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Jan 10, 2018 - Institute of Drug Research in Medicine Capital of China, Benxi 117000, P.R. China. •S Supporting Information. ABSTRACT: A ...
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Article Cite This: J. Org. Chem. 2018, 83, 1312−1319

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Lewis Acid Assisted Electrophilic Fluorine-Catalyzed Pinacol Rearrangement of Hydrobenzoin Substrates: One-Pot Synthesis of (±)-Latifine and (±)-Cherylline Hui Shi,†,‡ Chuan Du,†,‡ Xinhang Zhang,†,‡,§ Fukai Xie,†,‡ Xiaoyu Wang,†,‡ Shanshan Cui,†,‡ Xiaoshi Peng,†,‡ Maosheng Cheng,†,‡ Bin Lin,*,†,‡ and Yongxiang Liu*,†,‡,§ †

Key Laboratory of Structure-Based Drug Design and Discovery, Ministry of Education, and §Wuya College of Innovation, Shenyang Pharmaceutical University, Shenyang 110016, P.R. China ‡ Institute of Drug Research in Medicine Capital of China, Benxi 117000, P.R. China S Supporting Information *

ABSTRACT: A microwave-irradiated solvent-free pinacol rearrangement of hydrobenzoin substrates catalyzed by a combination of N-fluorobenzenesulfonimide and FeCl3·6H2O was developed. Its selectivity was first investigated by density functional theory (DFT) calculations. Then the functional group tolerance was examined by synthesizing a series of substrates designed based on the insight provided by the DFT calculations. The application of the methodology was demonstrated by the efficient one-pot synthesis of (±)-latifine and (±)-cherylline, both are 4-aryltetrahydroisoquinoline alkaloids isolated from Amaryllidacecae plants.



INTRODUCTION The pinacol rearrangement is a classic transformation of vicinal diols leading to carbonyl compounds. The uses of pinacol rearrangement and related transformations in the syntheses of natural products and bioactive molecules were explored widely for their unique abilities in constructing quaternary carbons and novel skeletons.1 Numerous efforts on developing mild and environmentally friendly catalysts to replace hazardous stoichiometric reagents have been made by utilizing solid acid catalysts and ionic liquids.2 However, circumvention of harsh conditions and cumbersome procedures, predictable selectivity of the reaction, and utility of readily available catalysts are still in urgent need for the further development of pinacol rearrangement under more mild and robust conditions. The precise control of selectivity of hydrobenzoin, a textbook substrate in various Brønsted acid, Lewis acid, and solid acid catalyzed pinacol rearrangements has remained a great challenge and has been influenced by many factors, such as catalysts, solvents, temperatures, etc.2f,3,2f,3,4b,c The resulting mixture of ketone and aldehyde, due to the similar migratory aptitude of the phenyl group and hydride, is difficult to isolate, thus limiting the applications of this type of pinacol rearrangement in organic synthesis. We intend to address this problem by tuning the electronic nature of the phenyl groups in hydrobenzoin derivatives. Normally, the rearrangement can proceed either via a concerted mechanism without a carbocation intermediate or via a stepwise mechanism with a carbocation intermediate followed by a migration of the © 2018 American Chemical Society

functional group in the presence of strong Brønsted acids such as H2SO4 and H3PO4 or Lewis acids such as AlCl3.4 However, when the migrating group is aryl, which facilitates the formation of a carbocation, the stepwise mechanism is predominantly favored.5 We therefore hypothesize that it is possible to orientate the formation of a carbocation by synthesizing the substrates, according to the mechanism of pinacol rearrangement, with one electron-rich phenyl ring: the phenyl-migrated aldehyde product could be generated by increasing the electron density of the other phenyl ring, and the hydride-migrated ketone product could be generated by decreasing electron density of the other phenyl ring.5 The hypothesis is supported by density functional theory (DFT) calculations performed by selecting two such types of substrates as examples prior to our pinacol rearrangement experiments.6 The computational results predicted that the formation of aldehyde product for the substrate was due to the migration of the phenyl ring, and the formation of ketone product for the substrate was due to the migration of the hydride based on the calculated energy differences and the stability of the products (Scheme 1). The experiments were designed with the substrates selected by following the computational predictions above. In the end, the experimental results confirmed both our experimental design and computational predictions. Received: November 8, 2017 Published: January 10, 2018 1312

DOI: 10.1021/acs.joc.7b02587 J. Org. Chem. 2018, 83, 1312−1319

Article

The Journal of Organic Chemistry Scheme 1. DFT Calculation on the Selectivity on the Pinacol Rearrangement of Hydrobenzoin Substrates

In recent studies, we have described a series of water-tolerant Lewis acid catalysts,7 among which N-fluorobenzenesulfonimide (NFSI) was identified as a novel catalyst under the assistance of Lewis acids to catalyze multiple kinds of transformations such as Meyer−Schuster rearrangement, Baeyer−Villiger oxidation, Beckmann rearrangement, etc.7d,e The robust electrophilic fluorine catalysts, developed by our group, were derived from the combination of NFSI and a variety of Lewis acids and could shorten the reaction time, improve the yields, and decrease the temperatures of a number of Lewis acid catalyzed reactions markedly compared with those reported with Lewis acids only.7e Our precedent investigation, which revealed the Lewis acidity of the combination of NFSI and Lewis acids and the action model between NFSI and Lewis acids, prompted us to examine the catalytic ability of this novel catalyst in some reactions, where harsh conditions are required, such as pinacol rearrangement described in this study. Based on the control of selectivity of hydrobenzoin substrates suggested by DFT calculations, we aimed to develop a practical and selectivity-controllable NFSI/ FeCl3·6H2O-catalyzed pinacol rearrangement of hydrobenzoin substrates and to apply it to the efficient one-pot synthesis of (±)-latifine and (±)-cherylline, both are 4-aryltetrahydroisoquinoline alkaloids isolated from Amaryllidacecae plants. Compared with the reported pinacol rearrangement of hydrobenzoin, this method provided a more mild and robust condition for the controllable synthesis of both 2,2-diarylacetaldehydes and 2-aryl-1-arylethanones only by changing the substituents of the substrates.

