Bifunctional Thiourea–Ammonium Salt Catalysts Derived from

Dec 22, 2017 - An efficient enantioselective aza-Henry reaction of aryl α-ketoester-derived ketimines has been realized by using bifunctional thioure...
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Article Cite This: J. Org. Chem. 2018, 83, 1486−1492

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Bifunctional Thiourea−Ammonium Salt Catalysts Derived from Cinchona Alkaloids: Cooperative Phase-Transfer Catalysts in the Enantioselective Aza-Henry Reaction of Ketimines Ning Lu, Yanhong Fang, Yuan Gao, Zhonglin Wei, Jungang Cao, Dapeng Liang, Yingjie Lin,* and Haifeng Duan* Department of Organic Chemistry, College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, China S Supporting Information *

ABSTRACT: An efficient enantioselective aza-Henry reaction of aryl α-ketoester-derived ketimines has been realized by using bifunctional thiourea−ammonium salt phase-transfer catalysts, which were derived from quinine. A variety of aryl αketoester-derived N-Ts ketimines were investigated, and the corresponding products were obtained in high to excellent yields (up to 99%) with good to high enantioselectivities (up to >99% ee).

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INTRODUCTION

of these molecules contain chiral centers. Although a variety of synthetic methods for α,β-diamino acids have been reported,2b,9 there are still limitations for asymmetric synthetic protocols.10 Thus, the development of new asymmetric synthetic methods for synthesizing chiral α,β-diamino acids is highly desirable. It is well-known that the catalytic asymmetric aza-Henry reaction is one of the strongest and most effective methods for constructing carbon−carbon bonds.11 By the asymmetric azaHenry reaction of ketimines,12 optically active compounds containing a quaternary chiral center can be obtained. The reaction of aryl α-ketoester-derived ketimines with nitroalkanes,12d,13 followed by a simple reduction of the nitro groups of the corresponding products, can give optically active Cα-tetrasubstituted α-amino-β2,2-amino-acid precursors. Recently, the aza-Henry reaction of aldimines was extensively studied;14 however, ketimines as substrates for the aza-Henry reaction were reported relatively less frequently due to their poor reactivity.15 On the basis of our previous work, we have successfully realized the asymmetric aza-Henry reaction of aldimines and ketimines using bifunctional phase-transfer catalysts.16 So far, successful asymmetric aza-Henry reactions of aryl α-ketoester-derived ketimines with nitromethane have not been reported. Herein, we would like to report the bifunctional phase-transfer-catalyzed enantioselective azaHenry reaction of N-p-tolylsulfonyl ketimines 2 with nitromethane.

α,β-Diamino acids and α,β-diamino esters are important structural motifs present in many biologically active compounds and natural products.1 In particular, the introduction of Cαtetrasubstituted α,β-diamino acids in molecules has important and special biological significance.2 For example, (R,R)dysibetaine and (S,R)-epidysibetaine (I) have been isolated from the aqueous extract of the marine sponge Dysidea herbacea collected from Yap, Micronesia;3 the immunomodulator peptide FR 900490 (II) is a bioactive drug molecule;4 the cell adhesion molecule VLA-4 (III) is an antagonist, mediates cell trafficking, and also regulates the activation and survival of a number of cell types;5 and roxifiban (IV), also referred to as DMP754 (the ester prodrug of XV459), is a potent and selective antagonist of the platelet glycoprotein IIb/IIIa receptor6 (Figure 1). Moreover, Cα-tetrasubstituted α,βdiamino acids are widely used in natural products such as (−)-cucurbitine7 and related derivatives, such as imidazolines8 and biologically active β-lactam antibiotics.2a In addition, most

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RESULTS AND DISCUSSION Initially, the reaction between N-p-tolylsulfonyl ketimine 2a and nitromethane in the presence of 5 equiv of LiOH·H2O at −20 Figure 1. Bioactive compounds and natural products bearing an α,βdiamino acid scaffold. © 2017 American Chemical Society

Received: December 6, 2017 Published: December 22, 2017 1486

DOI: 10.1021/acs.joc.7b03078 J. Org. Chem. 2018, 83, 1486−1492

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The Journal of Organic Chemistry °C in toluene (Table 1) was selected as the model reaction. Some well-behaved bifunctional phase-transfer catalysts,

catalysts (1k−1n).18 It is well-known that these kinds of quinine-derived thiourea−quaternary ammonium catalysts are rarely reported and applied in asymmetric transformations due to their difficult synthesis according to traditional synthetic methods.19 When catalysts 1k and 1l were examined in this reaction, they showed catalytic activities better than those of catalysts 1g and 1j (entry 11 vs 7 and entry 12 vs 10). Notably, catalyst 1l showed the best catalytic activity and gave one of the most satisfying results among all of the other catalysts (entry 12, 97% yield, 76% ee). However, when catalyst 1o, which lacks the quaternary ammonium center with respect to 1l, was used as the catalyst, the yield and enantioselectivity of the product decreased dramatically (entry 15, 35% yield, 19% ee). We chose catalyst 1l as the optimal catalyst. Next, we began to optimize other reaction conditions, including the base, solvent, and temperature (Table 2). During the optimization of

Table 1. Optimization of the Catalystsa

Table 2. Optimization of the Solvent and Temperaturea

entry

cat.

time (h)

yieldb (%)

ee (%)c

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

1a 1b 1c 1d 1e 1f 1g 1h 1i 1j 1k 1l 1m 1n 1o

24 24 24 24 24 24 24 24 24 24 24 24 24 24 24

97 96 96 98 87 94 87 69 65 70 95 97 98 80 35

9 4 32 27 14 3 36 7 3 38 55 76 64 32 19

entry

solvent

T

time (h)

yieldb (%)

ee (%)c

1 2 3 4 5 6 7 8 9 10 11 12 13d 14e

toluene CH2Cl2 CHCl3 m-xylene MTBE DCE Et2O THF m-xylene m-xylene m-xylene m-xylene m-xylene m-xylene

