Short Synthesis of the Monoterpene Indole Alkaloid (±)-Arbornamine

Note Cite This: J. Org. Chem. 2018, 83, 4867−4870

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Short Synthesis of the Monoterpene Indole Alkaloid (±)-Arbornamine Yu Zheng,†,‡ Bei-Bei Yue,†,‡ Kun Wei,† and Yu-Rong Yang*,† †

State Key Laboratory of Phytochemistry and Plant Resources in West China, Kunming Institute of Botany, Chinese Academy of Sciences, Kunming 650201, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: The first total synthesis of the monoterpene indole alkaloid (±)-arbornamine (1) has been completed, which proceeds in only 6 steps and 31% overall yield from three readily available, known compounds. The synthesis features a cascade involving a Pictet−Spengler cyclization/intramolecular ammonolysis to create the tetracyclic core of arbornamine (1) in a single chemical operation. The subsequent elaboration of 5 into 1 was effected by a key reductive Heck reaction and global reduction.

A

might be obtained during the above one-pot reaction if the free tryptamine was used.6 Our synthesis commenced with the key cascade cyclization. As shown in Scheme 2, tetracyclic δ-lactam 6 could be obtained in 73% isolated yield by heating the benzyl tryptamine 7 with dimethyl ester 8 and 2 equiv of TFA in refluxing toluene. In order to remove the benzyl protecting group from the Nbnitrogen atom and convert the δ-lactam 10 into the vinyl iodide present in 11, δ-lactam 6 was first subjected to hydrogenolysis conditions, at atmospheric pressure using Pearlman’s catalyst and then subsequently alkylated with readily available (Z)-1bromo-2-iodobut-2-ene7 providing vinyl iodide 11. Treatment of vinyl iodide 11 with a standard selenenylation−elimination protocol afforded a conjugated tetracyclic δ-lactam 5. With δlactam 5 in hand, the stage was set for the closure of the final ring. To this end, a reductive Heck cyclization mediated by Ni(cod)2 was utilized,4 and the desired pentacyclic product 4 was obtained in 91% yield. Lastly, pentacyclic δ-lactam 4 was reduced globally with lithium aluminum hydride to give arbornamine (1). During this process both δ-lactam and methyl ester have been reduced. The highly facial selectivity of δ-lactam reduction probably was attributed to the shielding effect to the top-side exerted by the C-3 methyl ester, leading to the bottom-side approach by the hydride. Our NMR data of the synthetic sample are in agreement with those in the literature. In conclusion, we have developed a concise first route for the total synthesis of arbornamine (1), a newly isolated monoterpene indole alkaloid. The synthesis was accomplished in only 6 steps and 31% overall yield. Each step is productive in building molecular complexity except the step for the cleavage of the N-benzyl protecting group, which is strategically employed in order to avoid the formation of the undesired

rbornamine (1) is a monoterpene indole alkaloid that was isolated in 2016 by Kam and co-workers from a Malayan Kopsia arborea.1 We were first attracted to arbornamine (1) by its unique 6/5/6/5/6 “arbornane” skeleton, which is distinct from those of the eburnane and tacaman classes (Figure 1).

Figure 1. Monoterpene indole alkaloids.

More specifically, arbornamine (1) has a quaternary center at C-3 and a pyrrolidine ring instead of a piperidine ring. Unlike the eburnane and tacaman classes, whose isolation and synthesis have been extensively investigated,2 the pentacyclic structure of arbornamine (1) is found in no other indole alkaloids and no synthesis has yet been reported. In this paper, we outline the development of an efficient strategy for assembling the ring system of arbornamine (1), which has culminated in the first total synthesis of this unique alkaloid.3 As outlined in Scheme 1, our retrosynthetic analysis of arbornamine (1) calls for a one-pot reaction to produce the aminal moiety and hydroxymethine group by a global reduction of pentacyclic lactam 4. Pentacyclic lactam 4 was envisioned to arise from a reductive Heck cyclization of the vinyl iodide 5.4 The unsaturated tetracyclic δ-lactam 5 could in turn be derived from tetracyclic δ-lactam 6, which would arise via a Pictet− Spengler cyclization/intramolecular ammonolysis between tryptamine derivative 75 and the dimethyl ester 8 of 2ketoglutaric acid.6 Tryptamine 7 contains a benzyl group at the Nb-nitrogen atom as a protecting group. This design is strategical because an undesired regioisomeric γ-lactam 9 © 2018 American Chemical Society