Scheme 2. Substrate Scope of Pinacol Rearrangement Resulting in the Formation of 2,2-Diarylacetaldehydes

rearrangement of substrate 1a at 40 °C in 15 min under microwave irradiation in the absence of solvents with a yield of 80%. In contrast, as a control catalyst, 1 mol % of FeCl3·6H2O did not catalyze the reaction at the same temperature at all and showed no trace of reaction even at 100 °C. When 5 mol % of NFSI was used as the only catalyst in the rearrangement reaction, lower yield (20%) was obtained under the same conditions. Those results were in agreement with our previous observation in Meyer−Schuster rearrangement and Beckmann rearrangement.7e It is worth mentioning that the NFSI/FeCl3· 6H2O system is superior to the same quantity of Brønsted acid such as H2SO4, which gives a trace amount of product under the same condition. The DFT calculations were carried out after we developed the very first model reaction and before we examined the full



RESULTS AND DISCUSSION At first, a model vicinal diol substrate 1a was synthesized to test the reaction potential of the combination of 5 mol % of NFSI and 1 mol % of FeCl3·6H2O, the optimal conditions obtained in our precedent study (Scheme 2). It was found that the combination of NFSI and FeCl3·6H2O catalyzed the pinacol 1313

DOI: 10.1021/acs.joc.7b02587 J. Org. Chem. 2018, 83, 1312−1319

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The Journal of Organic Chemistry Scheme 3. Substrate Scopes of Pinacol Rearrangement To Yield 2-Aryl-1-arylethanones

reaction will be initiated by the formation of a carbocation on the electron-rich benzyl position, and the migration of hydride or electron-deficient phenyl ring will result in aldehyde or ketone products. In the DFT calculations, the end product of the hydride migration pathway was more stable than that of the electron-deficient phenyl ring migration pathway (Scheme 1). In agreement with the theoretical calculations, in all cases, the exclusiveness of hydride migration yielding 2-aryl-1-arylethanone as the only products indicated that the electronic nature of the aryl rings could control the migratory aptitude to afford different products. Lastly, the tertiary pinacol substrates were examined under this reaction conditions. The unsymmetrical tertiary pinacol substrates 1q−1s were first prepared to test the scope. It was found that the benzyl carbocation was formed prior to the migration of a proximal group to give α-trisubstituted ketones 2q−2s. The symmetrical tertiary pinacol substrates 1t−1w were then designed to examine the scopes further, and the results were similar to those of substrates 1q−1s. The symmetrical tertiary substrates 1x−1z with four of the same aromatic substitutions also gave α-trisubstituted ketone products with excellent yields (Scheme 4). Based on the current results and our previous study,7e a plausible mechanism for the diverted pathway of unsymmetrical secondary vicinal diols was proposed in Scheme 5. Activation of hydroxyl groups of the substrates 1f and 1i by electrophilic fluorine promoted the formation of carbocations 1f-2 and 1i-2 on the electron-rich benzyl position in both substrates. The different migratory aptitudes of substituted phenyl groups and hydride led to the formation of the phenyl migration product 2f and the hydride migration product 2i, respectively. Inspired by the precise control of the products obtained in the NFSI/FeCl3·6H2O-catalyzed pinacol rearrangement, we designed a practical synthetic route to the naturally occurring isoquinoline alkaloids (±)-latifine and (±)-cherylline.9 The pinacol substrates 3a and 3b were synthesized readily by a Wittig/epoxidation/ring-opening one-pot protocol utilizing substituted benzaldehydes as starting materials. The syntheses of (±)-latifine (4a) and (±)-cherylline (4b) were achieved by the NFSI/FeCl3·6H2O-catalyzed pinacol rearrangement as the

scope of the newly developed methodology. It was predicted that without any substitutions on both phenyl groups on the substrate (case a), the reaction would need to cross the barrier of 24.78 kcal/mol (relative to the starting material) if the hydride migrated and the barrier of 15.61 kcal/mol if the phenyl group migrated. When both substitutions were electrondonating methoxy groups or electron-withdrawing nitro groups on both phenyl groups (cases b and d), the barrier was predicted to be much lower when the aryl group migrated than when the hydride migrated due to the reason that aryl groups had better migratory aptitude than hydride, even with one electron-withdrawing group on the phenyl group. Since the barrier was much lower for aryl migration, that should be the path that the reaction would follow. Indeed that was observed experimentally. When one substitution was an electrondonating methoxy group and the other was an electronwithdrawing nitro group on the phenyl group (case c), hydride migration was expected to happen because the ketone product was more thermodynamically stable than the aldehyde product. Such prediction was also consistent with the experimental observation (Scheme 1). Subsequently, the scope of the transformation under the NFSI/FeCl3·6H2O system was examined by using a series of synthetic secondary vicinal diol substrates, including both unsymmetrical and symmetrical ones, based on the predictions from the prior DFT calculations. The unsymmetrical secondary vicinal diols 1a−1e were prepared using methods reported in the literature8 and subjected to the standard conditions leading to, with yields of at least 80%, corresponding 2,2-diarylacetaldehydes 2a−2e with the migration of electron-rich phenyl rings, which can better stabilize the positively charged intermediates than electron-deficient ones. The symmetrical pinacol substrates 1f−1h with electron-rich substitutions on phenyl rings were then examined to afford the 2,2-diarylacetaldehydes 2f−2h with excellent yields, as expected (Scheme 2). Next, we prepared some unsymmetrical secondary vicinal diols that bear both electron-rich and electron-deficient phenyl groups shown in Scheme 3 to examine the migratory rules further. According to the general stepwise mechanism, the 1314

DOI: 10.1021/acs.joc.7b02587 J. Org. Chem. 2018, 83, 1312−1319

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The Journal of Organic Chemistry Scheme 4. Substrate Scopes of Pinacol Rearrangement To Yield α-Trisubstituted Ketones

Scheme 6. One-Pot Synthesis of (±)-Latifine and (±)-Cherylline Using Pinacol Rearrangement as the Key Step



CONCLUSIONS A readily available and environmentally friendly NFSI/FeCl3· 6H2O system for catalyzing pinacol rearrangement under mild and neat conditions was developed. The functional group tolerance of the methodology was examined by a series of designed substrates. The influence of electronic effect on migration sequence was probed by both DFT calculations and experimental results. The synthetic applications of the methodology were demonstrated by the one-pot syntheses of two alkaloid natural products (±)-latifine and (±)-cherylline, affording a unique and efficient method for the preparation

key step followed by one-pot reductive amination/Pictet− Spengler and hydrogenation reactions with excellent yields, in which four chemical bonds including two C−C and two C−N bonds were constructed, and only one isolation step was required (Scheme 6). The one-pot synthetic method afforded a rapid access to the isoquinoline natural products and analogues. Scheme 5. Proposed Mechanism for the Formation of 2f and 2i

