General procedure for the synthesis of methyl 3-methyl-4-hydroxybenzoate from methanol and 4-hydroxy-3-methylbenzoic acid: 4-hydroxy-3-methylbenzoic acid (342 g, 2.24 mol) was dissolved in methanol (3.5 L), and the solution was bubbled with hydrogen chloride gas (HCl) for 5 min. Subsequently, the reaction mixture was stirred at room temperature for 12 hours. Upon completion of the reaction, the reaction mixture was concentrated to afford methyl 3-methyl-4-hydroxybenzoate (372 g, quantitative yield). NMR hydrogen spectrum of the product (1H-NMR, CDCl3, δ ppm): 7.82 (1H, single peak), 7.78 (1H, double peak), 6.80 (1H, double peak), 3.86 (3H, single peak), 2.22 (3H, single peak).