Received: February 27, 2018 Published: March 28, 2018 4867

DOI: 10.1021/acs.joc.8b00529 J. Org. Chem. 2018, 83, 4867−4870

Note

The Journal of Organic Chemistry Scheme 1. Retrosynthetic Analysis of Arbornamine

reference to residual solvent signals [1H NMR, CDCl3 (7.26); 13C NMR, CDCl3 (77.0)]. Peak multiplicities were recorded as follows: s = singlet, d = doublet, t = triplet, m = multiplet or unresolved, brs = broad singlet. Infrared (IR) spectra were recorded on a Bruker Tensor27 Fourier transform infrared spectrometer with KBr pellets. High resolution mass spectral (HRMS) data were obtained at the mass spectrometry service operated at a Shimadzu UPLC-IT-TOF spectrometer for electrospray ionization (ESI), and mass-to-charge ratios (m/z) were reported. Melting points were measured on a WRX5A melting point apparatus. (±)-Methyl (R)-3-Benzyl-6-oxo-2,3,5,6-tetrahydro-1H-indolo[3,2,1-de][1,5]naphthyridine-3a(4H)-carboxylate (6). To a solution of compound 7 (2.50 g, 10 mmol, 1.0 equiv) in toluene (20 mL) at rt were added dimethyl 2-oxoglutarate (1.60 mL, 11 mmol, 1.1 equiv) and trifluoroacetic acid (1.49 mL, 20 mmol, 2.0 equiv) sequentially. After stirring for 24 h at 120 °C under reflux, the reaction mixture was cooled to rt, quenched by the addition of saturated aqueous sodium bicarbonate (20 mL), and extracted with ethyl acetate (3 × 20 mL). The combined organic extracts were washed with brine (10 mL), dried over anhydrous sodium sulfate, and concentrated in vacuo. The resulting crude residue was purified by column chromatography (petroleum ether/ethyl acetate = 10:1) to give compound 6 (2.73 g, 73%) as a white solid: mp 147−151 °C; Rf = 0.47 (petroleum ether/ ethyl acetate = 4:1); IR (KBr) ν 3418, 2950, 2843, 1718, 1646, 1455, 1381, 1360, 1316, 1243, 1166, 974, 808, 754, 739 cm−1; 1H NMR (400 MHz, CDCl3) δ 8.43 (d, J = 8.0 Hz, 1H), 7.45 (d, J = 7.6 Hz, 1H), 7.40−7.24 (m, 7H), 4.37 (d, J = 14.5 Hz, 1H), 3.77 (s, 3H), 3.42 (d, J = 14.6 Hz, 1H), 3.04 (dd, J = 12.2, 6.5 Hz, 1H), 2.94−2.81 (m, 3H), 2.80−2.76 (m, 1H), 2.75−2.70 (m, 1H), 2.66 (dd, J = 16.1, 4.7 Hz, 1H), 2.17−2.08 (m, 1H); 13C NMR (100 MHz, CDCl3) δ 170.8 (s), 167.6 (s), 139.6 (s), 135.1 (s), 132.8 (s), 129.2 (s), 128.4 (d), 128.1 (d), 127.0 (d), 124.9 (d), 124.0 (d), 118.4 (d), 116.3 (d), 114.8 (s), 62.6 (s), 53.7 (t), 52.1 (q), 45.0 (t), 31.3 (t), 30.3 (t), 21.0 (t); HRMS (ESI) m/z [M + Na]+ calcd for C23H22N2O3Na 397.1523, found 397.1520. (±)-Methyl (R)-6-Oxo-2,3,5,6-tetrahydro-1H-indolo[3,2,1-de][1,5]naphthyridine-3a(4H)-carboxylate (10). To a solution of compound 6 (1.20 g, 3.2 mmol, 1.0 equiv) in methanol (24 mL) and ethyl acetate (6 mL) was added Pd(OH)2 (0.45 g, 0.32 mmol, 0.1 equiv, palladium hydroxide 20% on carbon, wetted with ca. 50% water) at rt. The system was then evacuated and refilled with H2 three times. After stirring for 20 h at rt under atmospheric pressure of H2, the reaction was filtered through a short pad of silica gel with dichloromethane/methanol (10:1) and concentrated in vacuo. The resulting crude residue was purified by silica gel column chromatography (dichloromethane/methanol = 10:1) to give compound 10 (0.857 g, 94% yield) as a white solid: mp 118−123 °C; Rf = 0.71 (dichloromethane/methanol = 10:1); IR (KBr) ν 3435, 2954, 2923, 2842, 1730, 1704, 1641, 1442, 1379, 1317, 1244, 1193, 1171, 1086, 962, 810, 753 cm−1; 1H NMR (400 MHz, CDCl3) δ 8.41 (d, J = 8.0