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DOI: 10.1021/acs.joc.7b02587 J. Org. Chem. 2018, 83, 1312−1319

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The Journal of Organic Chemistry

(s, 1H), 6.79 (d, J = 8.6 Hz, 2H), 5.38 (d, J = 5.5 Hz, 1H), 5.17 (d, J = 5.4 Hz, 1H), 4.80 (t, J = 5.2 Hz, 1H), 4.61 (t, J = 5.1 Hz, 1H), 3.74 (s, 3H), 3.72 (s, 3H), 3.70 (s, 3H); 13C NMR (150 MHz, DMSO-d6) δ 158.7, 148.7, 148.2, 135.2, 134.3, 128.2, 115.0, 113.2, 113.0, 112.3, 76.0, 75.8, 56.2, 55.9, 55.3; IR (thin film, cm−1) 3424, 3000, 2923, 1607, 1508, 1251, 1157, 1030; HRMS (ESI) m/z [M − H]− calcd for C17H18BrO5 381.0343, found 381.0335. 1-(Benzo[d][1,3]dioxol-5-yl)-2-(3,4,5-trimethoxyphenyl)ethane1,2-diol (1d): Purified by a flash column chromatography on silica gel with petroleum ether/ethyl acetate (2:1) as an eluent to give the product 1d (306 mg) as a white solid with a yield of 44%; mp 102.7− 122.5 °C; the ratio of major isomer and minor isomer is 7.7:1; 1H NMR (600 MHz, DMSO-d6) (major isomer) δ 6.85 (d, J = 1.4 Hz, 1H), 6.78 (d, J = 7.9 Hz, 1H), 6.69 (dd, J = 8.0, 1.5 Hz, 1H), 6.53 (s, 2H), 5.96 (dd, J = 4.3, 0.9 Hz, 2H), 5.20−5.08 (m, 2H), 4.50−4.42 (m, 2H), 3.70 (s, 6H), 3.63 (s, 3H); (minor isomer) δ 6.75−6.72 (m, 2H), 6.59 (dd, J = 8.0, 1.4 Hz, 1H), 6.41 (s, 2H), 5.93 (dd, J = 5.5, 0.8 Hz, 2H), 5.30 (d, J = 4.0 Hz, 1H), 5.25 (d, J = 4.0 Hz, 1H), 4.49−4.44 (m, 2H), 3.65 (s, 6H), 3.60 (s, 3H); 13C NMR (150 MHz, DMSO-d6) (major isomer) δ 152.1, 146.5, 145.9, 139.1, 137.4, 136.2, 120.7, 107.9, 107.1, 104.6, 100.5, 77.1, 76.8, 60.0, 55.7; (minor isomer) 152.0, 146.5, 145.8, 138.2, 136.8, 136.1, 120.4, 107.6, 107.2, 104.4, 100.5, 77.7, 77.4, 59.8, 55.0; IR (thin film, cm−1) 3501, 3413, 2933, 2878, 1632, 1597, 1501, 1483, 1462, 1420, 1238, 1126, 1037; HRMS (ESI) m/z [M + Na]+ calcd for C18H20NaO7 371.1107, found 371.1097. 1-(4-Methoxyphenyl)-2-(4-nitrophenyl)ethane-1,2-diol (1i): Purified by a flash column chromatography on silica gel with petroleum ether/ethyl acetate (3:1) as an eluent to give the product 1i (260 mg) as a yellow oil with a yield of 45%; the ratio of major isomer and minor isomer is 1.3:1; 1H NMR (600 MHz, DMSO-d6) (major isomer) δ 8.05 (d, J = 8.7 Hz, 2H), 7.36 (d, J = 8.7 Hz, 2H), 7.01 (d, J = 8.6 Hz, 2H), 6.74 (d, J = 8.6 Hz, 2H), 5.65 (d, J = 4.4 Hz, 1H), 5.43 (d, J = 4.4 Hz, 1H), 4.76 (t, J = 5.0 Hz, 1H), 4.62 (t, J = 5.0 Hz, 1H), 3.68 (s, 3H); (minor isomer) 8.12 (d, J = 8.7 Hz, 2H), 7.48 (d, J = 8.7 Hz, 2H), 7.15 (d, J = 8.6 Hz, 2H), 6.82 (d, J = 8.6 Hz, 2H), 5.54 (d, J = 4.8 Hz, 1H), 5.30 (d, J = 4.8 Hz, 1H), 4.68 (t, J = 5.0 Hz, 1H), 4.55 (t, J = 5.0 Hz, 1H), 3.71 (s, 3H); 13C NMR (150 MHz, DMSO-d6) (major isomer) δ 158.6, 151.1, 146.7, 134.0, 128.7, 122.7, 113.1, 76.9, 76.9, 55.3; (minor isomer) 158.6, 151.8, 146.7, 135.1, 128.9, 128.6, 122.8, 113.2, 76.8, 76.6, 55.3; IR (thin film, cm−1) 3460, 2918, 2846, 1604, 1512, 1345, 1244, 1075, 1023; HRMS (ESI) m/z [M + Na]+ calcd for C15H15NNaO5 312.0842, found 312.0845. 1-(4-Methoxyphenyl)-2-(2-nitrophenyl)ethane-1,2-diol (1j): Purified by a flash column chromatography on silica gel with petroleum ether/ethyl acetate (3:1) as an eluent to give the product 1j (243 mg) as a yellow solid with a yield of 42%; mp 96−98 °C; 1H NMR (600 MHz, DMSO-d6) δ 7.75 (dd, J = 8.1, 0.8 Hz, 1H), 7.59−7.52 (m, 2H), 7.44−7.39 (m, 1H), 7.00 (d, J = 8.6 Hz, 2H), 6.75 (d, J = 8.7 Hz, 2H), 5.66 (d, J = 5.2 Hz, 1H), 5.36 (d, J = 5.3 Hz, 1H), 5.25 (t, J = 5.0 Hz, 1H), 4.64 (t, J = 5.0 Hz, 1H), 3.69 (s, 3H); 13C NMR (150 MHz, DMSO-d6) δ 158.6, 148.8, 137.3, 134.4, 132.4, 130.3, 128.2, 128.1, 123.7, 113.2, 75.9, 72.0, 55.3; IR (thin film, cm−1) 3418, 2920, 2360, 1609, 1516, 1345, 1249, 1030; HRMS (ESI) m/z [M + Na]+ calcd for C15H15NNaO5 312.0842, found 312.0845. 1-(4-Methoxyphenyl)-2-(3-nitrophenyl)ethane-1,2-diol (1k): Purified by a flash column chromatography on silica gel with petroleum ether/ethyl acetate (2:1) as an eluent to give the product 1k (225 mg) as a yellow solid with a yield of 39%; mp 69−71 °C; the ratio of major isomer and minor isomer is 3:1; 1H NMR (600 MHz, DMSO-d6) (major isomer) δ 8.03−8.01 (m, 1H), 7.95−7.94 (m, 1H), 7.52−7.45 (m, 2H), 7.02 (d, J = 8.6 Hz, 2H), 6.74 (d, J = 8.6 Hz, 2H), 5.66 (d, J = 4.6 Hz, 1H), 5.42 (d, J = 4.6 Hz, 1H), 4.79 (t, J = 4.9 Hz, 1H), 4.63 (t, J = 4.9 Hz, 1H), 3.68 (s, 3H); (minor isomer) 8.10−8.06 (m, 2H), 7.62−7.52 (m, 2H), 7.16 (d, J = 8.6 Hz, 2H), 6.82 (d, J = 8.6 Hz, 2H), 5.56 (d, J = 4.6 Hz, 1H), 5.30 (d, J = 4.6 Hz, 1H), 4.68 (t, J = 5.6 Hz, 1H), 4.55 (t, J = 5.6 Hz, 1H), 3.72 (s, 3H); 13C NMR (150 MHz, DMSO-d6) (major isomer) δ 158.2, 147.1, 145.0, 133.9, 133.7, 128.6, 128.2, 121.7, 121.6, 112.7, 76.4, 76.2, 55.0; (minor isomer) 158.3, 147.2, 145.7, 134.8, 134.3, 128.7, 128.3, 122.0, 121.6, 112.9, 76.4, 76.2, 55.0; IR (thin film, cm−1) 3414, 3089, 3004, 2916, 2838, 1612, 1528,

