Chiral aldehyde catalysis for the catalytic asymmetric activation of

a Key Laboratory of Applied Chemistry of Chongqing Municipality, School of ... b College of Pharmacy, Third Military Medical University, Chongqing 400...
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Article Cite This: J. Am. Chem. Soc. 2018, 140, 9774−9780

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Chiral Aldehyde Catalysis for the Catalytic Asymmetric Activation of Glycine Esters Wei Wen,†,§ Lei Chen,†,§ Ming-Jing Luo,† Yan Zhang,† Ying-Chun Chen,‡ Qin Ouyang,*,‡ and Qi-Xiang Guo*,† †

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Key Laboratory of Applied Chemistry of Chongqing Municipality, School of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, China ‡ College of Pharmacy, Third Military Medical University, Chongqing 400038, China S Supporting Information *

ABSTRACT: Chiral aldehyde catalysis is uniquely suitable for the direct asymmetric α-functionalization of N-unprotected amino acids, because aldehydes can reversibly form imines. However, there have been few successful reports of these transformations. In fact, only chiral aldehyde catalyzed aldol reactions of amino acids and alkylation of 2-amino malonates have been reported with good chiral induction. Here, we report a novel type of chiral aldehyde catalyst based on face control of the enolate intermediates. The resulting chiral aldehyde is the first efficient nonpyridoxal-dependent catalyst that can promote the direct asymmetric α-functionalization of N-unprotected glycine esters. Possible transition states and the proton transfer process were investigated by density functional theory calculations.

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INTRODUCTION The use of aldehyde catalysis in chemical reactions of amino acids can be traced back to the mid-1900s. For example, chiral pyridoxal-dependent enzyme catalysis1 has been successfully used in the aldol addition of amino acids to aldehydes, a biological process involved in amino acid metabolism.2 Inspired by this process, several biomimetic chiral pyridoxal analogues have been developed.3 However, reactions based on these chiral pyridoxal catalysts, and actual enzymes, cannot produce satisfactory chiral induction in reactions other than aldol addition. In 2011, chiral glyceraldehyde analogues were reported to be good tethering catalysts for intermolecular Cope-type hydroaminations of allylic amines.4 Recently, our group reported a chiral BINOL aldehyde catalyst for the asymmetric α-alkylation of 2-aminomalonate with 3-indolylmethanol.5 The activation of the α-C−H bond of an amino acid via the formation of an imine is a unique feature of chiral aldehyde catalysis.6 This feature shows great potential as a straightforward catalytic asymmetric α-functionalization of simple amino acids, especially glycine, for the synthesis of chiral amino acid derivatives. However, this priority has not been well used. Except for the aldol addition, the direct catalytic asymmetric α-functionalization of N-unprotected glycine derivatives has never been disclosed. An alternative method is the utilization of N-protect glycine derivatives, such as imino esters, as reactants.7 So, the development of novel chiral aldehyde catalysts and aldehyde catalyzed direct asymmetric α-functionalization of amino acids show good potential for applications in chiral aldehyde catalysis and amino acid chemistry. Here, we report our attempt to design a novel © 2018 American Chemical Society

chiral aldehyde catalyst and its application in the catalytic asymmetric α-nucleophilic addition of glycine esters to α, βunsaturated ketones.

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RESULTS AND DISCUSSION Catalyst Design. In initial studies, we used the known chiral BINOL aldehyde I as a catalyst, and the tert-butyl glycine ester 1a and chalcone 2a as model reactants. We hoped that a nucleophilic conjugated addition would take place in the promotion of chiral aldehydes, followed by intramolecular condensation to afford optically active Δ(1)-pyrrolines,8 a type of important molecule that can be widely found in biologically active compounds9 and used as an intermediate for the synthesis of pharmacologically active molecules.10 The target product was generated; however, the yield and enantioselectivity were moderate (Scheme 1). We attempted to improve the experimental results through optimizing the reaction conditions, but failed. Hence, we turned our attention to designing a new catalyst. As shown in Figure 1, when BINOL aldehyde I was used as a catalyst, intermediate I would be generated via sequential imine formation and deprotonation. The nucleophilic site (i.e., the α-carbon of the glycine ester) in this intermediate is far away from the chiral axis center and the aryl substituent at the 3′-position. This arrangement might explain the low enantioselectivity. Furthermore, the BINOL aldehyde I can successfully activate the 2-aminomalonate;5 however, the Received: June 25, 2018 Published: July 11, 2018 9774