Scheme 2. Total Synthesis of Arbornamine

regioisomer 9. Further studies toward the enantioselective total synthesis of (−)-arbornamine (1) using an asymmetric Pictet− Spengler cyclization8 are currently ongoing in our laboratory.

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EXPERIMENTAL SECTION

General Information. Unless otherwise stated, all oxygen or moisture sensitive reactions were conducted in flame-dried glassware under an atmosphere of nitrogen. All solvents were purified and dried according to standard methods prior to use. The compound 75 and (Z)-1-bromo-2-iodobut-2-ene7 were prepared according to the reported procedure. Reagents were purchased from commercial sources and were used without further purification. Chromatographic purification of products was accomplished using forced-flow chromatography on 200−300 mesh silica gel. The TLC glass plates were performed on 0.20 mm or 1.0 mm (preparative) silica gel GF254 plates and visualized with UV light (254 nm) or exposure to bismuth potassium iodide. 1H and 13C NMR spectra were acquired on a Bruker Avance III-400 spectrometer, and TMS was used as an internal standard. Chemical shifts were given in parts per million (ppm) with 4868

DOI: 10.1021/acs.joc.8b00529 J. Org. Chem. 2018, 83, 4867−4870

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The Journal of Organic Chemistry

31(2H)-carboxylate (4). Ni(cod)2 (550 mg, 2.0 mmol, 4.0 equiv) was dissolved in anhydrous MeCN (20 mL). (Caution! Ni(cod)2 was taken in a glovebox.) Compound 5 (230 mg, 0.5 mmol, 1.0 equiv) and triethyl amine (208 μL, 1.5 mmol, 3.0 equiv) were dissolved in anhydrous MeCN (12 mL), and the mixture was added to the Ni(cod)2 solution. The resulting solution was stirred at rt for 30 min before adding triethyl silane (159 μL, 1.0 mmol, 2.0 equiv). After stirring for an additional 2 h at rt, the reaction was quenched by the addition of saturated aqueous sodium carbonate (30 mL) and extracted with dichloromethane (3 × 20 mL). The combined organic extracts were washed with brine (10 mL), dried over anhydrous sodium sulfate, and concentrated in vacuo. The resulting crude residue was purified by column chromatography (petroleum ether/ethyl acetate = 1:1) to give compound 4 (152 mg, 91% yield) as a colorless oil: Rf = 0.23 (petroleum ether/ethyl acetate = 1:1); IR (KBr) ν 3433, 2949, 2937, 2850, 1734, 1703, 1628, 1453, 1376, 1321, 1254, 1204, 1151, 1122, 1042, 754 cm−1; 1H NMR (400 MHz, CDCl3) δ 8.38 (d, J = 8.0 Hz, 1H), 7.48 (d, J = 7.6 Hz, 1H), 7.38 (t, J = 7.3 Hz, 1H), 7.31 (t, J = 7.3 Hz, 1H), 5.43−5.28 (m, 1H), 3.81 (s, 3H), 3.60−3.47 (m, 3H), 3.37−3.20 (m, 3H), 3.11 (dd, J = 12.5, 2.1 Hz, 1H), 2.89 (ddd, J = 17.8, 10.5, 7.6 Hz, 1H), 2.61 (dd, J = 17.4, 7.0 Hz, 1H), 1.66 (d, J = 6.0 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ 171.2 (s), 166.8 (s), 135.7 (s), 135.5 (s), 130.1 (s), 129.5 (s), 125.2 (d), 124.1 (d), 119.9 (d), 118.6 (d), 116.6 (d), 114.5 (s), 66.8 (s), 54.1 (t), 53.1 (q), 44.4 (d), 41.7 (t), 34.1 (t), 15.8 (t), 13.8 (q); HRMS (ESI) m/z [M + H]+ calcd for C20H21N2O3 337.1547, found 337.1547. (±)-Arbornamine (1). To a solution of compound 4 (84 mg, 0.25 mmol, 1.0 equiv) in THF (2.5 mL) at 0 °C was added LiAlH4 (38 mg, 1.0 mmol, 4.0 equiv). After stirring at 0 °C for 10 min, the reaction was quenched by the cautious addition of water (5 mL) and was extracted with dichloromethane (3 × 10 mL). The combined organic extracts were washed with brine (5 mL), dried over anhydrous sodium sulfate, and concentrated in vacuo. The resulting crude residue was purified by column chromatography (dichloromethane/methanol = 10:1) to give compound 1 (71 mg, 92% yield) as a white solid: mp 163−166 °C; Rf = 0.33 (dichloromethane/methanol = 10:1); IR (KBr) ν 3411, 3051, 2932, 2855, 1620, 1458, 1324, 1304, 1276, 1205, 1065, 1019, 745 cm−1; 1H NMR (400 MHz, CDCl3) δ 7.50 (d, J = 8.1 Hz, 1H), 7.43 (d, J = 8.1 Hz, 1H), 7.21 (t, J = 7.8 Hz, 1H), 7.14 (t, J = 7.8 Hz, 1H), 5.82 (t, J = 2.8 Hz, 1H), 5.34 (q, J = 6.2 Hz, 1H), 4.07 (d, J = 10.8 Hz, 1H), 3.73 (d, J = 10.8 Hz, 1H), 3.50 (t, J = 8.9 Hz, 1H), 3.41−3.38 (m, 1H), 3.36 (d, J = 10.8 Hz, 1H), 3.31 (d, J = 10.8 Hz, 1H), 3.18 (dd, J = 14.7, 6.1 Hz, 1H), 2.86 (ddd, J = 15.6, 11.1, 6.2 Hz, 1H), 2.66 (ddd, J = 13.5, 8.1, 3.2 Hz, 1H), 2.61 (dd, J = 15.7, 6.1 Hz, 1H), 1.63 (d, J = 6.3 Hz, 3H), 1.05 (ddd, J = 13.5, 10.5, 2.7 Hz, 1H); 13 C NMR (100 MHz, CDCl3) δ 140.8 (s), 137.8 (s), 134.3 (s), 128.2 (s), 122.0 (d), 120.3 (d), 118.8 (d), 116.3 (d), 109.9 (d), 108.0 (s), 76.1 (d), 67.0 (t), 63.8 (s), 54.2 (t), 41.2 (t), 36.9 (t), 36.0 (d), 16.6 (t), 14.0 (q); HRMS (ESI) m/z [M + H]+ calcd for C19H23N2O2 311.1754, found 311.1756.