−20 −20 −20 −20 −20 −20 −20 −20 0 −10 −30 −40 −10 −10

24 20 20 24 24 18 18 20 3 3 24 24 3 3

97 96 95 98 95 53 94 97 99 99 95 53 97 96

76 80 72 82 80 60 79 8 74 94 76 83 86 84

a

Unless otherwise noted, reactions were carried out with 0.1 mmol of 2a and 10 mol % catalyst in 0.5 mL of solvent. bYield of isolated product. cDetermined by HPLC using a chiral stationary phase. d1 equiv of LiOH·H2O was used. e0.5 equiv of LiOH·H2O was used.

a

Unless otherwise noted, reactions were carried out with 0.1 mmol of 2a and 10 mol % catalyst in 0.5 mL of solvent. bYield of isolated product. cDetermined by HPLC using a chiral stationary phase.

the base, LiOH·H2O was identified as a better base than the other bases for this reaction (see SI). Subsequently, the effects of the eight solvents on the reaction were examined (Table 2, entries 1−8). As shown in Table 2, m-xylene as a solvent can give the best results (Table 2, entry 4, 98% yield, 82% ee). Finally, the reaction temperature was investigated (Table 2, entries 9−12). Pleasingly, when the reaction temperature was −10 °C, the enantioselectivity of the product was improved significantly and gave the highest ee of 94% with an almost quantitative yield. This allowed for a shortened reaction time of 3 h. In addition, reducing the amount of base needed for the reaction was also investigated; however, the reaction results were not improved (Table 2, entries 13 and 14). Accordingly, we chose 1l as the catalyst, m-xylene as the solvent, and a reaction temperature of −10 °C as the optimal reaction conditions. With the optimized conditions in hand, we explored the scope of the reaction (Table 3). Good-to-excellent enantiose-

including the multiple hydrogen-bonding donors 1a and 1b (Table 1, entries 1 and 2, respectively), were evaluated first. As can be seen in Table 1, the catalytic activity of this type of catalyst is poor. Next, catalyst 1c, reported by Dixon et al.,17 and catalysts 1d−1j, bearing seven different groups of the benzylic moiety, were tested (entries 3−10, respectively). Among this type of urea-quaternary amonium catalyst, catalyst 1j, which possesses a sterically hindered 3,5-di-tert-butyl benzyl moiety, proved to be the best and gave the product 3a in 70% yield with a 38% ee (entry 10). In order to improve the catalytic activity of chiral phase-transfer catalysts in this model reaction, on the basis of the superior quinine chiral skeleton and using a thiourea moiety in place of a urea moiety, we successfully synthesized several novel chiral thiourea quaternary ammonium 1487

DOI: 10.1021/acs.joc.7b03078 J. Org. Chem. 2018, 83, 1486−1492

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The Journal of Organic Chemistry Table 3. Substrate Scopea

entry

Ar

R

3

time (h)

yieldb (%)

eec (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Ph o-MeC6H4 m-MeC6H4 m-FC6H4 m-ClC6H4 m-CF3C6H4 p-MeC6H4 p-MeOC6H4 p-FC6H4 p-ClC6H4 p-BrC6H4 Ph α-naphthyl β-naphthyl

Me Me Me Me Me Me Me Me Me Me Me Et Me Me

3a 3b 3c 3d 3e 3f 3g 3h 3i 3j 3k 3l 3m 3n

3 3 3 3 3 3 3 3 3 3 3 3 3 3

99 94 96 93 85 84 89 96 90 98 99 98, 70d 78 86

94 90 94 86 85 72 86 98 86 95 >99 79, 94d 75 67

Figure 3. Plausible transition state.

ketoester-derived N-Ts ketimines. Further, the transition state results in the addition product with high enantioselectivity. Finally, the validity and practicability of this synthetic approach were proven by derivatization of the aza-Henry adduct α,β-diamino ester. An optically active product, such as 3a, was easily converted into the corresponding α,β-diamino ester 4a containing two different N-protected groups in good yield by zinc-mediated reduction without loss of enantioselectivity (Scheme 1). Scheme 1. Preparation of α,β-Diaminopropionic Ester 4a

a

Unless otherwise noted, reactions were carried out with 0.1 mmol of 2a and 10 mol % 1l in 0.5 mL of solvent. bYield of isolated product. c Determined by HPLC using a chiral stationary phase. dAfter crystallization.

lectivities were obtained using aryl α-ketoester-derived imines bearing either electron-withdrawing or electron-donating substituents (up to 99% yield, up to 99% ee; Table 3, entries 1−11). When the methyl groups of the ester moiety of the substrate were replaced by ethyl groups, the reaction also afforded the product 3l in excellent yield with good enantioselectivity (Table 3, entry 12). In addition, polyaromatic substrates such as 2m and 2n also gave corresponding products in good yields with good enantioselectivities (Table 3, entries 13 and 14). The absolute configuration of 3l was determined to be R based on single-crystal X-ray structure analysis20 (Figure 2). X-ray diffraction analysis of its single crystal prepared by

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CONCLUSIONS In summary, the first catalytic enantioselective aza-Henry reaction of aryl α-ketoester-derived N-Ts ketimines has been realized using a novel bifunctional thiourea−ammonium salt derived from quinine as a phase-transfer catalyst. In this catalytic system, only a short reaction time was needed, and the corresponding adducts bearing quaternary chiral centers were obtained in high to excellent yields (up to 99% yield) with good-to-high enantioselectivities (up to 99% ee). Using this asymmetric catalytic protocol, useful α,β-diamino esters could be synthesized. Further studies of the reaction mechanism and work to uncover the full capabilities of this new catalyst are ongoing.

