Dimethyl Sulfoxide → 4-Chloro-2-ethoxybenzaldehyde

Oxalyl chloride (4.87 g, 38.4 mmol) is added to dry dichloromethane (80 ml) cooled to below −60° C. under an atmosphere of nitrogen followed by dropwise addition of a solution of dry dimethyl sulphoxide (6.25 g, 80.10 mmol) in dry dichloromethane (25 ml) with stirring. After stirring for 15 min. at −78° C., a solution of 4-chloro-2-ethoxybenzyl alcohol (5.97 g, 32.0 mmol) in dry dichloromethane (35 ml) is added dropwise to give a white suspension. The suspension is stirred at −78° C. for 0.5 h then added triethylamine (16.18 g, 0.16 mol) rapidly. The mixture is stirred at room temperature for 1.5 h before quenching with aqueous saturated sodium bicarbonate (100 ml). The aqueous phase is separated and extracted with dichloromethane (2×). The combined organic phases is washed with aqueous HCl (1M) and then with brine, dried over magnesium sulphate, filtered and evaporated to a yellow solid. The solid is dissolved in diethyl ether and purified by eluting through a short plug of flash silica. Evaporate to give 4-chloro-2-ethoxybenzaldehyde as a pale yellow solid.