Hz, 1H), 7.46 (d, J = 7.5 Hz, 1H), 7.36 (t, J = 7.2 Hz, 1H), 7.30 (t, J = 7.4 Hz, 1H), 3.76 (s, 3H), 3.38 (dd, J = 12.5, 6.2 Hz, 1H), 3.03 (td, J = 11.5, 5.1 Hz, 1H), 2.87−2.67 (m, 4H), 2.37 (ddd, J = 13.1, 4.9, 2.3 Hz, 1H), 2.12 (td, J = 13.1, 6.0 Hz, 1H); 13C NMR (100 MHz, CDCl3) δ 174.4 (s), 167.6 (s), 135.2 (s), 132.4 (s), 129.2 (s), 125.0 (d), 124.0 (d), 118.5 (d), 116.3 (d), 114.0 (s), 59.5 (s), 52.9 (q), 41.4 (t), 32.7 (t), 31.4 (t), 21.1 (t); HRMS (ESI) m/z [M + H]+ calcd for C16H17N2O3 285.1234, found 285.1240. (±)-Methyl (R,Z)-3-(2-Iodobut-2-en-1-yl)-6-oxo-2,3,5,6-tetrahydro-1H-indolo[3,2,1-de][1,5]naphthyridine-3a(4H)-carboxylate (11). To a solution of compound 10 (0.85 g, 3.0 mmol, 1.0 equiv) in THF (15 mL) and DMF (15 mL) at rt were added 4 Å molecular sieves (0.85 g), (Z)-1-bromo-2-iodobut-2-ene (1.18 g, 4.5 mmol, 1.5 equiv), and Cs2CO3 (1.47 g, 4.5 mmol, 1.5 equiv). After stirring for 24 h at rt, the reaction was quenched by the addition of water (30 mL) and extracted with ethyl acetate (3 × 30 mL). The combined organic extracts were washed with water (3 × 10 mL) and brine (10 mL), dried over anhydrous sodium sulfate, and concentrated in vacuo. The resulting crude residue was purified by column chromatography (petroleum ether/ethyl acetate = 10:1 to 6:1) to give compound 11 (1.19 g, 86% yield) as a pale yellow solid: mp 177−179 °C; Rf = 0.45 (petroleum ether/ethyl acetate = 4:1); IR (KBr) ν 3437, 2956, 2947, 2929, 2835, 1731, 1643, 1456, 1429, 1382, 1363, 1320, 1233, 1175, 1083, 972, 750 cm−1; 1H NMR (400 MHz, CDCl3) δ 8.41 (d, J = 7.9 Hz, 1H), 7.46 (d, J = 7.5 Hz, 1H), 7.35 (dt, J = 7.3, 3.8 Hz, 1H), 7.30 (dt, J = 7.3, 3.8 Hz, 1H), 5.97 (q, J = 6.2 Hz, 1H), 3.93 (d, J = 14.8 Hz, 1H), 3.71 (s, 3H), 3.15 (d, J = 14.9 Hz, 2H), 2.90−2.68 (m, 6H), 2.23−2.15 (m, 1H), 1.83 (dd, J = 6.3, 1.3 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ 171.1 (s), 167.5 (s), 135.2 (s), 132.7 (s), 132.1 (d), 129.2 (s), 125.0 (d), 124.0 (d), 118.4 (d), 116.4 (d), 114.8 (s), 110.3 (s), 62.2 (s), 61.4 (t), 52.2 (q), 44.2 (t), 31.1 (t), 29.9 (t), 21.8 (q), 21.2 (t); HRMS (ESI) m/z [M + Na]+ calcd for C20H21IN2O3Na 487.0489, found 487.0497. (±)-Methyl (R,Z)-3-(2-Iodobut-2-en-1-yl)-6-oxo-2,3-dihydro-1Hindolo[3,2,1-de][1,5]naphthyridine-3a(6H)-carboxylate (5). To a solution of compound 