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EXPERIMENTAL SECTION

General Information. Unless otherwise stated, all reagents were purchased from commercial suppliers and used without purification. All solvents were obtained from commercial sources and were purified according to standard procedures. For thin-layer chromatography (TLC), silica gel plates (HSGF 254) were used and compounds were visualized by irradiation with UV light. Purification of reaction products was carried out by flash column chromatography using silica gel (200−300 mesh). 1H and 13C NMR spectra were recorded on a Varian Mercury-300BB (300 MHz) or a Bruker NMR (400 or 500 MHz) spectrometer. All chemical shifts (δ) were given in ppm. Chemical shifts are relative to the resonance of the deuterated solvent as the internal standard (CDCl3, δ 7.26 ppm for proton NMR, δ 77.16 ppm for carbon NMR; CD3OD-d4, δ 3.31 ppm for proton NMR, δ 49.00 ppm for carbon NMR; DMSO-d6, δ 2.50 ppm for proton NMR, δ 39.52 ppm for carbon NMR). Data are presented as follows: chemical shift, integration, multiplicity (br = broad, s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet), and coupling constant in hertz. Mass spectra were recorded on a Bruker Agilent 1290 MicrOTOF-Q II instrument. Melting points were measured on a melting point apparatus and were uncorrected. The enantioselectivity

Figure 2. X-ray structure of 3l. Hydrogen atoms are omitted for clarity, and ellipsoids are drawn at the 50% probability level.

recrystallization from an n-hexane/ethyl acetate solvent system at room temperature allowed us to assign the absolute configuration of 3l to be R. On the basis of the stereochemistry of the obtained products, we proposed a possible transition state model to rationalize the stereochemical results of the reaction21 (Figure 3). The ammonium motif pairs with the nitro compounds by electrostatic interaction, and the thiourea motif captures the ketimines by H-bond interaction. Such an assembly would direct the nucleophile to attack from the re face of the aryl α1488

DOI: 10.1021/acs.joc.7b03078 J. Org. Chem. 2018, 83, 1486−1492

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The Journal of Organic Chemistry

1H), 3.72 (t, J = 6.6 Hz, 2H), 3.60−3.44 (m, 1H), 3.37 (d, J = 11.2 Hz, 1H), 2.75 (d, J = 7.9 Hz, 1H), 2.33−2.02 (m, 3H), 1.97−1.78 (m, 3H), 1.35 (s, 18H). 13C NMR (101 MHz, CDCl3) δ: 179.3, 163.4, 163.0, 158.9, 152.7, 148.3, 145.8, 145.2, 143.4, 143.0, 136.0, 135.0, 132.1, 127.9, 127.3, 126.1, 125.8, 125.1, 124.2, 123.3, 122.7, 121.8, 120.9, 120.0, 118.9, 102.6, 70.2, 68.0, 67.5, 59.8, 58.6, 56.2, 53.1, 49.5, 37.3, 35.0, 31.4, 29.8, 27.2, 25.7. HRMS (ESI) calculated for C42H52N5O3S [M − Br]+: 706.3785, found 706.3784. IR (neat, cm−1): 3434, 2958, 1672, 1622, 1597, 1558, 1509, 1475, 1395, 1251, 1179, 1135, 1026, 853, 720. Catalyst 1n. Light yellow solid, 115 mg, 54% yield for two steps, mp 120−122 °C, [α]D25 = −55.7 (c = 0.37, CHCl3). 1H NMR (300 MHz, CD3OD) δ: 8.82 (d, J = 4.7 Hz, 1H), 8.15 (t, J = 7.9 Hz, 1H), 8.03 (d, J = 9.2 Hz, 1H), 7.94 (s, 1H), 7.72 (d, J = 4.8 Hz, 1H), 7.65 (d, J = 9.6 Hz, 2H), 7.53 (dd, J = 9.2, 2.4 Hz, 1H), 7.43 (d, J = 1.3 Hz, 2H), 7.28 (d, J = 10.6 Hz, 1H), 5.30 (d, J = 12.6 Hz, 1H), 5.00 (t, J = 11.4 Hz, 1H), 4.82 (br, 1H), 4.52 (d, J = 12.7 Hz, 1H), 4.08 (s, 3H), 3.97−3.80 (m, 1H), 3.28−3.18 (m, 1H), 2.27−2.05 (m, 2H), 2.04− 1.75 (m, 4H), 1.65−1.39 (m, 3H), 1.36 (s, 18H), 1.29 (d, J = 4.5 Hz, 2H), 1.14 (dd, J = 26.1, 11.9 Hz, 1H), 0.88 (t, J = 7.3 Hz, 3H). 