DOI: 10.1021/jacs.8b06676 J. Am. Chem. Soc. 2018, 140, 9774−9780

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Journal of the American Chemical Society

and stereoselective controlling abilities in our model reaction. As expected, catalyst 4a gave product 3a with good enantioselectivity, but in low yield (Table 1, entry 1). In

Scheme 1. Initial Studies

Table 1. Screening of Catalysts and Optimization of Reaction Conditionsa

entry 1 2 3 4 5 6 7e 8e 9e 10e 11e 12e 13e 14e 15e 16e,f 17e,f,g 18e,g,h 19e,g,h,i 20e,g,h,i

Figure 1. Catalyst redesign.

activation of the glycine ester becomes difficult because of the low carbon acidity of the glycine ester. To overcome the above challenges, we examined a modified BINOL aldehyde II as our candidate catalyst. In this type of aldehyde, formyl is adjacent to the chiral axis center, and the neighboring aryl group (Ar) in the resulting intermediate II can control the face selectivity of enolate better than that in intermediate I. Another important feature is that an electron-withdrawing group can be introduced at the 4-position by hydroxyl-directed electrophilic substitution, which can then improve the activity of the catalyst. With these ideas in mind, we prepared this novel chiral aldehyde. As shown in Scheme 2, chiral aldehydes 4a−f could be synthesized in good yield from (S)-[1,1′-binaphthalene]-2,2′-diol in 8−9 steps, and the molecular structure of 4b was confirmed by single crystal X-ray analysis (see Supporting Information).11 Optimization of Reaction Conditions. With these chiral aldehydes in hand, we then examined their catalytic activation

4 4a 4b 4c 4d 4e 4f 4b 4b 4b 4b 4b 4b 4b 4b 4b 4b 4b 4b 4b 4d

solvent CHCl3 CHCl3 CHCl3 CHCl3 CHCl3 CHCl3 CHCl3 DCE THF PhCH3 PhCl o-xylene m-xylene p-xylene mesitylene PhCH3 PhCH3 PhCH3 PhCH3 PhCH3

time (h) 72 8 10 11 5 11 30 52 27 48 43 50 50 50 50 50 24 24 19 24

yield (%)b 22 70 75 82 50 69 61 37 83 67 72 59 57 56 58 59 78 84 93 90

drc d

ND 85/15 86/14 89/11 86/14 87/13 86/14 86/14 90/10 86/14 88/12 84/16 85/15 84/16 85/15 89/11 88/12 89/11 89/11 90/10

ee (%)c 74 87 86 88 86 82 82 83 79 88 84 88 88 88 88 91 95 96 95 96

a

1a (0.2 mmol), 2a (0.1 mmol), 4 (0.02 mmol), 4-MeOC6H4CO2H (0.03 mmol), DBU (0.1 mmol), and solvent (0.5 mL) at 50 °C. b Isolated yield. cDetermined by chiral HPLC. dNot determined. e10 mol % 4b or 4d. f15 mol % 2,6-pyridinedicarboxylic acid instead of 4MeOC6H4CO2H. g1 equiv of TBD instead of DBU. h25 mol % 2,6pyridinedicarboxylic acid instead of 4-MeOC6H4CO2H. iUsing 0.33 mL of toluene.

comparison, catalyst 4b, bearing a bromine at its 4-position, generated product 3a with high yield and enantioselectivity in a short reaction time (Table 1, entry 2). Catalysts 4c−f were then examined, and all gave the product in moderate to high yields with good diastereo- and enantioselectivities (Table 1, entries 3−6). We selected 4b as a catalyst for further optimization of the reaction conditions. The catalyst loading had little influence on the experimental outcomes. With a 10 mol % loading of 4b, both the yield and enantioselectivity of 3a decreased slightly (Table 1, entry 7). We performed subsequent tests with 0.1 equiv of catalyst 4b. Solvent screening results indicated that toluene was the best choice of solvent in terms of enantioselectivity (Table 1, entry 10). Acid and base additives were then investigated (see Supporting Information, Tables S2−S3). We found that 2,6-pyridinedicarboxylic acid (Table 1, entry 16) and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) (Table 1, entry 17) slightly improved the enantioselectivity. The combination of 1 equiv of TBD and 0.25 equiv of 2,6-pyridinedicarboxylic acid was found to be appropriate for this reaction (Table 1, entry 18). When this reaction was performed at a concentration of 0.3 M (based on chalcone), the yield of 3a improved to 93%, and the diastereoand enantioselectivity remained at a high level (Table 1, entry 19). Aldehyde 4d was determined to be the best catalyst from

