[CH:1]([C:3]1[CH:13]=[CH:12][C:6]([C:7]([O:9][CH2:10][CH3:11])=[O:8])=[C:5]([CH3:14])[CH:4]=1)=O.[CH:15](C1C=CC(C(O)=O)=C(C)C=1)=O.OS(O)(=O)=O>CCO>[CH3:14][C:5]1[CH:4]=[C:3]([CH:1]=[CH2:15])[CH:13]=[CH:12][C:6]=1[C:7]([O:9][CH2:10][CH3:11])=[O:8]Ethyl 4-formyl-2-methylbenzoate (AI16). To a stirred solution of 4-formyl-2-methylbenzoic acid (3 g, 18.2 mmol) in EtOH (30 mL) was added conc. H2SO4 (2 mL) and the reaction mixture was heated at 80° C. for 18 h. The reaction mixture was cooled to 25° C. and concentrated under reduced pressure. The residue was diluted with EtOAc and washed with water. The combined EtOAc extracts were washed with brine, dried over Na2SO4 and concentrated under reduced pressure to afford the title compound as a solid (2.8 g, 80%): 1H NMR (400 MHz, CDCl3) δ 10.05 (s, 1H), 8.04 (m, 1H), 7.75 (m, 2H), 4.43 (m, 2H), 2.65 (s, 3H), 1.42 (m, 3H).
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