, 839, 781, 699 cm-1.

General procedure for the preparation of Chiral Schiff base ligands (3)

To a solution of salicylaldehyde (0.61 g, 5 mmol) in anhydrous EtOH (100 mL) was added 2 (5 mmol). The reaction was refluxed for 3 h. The EtOH was removed and the residue was purified by column chromatography.

(S)-2-[(3-Ethyl-3-hydroxy-1-phenylpentan-2- ylimino)methyl]phenol (3a).

Yield: 93%; a yellow oil; [(Alpha) ]D 23 -250.3 (c 0.004, CH2Cl2); Rf = 0.45 (petroleum ether-ethyl acetate, 5:1). IR (KBr): 3455, 2967, 1627, 1495, 1455, 1277, 1207, 1151, 1082, 1029, 956, 843, 754, 699, 647 cm-1. 1H NMR (500MHz, CDCl3): (delta) = 13.42 (s, br., 1H, PhOH), 7.59 (s, 1H, CH=N), 7.29-6.76 (m, 9H, Ph-H), 3.31-3.28 (m, 1H, CH), 3.17-3.14 (d, 1H, PhCH2, J = 13 Hz), 2.83-2.77 (d, 1H, PhCH2, J = 13 Hz), 1.75-1.71 (m, 4H, CH2), 1.25 (s, 1H, OH), 0.99-0.94 (m, 6H, CH3). 13C NMR (125MHz, CDCl3): (delta) = 165.7, 161.1, 139.1, 132.2, 131.3, 129.6, 128.2, 126.1, 118.5, 118.3, 116.8, 78.0, 77.2, 77.1, 76.7, 75.4, 36.7, 28.4, 27.8, 7.5, 7.3. MS (EI, 70 eV): m/z = 311 [M+], 225, 183, 134,121, 104, 91, 77.

(S)-2-[(1-Hydroxy-1,1,3-triphenylpropan-2- ylimino)methyl]phenol (3b).

Yield: 95%; a yellow crystal, mp 160AdegC; [(Alpha) ]D23 -59.6 (c 0.01, CH2Cl2); Rf = 0.65 (petroleum ether-ethyl acetate, 5:1). IR (KBr): 3566, 3058, 3026, 2887, 1627, 1580, 1494, 1447, 1415, 1117, 1082, 956, 786, 631, 586 cm-1. 1H NMR (500MHz, CDCl3): (delta) = 12.69 (br., 1H, PhOH), 7.63 (s, 1H, CH=N), 7.65- 6.71 (m, 19H, Ph-H), 4.36-4.33 (m, 1H, CH), 3.05- 3.05 (d, 1H, CH2Ph, J = 14 Hz), 2.98 (s, 1H, OH), 2.86-2.82 (d, 1H, CH2Ph, J = 14 Hz). 13C NMR (125MHz, CDCl3): (delta) = 166.6, 160.6, 145.2, 143.9, 138.8, 132.7, 131.6, 129.7, 128.5, 128.4, 128.3, 127.1, 127.0, 126.4, 126.1, 126.1, 118.7, 118.1, 116.8, 79.7, 78.5, 37.4. MS (EI, 70 eV): m/z = 408 [M+H]+, 225, 207, 183, 165, 134, 122, 106, 77.

(S)-2-[(3-Benzyl-3-hydroxy-1,4-diphenylbutan-2- ylimino)methyl]phenol (3c).