Scheme 2. Catalyst Synthesisa

a Reaction conditions: (a) Tf2O, iPr2NEt, CH2Cl2, −78 °C to rt, 96%; (b) ArMgBr, NiCl2(dppe), THF, reflux, 90−98%; (c) Tf2O, iPr2NEt, CH2Cl2, −78 °C to rt, 80−86%; (d) HCOOPh, Pd(OAc)2, dppp, i Pr2NEt, 120 °C; (e) nPrOH, NaH, THF, two steps: 70−77%; (f) Mg(TMP)2, B(OMe)3, THF, then 1 M HCl; (g) H2O2, CHCl3, two steps: 60−66%; (h) DIBAL-H, CH2Cl2, −78 °C, 46−50%; (i) (for 4b, 4d−f) Br2, CHCl3, 73−80%; (for 4c) I2/KIO3, AcOH, H3PO4, EtOH, 99%.

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Table 2. Substrate Scope of α,β-Unsaturated Ketones (I)a

the catalyst screening (Table 1, entry 4) and was further examined under the above reaction conditions determined for 4b. Through the promotion of 4d, product 3a was generated smoothly in excellent yield (90%), good diastereoselectivity (90:10), and excellent enantioselectivity (96% ee) (Table 1, entry 20). On the basis of these results, the optimal reaction conditions were determined. Substrate Scope. With the optimal reaction conditions in hand, we then examined the substrate scope of the reaction. First, various substituents, including aryl, heteroaryl, alkyl, alkenyl, and alkynyl groups, were introduced at the end of the enone CC bonds. Aryl substituents at this site were well tolerated in this reaction. For example, enones bearing ortho-, meta-, and para-substituted phenyls were good reaction partners for this transformation, giving products 3b−p in good to excellent yields, diastereoselectivities, and enantioselectivities. The electronic properties and position of substituents on the phenyl ring did not obviously affect the results. Notably, substrates containing double α,β-unsaturated carbonyl units could react with 2 equiv of tert-butyl glycinate to afford product 3q bearing two Δ(1)-pyrroline units in 72% yield and 99% enantioselectivity. Furthermore, phenyl and 2-naphthyl groups were well tolerated at this site (Table 2, 3r). Heteroaryls, such as 2-furyl, 2-thienyl, 2-pyridyl, 2-quinolyl, and 3-indolyl, were also connected to the end site of CC bonds of Michael acceptors. Excellent stereoselective outcomes were obtained from these heteroaryl substituted enones (Table 2, 3s−3x). We noted that 2-pyridyl and 2-quinolyl showed lower yields, likely because the pyridine unit changed the basicity of this reaction system (Table 2, 3u−3v). Alkyl groups linked to the CC bonds also showed good tolerance in this reaction. Methyl, ethyl, cyclopropyl, and n-propyl substituted enones could produce Δ(1)-pyrroline with good results (Table 2, 3y−3ab). An alkyl longer than 3 carbons decreased the yield slightly, but the diastereo- and enantioselectivities still remained at a high level (Table 2, 3ac−3af). Alkyl substituents containing an ether or amino group participated in this reaction and gave products 3ag and 3ah in moderate yields, good diastereoselectivities, and excellent enantioselectivities. The yield of 3ah increased to 56% after we increased the catalyst loading to 20 mol %. Some alkenyls were also tolerated in this reaction, and the steric effect could affect the yield. 1Methyl propenyl substituted enone gave product 3ai in 37% yield, while 1-cyclohexyl and styryl-substituted ones provided products 3aj and 3ak in yields of 64% and 79%, respectively. Excellent enantioselectivities were observed in these alkenyl substituted Δ(1)-pyrrolines. However, phenylethynyl markedly decreased the reaction effectiveness with a reduced yield, diastereoselectivity, and enantioselectivity (Table 2, 3al). Second, we examined the substrate scope with respect to the substituent connected to the carbonyl, including aryl, heteroaryl, alkyl, and alkenyl substituents. When a substituted phenyl was connected to the carbonyl of a Michael acceptor, the position and electronic properties of the substituent affected the experimental outcome. For example, enones bearing ortho- or meta-substituted phenyls at this site provided corresponding products in good yields (Table 3, 5a−5d). By comparison, Michael acceptors with para-electron-withdrawing or weak electron-donating group substituted phenyls gave Δ(1)-pyrroline products in excellent yields (Table 3, 5e−5h). As a strong electron-donating group, dimethyl amino at the para-position of phenyl markedly decreased the yields (Table 3, 5j). Fortunately, all aryl substituted enones gave the target