of isoquinoline derivatives. The efforts for preparing bioactive small molecules by the one-pot method are currently underway in our laboratory.



EXPERIMENTAL SECTION

General Experimental Methods. Reagents were obtained commercially and used without further purification. Tetrahydrofuran was distilled from sodium under a nitrogen atmosphere. TLC analysis of reaction mixtures was performed on silica gel F-254 TLC plates. Flash chromatography was performed using silica gel (200−300 mesh). 1H and 13C NMR spectra were recorded in CDCl3, DMSO-d6, or CD3OD on a 600 MHz instrument. HR-ESIMS was carried out with electrospray ionization (ESI) using a Q-TOF mass spectrometer. IR spectra were recorded as thin film on an FT-IR spectrometer. Melting points (mp) were tested on a capillary melting point apparatus. Microwave reactions were performed using a microwave oven (CEM Discover) in sealed reaction vessels. The temperature was monitored using an internal vertically focused IR temperature sensor. General Procedures for the Synthesis of Propargylic Alcohols 1a−1e and 1i−1p. Under argon atmosphere, the butyllithium (2.5 M in hexanes, 0.54 mL, 1.3 mmol) was added to a solution of phosphonium bromide salts (1.3 mmol) in anhydrous THF at −20 °C, and the resulting red solution was stirred at this temperature for 1 h. A solution of the aldehydes (1.3 mmol) in anhydrous THF was added dropwise, and the resulting solution was allowed to warm to room temperature for 2−6 h. The resulting suspension was poured into water and extracted with ethyl acetate. The combined organic layers were washed with a saturated aqueous solution of NH4Cl and brine; after being dried over anhydrous Na2SO4, the solvent was removed in vacuo to give a crude material, which was used without further purification in the next step. The cis/trans-stilbenes (2 mmol) obtained above were dissolved in dichloromethane (20 mL), and the solution was cooled to 0 °C, then m-CPBA (863 mg, 5 mmol) was added. After being stirred at room temperature for 15 h, the mixture was extracted with dichloromethane. The combined organic layers were washed with 5% NaHCO3 solution and brine. After being dried over anhydrous Na2SO4, the solvent was removed in vacuo to give a crude material. The crude stilbene oxides were dissolved in THF (10 mL), and then a solution of sodium hydroxide (240 mg, 6 mmol) in water (5 mL) was added. After being stirred at room temperature for 3 h, the mixture was extracted with ethyl acetate. The combined organic layers were washed with a saturated aqueous solution of NH4Cl and brine. After being dried over anhydrous Na2SO4, the solvent was removed in vacuo to give a crude material, which was purified by a flash column chromatography on silica gel to afford the products 1a−1e and 1i−1p. 1-(Benzo[d][1,3]dioxol-5-yl)-2-(4-methoxyphenyl)ethane-1,2-diol (1b): Purified by a flash column chromatography on silica gel with petroleum ether/ethyl acetate (3:1) as an eluent to give the product 1b (277 mg) as a white solid with a yield of 48%; mp 105.9−107.3 °C; the ratio of major isomer and minor isomer is 5.6:1; 1H NMR (600 MHz, DMSO-d6) (major isomer) δ 7.01 (d, J = 8.7 Hz, 2H), 6.74 (d, J = 8.7 Hz, 2H), 6.75−6.73 (m, 2H), 6.49 (dd, J = 8.0, 1.5 Hz, 1H), 5.92 (dd, J = 3.5, 0.8 Hz, 2H), 5.26 (d, J = 3.8 Hz, 1H), 5.21 (d, J = 3.7 Hz, 1H), 4.48−4.44 (m, 2H), 3.68 (s, 3H); (minor isomer) δ 7.14 (d, J = 8.7 Hz, 2H), 6.82−6.80 (m, 2H), 6.74 (d, J = 8.7 Hz, 2H), 6.64 (dd, J = 8.0, 1.5 Hz, 1H), 5.95 (s, 2H), 5.11 (d, J = 4.4 Hz, 1H), 5.06 (d, J = 4.4 Hz, 1H), 4.48−4.44 (m, 2H), 3.71 (s, 3H); 13C NMR (150 MHz, DMSO-d6) (major isomer) δ 158.0, 146.4, 145.8, 136.5, 134.3, 128.3, 120.5, 112.7, 107.6, 107.2, 100.5, 77.5, 77.2, 54.9; (minor isomer) 158.1, 146.5, 145.8, 137.4, 135.3, 128.4, 120.6, 112.7, 107.8, 107.1, 100.5, 76.8, 76.5, 55.0; IR (thin film, cm−1) 3394, 3071, 3009, 2900, 2837, 1612, 1516, 1488, 1442, 1250, 1178, 1037; HRMS (ESI) m/z [M + Na]+ calcd for C16H16NaO5 311.0895, found 311.0891. 1-(4-(Benzyloxy)phenyl)-2-(3-bromo-4,5-dimethoxyphenyl)ethane-1,2-diol (1c): Purified by a flash column chromatography on silica gel with petroleum ether/ethyl acetate (2:1) as an eluent to give the product 1c (429 mg) as a yellow oil with a yield of 56%; 1H NMR (600 MHz, DMSO-d6) δ 7.16 (d, J = 8.6 Hz, 2H), 7.08 (s, 1H), 6.95 1316