Yield: 90%; a yellow solid, mp 86AdegC; [(Alpha) ] D23 -182.2 (c 0.01, CH2Cl2); Rf = 0.55 (petroleum ether- ethyl acetate, 5:1). IR (KBr): 3564, 3050, 3026, 2929, 1627, 1580, 1494, 1454, 1277, 1207, 1151, 1116, 1079, 1030, 852, 752, 726, 700, 653 cm-1; 1H NMR (500MHz, CDCl3): (delta) = 13.54 (s, 1H, PhOH), 7.39- 6.77 (m, 19H, Ph-H), 3.40-3.37 (m, 1H, CH), 3.22- 3.17 (m, 2H, PhCH2), 3.11-3.08 (d, 1H, PhCH2C, J = 14 Hz), 3.01-2.99 (d, 1H, PhCH2C, J = 14 Hz), 2.86-2.75 (m, 2H, PhCH2), 1.77 (s, 1H, OH); 13C NMR (125MHz, CDCl3): (delta) = 166.9, 161.1, 138.7, 136.8, 136.3, 132.4, 131.5, 130.9, 130.7, 129.7, 128.3, 128.3, 126.7, 126.6, 118.6, 118.4, 116.9, 43.2, 42.3, 36.9; MS (EI, 70 eV): m/z = 435 [M+], 304, 226, 225, 183, 121, 104, 91.

Salicylaldehyde-based chiral (Beta)-amino alcohols 4; General procedure

The Schiff base 3 (5mmol) was dissolved in anhydrous EtOH (25mL) and then treated with NaBH4 (3 equiv). The mixture was stirred under an argon atmosphere at room temperature. EtOH was removed. Saturated NH4Cl aqueous solution was added to the residue. Then the mixture was extracted with dichloromethane, dried with anhydrous Na2SO4. The solvent was evaporated and the residue was purified by column chromatography.

(S)-2-[(3-Ethyl-3-hydroxy-1-phenylpentan-2- ylamino)methyl]phenol (4a).

Yield: 56%; a pale yellow oil; [(Alpha) ] D23 -51.3 (c 0.004, CH2Cl2); Rf = 0.25 (petroleum ether-ethyl acetate, 3:1). IR (KBr): 3423, 3026, 2967, 2937, 2881, 1590, 1491, 1456, 1255, 1099, 1035, 951, 753, 699 cm-1. 1H NMR (500MHz, CDCl3): (delta) = 7.31-6.63 (m, 9H, Ph-H), 4.75 (s, 1H, NH), 3.62-3.60(d, 1H, CH2N, J = 13 Hz), 3.31-3.28 (d, 1H, CH2N, J=13 Hz), 3.06-3.02 (d, 1H, PhCH2, J = 14.5 Hz), 2.91- 2.87 (m, 1H, CH), 2.52-2.47 (d, 1H, PhCH2, J = 14.5 Hz), 1.71-1.47 (m, 4H, CH2), 1.25 (s, 1H, OH), 0.96-0.94 (m, 6H, CH3). 13C NMR (125MHz, CDCl3): (delta) = 157.6, 139.7, 129.1, 128.7, 128.7, 128.2, 126.5, 123.6, 119.0, 116.1, 64.8, 60.4, 53.4, 37.2, 28.6, 27.6, 21.0, 14.1, 7.5, 7.5. MS (EI, 70 eV): m/z = 226, 178, 120, 103, 91, 77.

(S)-2-[(1-Hydroxy-1,1,3-triphenylpropan-2- ylamino)methyl]phenol (4b).

Yield: 70%; a white solid, mp 116 AdegC; [(Alpha) ] 23 -49.8 (c 0.004, CH2Cl2); Rf = 0.35 (petroleum ether- ethyl acetate, 3:1). IR (KBr): 3463, 3329, 3058, 3024, 2924, 2858, 1588, 1492, 1448, 1249, 1053, 1029, 752, 741, 702 cm-1; 1H NMR (500MHz, CDCl3): (delta) = 7.65-6.48 (m, 19H, Ph-H), 3.96-3.94 (m, 1H, CH), 3.39-3.36 (d, 1H, CH2N, J = 12.5 Hz), 3.15-3.12 (d, 1H, CH2N, J = 12.5 Hz), 3.06-3.03 (d, 1H, PhCH2, J = 14 Hz), 2.61-2.56 (d, 1H, PhCH2, J = 14 Hz), 1.28 (s, 1H, OH). 13C NMR (125MHz, CDCl3): (delta) = 157.1, 145.6, 144.6, 139.4, 129.1, 128.8, 128.8, 128.7, 128.5, 127.3, 127.1, 126.6, 125.6, 125.6, 123.4, 119.1, 116.2, 81.0, 66.2, 52.6, 37.7. MS (EI, 70 eV): m/z = 410 [M+H]+, 304, 226, 227, 212, 167, 134, 121, 107, 93, 77.