a 1a (0.2 mmol), Michael acceptor (0.1 mmol), 4d (0.01 mmol), 2,6pyridinedicarboxylic acid (0.025 mmol), TBD (0.1 mmol), and toluene (0.33 mL) at 50 °C. bIsolated yield. cDetermined by 1H NMR. dDetermined by chiral HPLC. eUsing 15 mol % 4d and 4 equiv of tert-butyl glycine ester. fUsing 20 mol % 4d.

products in good diastereoselectivities and excellent enantioselectivities. Electron-rich heteroaryls 2-furyl and 3-thienyl reacted with tert-butyl glycine ester under the optimal reaction conditions; however, only small amounts of products were observed. After we increased the catalyst loading, corresponding Δ(1)-pyrrolines 5o−5p were obtained in 40% and 71% yields, respectively. N-Bs (PhSO2-) protected 3-indolyl and ferrocenyl substituted enones were suitable acceptors and gave products 5q−5r in moderate yields and good stereoselectivities. With respect to the alkyl substituent, steric effects 9776

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Journal of the American Chemical Society Table 3. Substrate Scope of α,β-Unsaturated Ketones (II)a

Table 4. Substrate Scope of Glycine Derivativesa

a

Glycine derivative (0.2 mmol), Michael acceptor (0.1 mmol), 4d (0.01 mmol), K2CO3 (0.1 mmol), 2,6-pyridinedicarboxylic acid (0.025 mmol), TBD (0.1 mmol), and toluene (0.33 mL) at 50 °C. b Isolated yield. cDetermined by 1H NMR. dDetermined by chiral HPLC. eNeutral glycine derivative was used as the donor, and no K2CO3 was added. fAt 70 °C.

instability of the corresponding glycine ester lowered the yield of the product. Nevertheless, the diastereoselectivities achieved with the glycine methyl or ethyl esters were much better than that with the glycine tert-butyl ester. Xu’s work8a indicated that cis-Δ(1)-pyrroline could be converted into its trans isomer through promotion by DBU. Similarly, we suggest that the improvement of the diastereoselectivity was likely caused by the TBD-promoted cis to trans transformation. A smaller alkoxyl group enabled an easier transformation. We designed a comparison experiment to confirm this hypothesis. Under the standard reaction conditions, cis-6a converted to its trans form completely; however, the cis-3a gave a mixture with 88% trans isomer. These results indicated that we could improve the diastereoselectivity through tuning the alkoxy group. Thus, several enones that did not show excellent diastereoselectivities in Tables 2 and 3 were tested in a reaction with glycine ethyl ester hydrochloride. As expected, all these reactions gave products in excellent yields and stereoselectivities (Table 4, 6e−6l). Glycine derived amides and dipeptide were also tested here. The reaction between dimethyl amine, pyrrolidine, and morpholine derived glycinamides and chalcone took place slowly under the optimal reaction conditions. After we raised the reaction temperature, products 6m−6o could be obtained in moderate yields and moderate to good enantioselectivities. The moderate yields might be attributed to the low carbon acidity of the corresponding glycinamide donors. Glycine dipeptide could produce the desired product in good yield; however, the enantioselectivity was moderate (Table 4, 6p). Nevertheless, this reaction showed potential for use as a chiralaldehyde catalytic strategy for modifying glycine derived peptides. The absolute configurations of compound 3a (2S, 3R) were determined by a comparison of the specific rotation with a literature value (see Supporting Information). The stereo-

a 1a (0.2 mmol), Michael acceptor (0.1 mmol), 4d (0.01 mmol), 2,6pyridinedicarboxylic acid (0.025 mmol), TBD (0.1 mmol), and toluene (0.33 mL) at 50 °C. bIsolated yield. cDetermined by 1HNMR. d Determined by chiral HPLC. eUsing 20 mol % 4d. fAt 70 °C.

