Methyl 5-formyl-2-methylbenzoate (268 mg, 1.5 mmol) was used as starting material and dissolved in N,N-dimethylformamide (DMF, 5 mL). Subsequently, Oxone (1.110 g, 1.8 mmol) was added to the solution and the reaction mixture was stirred at room temperature for 18 hours. After completion of the reaction, the reaction mixture was diluted with ethyl acetate (EtOAc, 10 mL) followed by 0.5 M aqueous hydrochloric acid (20 mL). The organic phase was separated, dried with anhydrous sodium sulfate (Na2SO4), filtered and concentrated under reduced pressure to afford the target product 3-(methoxycarbonyl)-4-methylbenzoic acid as an off-white powder (247 mg, 1% yield). The structure of the product was confirmed by 1H NMR (500 MHz, DMSO-d6): δ 8.38 (d, J = 1.8 Hz, 1H), 8.01 (dd, J = 7.9, 1.9 Hz, 1H), 7.48 (d, J = 8.0 Hz, 1H), 3.86 (s, 3H), 2.59 (s, 3H).