11 (930 mg, 2.0 mmol, 1.0 equiv) in THF (10 mL) at −78 °C was added LiHMDS (8.0 mL, 1.0 M, 8.0 mmol, 4.0 equiv) dropwise over 10 min. The resulting solution was stirred at the same temperature for 2 h before adding a solution of PhSeBr (708 mg, 3.0 mmol, 1.5 equiv) in THF (2 mL) at −78 °C. The mixture was stirred for 2 h at −78 °C and allowed to warm to rt. Upon complete consumption of the starting material, the reaction was quenched by the addition of saturated aqueous ammonium chloride (10 mL) and extracted with ethyl acetate (3 × 10 mL). The combined organic extracts were washed with brine (10 mL), dried over anhydrous sodium sulfate, and concentrated in vacuo. To a solution of the above crude residue in DCM (20 mL) was added saturated aq NH4Cl (2.0 mL, 1.0 mL/mmol) at 0 °C. Then a solution of H2O2 (1.0 mL, 0.5 mL/mmol, 30% in H2O) was added dropwise. After stirring for 30 min at 0 °C, the reaction was quenched by the addition of saturated aqueous sodium thiosulfate (10 mL) and extracted with dichloromethane (3 × 10 mL). The combined organic extracts were washed with brine (10 mL), dried over anhydrous sodium sulfate, and concentrated in vacuo. The resulting crude residue was purified by column chromatography (petroleum ether/ethyl acetate = 10:1 to 6:1) to give compound 5 (583 mg, 63% yield) as a pale yellow oil: Rf = 0.48 (petroleum ether/ethyl acetate = 4:1); IR (KBr) ν 3437, 2951, 2920, 2847, 1736, 1699, 1640, 1453, 1385, 1327, 1209, 1145, 1125, 1095, 1013, 821, 755 cm−1; 1H NMR (400 MHz, CDCl3) δ 8.39 (d, J = 8.0 Hz, 1H), 7.50 (d, J = 7.6 Hz, 1H), 7.39 (t, J = 7.3 Hz, 1H), 7.33 (t, J = 7.3 Hz, 1H), 6.99 (d, J = 9.9 Hz, 1H), 6.31 (d, J = 9.9 Hz, 1H), 5.99 (q, J = 6.3 Hz, 1H), 3.71 (s, 3H), 3.33 (m, 3H), 3.23−3.14 (m, 1H), 2.89 (ddd, J = 17.7, 15.5, 10.1 Hz, 1H), 2.61 (dd, J = 17.0, 5.6 Hz, 1H), 1.82 (d, J = 6.3 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ 169.9 (s), 159.7 (s), 141.3 (d), 135.6 (s), 132.6 (d), 131.5 (s), 128.6 (s), 127.8 (d), 125.1 (d), 124.0 (d), 118.7 (d), 116.2 (d), 115.0 (s), 108.3 (s), 65.4 (s), 60.8 (t), 53.5 (q), 44.1 (t), 21.7 (q), 17.5 (t); HRMS (ESI) m/z [M + Na]+ calcd for C20H19IN2O3Na 485.0333, found 485.0331. (±)-Methyl (31R,12aS,E)-1-Ethylidene-11-oxo-1,4,5,11,12,12ahexahydroindolo[3,2,1-de]pyrrolo[3,2,1-ij][1,5]naphthyridine-