notably influenced the experimental outcomes. For example, methyl substituted enone produced a mixture under the optimal reaction conditions, partially because this acceptor could be converted into its enolate form easily, which increased the complexity of this reaction. To overcome this issue, bulky alkyl groups, including isopropyl and cyclohexyl were introduced at this site. Satisfactorily, after we increased the reaction temperature, these two substrates gave corresponding products in good yields and stereoselectivities (Table 3, 5t−u). Alkenyl substituents connected to the carbonyl could affect the yields of the product. For example, Δ(1)-pyrrolines 5v and 5w were produced in yields of 53% and 32%, respectively, although their stereoselectivities remained at a high level. Third, several glycine derivatives were introduced as donors in this reaction. The halogen acid salts of amino acid esters are more stable and cheaper than the neutral form. Thus, to improve the practicability of our method, we introduced glycine ester hydrochlorides as donors. K2CO3 (1 equiv) was added to the reaction system in these experiments. Notably, the widely used glycine ester hydrochlorides, including glycine methyl, ethyl, and isopropyl ester hydrochlorides, were found to be suitable donors for this reaction, giving the corresponding products in excellent enantioselectivities (Table 4, 6a−6c). The yields were clearly affected by the alkoxyl group. We found the Michael acceptors could not be consumed completely, and the methyl glycine ester disappeared after 24 h, even when 2 equiv of this donor were added. Thus, we posit that the 9777

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to the O atom of the aldehyde becomes the main pathway. When 4-Br substituted chiral aldehyde 4b was used as a catalyst, the energy of S-TS 2 (25.3 kcal/mol) was further reduced to 23.3 kcal/mol (S-TS 2-4b). Thus, the bromine substituent greatly accelerated the reaction speed. After the formation of the Schiff base, TBD removed a hydrogen atom at the α-position to form INT1. The energy barriers (S-TS3) for the forward and reverse reaction were considerably lower, meaning that the hydrogen atoms at the α-position could easily exchange in the reaction system. We further conducted comprehensive computational calculations to elucidate the reason for the stereoselective addition and subsequent proton transfer in stage II. According to the initial computational exploration of the configuration of 2a and INT1, we considered both Re-face and Si-face attack by different arrangements, corresponding to paths A−D (Figure 2). For the attack orientation, the Si-face attack via TS1A and TS1B formed stereoselective intermediates (SR)-INT1A or (SS)-INT1B. These intermediates could undergo further proton transfer via (SR)-TS2A or (SS)-TS2B to form (SR)INT2A or (SS)-INT2B. Thus, complex (SR)-3a or (SS)-3a would be produced through a continuous process of proton transfer, hydrolysis, and cyclization. Similarly, complex (RS)-3a or (RR)-3a could be obtained through path C or D by Re-face attack. The relative Gibbs free energy barriers of TS1A, TS1B, TS1C, and TS1D were 13.9, 20.9, 21.6, and 18.8 kcal/mol, respectively. Among these TSs, hydrogen bonds between 2a and INT1 were found in TS1A and TS1D, which might explain their lower energies. Furthermore, the shape matching to reduce steric repulsion and the π−π interactions between the aryl ring 2a and binaphthyl in TS1A might also contribute to the lower energy barriers. Similarly, the energies of (SR)TS2A and (SR)-INT2 with values of 14.8 and −2.0 kcal/mol were respectively lower than those of the corresponding pathways (Figure 3). Because the stereoselectivity was determined in the first addition step and the proton transfer might be accelerated by additives, the energy barriers of TS1A−D suggested pathway A, with TS1A and (SR)-TS2A leading to a final product with an (SR)-configuration, for the more favorable transition state structures, as the calculated Boltzmann distribution of product based on the energy barriers of TS1A−D suggested that >99% products should be the (SR)-configuration. From the qualitative perspective, the RR3a should be the other main isomer because the energy barrier of 18.8 kcal/com was consistent with our experimental results that the enantioselectivity is better than the diastereoselectivity for the reaction of 1a and 2a.