(S)-2-[(3-Benzyl-3-hydroxy-1,4-diphenylbutan-2- ylamino)methyl]phenol (4c).

Yield: 60%; a white solid, mp 98 AdegC; [(Alpha) ] D23 -68.4 (c 0.004, CH2Cl2); Rf = 0.55 (petroleum ether- ethyl acetate, 5:1). IR (KBr): 3424, 3025, 2922, 1603, 1492, 1453, 1711, 1074, 1033, 751, 729, 699 cm-1. 1H NMR (500MHz, CDCl3): (delta) = 7.36-6.47 (m, 19H, Ph- H), 3.41-3.37 (d, 1H, PhCH2C, J = 12 Hz), 3.22- 3.19 (d, 1H, PhCH2, J = 14 Hz), 3.11-3.08 (d, 1H, PhCH2, J = 14 Hz), 3.28-3.26 (d, 1H, CH2N, J = 13 Hz), 3.06-3.02 (d, 1H, CH2N, J = 13Hz), 3.05-3.03 (d, 1H, PhCH2, J = 14 Hz), 2.96-2.93(d, 1H, PhCH2, J = 14 Hz), 2.85-2.83 (d, 1H, PhCH2C, J = 12 Hz), 1.26 (s, 1H, OH). 13C NMR (125MHz, CDCl3): (delta) = 157.6, 138.9, 136.5, 136.1, 131.5, 130.5, 129.7, 129.1, 128.8, 128.7, 128.6, 128.5, 128.5, 128.3, 128.1, 127.6, 126.9, 126.7, 126.6, 124.1, 118.8, 116.1, 64.2, 53.8, 43.2, 41.6, 37.6. MS (EI, 70 eV): m/z = 437 [M+], 304, 226, 225, 183, 121, 104, 91.

Typical procedure of asymmetric epoxidation of chalcone

To a stirred solution of 5a (1.0 g, 4.8 mmol) in toluene (20 mL), 3c (0.195 g, 10 mol%) was added. Then 30 equivalent of H2O2 (30%, aq) and tetrabutylammonium bromide (0.46 g, 30 mol%) were added. After stirring for 20 minutes at room temperature, 6 equive NaOH (1 M) was dropped slowly. The reaction was monitored by TLC. For the work-up the mixture was poured slowly into a cold NaHSO3 solution (saturated). The organic layer was separated and the solvent was removed under reduced pressure. The residue was analyzed by HPLC to give the yield and e.e. value.

Conclusion

We have conveniently synthesized chiral Schiff bases and salicylaldehyde-based chiral (Beta)- amino alcohols from L-phenylalanine. Ligands 3a and 3b were found to be highly efficient catalysts for asymmetric epoxidation of chalcone 5a. Further experiments indicated that compound 3a was an efficient catalyst for the asymmetric epoxidation of different chalcones. Further work is undergoing to extend the variety of this type of chiral ligands and the applications of their metallic complexes to other types of asymmetric reactions.

Acknowledgment

This work was financially supported by the National Science Foundation of Zhejiang Province (LY12B02016) and the State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, Zhejiang University of Technology (People's Republic of China).

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State Key Laboratory Breeding Base of Green Chemistry - Synthesis Technology, Zhejiang, University of Technology, Hangzhou 310014, P. R. China. qbsong@zjut.edu.cn
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Author:Shen, Tianhua; Li, Junqi; Cui, Funa; Zhou, Xiaocong; Chen, Xiaoli; Song, Qingbao
Publication:Journal of the Chemical Society of Pakistan
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Date:Oct 31, 2013
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