A 500 mL side arm round bottom flask with a stirrer bar was placed under argon and charged with 4-tert-butylphenol (3.00 g, 20.0 mmol) and dry acetonitrile (100 mL). To this was added dry Et₃N (10.4 mL 75.0 mmol), anhydrous MgCl₂ (2.86 g, 30.0 mmol) and the solution was stirred for 30 min. Dry paraformaldehyde (4.05 g 135.0 mmol) was added and a Schlenk condenser fitted to the round bottom flask. The mixture was heated to reflux under argon for ca 3h. The resulting yellow solution was allowed to cool to room temperature and was added to 5 % HCl (aq.) (200 mL) followed by stirring for 30 min. This was followed by extraction with diethyl ether (7 × 25 mL). The ether fractions were combined, washed with saturated NaCl (aq.) (3 × 20 mL) and the organic layer dried over anhydrous NaSO₄ followed by filtration. All volatiles were removed under reduced pressure to yield a yellow oil which ¹H NMR spectra showed to be 5-tert-butyl-2- hydroxybenzaldehyde (2.97 g, 83 %).

1H NMR (300 MHz, CDCl3): δ ppm 10.86 (s, 1H, ArOH), 9.87 (s, 1H, HC=O), 7.57 (dd, 1H, ArH) 7.50 (d, 1H, ArH), 6.92 (d, 1H, ArH), 1.31 (s, 9H, CMe3).

MS (ESI): m/z 178 [M]⁺, 163 [M-CH3]⁺, 161 [M-OH]⁺, 148 [M-(CH3)2]⁺, 133 [M-(CH3)3]⁺