13C NMR (101 MHz, CDCl3) δ: 180.2, 163.4, 163.1, 159.0, 152.7, 148.3, 145.2, 143.2, 141.0, 132.0, 131.6, 131.3, 127.9, 127.3, 126.2, 125.0, 124.6, 123.5, 122.9, 121.9, 120.0, 118.3, 118.0, 115.4, 102.5, 70.3, 67.6, 61.9, 56.2, 53.5, 49.5, 35.5, 35.0, 31.3, 27.1, 25.1, 24.2, 11.0. HRMS (ESI) calculated for C44H53F6N4OS [M − Br]+: 799.3839, found 799.3837. IR (neat, cm−1): 3421, 2967, 1675, 1623, 1558, 1508, 1475, 1386, 1279, 1179, 1136, 1030, 886, 720, 681. General Procedure for the Nitro-Mannich Reaction of Ketimines and Characterization of aza-Henry Addition Products 3a−3n. Ketimines 2 (0.1 mmol) and catalyst 1l (8.8 mg, 0.01 mmol, 10 mol %) were dissolved in dry m-xylene, and nitoalkane (1 mmol, 10 equiv) was added. The mixture was cooled to −10 °C. Freshly ground LiOH·H2O (21 mg, 0.5 mmol, 5 equiv) was added in one portion, and the resulting suspension was vigorously stirred for 3 h. To the mixture was added 1 mL sat. aq. NH4Cl, and the solution was allowed to warm to room temperature. The aqueous layer was extracted with EA (3 × 5 mL). Then, the organic layer was dried over Na2SO4, filtered, and concentrated under reduced pressure. The crude product was purified by flash chromatography (PE/EA = 5:1). Methyl (R)-2-[(4-Methylphenyl)sulfonamide]-3-nitro-2-phenylpropanoate (3a). White solid, 36.7 mg, 97% yield, mp 115−116 °C, [α]D25 = −55 (c = 0.64, CHCl3). The ee value was 94% (Chiralpak AD-H, hexane/i-PrOH = 80:20, 230 nm, 1 mL/min, tmajor = 20.12 min, tminor = 30.20 min). 1H NMR (300 MHz, CDCl3) δ: 7.29−7.14 (m, 3H), 7.08 (t, J = 7.5 Hz, 2H), 6.99 (d, J = 8.2 Hz, 4H), 6.37 (s, 1H), 5.96 (d, J = 14.2 Hz, 1H), 5.45 (d, J = 14.2 Hz, 1H), 3.74 (s, 3H), 2.33 (s, 3H). 13C NMR (101 MHz, CDCl3) δ: 170.1, 143.1, 138.3, 132.5, 129.3, 129.2, 128.9, 126.7, 126.3, 78.2, 64.6, 54.5, 21.5. HRMS (ESI) calculated for C17H19N2O6S [M + H]+: 379.0958, found 379.0959. IR (neat, cm−1): 3283, 1748, 1568, 1384, 1340, 1268, 1233, 1165, 1147, 1091. Methyl (R)-2-[(4-Methylphenyl)sulfonamide]-3-nitro-2-(o-tolyl)propanoate (3b). White solid, 36.9 mg, 94% yield, mp 132−133 °C, [α]D25 = +15 (c = 0.2, CHCl3). The ee value was 90% (Chiralpak AD-H, hexane/i-PrOH = 80:20, 230 nm, 1 mL/min, tmajor = 23.08 min, tminor = 18.27 min). 1H NMR (300 MHz, CDCl3) δ: 7.31−7.23 (m, 1H), 7.18 (dd, J = 6.7, 4.4 Hz, 2H), 7.02 (d, J = 8.3 Hz, 2H), 6.93 (d, J = 8.5 Hz, 1H), 6.64 (d, J = 7.3 Hz, 1H), 6.22 (s, 1H), 6.15 (d, J = 11.3 Hz, 1H), 5.31 (d, J = 11.2 Hz, 1H), 3.79 (s, 3H), 2.33 (s, 3H), 1.65 (s, 3H). 13C NMR (126 MHz, CDCl3) δ: 170.0, 143.1, 137.8, 137.4, 132.8, 130.5, 129.5, 129.0, 127.8, 126.8, 126.4, 81.6, 64.8, 54.6, 21.6, 19.9. HRMS (ESI) calculated for C18H21N2O6S [M + H]+: 393.1115, found 393.1119. IR (neat, cm−1): 3317, 1742, 1556, 1375, 1344, 1266, 1226, 1163, 1117, 1089. Methyl (R)-2-[(4-Methylphenyl)sulfonamide]-3-nitro-2-(m-tolyl)propanoate (3c). White solid, 37.7 mg, 96% yield, mp 126−127 °C, [α]D25 = −29 (c = 0.31, CHCl3). The ee value was 94% (Chiralpak AD-H, hexane/i-PrOH = 80:20, 230 nm, 1 mL/min, tmajor = 15.89 min, tminor = 23.01 min). 1H NMR (400 MHz, CDCl3) δ: 7.17 (d, J = 8.2 Hz, 2H), 7.09−6.93 (m, 4H), 6.84 (d, J = 7.5 Hz, 1H), 6.64 (s, 1H),