DOI: 10.1021/acs.joc.7b02587 J. Org. Chem. 2018, 83, 1312−1319

Article

The Journal of Organic Chemistry 1351, 1249, 1176; HRMS (ESI) m/z [M + Na]+ calcd for C15H15NNaO5 312.0842, found 312.0845. 4-(1,2-Dihydroxy-2-(4-methoxyphenyl)ethyl)benzonitrile (1l): Purified by a flash column chromatography on silica gel with petroleum ether/ethyl acetate (2:1) as an eluent to give the product 1l (231 mg) as a white solid with a yield of 43%; mp 63−65 °C; the ratio of major isomer and minor isomer is 1.6:1; 1H NMR (600 MHz, DMSO-d6) (major isomer) δ 7.66−7.61 (m, 2H), 7.27 (d, J = 8.2 Hz, 2H), 6.99 (d, J = 8.6 Hz, 2H), 6.76−6.72 (m, 2H), 5.58 (d, J = 4.5 Hz, 1H), 5.39 (d, J = 4.3 Hz, 1H), 4.70−4.67 (m, 1H), 4.60−4.56 (m, 1H), 3.68 (s, 3H); (minor isomer) 7.71−7.67 (m, 2H), 7.39 (d, J = 8.2 Hz, 2H), 7.12 (d, J = 8.6 Hz, 2H), 6.85−6.78 (m, 2H), 5.46 (d, J = 4.8 Hz, 1H), 5.26 (d, J = 4.6 Hz, 1H), 4.64−4.60 (m, 1H), 4.55−4.49 (m, 1H), 3.71 (s, 3H); 13C NMR (150 MHz, DMSO-d6) (major isomer) δ 158.2, 148.4, 133.7, 131.2, 128.2, 128.1, 119.1, 112.7, 109.3, 76.8, 76.5, 54.9; (minor isomer) 158.2, 149.2, 134.7, 131.2, 128.4, 128.3, 119.1, 112.8, 109.3, 76.6, 76.2, 55.0; IR (thin film, cm−1) 3362, 3002, 2918, 2842, 2227, 1610, 1512, 1247, 1027; HRMS (ESI) m/z [M − H]− calcd for C16H14NO3 268.0979, found 268.0978. 1-(2,3-Dichlorophenyl)-2-(4-methoxyphenyl)ethane-1,2-diol (1m): Purified by a flash column chromatography on silica gel with petroleum ether/ethyl acetate (3:1) as an eluent to give the product 1m (318 mg) as a colorless oil with a yield of 51%; the ratio of major isomer and minor isomer is 6.3:1; 1H NMR (600 MHz, DMSO-d6) (major isomer) δ 7.48 (d, J = 7.8 Hz, 1H), 7.30 (d, J = 7.8 Hz, 1H), 7.25 (t, J = 7.8 Hz, 1H), 7.11 (d, J = 8.5 Hz, 2H), 6.80 (d, J = 8.5 Hz, 2H), 5.53 (d, J = 4.4 Hz, 1H), 5.31 (d, J = 4.4 Hz, 1H), 5.11 (t, J = 5.2 Hz, 1H), 4.62 (t, J = 5.2 Hz, 1H), 3.71 (s, 3H); (minor isomer) δ 7.58 (d, J = 7.8 Hz, 1H), 7.50 (d, J = 7.8 Hz, 1H), 7.35 (t, J = 7.8 Hz, 1H), 7.19 (d, J = 8.5 Hz, 2H), 6.82 (d, J = 8.5 Hz, 2H), 5.58 (d, J = 5.6 Hz, 1H), 5.30 (d, J = 5.6 Hz, 1H), 5.01 (t, J = 5.0 Hz, 1H), 4.65 (t, J = 5.0 Hz, 1H), 3.71 (s, 3H); 13C NMR (150 MHz, DMSO-d6) (major isomer) δ 158.2, 143.4, 134.6, 130.9, 129.3, 128.8, 128.3, 127.8, 127.5, 112.9, 74.9, 74.1, 56.0; (minor isomer) 158.3, 143.6, 134.3, 130.8, 130.1, 128.7, 128.6, 127.3, 127.4, 112.7, 75.4, 73.6, 56.0; IR (thin film, cm−1) 3411, 3000, 2955, 2933, 1612, 1513, 1250, 1177, 1035; HRMS (ESI) m/z [M + Na]+ calcd for C15H14Cl2NaO3 335.0212, found 335.0214. 1-(2-Bromo-5-fluorophenyl)-2-(4-methoxyphenyl)ethane-1,2-diol (1o): Purified by a flash column chromatography on silica gel with petroleum ether/ethyl acetate (3:1) as an eluent to give the product 1o (292 mg) as a yellow solid with a yield of 43%; mp 56−58 °C; 1H NMR (600 MHz, DMSO-d6) δ 7.53 (dt, J = 10.0, 5.0 Hz, 1H), 7.06− 6.97 (m, 4H), 6.80−6.76 (m, 2H), 5.57 (d, J = 4.8 Hz, 1H), 5.34 (d, J = 4.4 Hz, 1H), 4.97 (t, J = 4.5 Hz, 1H), 4.63 (t, J = 4.7 Hz, 1H), 3.70 (s, 3H); 13C NMR (150 MHz, DMSO-d6) δ 161.2 (d, 1JCF = 243.3 Hz), 158.3, 144.8 (d, 3JCF = 7.1 Hz), 133.9, 133.2 (d, 3JCF = 7.9 Hz), 128.6, 117.1 (d, 4JCF = 2.7 Hz), 116.2 (d, 2JCF = 23.5 Hz), 115.8 (d, 2 JCF = 22.7 Hz), 112.7, 75.2, 75.1, 55.0; IR (thin film, cm−1) 3415, 3073, 2934, 2837, 1612, 1580, 1250, 1176, 1027; HRMS (ESI) m/z [M + Na]+ calcd for C15H14BrFNaO3 363.0003, found 363.0011. 1-(2-Bromo-5-chlorophenyl)-2-(4-methoxyphenyl)ethane-1,2diol (1p): Purified by a flash column chromatography on silica gel with petroleum ether/ethyl acetate (3:1) as an eluent to give the product 1p (313 mg) as a yellow oil with a yield of 44%; 1H NMR (600 MHz, DMSO-d6) δ 7.52 (d, J = 8.5 Hz, 1H), 7.25 (d, J = 2.6 Hz, 1H), 7.21 (dd, J = 8.5, 2.7 Hz, 1H), 7.06 (d, J = 8.6 Hz, 2H), 6.79 (d, J = 8.6 Hz, 2H), 5.57 (d, J = 4.8 Hz, 1H), 5.34 (d, J = 4.2 Hz, 1H), 4.96 (t, J = 5.0 Hz, 1H), 4.62 (d, J = 4.2 Hz, 1H), 3.71 (s, 3H); 13C NMR (150 MHz, DMSO-d6) δ 158.7, 144.7, 134.4, 133.6, 132.3, 129.7, 128.9, 128.8, 121.5, 113.1, 75.7, 75.5, 55.4; IR (thin film, cm−1) 3369, 3003, 2918, 2842, 1644, 1512, 1459, 1247, 1026; HRMS (ESI) m/z [M − H]− calcd for C15H13BrClO3 354.9742, found 354.9738. General Procedures for the Synthesis of Propargylic Alcohols 1f−1g. A suspension of benzaldehydes (1.89 mmol) and magnesium turnings (1.0 g, 42 mmol) in ammonium chloride aqueous solution (0.1 M, 10 mL) was stirred overnight under an atmosphere of air at room temperature. The reaction was quenched with dilute HCl solution and extracted with ethyl acetate three times. The combined organic layers were washed with a saturated aqueous solution of

