Visible-Light-Mediated Synthesis of Pyrazines from Vinyl Azides Utilizing a Photocascade Process

Asik Hossain; Santosh K. Pagire; Oliver Reiser

doi:10.1055/s-0036-1590888

Synlett, Table of Contents

Synlett 2017; 28(14): 1707-1714
DOI: 10.1055/s-0036-1590888

cluster

Visible-Light-Mediated Synthesis of Pyrazines from Vinyl Azides Utilizing a Photocascade Process

Asik Hossain

Institut für Organische Chemie, Universität Regensburg, Universitätsstraße 31, 93053 Regensburg, Germany Email: oliver.reiser@chemie.uni-regensburg.de

,

Santosh K. Pagire

Institut für Organische Chemie, Universität Regensburg, Universitätsstraße 31, 93053 Regensburg, Germany Email: oliver.reiser@chemie.uni-regensburg.de

,

Oliver Reiser *

Institut für Organische Chemie, Universität Regensburg, Universitätsstraße 31, 93053 Regensburg, Germany Email: oliver.reiser@chemie.uni-regensburg.de

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Abstract

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Dedicated to Professor Albert Padwa on the occasion of his 80^th birthday

Published as part of the ISHC Conference Special Section

Abstract

A convenient method for the synthesis of substituted pyrazines from vinyl azides has been developed. This method is enabled by a dual-energy and electron-transfer strategy by visible-light photocatalysis. Initially, vinyl azides are activated by a triplet sensitization process from an excited ruthenium photocatalyst in the presence of water to form dihydropyrazines, followed by a single-electron-transfer (SET) process under oxygen (air) atmosphere that leads to the tetrasubstituted pyrazines in good to excellent yields.

Keywords

visible-light photocatalysis - pyrazines - synthetic methods - vinyl azides - photocascade process

