Compound S1

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Synthetic procedure: See article for the definitive version of this procedure and for full experimental details.

7-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-1H-indole (4.30 g, 1.0 equiv) and 2,6-lutidine (5.6 mL, 2.8 equiv) were dissolved in THF (59 mL) and the solution was stirred at 0 ºC. Oxalyl chloride (2.2 mL, 1.4 equiv) was added in one portion, and the solution was warmed to room temperature and stirred for 90 minutes. When the starting material was consumed as judged by TLC, the solution was cooled back to 0 ºC and methylamine (2M in THF, 26.5 mL, 3 equiv) was added until an aliquot showed a pH>10 on a wetted piece of pH paper. The reaction was stirred for 5 minutes, then was diluted with water (20 mL) and stirred vigorously as it warmed to room temperature. 1 N HCl_(aq) (25 mL) was added until an aliquot showed a pH<2 on a wetted piece of pH paper. Sodium periodate (9.7 g, 2.3 equiv) was added and the reaction was stirred vigorously at room temperature for 6 hours. The reaction was diluted with 100 mL of hexanes and 100 mL of water and cooled to 0 ºC. The solids were filtered off, and washed thoroughly with water, and then with diethyl ether. The solids were then dried in vacuo to afford S1 (3.578 g, 82% yield) as a beige solid. ¹H NMR (500 MHz, DMSO-d₆) δ 11.35 (s, 1H), 8.72 (d, J = 3.4 Hz, 1H), 8.69 (q, J = 4.9 Hz, 1H), 8.58 (s, 2H), 8.31 (dd, J = 7.9, 1.3 Hz, 1H), 7.76 (dd, J = 7.1, 1.3 Hz, 1H), 7.25 (t, J = 7.5 Hz, 1H), 2.74 (d, J = 4.8 Hz, 3H). ¹³C NMR (125 MHz, DMSO-d₆) δ 181.94, 164.07, 140.39, 138.98, 130.15, 125.60, 123.66, 121.99, 111.55, 25.57. IR (thin film, cm^-1) 3312.1, 1671.7, 1622.0, 1605.8, 1502.2, 1409.8, 1335.9, 1219.9, 899.0, 787.6. HRMS (ESI-TOF) calculated for C₁₁H₁₂BN₂O₄ [M+H]⁺ m/z 247.08901, found 247.09014, difference 4.6 ppm.