NaHCO3 and brine. After being dried over anhydrous Na2SO4, the solvent was removed in vacuo, and the residue was purified by a flash column chromatography on silica gel to afford the products 1f−1g. General Procedures for the Synthesis of Propargylic Alcohol 1h. The solution of piperonyl aldehyde (1.5 g, 10 mmol) and magnesium turnings (1.2 g, 50 mmol) in methanol was added in a flask, which was fitted with a reflux condenser. One seed iodine was added while the solution was stirred vigorously at room temperature. A vigorous reaction occurred after a short time. After the reaction was complete (monitored by TLC), the mixture was quenched with 2 M acetic acid and extracted with ethyl acetate three times. The combined organic layers were washed with a saturated aqueous solution of NaHCO3 and brine. After being dried over anhydrous Na2SO4, the solvent was removed in vacuo, and the residue was purified by a flash column chromatography on silica gel to afford the product 1h. General Procedures for the Synthesis of Propargylic Alcohols 1t−1w. The solution of aryl ketones (10 mmol), aluminum powder (0.81 g, 30 mmol), and KOH (5.0 g, 90 mmol) in methanol (10 mL) was added in a flask, which was fitted with a reflux condenser. The mixture was stirred at room temperature for 3 h. After the reaction was complete (monitored by TLC), the reaction mixture was filtered to remove unreacted aluminum powder and water was added to the filtrate. The filtrate was extracted with dichloromethane three times, and the combined organic layers were washed with brine. After being dried over anhydrous Na2SO4, the solvent was removed in vacuo, and the residue was purified by a flash column chromatography on silica gel to afford the products 1t−1w. General Procedures for the Synthesis of Propargylic Alcohols 1x−1z. A mixture of diaryl ketones (10 mmol), Zn powder (5.2 g, 80 mmol), ZnCl2 (1.1 g, 8 mmol), and 50% aqueous solution of THF (10 mL) was added in a flask and stirred at room temperature for 3 h. After the reaction was complete (monitored by TLC), the reaction mixture was combined with 3 N HCl (5 mL) and filtered to remove the Zn powder. The filtrate was extracted with ethyl acetate three times, and the combined organic layers were washed with brine. After being dried over anhydrous Na2SO4, the solvent was removed in vacuo to give a crude material, which was purified by a flash column chromatography on silica gel to afford the products 1x−1z. General Procedures for the Synthesis of Propargylic Alcohols 1q−1s. To a 100 mL round-bottomed flask equipped with a dropping funnel was added magnesium turnings (0.6 g, 25 mmol) in dry DMF (25 mL). After the flask was cooled in an ice−salt bath, a mixture of aromatic ketone (5 mmol), acetone (2.9 g, 50 mmol), and trimethylchlorosilane (2.7 g, 25 mmol) in DMF (20 mL) was added under magnetic stirring over 30 min. The reaction mixture was allowed to stir at room temperature overnight and was extracted with ethyl acetate. After evaporation of the solvent under reduced pressure, the crude material was dissolved into methanol with stirring. A 5% aqueous solution of sulfuric acid was carefully added into the methanol solution, and the reaction mixture was stirred for 6 h at room temperature. After neutralization, the reaction mixture was extracted with ethyl acetate. The combined organic layers were washed with a saturated aqueous solution of NaHCO3 and brine. The combined extracts were dried over anhydrous Na2SO4, the solvent was removed in vacuo, and the residue was purified by a flash column chromatography on silica gel to afford the products 1q−1s. General Procedures for the Synthesis of Aldehydes and Ketones 2a−2z. A mixture of FeCl3·6H2O (0.3 mg, 1 μmol), NFSI (1.6 mg, 5 μmol), and diols 1a−1z (0.1 mmol) was stirred for 5 min. Then the mixture was irradiated under microwaves for 15 min until the starting material disappeared (monitored by TLC). The crude mixture was purified by a flash column chromatography on silica gel to afford the products 2a−2z. 2-(Benzo[d][1,3]dioxol-5-yl)-2-(4-methoxyphenyl)acetaldehyde (2b): Purified by a flash column chromatography on silica gel with petroleum ether/ethyl acetate (20:1) as an eluent to give the product 2b (24 mg) as a colorless oil with a yield of 88%; 1H NMR (600 MHz, CDCl3) δ 9.87 (d, J = 2.4 Hz, 1H), 7.15−7.10 (m, 2H), 6.94−6.89 (m, 2H), 6.80 (d, J = 7.9 Hz, 1H), 6.69−6.64 (m, 2H), 5.95 (s, 2H), 4.75 (d, J = 2.4 Hz, 1H), 3.81 (s, 3H); 13C NMR (150 MHz, CDCl3) δ 1317

