General procedure for the synthesis of 3-hydroxy-5-methylbenzoic acid from the compound (CAS:100118-65-8): the oxo compound (57 g, 0.21 mol) was added to water (450 mL) together with magnesium oxide (38.1 g, 0.945 mol). The reaction mixture immediately changed to a dark reddish-orange color, followed by a gradual change to a light brown color over about 15 minutes. After continuous stirring and heating for 30 minutes, the excess magnesium oxalate and magnesium oxide were removed by filtration. The residue was washed with warm water, the filtrates were combined and concentrated by distillation under vacuum to about 30 mL. The concentrate was slowly added to a mixture consisting of concentrated hydrochloric acid and water (1:1, v/v, 50 mL), whereupon a precipitate was produced. The precipitate was filtered, washed with cold water and dried. Finally, purification by recrystallization in water afforded 3-hydroxy-5-methylbenzoic acid as a light brown solid (13.43 g, 42% yield). Melting point: 202-203 °C (literature value 207-208 °C); 1H NMR (D2O) δ: 2.19 (s, 3H), 6.82 (s, 1H), 7.07 (s, 1H), 7.20 (s, 1H); 13C NMR (D2O) δ: 20.20, 113.00, 121.03, 122.25, 130.65, 140.55, 155.34, 170.26.
