To a solution of 3-ethylphenol (6.11 g, 50 mmol) was sequentially added titanium (IV) chloride (100 ml, 100 mmol, 1 M solution in dichloromethane) and 1,1-dichlorodimethyl ether (7.47 ml, 82.5 mmol) at 0 °C. The reaction mixture was stirred at 0°C for 45 minutes. After completion of the reaction, the mixture was poured into ice water and extracted with ethyl acetate (EtOAc). The organic layer was washed with saturated saline, dried over anhydrous sodium sulfate and concentrated under reduced pressure to give a bright pink oil. Purification by fast column chromatography (eluent: hexane/ethyl acetate, 0 to 10% gradient) afforded 4-ethyl-2-hydroxybenzaldehyde (4.9 g, 65% yield) by first elution, followed by 2-ethyl-4-hydroxybenzaldehyde (1.32 g, 18% yield) by subsequent elution. 1H NMR (400 MHz, CDCl3) data of product A: δ 1.25 (t, J=7.6 Hz, 3H), 2.67 (d, J=7.6 Hz, 2H), 6.82 (s, 1H), 6.85 (d, J=8.0 Hz, 1H), 7.46 (t, J=8.0 Hz, 1H), 9.83 (s, 1H), 11.05 (s, 1H). 1H NMR (400 MHz, CDCl3) data of product B: δ 1.27 (t, J=7.6Hz, 3H), 3.03 (d, J=7.6Hz, 2H), 6.68 (s, 1H), 6.78 (s, 1H), 6.82 (d, J=8.3Hz, 1H), 7.77 (t, J=8.3Hz, 1H), 10.10 (s, 1H).