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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.8b00529. 1

H and 13C NMR spectra of all new compounds (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yu-Rong Yang: 0000-0001-6874-109X Notes

The authors declare no competing financial interest. 4869

DOI: 10.1021/acs.joc.8b00529 J. Org. Chem. 2018, 83, 4867−4870

Note

The Journal of Organic Chemistry

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Bioinspired Collective Syntheses of Iboga-Type Indole Alkaloids. Org. Lett. 2016, 18, 2447−2450. (7) (a) Sirasani, G.; Andrade, R. B. Total Synthesis of (−)-Leuconicine A and B. Org. Lett. 2011, 13, 4736−4737. (b) Yin, W.; Kabir, M. S.; Wang, Z.; Rallapalli, S. K.; Ma, J.; Cook, J. M. Enantiospecific Total Synthesis of the Important Biogenetic Intermediates along the Ajmaline Pathway, (+)-Polyneuridine and (+)-Polyneuridine Aldehyde, as well as 16-Epivellosimine and Macusine A. J. Org. Chem. 2010, 75, 3339−3349. (8) (a) Taylor, M. S.; Jacobsen, E. N. Highly Enantioselective Catalytic Acyl-Pictet-Spengler Reactions. J. Am. Chem. Soc. 2004, 126, 10558−10559. (b) Seayad, J.; Seayad, A. M.; List, B. Catalytic Asymmetric Pictet-Spengler Reaction. J. Am. Chem. Soc. 2006, 128, 1086−1087. (c) Wanner, M. J.; van der Haas, R. N. S.; de Cuba, K. R.; van Maarseveen, J. H.; Hiemstra, H. Catalytic Asymmetric PictetSpengler Reactions via Sulfenyliminium Ions. Angew. Chem., Int. Ed. 2007, 46, 7485−7487. (d) Huang, D.; Xu, F.; Lin, X.; Wang, Y. Highly Enantioselective Pictet-Spengler Reaction Catalyzed by SPINOLphosphoric Acids. Chem. - Eur. J. 2012, 18, 3148−3152.

ACKNOWLEDGMENTS We thank the Key Research Program of Frontier Sciences of the CAS (QYZDB-SSW-SMC026), the NSFC (21672224 and 21472200), and the Government of Yunan Province (2017FA002) for financial support.

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REFERENCES

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DOI: 10.1021/acs.joc.8b00529 J. Org. Chem. 2018, 83, 4867−4870