(ee) value determination was carried out using chiral HPLC (Waters) instrumentation with a Chiracel AD-H column. Optical rotations were measured on a Shanghai ShenGuang SGW-2 polarimeter at λ = 589 nm. Optical rotations are reported as follows: [α]D25 (c = g/100 mL, solvent). Starting Materials. Urea 1o and compound 1i′ were prepared according to reported procedures.22 Aryl α-ketoester-derived N-Ts ketimines 2 were prepared according to literature procedures.12d,23 All phase-transfer catalysts (1a and 1b16c and 1c−1j17) were synthesized according to procedures reported previously. m-Xylene was dried over 4 Å molecular sieves. General Procedure for the Preparation of Catalysts 1k−1n. Under argon protection, 1i′22b (500 mg, 1.18 mmol, 1 equiv) was dissolved in anhydrous THF (8 mL). Benzyl bromide (1.3 mmol, 1.1 equiv) was added to the mixture. The mixture was heated to reflux, and after 12 h, the mixture was concentrated under reduced pressure and purified by flash chromatography (Et2O/MeOH = 10:1 to 8:1) to afford the desired product 1i′′. Under argon protection, TFA (2.4 mL) was added to a solution of 1i′′ (0.24 mmol) in 2.4 mL of anhydrous CH2Cl2 while being stirred. The mixture was stirred overnight and was concentrated to dryness under reduced pressure. The residue was redissolved in 10 mL of CH2Cl2; the mixture was adjusted to pH 7−8 by aqueous ammonia, and the organic phases were extracted by CH2Cl2 (2 × 5 mL). The organic phases were combined, dried over anhydrous Na2SO4, and evaporated under reduced pressure. The crude free amine was dried under a vacuum and dissolved in 2.4 mL of anhydrous CH2Cl2. Aryl isothiocyanate (0.26 mmol, 1.1 equiv) was added, and the resulting mixture was stirred at rt overnight. After evaporation of the volatiles, the crude reaction mixture was purified by flash column chromatography (DCM/MeOH = 50:1 to 30:1) to give catalysts 1k−1n. Catalyst 1k. Light yellow solid, 140 mg, 70% yield for two steps, mp 130−131 °C, [α]D25 = −94.7 (c = 0.5, CHCl3). 1H NMR (300 MHz, CD3OD) δ: 8.81 (d, J = 4.8 Hz, 1H), 8.22 (d, J = 2.6 Hz, 1H), 8.05−7.94 (m, 2H), 7.78 (d, J = 4.8 Hz, 1H), 7.65 (s, 1H), 7.60−7.43 (m, 3H), 7.35 (d, J = 10.8 Hz, 1H), 7.24−7.03 (m, 2H), 5.96−5.78 (m, 1H), 5.36−5.10 (m, 3H), 5.04−4.92 (m, 1H), 4.88−4.79 (m, 2H), 4.69 (d, J = 12.6 Hz, 1H), 4.08 (s, 3H), 3.92 (s, 3H), 3.83−3.67 (m, 1H), 3.55−3.39 (m, 2H), 2.78−2.62 (m, 1H), 2.25−2.03 (m, 3H), 1.91 (s, 1H), 1.27−1.11 (m, 2H). 13C NMR (126 MHz, CD3OD) δ: 182.0, 160.5, 160.4, 148.7, 145.8, 145.3, 142.5, 137.5, 136.5, 134.0, 132.7 (q, JCF = 34 Hz), 131.8, 128.7, 124.5, 122.4 (q, JCF = 264 Hz), 122.2, 121.2, 118.8, 118.1, 117.0, 113.2, 104.0, 70.7, 62.5, 61.1, 56.8, 56.1, 55.3, 51.0, 38.6, 28.6, 27.6, 25.6. HRMS (ESI) calculated for C37H37F6N4O2S [M − Br]+: 715.2536, found 715.2538. IR (neat, cm−1): 2968, 2361, 1674, 1622, 1559, 1508, 1475, 1386, 1279, 1179, 1134, 1029, 886, 760, 720, 680. Catalyst 1l. Light yellow solid, 126 mg, 60% yield for two steps, mp 146−147 °C, [α]D25 = −60.6 (c = 0.4, CHCl3). 1H NMR (300 MHz, CD3OD) δ: 8.82 (d, J = 4.7 Hz, 1H), 8.17 (d, J = 2.4 Hz, 1H), 8.03 (d, J = 9.2 Hz, 1H), 7.97 (s, 2H), 7.74 (d, J = 4.8 Hz, 1H), 7.65 (d, J = 6.3 Hz, 2H), 7.52 (dd, J = 9.2, 2.5 Hz, 1H), 7.44 (d, J = 1.5 Hz, 2H), 7.30 (d, J = 10.5 Hz, 1H), 6.02−5.82 (m, 1H), 5.40−5.17 (m, 3H), 5.08− 4.90 (m, 2H), 4.58 (d, J = 12.8 Hz, 1H), 3.96−3.80 (m, 1H), 3.63− 3.45 (m, 1H), 3.42−3.34 (m, 1H), 2.75 (d, J = 8.4 Hz, 2H), 2.29−1.98 (m, 3H), 1.92 (s, 1H), 1.35 (s, 18H), 1.30−1.08 (m, 2H). 13C NMR (101 MHz, CD3OD) δ: 182.0, 160.6, 153.5, 148.7, 145.9, 145.0, 142.3, 137.5, 133.0, 132.6, 131.9, 129.1, 128.6, 127.8, 125.9, 124.7, 124.6, 120.9, 119.0, 118.2, 103.8, 70.6, 67.7, 61.6, 56.9, 55.5, 50.9, 49.6, 38.4, 35.9, 31.7, 28.9, 28.0, 25.4. HRMS (ESI) calculated for C44H51F6N4OS [M − Br]+: 797.3682, found 797.3696. IR (neat, cm−1): 3420, 2966, 1673, 1623, 1559, 1508, 1476, 1385, 1279, 1179, 1135, 1029, 885, 720, 680. Catalyst 1m. Light yellow solid, 128 mg, 68% yield for two steps, mp 145−146 °C, [α]D25 = −33.7 (c = 0.35, CHCl3). 1H NMR (400 MHz, CD3OD) δ: 8.83 (d, J = 4.7 Hz, 1H), 8.16 (t, J = 4.4 Hz, 1H), 8.07 (d, J = 9.1 Hz, 2H), 8.00 (d, J = 8.9 Hz, 1H), 7.82−7.70 (m, 1H), 7.69−7.59 (m, 2H), 7.52 (dd, J = 9.2, 2.5 Hz, 1H), 7.44 (s, 2H), 7.31 (d, J = 10.6 Hz, 1H), 6.01−5.80 (m, 1H), 5.39−5.12 (m, 3H), 5.08− 4.89 (m, 2H), 4.59 (d, J = 12.7 Hz, 1H), 4.09 (s, 3H), 3.94−3.79 (m, 1489

