phenol (1 equiv.)
ninhydrin (1 equiv.)
4-methylaniline (2 equiv.)
acetic acid (100 ml / 0.1 mol)
2N sodium hydroxide (30 ml / 0.05 mol)
H₂O (10 ml / 0.05 mol)
6N hydrochloric acid (10 ml / 0.05 mol)

The phenol is added to a hot solution of ninhydrin in acetic acid and the mixture heated at reflux for 2h. After cooling to room temperature the resulting precipitate was isolated by vacuum filtration (1), washed with ice-cold acetic acid and dried in vacuo, 2nd crop can be collected. 4-Methylaniline is added to a solution of (1) in acetic acid and stirred under reflux for 12h The mixture is allowed to cool to room temperature before the resulting precipitate was isolated by vacuum filtration, washed with ice-cold acetic acid and dried in vacuo, 2nd crop can be collected. (2) (2) is added to aqueous 2N NaOH and refluxed for 15 min, resulting white ppt was isolated by filtratrion and washed with water. Yellow filtrate and washing were then cooled and acidified with 6N HCl, resulting white ptp is isolated by vacuum filtration, washed with cold water and recrystallized from ethanol/water to give a white solid.

Depends on the starting phenol. 3-iso-Propyl-2-hydroxybenzoic acid ¹H NMR (CDCl₃): 10.91 (brs, 1H, OH), 7.82 (d, J = 6, 1H, Ar-H), 7.45 (d, J = 6, 1H, Ar-H), 6.88 (t, J = 6, m1H, Ar-H), 4.71 (brs, 1H, OH), 3.38 (sept, J = 7, 1H, CHMe2), 1.23 (d, J = 7, 6H, CHMe2).

Schmitt, G. Synthesis 1984, 758.