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References and Notes

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27 Photochemical Synthesis of Pyrazines: General Procedure An oven-dried Schlenk tube (10 mL size) equipped with a stirring bar was charged with [Ru(bpy)₃]Cl₂·6H₂O (3.7 mg, 0.01 equiv, 1 mol%) followed by 1.60 mL MeCN and 0.40 mL distilled water. The resulting solution was degassed via nitrogen bubbling for 5 min using a syringe needle. Then vinyl azide 2 (0.50 mmol, 1 equiv) was added under nitrogen. Irradiation with a blue LED (λ_max = 455 nm) was carried out for 24 h at rt. The reaction mixture was transferred to a separatory funnel containing distilled water, and extracted three times with EtOAc. After drying the combined organic layers (Na₂SO₄), the resulting solution was concentrated in vacuo. The pure product was obtained either by recrystallization from CH₂Cl₂/pentane or by silica gel column chromatography using hexanes and EtOAc as eluents. Diethyl 3,6-Diphenylpyrazine-2,5-dicarboxylate (3a) Following the general procedure, 3a was prepared from ethyl (Z)-2-azido-3-phenylacrylate (2a, 109 mg, 0.50 mmol, 1.00 equiv) and [Ru(bpy)₃]Cl₂·6H₂O (3.7 mg, 0.01 equiv, 1 mol%). The crude product was purified by column chromatography (silica gel, hexanes–EtOAc = 9:1, R_f = 0.20) to afford 3a as a white solid (57 mg, 61% yield). ¹H NMR (300 MHz, CDCl₃): δ = 7.74–7.71 (m, 4 H), 7.49–7.47 (m, 6 H), 4.31 (q, J = 7.1 Hz, 4 H), 1.16 (t, J = 7.1 Hz, 6 H). ¹³C NMR (75 MHz, CDCl₃): δ = 166.2, 149.9, 144.8, 136.2, 130.0, 128.9, 128.7, 62.5, 13.8. ESI-HRMS: m/z calcd for C₂₂H₂₀N₂O₄ [M + H]⁺: 377.1423; found: 377.1502. IR (neat): 3059, 2986, 2925, 2854, 1731, 1449, 1405, 1380, 1293, 1173, 1137, 1094, 1057, 1021, 856, 766, 705 cm^–1. Diethyl 3,6-Di-p-tolylpyrazine-2,5-dicarboxylate (3b) Following the general procedure, 3b was prepared from ethyl (Z)-2-azido-3-(p-tolyl)acrylate (2b, 115 mg, 0.50 mmol, 1.00 equiv) and [Ru(bpy)₃]Cl₂·6H₂O (3.7 mg, 0.01 equiv, 1 mol%). The crude product was dissolved in a minimum amount of CH₂Cl₂ followed by the addition of n-pentane to initiate crystallization. After 30 min, the solvent mixture was decanted, and the solid was washed twice with n-pentane. The residual solvents were removed in vacuo which afforded 3b as white solid (83 mg, 82%). ¹H NMR (300 MHz, CDCl₃): δ = 7.62 (d, J = 8.1 Hz, 4 H), 7.27 (d, J = 7.9 Hz, 4 H), 4.32 (q, J = 7.1 Hz, 4 H), 2.41 (s, 6 H), 1.19 (t, J = 7.1 Hz, 6 H). ¹³C NMR (75 MHz, CDCl₃): δ = 166.4, 149.4, 144.6, 140.2, 133.3, 129.5, 128.7, 62.4, 21.5, 13.9. ESI-HRMS: m/z calcd for C₂₄H₂₄N₂O₄ [M + H]⁺: 405.1736; found: 405.1814. IR (neat): 2986, 2871, 1723, 1609, 1462, 1442, 1408, 1292, 1247, 1174, 1137, 1095, 1057, 1012, 853, 828, 775, 700 cm^–1. Diethyl 3,6-Di(naphthalen-2-yl)pyrazine-2,5-dicarboxylate (3c) Following the general procedure, 3c was prepared from ethyl (Z)-2-azido-3-(naphthalen-2-yl)acrylate (2c, 134 mg, 0.50 mmol, 1 equiv) and [Ru(bpy)₃]Cl₂·6H₂O (3.7 mg, 0.01 equiv, 1 mol%). The crude product was purified by column chromatography (silica gel, hexanes–EtOAc = 4:1, R_f = 0.30) to afford 3c as a white solid (81 mg, 70% yield). ¹H NMR (400 MHz, CDCl₃): δ = 8.28 (s, 2 H), 7.97–7.85 (m, 8 H), 7.59–7.53 (m, 4 H), 4.33 (d, J = 7.1 Hz, 4 H), 1.13 (t, J = 7.1 Hz, 6 H). ¹³C NMR (100 MHz, CDCl₃): δ = 166.4, 149.6, 145.1, 134.0, 133.5, 133.2, 129.0, 128.8, 128.6, 127.9, 127.4, 126.8, 125.9, 62.6, 13.9. ESI-HRMS: m/z calcd for C₃₀H₂₄N₂O₄ [M + H]⁺: 477.1736; found : 477.1821. IR (neat):3056, 2974, 2927, 2854, 1729, 1686, 1462, 1405, 1381, 1304, 1236, 1169, 1140, 1004, 906, 871, 852, 830, 750 cm^–1. Diethyl 3,6-Fis(4-methoxyphenyl)pyrazine-2,5-dicarboxylate (3d) Following the general procedure, 3d was prepared from ethyl (Z)-2-azido-3-(4-methoxyphenyl)acrylate (2d, 124 mg, 0.50 mmol, 1 equiv) and [Ru(bpy)₃]Cl₂·6H₂O (3.7 mg, 0.01 equiv, 1 mol%). The crude product was purified by column chromatography (silica gel, hexanes–EtOAc = 3:1, R_f = 0.28) to afford 3d as a white solid (53 mg, 49% yield). ¹H NMR (300 MHz, CDCl₃): δ = 7.70–7.67 (m, 4 H), 6.99–6.96 (m, 4 H), 4.33 (q, J = 7.1 Hz, 4 H), 3.85 (s, 6 H), 1.21 (t, J = 7.1 Hz, 6 H). ¹³C NMR (75 MHz, CDCl₃): δ = 166.5, 161.2, 148.5, 144.1, 130.3, 128.5, 114.2, 62.4, 55.5, 13.9. ESI-HRMS: m/z calcd for C₂₄H₂₄N₂O₆ [M + H]⁺: 437.1634; found: 437.1740. IR (neat): 2980, 2955, 2926, 2851, 1732, 1607, 1580, 1518, 1414, 1295, 1253, 1177, 1148, 1060, 1017, 841, 811, 786, 752 cm^–1. Diethyl 3,6-Bis(2-methoxyphenyl)pyrazine-2,5-dicarboxylate (3e) Following the general procedure, 3e was prepared from ethyl (Z)-2-azido-3-(2-methoxyphenyl)acrylate (2e, 124 mg, 0.50 mmol, 1.00 equiv) and [Ru(bpy)₃]Cl₂·6H₂O(3.7 mg, 0.01 equiv, 1 mol%). The crude product was recrystallized as described for 3b to afford 3e as white solid (86 mg, 79%). ¹H NMR (300 MHz, CDCl₃): δ = 7.78 (dd, J ₁ = 7.5 Hz, J ₂ = 1.6 Hz, 2 H), 7.44–7.38 (m, 2 H), 7.14–7.09 (m, 2 H), 6.89 (d, J = 8.2 Hz, 2 H), 4.25 (q, J = 7.1 Hz, 4 H), 3.73 (s, 6 H), 1.14 (t, J = 7.1 Hz, 6 H). ¹³C NMR (75 MHz, CDCl₃): δ = 165.5, 156.5, 147.6, 145.9, 131.4, 131.2, 126.5, 121.4, 110.1, 61.8, 55.1, 13.9. ESI-HRMS: m/z calcd for C₂₄H₂₄N₂O₆ [M + H]⁺: 437.1634; found: 437.1739. IR (neat): 2968,2924, 2843, 1733, 1601, 1495, 1465, 1439, 1411, 1390, 1280, 1248, 1226, 1182, 1149, 1111, 1061, 1014, 854, 759, 697 cm^–1. Diethyl 3,6-Bis(2-chloro-3,4-dimethoxyphenyl)pyrazine-2,5-dicarboxylate (3f) Following the general procedure, 3f was prepared from ethyl (Z)-2-azido-3-(2-chloro-3,4-dimethoxyphenyl)acrylate (2f, 156 mg, 0.50 mmol, 1.00 equiv) and [Ru(bpy)₃]Cl₂·6H₂O (3.7 mg, 0.01 equiv, 1 mol%). The crude product was recrystallized as described for 3b to afford 3f as white solid (122 mg, 87%). ¹H NMR (400 MHz, CDCl₃): δ = 7.36 (d, J = 8.5 Hz, 2 H), 6.98 (d, J = 8.6 Hz, 2 H), 4.28 (q, J = 7.1 