chemistries of compounds 3b−3al, 5a−5w, and 6a−6p were assigned by analogy with those of 3a. The Δ(1)-pyrroline products could be readily converted into other polysubstituted amino esters. For example, compound 3a reacted with 1-benzyl-1H-pyrrole-2,5-dione 7 to give polycyclic product 8 in good yield. Compound 3a could also be converted into a linear amino ester 10 in excellent yield under mild reaction conditions. In both cases, the enantioselectivity of 3a was completely transferred to the products (Scheme 3). Scheme 3. Synthetic Application

Mechanism Investigation. Three stages are included in this reaction. Stage I is the formation of a Schiff base from the chiral aldehyde catalyst and glycine ester. Stage II is a nucleophilic addition of the formed Schiff base to the enone, during which all chiral centers are generated. The third stage is an intramolecular condensation leading to Δ(1)-pyrroline products. Control experiments indicated that this reaction could not take place and we recovered most of the starting materials if the hydroxyl of catalyst 4 was protected (4g) or removed (4h). Additionally, Bromine substitution of chiral aldehyde 4b improved the yield of 3a greatly (Scheme 4). Scheme 4. Control Experiment

These results indicated that both the hydroxyl and bromine groups were crucial for this reaction. Another important fact is that the acids and bases greatly affected the product yield; however, these had little influence on the stereoselectivities (see Supporting Information). Thus, the acid and base could be removed from the transition states of stage II. On the basis of these results, the process of stages I and II was rationalized by density functional theory (DFT) computational calculations (see Supporting Information). First, we found the acid additive could reduce the energy barriers of the transition states for both steps, the N atom attacking the carbonyl and the removal of water in stage I, resulting in more rapid formation of the Schiff base intermediate. Our investigations of the proton transfer process indicated that the hydroxyl group of the catalyst plays an important role in reducing the energy barrier for the reaction at the neighboring carbonyl via the formation of an intramolecular hydrogen bond; thus, proton transfer from the −OH

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CONCLUSION A novel type of chiral aldehyde catalyst was rationally designed and applied to the catalytic activation of glycine esters in this work. Chiral aldehyde 4d exhibited excellent catalytic activity and stereoselective control over the nucleophilic addition of glycine esters to conjugated enones. This catalytic system showed a broad substrate scope. Various polysubstituted Δ(1)pyrrolines were generated in excellent yields, distereoselectivities, and enantioselectivities. DFT calculations indicated that the chiral aldehyde 4 acted as a bifunctional catalyst and that the hydroxyl group of 4 was crucial for the proton transfer process, in both the imino ester formation and the nucleophilic addition stages. In principle, this catalytic model disclosed here could be used for other carbon nucleophilic reactions of 9778

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Figure 2. Different approaches of stereoselective addition and subsequent proton transfer from INT1 to produce the corresponding stereochemical outcomes.

Figure 3. Computed potential energy surface for the stereoselective addition between INT1 and 2a. Energies are obtained at the M06-2x/6-31+ +G(d,p) (toluene) level and are given in kcal/mol relative to reactants.

glycines. Further investigations to this end are currently underway in our lab.

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Notes

The authors declare no competing financial interest.

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ASSOCIATED CONTENT

ACKNOWLEDGMENTS We are grateful for financial support from NSFC (21472150), the Program for New Century Excellent Talents in Universities (NCET-12-0929), and the National Key Research and Development Program of China (2018YFA0507900).

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b06676.

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Copies of 1H and 13C NMR spectra (PDF) Representative experimental procedures and analytical data for all new compounds (PDF) Crystallographic data for determination of the configuration of product (S)-4b (CIF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Ying-Chun Chen: 0000-0003-1902-0979 Qin Ouyang: 0000-0002-1161-5102 Qi-Xiang Guo: 0000-0002-0405-7958 Author Contributions §

W.W. and L.C. contributed equally. 9779

DOI: 10.1021/jacs.8b06676 J. Am. Chem. Soc. 2018, 140, 9774−9780

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DOI: 10.1021/jacs.8b06676 J. Am. Chem. Soc. 2018, 140, 9774−9780