DOI: 10.1021/acs.joc.7b02587 J. Org. Chem. 2018, 83, 1312−1319

Article

The Journal of Organic Chemistry

give the crude product 3a-3 or 3b-3 to be used in the next step without further purification. A mixture of the amine 3a-3 or 3b-3 (1.5 mmol), formaldehyde (69 mg, 2.3 mmol), and acetic acid (8 mL) was stirred at 90 °C under a nitrogen atmosphere for 4 h. After completion of the reaction (monitored by TLC), it was cooled to room temperature. The reaction mixture was basified with a saturated aqueous solution of NaHCO3. This basified solution was extracted by ethyl acetate. The combined organic layers were washed with a saturated aqueous solution of NH4Cl and brine. After being dried over anhydrous Na2SO4, the solvent was removed in vacuo to give a crude product 3a-4 or 3b-4 to be used in the next step without further purification. To a solution of the crude product 3a-4 or 3b-4 (1.5 mmol) obtained above in EtOH (5 mL) was added 10% Pd/C (14.5 mg) at room temperature, and the mixture was stirred under a hydrogen atmosphere at the same temperature for 6 h. After the reaction completed, the insoluble material was removed by filtration. The filtrate was concentrated and purified by a flash column chromatography on silica gel with CHCl3/MeOH (10:1, v/v) as eluent to give the product 4a or 4b. The spectroscopic data are identical with those reported.9a,b 4-(4-Hydroxyphenyl)-6-methoxy-2-methyl-1,2,3,4-tetrahydroisoquinolin-5-ol, Acetate Salt (4a): Purified by a flash column chromatography on silica gel with dichloromethane/methyl alcohol (10:1) as an eluent to give the product 4a (256 mg) as a colorless oil with a yield of 60%; 1H NMR (600 MHz, CD3OD) δ 6.90 (t, J = 8.1 Hz, 3H), 6.68 (dd, J = 11.8, 8.5 Hz, 3H), 4.43 (t, J = 6.2 Hz, 1H), 4.07 (d, J = 14.5 Hz, 1H), 3.90 (d, J = 14.6 Hz, 1H), 3.82 (s, 3H), 3.36 (dd, J = 12.1, 6.2 Hz, 1H), 3.08 (dd, J = 12.1, 6.3 Hz, 1H), 2.65 (s, 3H), 1.92 (s, 3H); 13C NMR (150 MHz, CD3OD) δ 177.4, 157.0, 148.3, 145.7, 135.1, 129.9, 125.3, 122.7, 117.9, 116.1, 111.6, 61.6, 57.5, 56.5, 44.7, 39.8, 22.2. 4-(4-Hydroxyphenyl)-6-methoxy-2-methyl-1,2,3,4-tetrahydroisoquinolin-7-ol, Acetate Salt (4b): Purified by a flash column chromatography on silica gel with dichloromethane/methyl alcohol (10:1) as an eluent to give the product 4b (258 mg) as a colorless oil with a yield of 61%; 1H NMR (600 MHz, CD3OD) δ 7.02 (d, J = 8.3 Hz, 2H), 6.76 (d, J = 8.3 Hz, 2H), 6.59 (s, 1H), 6.35 (s, 1H), 4.27 (dd, J = 10.5, 6.0 Hz, 1H), 4.00 (d, J = 14.6 Hz, 1H), 3.87 (d, J = 14.6 Hz, 1H), 3.60 (s, 3H), 3.34 (d, J = 6.0 Hz, 1H), 2.83 (t, J = 11.4 Hz, 1H), 2.67 (s, 3H), 1.92 (s, 2H); 13C NMR (150 MHz, CD3OD) δ 178.5, 157.7, 148.6, 146.7, 134.5, 131.1, 128.5, 125.3, 116.5, 113.5, 112.8, 61.5, 57.5, 56.2, 44.6, 44.4, 22.8. Computational Methods. All of the DFT calculations conducted in this study were carried out using the Gaussian 09 series of programs. DFT method B3LYP with a standard 6-31G (d) basis set was used for the geometry optimizations. The local minima and transition state were confirmed by frequency analysis with zero negative frequency and one negative frequency, respectively.