DOI: 10.1021/acs.joc.7b03078 J. Org. Chem. 2018, 83, 1486−1492

Article

The Journal of Organic Chemistry

Methyl (R)-2-(4-Fluorophenyl)-2-[(4-methylphenyl)sulfonamide]3-nitropropanoate (3i). White solid, 35.7 mg, 90% yield, mp 153−154 °C, [α]D25 = −28.2 (c = 0.22, CHCl3). The ee value was 86% (Chiralpak AD-H, hexane/i-PrOH = 80:20, 230 nm, 1 mL/min, tmajor = 20.49 min, tminor = 29.43 min). 1H NMR (400 MHz, CDCl3) δ: 7.22 (d, J = 8.1 Hz, 2H), 7.14−6.91 (m, 4H), 6.76 (t, J = 8.6 Hz, 2H), 6.35 (s, 1H), 5.92 (d, J = 14.3 Hz, 1H), 5.41 (d, J = 14.3 Hz, 1H), 3.75 (s, 3H), 2.36 (s, 3H). 13C NMR (101 MHz, CDCl3) δ: 169.9, 164.2, 161.7, 143.5, 138.3, 129.3, 128.3 (d, J = 8.6 Hz), 126.7, 115.8 (d, J = 22.0 Hz), 78.1, 64.0, 54.6, 21.6. HRMS (ESI) calculated for C17H18FN2O6S [M + H]+: 397.0864, found 397.0865. IR (neat, cm−1): 3286, 1747, 1566, 1381, 1344, 1268, 1232, 1187, 1162, 1090. Methyl (R)-2-(4-Chlorophenyl)-2-[(4-methylphenyl)sulfonamide]3-nitropropanoate (3j). White solid, 42.1 mg, 98% yield, mp 158− 159 °C, [α]D25 = −66.7 (c = 0.27, CHCl3). The ee value was 95% (Chiralpak AD-H, hexane/i-PrOH = 80:20, 230 nm, 1 mL/min, tmajor = 21.31 min, tminor = 34.04 min). 1H NMR (400 MHz, CDCl3) δ: 7.21 (d, J = 8.2 Hz, 2H), 7.03 (dd, J = 13.3, 8.4 Hz, 4H), 6.90 (d, J = 8.7 Hz, 2H), 6.40 (s, 1H), 5.92 (d, J = 14.1 Hz, 1H), 5.41 (d, J = 14.1 Hz, 1H), 3.75 (s, 3H), 2.38 (s, 3H). 13C NMR (101 MHz, CDCl3) δ: 169.8, 143.6, 138.2, 135.7, 131.0, 129.4, 128.9, 127.7, 126.7, 77.8, 64.0, 54.6, 21.6. HRMS (ESI) calculated for C17H18ClN2O6S [M + H]+: 413.0569, found 413.0577. IR (neat, cm−1): 3282, 1749, 1565, 1345, 1327, 1270, 1237, 1164, 1153, 1090. Methyl (R)-2-(4-Bromophenyl)-2-[(4-methylphenyl)sulfonamide]3-nitropropanoate (3k). White solid, 45.3 mg, 99% yield, mp 154− 156 °C, [α]D25 = −116 (c = 0.1, CHCl3). The ee value was >99% (Chiralpak AD-H, hexane/i-PrOH = 80:20, 230 nm, 1 mL/min, tmajor = 20.97 min, tminor = 33.65 min). 1H NMR (400 MHz, CDCl3) δ: 7.21 (d, J = 8.2 Hz, 2H), 7.03 (dd, J = 13.1, 8.4 Hz, 4H), 6.90 (d, J = 8.6 Hz, 2H), 6.40 (s, 1H), 5.92 (d, J = 14.1 Hz, 1H), 5.41 (d, J = 14.1 Hz, 1H), 3.75 (s, 3H), 2.38 (s, 3H). 13C NMR (101 MHz, CDCl3) δ: 169.8, 143.5, 138.2, 135.7, 131.0, 129.4, 128.9, 127.7, 126.7, 77.9, 64.0, 54.6, 21.6. HRMS (ESI) calculated for C17H18BrN2O6S [M + H]+: 457.0063, found 457.0066. IR (neat, cm−1): 3282, 1747, 1564, 1346, 1326, 1269, 1238, 1164, 1152, 1090. Methyl (R)-2-[(4-Methylphenyl)sulfonamide]-3-nitro-2-phenylpropanoate (3l). White solid, 38.5 mg, 98% yield, mp 125−126 °C, [α]D25 = −21.5 (c = 0.68, CHCl3). The ee value was 79% (Chiralpak AD-H, hexane/i-PrOH = 80:20, 230 nm, 1 mL/min, tmajor = 16.79 min, tminor = 22.80 min). 1H NMR (300 MHz, CDCl3) δ: 7.25−7.14 (m, 3H), 7.13−7.04 (m, 2H), 7.03−6.92 (m, 4H), 6.37 (s, 1H), 5.95 (d, J = 14.3 Hz, 1H), 5.45 (d, J = 14.2 Hz, 1H), 4.34−4.21 (m, 1H), 4.21− 4.08 (m, 1H), 2.33 (s, 3H), 1.14 (t, J = 7.1 Hz, 3H). 13C NMR (101 MHz, CDCl3) δ: 169.5, 143.1, 138.4, 132.7, 129.3, 129.2, 128.8, 126.7, 126.3, 78.1, 64.5, 63.9, 21.6, 13.8. HRMS (ESI) calculated for C18H21N2O6S [M + H]+: 393.1115, found 393.1125. IR (neat, cm−1): 3248, 1754, 1560, 1377, 1337, 1265, 1224, 1169, 1151, 1089. Methyl (R)-2-[(4-Methylphenyl)sulfonamide]-2-(naphthalen-1yl)-3-nitropropanoate (3m). White solid, 33.4 mg, 78% yield, mp 134−135 °C, [α]D25 = −10.7 (c = 0.43, CHCl3). The ee value was 75% (Chiralpak AD-H, hexane/i-PrOH = 80:20, 230 nm, 1 mL/min, tmajor = 28.71 min, tminor = 30.20 min). 1H NMR (400 MHz, CDCl3) δ: 7.76 (d, J = 8.1 Hz, 1H), 7.65 (d, J = 8.7 Hz, 1H), 7.57 (d, J = 8.0 Hz, 1H), 7.45 (t, J = 7.8 Hz, 1H), 7.36 (d, J = 7.2 Hz, 1H), 7.26 (dd, J = 8.5, 6.3 Hz, 1H), 7.11 (t, J = 7.8 Hz, 1H), 6.73 (d, J = 8.3 Hz, 2H), 6.45 (s, 1H), 6.39 (d, J = 8.1 Hz, 2H), 6.31 (d, J = 11.2 Hz, 1H), 5.48 (d, J = 11.2 Hz, 1H), 3.68 (s, 3H), 2.02 (s, 3H). 13C NMR (101 MHz, CDCl3) δ: 170.7, 142.4, 136.1, 134.2, 131.3, 130.8, 128.9, 128.2, 127.7, 127.2, 126.3, 125.6, 125.6, 124.5, 123.1, 81.6, 64.8, 54.7, 21.2. HRMS (ESI) calculated for C21H21N2O6S [M + H]+: 429.1115, found 429.1120. IR (neat, cm−1): 3273, 1747, 1558, 1380, 1347, 1261, 1242, 1185, 1165, 1090. Methyl (R)-2-[(4-Methylphenyl)sulfonamide]-2-(naphthalen-2yl)-3-nitropropanoate (3n). White solid, 36.9 mg, 86% yield, mp 146−147 °C, [α]D25 = −21.2 (c = 0.1, CHCl3). The ee value was 67% (Chiralpak AD-H, hexane/i-PrOH = 80:20, 230 nm, 1 mL/min, tmajor = 19.84 min, tminor = 28.58 min). 1H NMR (400 MHz, CDCl3) δ: 7.71 (d, J = 7.3 Hz, 1H), 7.61 (d, J = 7.1 Hz, 1H), 7.57−7.38 (m, 4H), 7.03 (d, J = 8.0 Hz, 2H), 6.95 (d, J = 8.6 Hz, 1H), 6.61 (d, J = 7.9 Hz, 2H),