Hz, 4 H), 3.94 (s, 6 H), 3.87 (s, 6 H), 1.17 (t, J = 7.1 Hz, 6 H). ¹³C NMR (100 MHz, CDCl₃): δ = 164.6, 154.8, 149.8, 145.5, 145.1, 129.5, 127.8, 126.4, 110.9, 62.5, 60.8, 56.3, 13.9. ESI-HRMS: m/z calcd for C₂₆H₂₆Cl₂N₂O₈ [M + H]⁺: 565.1066; found: 565.1143. IR (neat): 3090, 2980, 2938, 2841, 1735, 1590, 1490, 1447, 1412, 1392, 1293, 1269, 1220, 1202, 1099, 1038, 1003, 819, 790, 778 cm^–1. Diethyl 3,6-Bis(4-fluorophenyl)pyrazine-2,5-dicarboxylate (3g) Following the general procedure, 3g was prepared from ethyl (Z)-2-azido-3-(4-fluorophenyl)acrylate (2g, 118 mg, 0.50 mmol, 1.00 equiv) and [Ru(bpy)₃]Cl₂·6H₂O (3.7 mg, 0.01 equiv, 1 mol%). The crude product was recrystallized as described for 3b to afford 3g as white solid (87 mg, 85%). ¹H NMR (300 MHz, CDCl₃): δ = 7.74–7.70 (m, 4 H), 7.20–7.15 (m, 4 H), 4.33 (q, J = 7.1 Hz, 4 H), 1.20 (t, J = 7.1 Hz, 6H). ¹³C NMR (75 MHz, CDCl₃): δ = 165.9, 164.1 (d, ¹ J _C–F = 250.6 Hz), 148.8, 144.5, 132.1 (d, ⁴ J _C–F = 3.1 Hz), 130.9 (d, ³ J _C–F = 8.6 Hz), 116.0 (d, ² J _C–F = 21.8 Hz), 62.7, 13.9. ¹⁹F NMR (282 MHz, CDCl₃): δ = –110.9. ESI-HRMS: m/z calcd for C₂₂H₁₈F₂N₂O₄ [M + H]⁺: 413.1235; found: 413.1320. IR (neat): 3084, 2985, 2925, 2854, 1733, 1599, 1512, 1409, 1383, 1289, 1227, 1179, 1155, 1096, 1053, 1012, 845, 815, 766 cm^–1. Diethyl 3,6-Bis(2-fluorophenyl)pyrazine-2,5-dicarboxylate (3h) Following general procedure, 3h was prepared from ethyl (Z)-2-azido-3-(2-fluorophenyl)acrylate (2h, 118 mg, 0.50 mmol, 1.00 equiv) and [Ru(bpy)₃]Cl₂·6H₂O (3.7 mg, 0.01 equiv, 1 mol%). The crude product was recrystallized as described for 3b to afford 3h as white solid (83 mg, 81%). ¹H NMR (300 MHz, CDCl₃): δ = 7.86–7.80 (m, 2 H), 7.51–7.44 (m, 2 H), 7.35–7.30 (m, 2 H), 7.16–7.09 (m, 2 H), 4.35 (q, J = 7.1 Hz, 4 H), 1.21 (t, J = 7.1 Hz, 6 H). ¹³C NMR (75 MHz, CDCl₃): δ = 164.7, 160.1 (d, ¹ J _C–F = 248.7 Hz), 146.5, 145.4 (d, J = 1.9 Hz), 132.0 (d, ³ J _C–F = 8.5 Hz), 131.7 (d, ⁴ J _C–F = 2.3 Hz), 124.9 (d, ³ J _C–F = 3.3 Hz), 124.7, 115.4 (d, ² J _C–F = 21.7 Hz), 62.5, 13.9. ¹⁹F NMR (282 MHz, CDCl₃): δ = –116.6. ESI-HRMS: m/z calcd for C₂₂H₁₈F₂N₂O₄ [M + H]⁺: 413.1235; found: 413.1320. IR (neat): 3071, 2985, 2939, 2906, 1730, 1613, 1581, 1490, 1454, 1399, 1384, 1245, 1172, 1140, 1093, 1059, 1014, 859, 757 cm^–1. Diethyl 3,6-Bis(4-chlorophenyl)pyrazine-2,5-dicarboxylate (3i) Following the general procedure, 3i was prepared from ethyl (Z)-2-azido-3-(4-chlorophenyl)acrylate (2i, 126 mg, 0.50 mmol, 1.00 equiv) and [Ru(bpy)₃]Cl₂·6H₂O (3.7 mg, 0.01 equiv, 1 mol%). The crude product was recrystallized as described for 3b to afford 3i as white solid (63 mg, 57%). ¹H NMR (300 MHz, CDCl₃): δ = 7.68–7.66 (m, 4 H), 7.47–7.45 (m, 4 H), 4.34 (q, J = 7.1 Hz, 4 H), 1.22 (t, J = 7.1 Hz, 6 H). ¹³C NMR (75 MHz, CDCl₃): δ = 165.8, 148.9, 144.5, 136.7, 134.4, 130.2, 129.1, 62.8, 13.9. ESI-HRMS: m/z calcd for C₂₂H₁₈Cl₂N₂O₄ [M + H]⁺: 445.0644; found: 445.0713. IR (neat): 2985, 2937, 1728, 1595, 1494, 1426, 1401, 1378, 1277, 1234, 1187, 1158, 1109, 1090, 1053, 1010, 854, 830, 778, 745 cm^–1. Diethyl 3,6-Bis(4-bromophenyl)pyrazine-2,5-dicarboxylate (3j) Following the general procedure, 3j was prepared from ethyl (Z)-2-azido-3-(4-bromophenyl)acrylate (2j, 148 mg, 0.50 mmol, 1 equiv) and [Ru(bpy)₃]Cl₂·6H₂O (3.7 mg, 0.01 equiv, 1 mol%). The crude product was purified by column chromatography (silica gel, hexanes–EtOAc = 3:1, R_f = 0.22) to afford 3j as a white solid (73 mg, 55% yield). ¹H NMR (300 MHz, CDCl₃): δ = 7.64–7.58 (m, 8 H), 4.34 (q, J = 7.2 Hz, 4 H), 1.22 (t, J = 7.2 Hz, 6 H). ¹³C NMR (75 MHz, CDCl₃): δ = 165.8, 149.0, 144.5, 134.9, 132.0, 130.4, 125.0, 62.8, 13.9. ESI-HRMS: m/z calcd for C₂₂H₁₈Br₂N₂O₄ [M + H]⁺: 532.9633; found: 532.9708. IR (neat): 2983, 2925, 2854, 1728, 1588, 1474, 1423, 1399, 1376, 1279, 1234, 1186, 1159, 1051, 1006, 854, 826, 776, 734 cm^–1. Diethyl 3,6-Bis(4-(trifluoromethyl)phenyl)pyrazine-2,5-dicarboxylate (3k) Following the general procedure, 3k was prepared from ethyl (Z)-2-azido-3-(4-(trifluoromethyl)phenyl)acrylate (2k, 143 mg, 0.50 mmol, 1 equiv) and [Ru(bpy)₃]Cl₂·6H₂O (3.7 mg, 0.01 equiv, 1 mol%). The crude product was purified by column chromatography (silica gel, hexanes–EtOAc = 3:1, R_f = 0.30) to afford 3k as a white solid (48 mg, 38% yield). ¹H NMR (300 MHz, CDCl₃): δ = 7.86 (d, J = 8.1 Hz, 4 H), 7.76 (d, J = 8.3 Hz, 4 H), 4.34 (q, J = 7.1 Hz, 4 H), 1.20 (t, J = 7.1 Hz, 6 H). ¹³C NMR (75 MHz, CDCl₃): δ = 165.4, 149.4, 144.9, 139.4, 132.1 (q, J = 33.1 Hz), 129.4, 125.7 (q, J = 3.6 Hz), 122.1, 62.9, 13.8. ¹⁹F NMR (282 MHz, CDCl₃): δ = –63.2. ESI-HRMS: m/z calcd for C₂₄H₁₈F₆N₂O₄ [M + H]⁺: 513.1171; found: 513.1254. IR (neat): 2982, 2965, 2927, 2856, 1721, 1414, 1324, 1254, 1138, 1105, 1072, 1053, 1016, 858, 832, 790, 717 cm^–1. Diethyl 3,6-Bis(4-nitrophenyl)pyrazine-2,5-dicarboxylate (3l) Following the general procedure, 3l was prepared from ethyl (Z)-2-azido-3-(4-nitrophenyl)acrylate (2l, 131 mg, 0.50 mmol, 1.00 equiv) and [Ru(bpy)₃]Cl₂·6H₂O (3.7 mg, 0.01 equiv, 1 mol%). The crude product was recrystallized as described for 3b to afford 3l as yellow solid (99 mg, 86%). ¹H NMR (300 MHz, CDCl₃): δ = 8.38–8.35 (m, 4 H), 7.93–7.90 (m, 4 H), 4.37 (q, J = 7.1 Hz, 4 H), 1.24 (t, J = 7.1 Hz, 6 H). ¹³C NMR (75 MHz, CDCl₃): δ = 164.9, 149.08, 149.00, 144.8, 141.8, 130.1, 124.0, 63.2, 14.0. ESI-HRMS: m/z calcd for C₂₂H₁₈N₄O₈ [M + H]⁺: 467.1125; found: 467.1198. IR (neat): 3110, 2985, 2958, 2919, 2851, 1721, 1601, 1516, 1466, 1408, 1344, 1296, 1259, 1144, 1100, 1009, 850, 800, 749, 689 cm^–1. Diethyl 3,6-Di(thiophen-2-yl)pyrazine-2,5-dicarboxylate (3m) Following the general procedure, 3m was prepared from ethyl (Z)-2-azido-3-(thiophen-2-yl)acrylate (2m, 112 mg, 0.50 mmol, 1 equiv) and [Ru(bpy)₃]Cl₂·6H₂O (3.7 mg, 0.01 equiv, 1 mol%). The crude product was purified by column chromatography (silica gel, hexanes–EtOAc = 5:1, R_f = 0.52) to afford 3m as a pale yellow solid (25 mg, 26%). ¹H NMR (300 MHz, CDCl₃): δ = 7.54–7.50 (m, 4 H), 7.12–7.09 (m, 2 H), 4.48 (q, J = 7.1 Hz, 4 H), 1.37 (t, J = 7.1 Hz, 6 H). ¹³C NMR (75 MHz, CDCl₃): δ = 165.9, 142.5, 141.9, 139.1, 130.4, 128.6, 128.4, 62.8, 14.0. ESI-HRMS: m/z calcd for C₁₈H₁₆N₂O₄S₂ [M + H]⁺: 389.0551; found: 389.0644. IR (neat): 3097, 2991, 2963, 2924, 2853, 1719, 1531, 1435, 1418, 1381, 1330, 1270, 1145, 1109, 852, 830, 706 cm^–1. Diethyl 3-(2-Chloro-3,4-dimethoxyphenyl)-6-(p-tolyl)pyrazine-2,5-dicarboxylate (3fb) Following the general procedure, 3fb was prepared from ethyl (Z)-2-azido-3-(2-chloro-3,4-dimethoxyphenyl)acrylate (2f, 78 mg, 0.25 mmol, 1 equiv), ethyl (Z)-2-azido-3-(p-tolyl)acrylate (2b, 57.7 mg, 0.25 mmol, 1.00 equiv) and [Ru(bpy)₃]Cl₂·6H₂O (3.7 mg, 0.01 equiv, 1 mol%). The crude product was purified by column chromatography (silica gel, hexanes–EtOAc = 3:1, R_f = 0.25) to afford 3fb as a white solid (37 mg, 31% yield). ¹H NMR (300 MHz, CDCl₃): δ = 7.66 (d, J = 8.1 Hz, 2 H), 7.33–7.27 (m, 3 H), 6.97 (d, J = 8.6 Hz, 1 H), 4.36–4.25 (m, 4 H), 3.93 (s, 3 H), 3.87 (s, 3 H), 2.42 (s, 3 H), 1.22–1.16 (m, 6 H). ¹³C NMR (75 MHz, CDCl₃): δ = 166.2, 164.7, 154.7, 150.5, 148.7, 145.5, 144.9, 144.8, 140.5, 133.2, 129.6, 129.5, 128.9, 127.8, 126.3, 110.8, 62.6, 62.4, 60.8, 56.3, 21.6, 13.97, 13.94. ESI-HRMS: m/z calcd for C₂₅H₂₅ClN₂O₆ [M + H]⁺: 485.1401; found: 485.1494. IR (neat): 2982, 2927, 2854, 1739, 1591, 1487, 1450, 1397, 1291, 1277, 1236, 1224, 1198, 1153, 1120, 1077, 1030, 1013, 964, 809, 755 cm^–1. Diethyl 3-(2-Chloro-3,4-dimethoxyphenyl)-6-(2-fluorophenyl)pyrazine-2,5-dicarboxylate (3fh) Following the general procedure, 3fh was prepared from ethyl (Z)-2-azido-3-(2-chloro-3,4-dimethoxyphenyl)acrylate (2f, 78 mg, 0.25 mmol, 1 equiv), ethyl (Z)-2-azido-3-(2-fluorophenyl)acrylate (2h, 59 mg, 0.25 mmol, 1 equiv) and [Ru(bpy)₃]Cl₂·6H₂O (3.7 mg, 0.01 equiv, 1 mol%). The crude product was purified by column chromatography (silica gel, hexanes–EtOAc = 3:1, R_f = 0.34) to afford 3fh as a white solid (45 mg, 37% yield). ¹H NMR (300 MHz, CDCl₃): δ = 7.87–7.82 (m, 1 H), 7.59–7.30 (m, 3 H), 7.16–7.09 (m, 1 H), 6.99–6.97 (m, 1 H), 4.38–4.26 (m, 4 H), 3.93 (s, 3 H), 3.87 (s, 3 H), 1.23–1.16 (m, 6 H). ¹³C NMR (75 MHz, CDCl₃): δ = 164.8, 164.5, 160.1 (d, ¹ J _C–F = 248.3 Hz), 154.8, 149.7, 146.5, 145.5, 145.1, 132.0 (d, ³ J _C–F = 8.5 Hz), 131.7 (d, ⁴ J _C–F = 2.2 Hz), 129.4, 127.7, 126.4, 125.0, 124.9 (d, ³ J _C–F = 3.3 Hz), 124.9, 115.4 (d, ² J _C–F = 21.6 Hz), 110.9, 62.5 (2×), 60.8, 56.3, 13.95, 13.92. ¹⁹F NMR (282 MHz, CDCl₃): δ = –116.6. ESI-HRMS: m/z calcd for C₂₄H₂₂ClFN₂O₆ [M + H]⁺: 489.1150; found: 489.1257. IR (neat): 2981, 2934, 2843, 1723, 1616, 1558, 1490, 1450, 1408, 1296, 1269, 1248, 1224, 1173, 1142, 1099, 1075, 1040, 1016, 812, 758 cm^–1.

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