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Sulfoxide chloride (SOCl2, 0.6 mL) was slowly added dropwise to anhydrous methanol (MeOH, 10 mL) at 0 °C. The reaction mixture was stirred continuously for 0.5 h at this temperature. Subsequently, 3-hydroxy-2-nitrobenzoic acid (0.6 g, 3.2 mmol) was added to the mixture. The reaction system was heated to reflux and maintained for 4 hours. The progress of the reaction was monitored by thin layer chromatography (TLC, unfolding agent ratio of petroleum ether: ethyl acetate = 1:1) to confirm the completion of the reaction. After completion of the reaction, the mixture was concentrated under vacuum to afford the target product methyl 2-nitro-3-hydroxybenzoate (0.63 g, 100% yield) as a brown solid.

[1] Patent: WO2007/125405, 2007, A2. Location in patent: Page/Page column 24; 72-73
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[3] Chem.Abstr., 1954, p. 3959
[4] Patent: US2002/52397, 2002, A1
[5] Patent: US5756510, 1998, A