6.37 (s, 1H), 5.96 (d, J = 14.1 Hz, 1H), 5.44 (d, J = 14.1 Hz, 1H), 3.74 (s, 3H), 2.33 (s, 3H), 2.03 (s, 3H). 13C NMR (101 MHz, CDCl3) δ: 170.2, 143.0, 138.8, 138.3, 132.0, 130.1, 129.1, 128.8, 127.3, 126.7, 123.1, 78.3, 64.4, 54.5, 21.5, 21.2. HRMS (ESI) calculated for C18H21N2O6S [M + H]+: 393.1115, found 393.1122. IR (neat, cm−1): 3276, 1748, 1560, 1378, 1342, 1268, 1226, 1159, 1152, 1090. Methyl (R)-2-(3-Fluorophenyl)-2-[(4-methylphenyl)sulfonamide]3-nitropropanoate (3d). White solid, 36.9 mg, 93% yield, mp 156− 157 °C, [α]D25 = −41.6 (c = 0.38, CHCl3). The ee value was 86% (Chiralpak AD-H, hexane/i-PrOH = 80:20, 230 nm, 1 mL/min, tmajor = 20.12 min, tminor = 35.91 min). 1H NMR (300 MHz, CDCl3) δ: 7.26−7.21 (m, 1H), 7.14 (td, J = 8.1, 5.9 Hz, 1H), 7.04 (d, J = 8.0 Hz, 2H), 6.97−6.79 (m, 2H), 6.63 (dt, J = 9.8, 2.2 Hz, 1H), 6.34 (s, 1H), 5.91 (d, J = 14.2 Hz, 1H), 5.42 (d, J = 14.2 Hz, 1H), 3.75 (s, 3H), 2.35 (s, 3H). 13C NMR (101 MHz, CDCl3) δ: 169.6, 163.9, 161.5, 143.6, 138.2, 134.8, 130.5 (d, J = 8.2 Hz), 116.5 (d, J = 21.0 Hz), 129.4, 126.6, 121.9, 121.8, 116.6, 116.4, 114.1 (d, J = 23.7 Hz), 77.9, 64.2, 54.7, 21.5. HRMS (ESI) calculated for C17H18FN2O6S [M + H]+: 397.0864, found 397.0873. IR (neat, cm−1): 3337, 1746, 1562, 1378, 1350, 1271, 1244, 1167, 1154, 1088. Methyl (R)-2-(3-Chlorophenyl)-2-[(4-methylphenyl)sulfonamide]3-nitropropanoate (3e). White solid, 35.1 mg, 85% yield, mp 152− 153 °C, [α]D25 = −20.7 (c = 0.27, CHCl3). The ee value was 85% (Chiralpak AD-H, hexane/i-PrOH = 80:20, 230 nm, 1 mL/min, tmajor = 18.90 min, tminor = 26.50 min). 1H NMR (400 MHz, CDCl3) δ: 7.27−7.10 (m, 4H), 7.06 (d, J = 8.0 Hz, 2H), 6.98 (d, J = 7.6 Hz, 1H), 6.84 (s, 1H), 6.38 (s, 1H), 5.95 (d, J = 14.1 Hz, 1H), 5.44 (d, J = 14.1 Hz, 1H), 3.79 (s, 3H), 2.38 (s, 3H). 13C NMR (101 MHz, CDCl3) δ: 169.6, 143.7, 138.0, 135.3, 134.1, 130.1, 129.6, 129.5, 127.1, 126.5, 124.2, 77.9, 64.1, 54.7, 21.6. HRMS (ESI) calculated for C17H18ClN2O6S [M + H]+: 413.0569, found 413.0573. IR (neat, cm−1): 3335, 1747, 1562, 1378, 1351, 1271, 1244, 1166, 1153, 1087. Methyl (R)-2-[(4-Methylphenyl)sulfonamide]-3-nitro-2-[3(trifluoromethyl)phenyl]propanoate (3f). White solid, 37.5 mg, 84% yield, mp 122−123 °C, [α]D25 = −31.3 (c = 0.3, CHCl3). The ee value was 72% (Chiralpak AD-H, hexane/i-PrOH = 80:20, 230 nm, 1 mL/min, tmajor = 12.91 min, tminor = 15.63 min). 1HNMR (400 MHz, CDCl3) δ: 7.47 (d, J = 7.3 Hz, 1H), 7.37−7.27 (m, 1H), 7.22−7.11 (m, 1H), 7.00 (d, J = 8.1 Hz, 1H), 6.41 (s, 1H), 6.00 (d, J = 14.1 Hz, 1H), 5.46 (d, J = 14.1 Hz, 1H), 3.77 (s, 1H), 2.32 (s, 1H). 13C NMR (101 MHz, CDCl3) δ: 169.6, 143.8, 138.0, 133.7, 131.7, 129.6, 129.5, 126.4, 126.3, 126.2, 123.5, 77.9, 64.2, 54.8, 21.5. HRMS (ESI) calculated for C18H18F3N2O6S [M + H]+: 447.0832, found 447.0835. IR (neat, cm−1): 3282, 1748, 1566, 1384, 1331, 1270, 1239, 1166, 1122, 1091. Methyl (R)-2-[(4-Methylphenyl)sulfonamide]-3-nitro-2-(p-tolyl)propanoate (3g). White solid, 34.9 mg, 89% yield, mp 128−129 °C, [α]D25 = −33.1 (c = 0.26, CHCl3). The ee value was 86% (Chiralpak AD-H, hexane/i-PrOH = 80:20, 230 nm, 1 mL/min, tmajor = 17.95 min, tminor = 26.34 min). 1H NMR (400 MHz, CDCl3) δ: 7.19 (d, J = 8.3 Hz, 2H), 6.98 (d, J = 8.1 Hz, 2H), 6.85 (s, 4H), 6.35 (s, 1H), 5.93 (d, J = 14.2 Hz, 1H), 5.41 (d, J = 14.2 Hz, 1H), 3.73 (s, 3H), 2.35 (s, 3H), 2.25 (s, 3H). 13C NMR (101 MHz, CDCl3) δ: 170.2, 143.0, 139.5, 138.4, 129.5, 129.4, 129.1, 126.8, 126.2, 78.1, 64.3, 54.4, 21.6, 21.1. HRMS (ESI) calculated for C18H21N2O6S [M + H]+: 393.1115, found 393.1117. IR (neat, cm−1): 3293, 1745, 1561, 1392, 1347, 1328, 1270, 1235, 1157, 1089. Methyl (R)-2-(4-Methoxyphenyl)-2-[(4-methylphenyl)sulfonamide]-3-nitropropanoate (3h). White solid, 39.2 mg, 96% yield, mp 126−127 °C, [α]D25 = −44 (c = 0.25, CHCl3). The ee value was 98% (Chiralpak AD-H, hexane/i-PrOH = 80:20, 230 nm, 1 mL/ min, tmajor = 24.82 min, tminor = 31.66 min). 1H NMR (400 MHz, CDCl3) δ: 7.20 (d, J = 8.2 Hz, 2H), 7.00 (d, J = 8.0 Hz, 2H), 6.87 (d, J = 8.8 Hz, 2H), 6.54 (d, J = 8.8 Hz, 2H), 6.39 (s, 1H), 5.92 (d, J = 14.2 Hz, 1H), 5.40 (d, J = 14.2 Hz, 1H), 3.74 (s, 6H), 2.34 (s, 3H). 13C NMR (101 MHz, CDCl3) δ: 170.3, 160.2, 142.9, 138.5, 129.2, 127.6, 126.8, 124.3, 114.1, 78.3, 64.1, 55.4, 54.4, 21.6. HRMS (ESI) calculated for C18H21N2O7S [M + H]+: 409.1064, found 409.1072. IR (neat, cm−1): 3297, 1740, 1561, 1348, 1324, 1261, 1236, 1169, 1156, 1089. 1490

