Selective Deprotection of N-Tosyl Alkoxyamines Using Bistrifluoromethane Sulfonimide: Formation of Oxime Ethers

Abstract

The detosylation of N-tosyl alkoxyamines was realized by treatment with benzaldehyde and bistrifluoromethane sulfonimide as the catalyst to afford the corresponding oxime ethers. The reaction is chemoselective as N-tosyl amines are not deprotected. A mechanism is proposed for this deprotection.

Key words

N-detosylation - bistrifluoromethane sulfonimide - N-tosyl-alkoxyamine - benzaldehyde - oxime ethers

• References and Notes

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• 16 Compound 9 was isolated as a mixture of E- and Z-isomers in a ratio 96:4.
• 17 Spectral Data of (E)-9 IR: ν = 2923, 1640, 1446, 1373, 1335, 1210, 1089, 967, 910, 900 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 8.06 (s, 1 H), 7.60–7.50 (m, 2 H), 7.40–7.30 (m, 3 H), 5.85 (m, 1 H), 5.04 (dq_app, J = 17.0, 1.8 Hz, 1 H), 4.97 (m, 1 H), 4.32 (m, 1 H), 2.18 (m, 2 H), 1.83 (m, 1 H), 1.59 (m, 1 H), 1.30 (d, J = 6.2 Hz, 3 H). ¹³C NMR (100 MHz, CDCl₃): δ = 145.2, 138.4, 130.8, 129.7, 128.4 (2 C), 126.9 (2 C), 114.6, 79.9, 34.8, 29.7, 19.2. MS (EI): m/z = 203 (13) [M^+•], 202 (40), 188 (8), 158 (8), 132 (3), 122 (7), 121 (10), 120 (9), 104 (42), 94 (4), 89 (5), 82 (6), 78 (9), 77 (47), 67 (15), 65 (8), 55 (100), 51 (13).
• 18 General Procedure for the Synthesis of Oxime Ethers from N-Tosyl Alkoxyamines In a round-bottom flask, a mixture of a solution of N-tosyl alkoxyamide (0.2 mmol, 1 equiv) in anhydrous CH₂Cl₂ (c = 0.25 M), aldehyde (2 equiv), and a solution of HNTf₂ (0.1 equiv) in anhydrous CH₂Cl₂ was stirred at 40 °C for 18 h. The reaction mixture was cooled to r.t. and saturated aqueous Na₂CO₃ was added. The two phases were separated, and the aqueous layer was extracted four times with CH₂Cl₂. The combined organic layers were washed with brine, dried over MgSO₄, filtered, and concentrated in vacuo. The crude product was purified by flash chromatography on silica gel (petroleum ether/ethyl acetate) to obtain the desired oxime ether.
• 19 Spectroscopic Data for 17 ¹H NMR (400 MHz, CDCl₃): δ = 8.02 (s, 1 H), 7.52–7.49 (m, 2 H), 6.90–6.70 (m, 2 H), 5.85 (m, 1 H), 5.04 (dq_app, J = 17.1, 1.8 Hz, 1 H), 4.96 (m, 1 H), 4.29 (m, 1 H), 3.82 (s, 3 H), 2.20–2.15 (m, 2 H), 1.81 (m, 1 H), 1.60 (m, 1 H), 1.29 (d, J = 6.2 Hz, 3 H). ¹³C NMR (100 MHz, CDCl₃): δ = 160.7, 147.5, 138.5, 128.3 (2 C), 125.4, 114.5, 114.1 (2C), 78.7, 55.3, 34.9, 29.7, 19.8. MS (EI): m/z = 233 (22) [M^+•], 232 (33), 218 (26), 188 (11), 174 (13), 162 (10), 151 (41), 150 (22), 147 (15), 146 (9), 136 (20), 135 (58), 134 (78), 108 (50), 107 (20), 92 (19), 91 (12), 77 (36), 67 (6), 55 (100), 51 (7). HRMS (ESI): m/z calcd for C₁₄H₂₀NO₂ [M + H]⁺: 234.1489; found: 234.1485.
• 20 Spectroscopic Data for (E)-18 IR: ν = 3077, 2969, 2936, 2838, 1607, 1572, 1504, 1463, 1438, 1418, 1372, 1311, 1283, 1270, 1208, 1159, 1120, 1107, 1068, 1034, 993, 965 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 8.37 (s, 1 H), 7.72 (d, J = 8.6 Hz, 1 H), 6.48 (dd, J = 8.6, 0.5 Hz, 1 H), 6.42 (d, J = 2.3 Hz, 1 H), 5.85 (m, 1 H), 5.03 (dq_app, J = 17.1, 1.8 Hz, 1 H), 4.95 (m, 1 H), 4.28 (m, 1 H), 3.82 (s, 3 H), 3.81 (s, 3 H), 2.17 (m, 2 H), 1.81 (m, 1 H), 1.60 (m, 1 H), 1.29 (d, J = 6.2 Hz, 3 H). ¹³C NMR (100 MHz, CDCl₃): δ = 162.1, 158.7, 143.9, 138.6, 127.2, 114.4, 114.3, 105.3, 98.2, 78.5, 55.5, 55.4, 34.9, 29.7, 19.8. MS (EI) m/z: 263 (M^+., 12), 204 (10), 192 (11), 177 (14), 166 (10), 164 (37), 163 (15), 150 (18), 149 (100), 137 (12), 134 (14), 122 (14), 121 (44), 120 (18), 107 (16), 92 (10), 91 (10), 79 (11), 77 (22), 67 (10), 55 (59). HRMS (ESI): m/z calcd for C₁₅H₂₂NO₃ [M + H]⁺: 264.1594; found: 264.1591.
• 21 Spectroscopic Data for 13 IR: ν = 3073, 2925, 1573, 1448, 1340, 1273, 1210, 1046, 944, 913 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 8.09 (s, 1 H), 7.60–7.55 (m, 2 H), 7.40–7.30 (m, 3 H), 5.87 (m, 1 H), 5.06–5.00 (m, 2 H), 4.04 (s, 2 H), 2.17 (dt_app, J = 7.5 Hz, 0.9 Hz, 2 H), 1.43 (m, 10 H). ¹³C NMR (100 MHz, CDCl₃): δ = 147.7, 134.8, 132.5, 129.5, 128.6 (2 C), 126.8 (2 C), 117.3, 79.5, 40.1, 37.2, 32.7 (2 C), 26.2, 21.4 (2 C). MS (EI): m/z = 257 (14), 256 (36), 132 (5), 122 (62), 106 (100), 104 (58), 95 (30), 93 (12), 81 (64), 79 (20), 69 (13), 55 (30), 51 (13).