198.7, 159.3, 148.4, 147.3, 130.5, 130.4, 128.4, 122.5, 114.7, 109.6, 108.8, 101.4, 63.0, 55.5; IR (thin film, cm−1) 3423, 2927, 2837, 2721, 1723, 1649, 1632, 1609, 1511, 1487, 1440, 1253; HRMS (ESI) m/z [M + Na]+ calcd for C16H14NaO4 293.0790, found 293.0787. 2-(3-Bromo-4,5-dimethoxyphenyl)-2-(4-methoxyphenyl)acetaldehyde (2c): Purified by a flash column chromatography on silica gel with petroleum ether/ethyl acetate (30:1) as an eluent to give the product 2c (34 mg) as a colorless oil with a yield of 94%; 1H NMR (600 MHz, CDCl3) δ 9.94 (s, 1H), 7.27 (s, 1H), 7.15 (d, J = 8.7 Hz, 2H), 7.10 (s, 1H), 6.93 (d, J = 8.7 Hz, 2H), 6.61 (s, 1H), 5.34 (s, 1H); 13 C NMR (150 MHz, CDCl3) δ 198.2, 159.1, 148.9, 148.5, 130.4, 128.3, 127.1, 115.8, 115.4, 114.4, 113.1, 62.1, 56.1, 56.0, 55.2; IR (thin film, cm−1) 3447, 2921, 2850, 1646, 1507, 1458, 1383, 1260; HRMS (ESI) m/z [M + Na]+ calcd for C17H17BrNaO4 387.0202, found 387.0203. 2-(Benzo[d][1,3]dioxol-5-yl)-2-(3,4,5-trimethoxyphenyl)acetaldehyde (2d): Purified by a flash column chromatography on silica gel with petroleum ether/ethyl acetate (15:1) as an eluent to give the product 2d (29 mg) as a colorless oil with a yield of 81%; 1H NMR (600 MHz, CDCl3) δ 9.87 (d, J = 2.5 Hz, 1H), 6.82 (d, J = 7.9 Hz, 1H), 6.71−6.66 (m, 2H), 6.40 (s, 2H), 5.97 (s, 2H), 4.73 (d, J = 2.5 Hz, 1H), 3.84 (s, 3H), 3.82 (s, 6H); 13C NMR (150 MHz, CDCl3) δ 198.3, 153.9, 148.4, 147.4, 137.7, 131.9, 129.9, 122.6, 109.6, 108.9, 106.4, 101.5, 63.8, 61.1, 56.4; IR (thin film, cm−1) 3432, 2937, 2838, 2724, 1723, 1651, 1632, 1609, 1589, 1488, 1419, 1245; HRMS (ESI) m/z [M + Na]+ calcd for C18H18NaO6 353.1001, found 353.0997. 2-(4-Methoxyphenyl)-2-(3,4,5-trimethoxyphenyl)acetaldehyde (2e): Purified by a flash column chromatography on silica gel with petroleum ether/ethyl acetate (10:1) as an eluent to give the product 2e (30 mg) as a colorless oil with a yield of 96%; 1H NMR (600 MHz, DMSO-d6) δ 9.90 (d, J = 2.0 Hz, 1H), 7.24−7.19 (m, 1H), 6.96−6.90 (m, 1H), 6.59 (d, J = 11.1 Hz, 1H), 4.94 (d, J = 1.8 Hz, 1H), 3.73 (s, 6H), 3.73 (s, 3H), 3.63 (s, 3H); 13C NMR (150 MHz, DMSO-d6) δ 200.3, 158.8, 153.5, 137.0, 133.5, 130.5, 129.5, 114.5, 106.8, 62.6, 60.4, 56.3, 55.4; IR (thin film, cm−1) 3448, 2935, 2839, 1725, 1648, 1586, 1508, 1460, 1414, 1254; HRMS (ESI) m/z [M − H]− calcd for C18H19O5 315.1238, found 315.1241. 2,2-Bis(3,4,5-trimethoxyphenyl)acetaldehyde (2g): Purified by a flash column chromatography on silica gel with petroleum ether/ethyl acetate (20:1) as an eluent to give the product 2g (36 mg) as a colorless oil with a yield of 95%; 1H NMR (600 MHz, DMSO-d6) δ 9.93 (d, J = 2.1 Hz, 1H), 6.65 (s, 4H), 4.89 (d, J = 2.0 Hz, 1H), 3.75 (s, 12H), 3.63 (s, 6H); 13C NMR (150 MHz, DMSO-d6) δ 200.3, 153.4, 137.1, 133.3, 106.8, 63.5, 60.4, 56.4; IR (thin film, cm−1) 3425, 2920, 2850, 1723, 1590, 1506, 1461, 1417; HRMS (ESI) m/z [M − H]− calcd for C20H23O7 375.1449, found 375.1447. 1-(2-Bromo-5-chlorophenyl)-2-(4-methoxyphenyl)ethanone (2p): Purified by a flash column chromatography on silica gel with petroleum ether/ethyl acetate (20:1) as an eluent to give the product 2p (21 mg) as a colorless oil with a yield of 63%; 1H NMR (600 MHz, CDCl3) δ 7.54−7.51 (m, 1H), 7.28−7.23 (m, 2H), 7.17−7.13 (m, 2H), 6.89−6.85 (m, 2H), 4.16 (s, 2H), 3.80 (s, 3H); 13C NMR (150 MHz, CDCl3) δ 200.6, 158.8, 142.7, 134.6, 133.6, 130.7, 128.4, 124.8, 116.4, 114.1, 55.2, 48.4; IR (thin film, cm−1) 3447, 3066, 3002, 2957, 2930, 2837, 1701, 1663, 1607, 1512, 1458, 1250, 1175; HRMS (ESI) m/z [M − H]− calcd for C15H11BrClO2 336.9636, found 336.9639. Procedures for the One-Pot Synthesis of (±)-Latifine and (±)-Cherylline. The aldehydes 3a-1 and 3b-1 were prepared according to the general procedures for the synthesis of aldehydes and ketones 2a−2z. To a solution of the acetaldehyde 3a-1 or 3b-1 (1.5 mmol) in a mixture of anhydrous DCM and MeOH (20 mL, 1:1, v/v) was added a solution of methylamine in THF (2 M, 3.8 mL, 7.5 mmol) at 0 °C, and the resulting transparent solution was stirred under a nitrogen atmosphere for 2 h at room temperature. Then NaBH4 (113 mg, 3 mmol) was added, and the mixture was stirred at room temperature for 2 h. After the reaction was complete (monitored by TLC), the mixture was diluted with DCM. The combined organic layers were washed with saturated aqueous NH4Cl solution and brine. After being dried over anhydrous Na2SO4, the solvent was removed in vacuo to



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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.7b02587. 1



H and 13C NMR spectra for all compounds and computational details (PDF)

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Corresponding Authors

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Yongxiang Liu: 0000-0003-0364-0137 Notes

The authors declare no competing financial interest. 1318

DOI: 10.1021/acs.joc.7b02587 J. Org. Chem. 2018, 83, 1312−1319

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The Journal of Organic Chemistry



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ACKNOWLEDGMENTS We are grateful to the Natural Science Foundation of Liaoning Province of China (No. 20170540855) and the foundation of Liaoning Province Education Administration of China (No. 2017LQN08) for financial support. We acknowledge the program for innovative research team of the Ministry of Education and the program for Liaoning innovative research team in university.



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DOI: 10.1021/acs.joc.7b02587 J. Org. Chem. 2018, 83, 1312−1319