DOI: 10.1021/acs.joc.7b03078 J. Org. Chem. 2018, 83, 1486−1492

Article

The Journal of Organic Chemistry 6.47 (s, 1H), 6.13 (d, J = 14.0 Hz, 1H), 5.58 (d, J = 14.0 Hz, 1H), 3.75 (s, 3H), 2.10 (s, 3H). 13C NMR (101 MHz, CDCl3) δ: 170.1, 143.0, 138.0, 133.2, 132.6, 129.2, 128.9, 128.8, 128.5, 127.5, 127.4, 126.9, 126.6, 126.3, 122.9, 78.2, 64.5, 54.5, 21.3. HRMS (ESI) calculated for C21H21N2O6S [M + H]+: 429.1115, found 429.1116. IR (neat, cm−1): 3301, 1741, 1563, 1380, 1352, 1269, 1240, 1211, 1163, 1090. Preparation of α,β-Diaminopropionic Ester 4a. To a suspension of 3a (94% ee, 38 mg, 0.10 mmol, 1 equiv) in HOAc (2 mL) was added zinc dust (130 mg, 2 mmol, 20 equiv) in small portions, and the mixture was stirred overnight. The resulting reaction was quenched with saturated aqueous Na2CO3, and the mixture was filtered through a short pad of Celite and washed with CHCl3 (10 mL). The filtrate was further extracted with CHCl3 (3 × 5 mL), and the combined organic layers were dried over anhydrous Na2SO4 and concentrated under reduced pressure. The residue was dissolved in CH2Cl2 (2 mL), and then Ac2O (14 μL, 0.15 mmol, 1.5 equiv) and Et3N (28 μL, 0.20 mmol, 2 equiv) were added. After the mixture was stirred for 1 h at room temperature, the reaction was quenched with saturated aqueous NH4Cl and the mixture extracted with CH2Cl2 (3 × 5 mL). The combined organic extracts were dried over Na2SO4 and concentrated under reduced pressure. The residue was purified by column chromatography (PE/EtOAc) to afford product 3t as a white solid (31.6 mg, 81% yield, 97% ee). Methyl (R)-3-Acetamido-2-[(4-methylphenyl)sulfonamide]-2phenylpropanoate (4a). White solid, 31.6 mg, 81% yield for two steps, mp 150−151 °C, [α]D25= −42 (c = 0.4, CHCl3). The ee value was 97% (Chiralpak AD-H, hexane/i-PrOH = 90:10, 220 nm, 1 mL/ min, tmajor = 11.62 min, tminor = 14.12 min). 1H NMR (400 MHz, CDCl3) δ: 7.30 (d, J = 8.3 Hz, 2H), 7.19−7.12 (m, 1H), 7.11−6.97 (m, 6H), 6.28 (s, 1H), 6.18 (d, J = 4.4 Hz, 1H), 4.66 (dd, J = 13.6, 8.5 Hz, 1H), 4.17 (dd, J = 13.6, 4.1 Hz, 1H), 3.60 (s, 3H), 2.34 (s, 3H), 1.96 (s, 3H). 13C NMR (101 MHz, CDCl3) δ: 172.0, 170.3, 143.2, 137.9, 135.3, 129.3, 128.5, 128.4, 126.9, 126.5, 67.1, 53.8, 43.4, 23.3, 21.5. HRMS (ESI) calculated for C19H23N2O5S [M + H]+: 391.1322, found 391.1325. IR (neat, cm−1): 3143, 1741, 1666, 1522, 1427, 1333, 1280, 1233, 1162, 1089.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.7b03078. X-ray crystallography data (CIF) 1 H and 13C NMR spectra and spectral data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Ning Lu: 0000-0003-1424-5714 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China for its financial support (51373067). REFERENCES

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DOI: 10.1021/acs.joc.7b03078 J. Org. Chem. 2018, 83, 1486−1492

Article

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DOI: 10.1021/acs.joc.7b03078 J. Org. Chem. 2018, 83, 1486−1492