C(=O)([O-])[O-].[K+].[K+].[CH2:7]([O:9][C:10]([C:12]1[C:16](Br)=[C:15]([N+:18]([O-:20])=[O:19])[S:14][CH:13]=1)=[O:11])[CH3:8].[CH2:21]([SH:28])[C:22]1[CH:27]=[CH:26][CH:25]=[CH:24][CH:23]=1>C(O)C>[CH2:7]([O:9][C:10]([C:12]1[C:16]([S:28][CH2:21][C:22]2[CH:27]=[CH:26][CH:25]=[CH:24][CH:23]=2)=[C:15]([N+:18]([O-:20])=[O:19])[S:14][CH:13]=1)=[O:11])[CH3:8]An aqueous solution of potassium carbonate (9.90 g, 71.6 mmol, dissolved in 40 mL water) was added to a suspension of 4-bromo-5-nitro-thiophene-3-carboxylic acid ethyl ester (20.06 g, 71.6 mmol) in ethanol at 25° C. Benzyl mercaptan (8.41 mL, 71.6 mmol) was added via pipette, and the dark red reaction mixture was stirred at 25° C. for 4 h and then was concentrated in vacuo to near-dryness. The remaining orange-brown solid was triturated with water (200 mL) and was collected by filtration. After washing with water (200 mL), the resulting solid was air-dried overnight to afford the desired product, 4-benzylsulfanyl-5-nitro-thiophene-3-carboxylic acid ethyl ester (22.63 g, 70.0 mmol, 98%), as a yellow/brown solid. 1H NMR (400 MHz, CDCl3) δ: 1.41 (3H, t, J=7.1 Hz), 4.25 (2H, s), 4.39 (2H, q, J=7.0 Hz), 7.18-7.23 (5H, m), 8.07 (1H, s).
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