Condensation of phenols with ninhydrin; Substituted salicylic acidsSyntheticPage 6 (2001)
Submitted 11th Jun 2001, published 11th Jun 2001
A contribution from the Peter Scott
phenol (1 equiv.), ninhydrin (1 equiv.), 4-methylaninine (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.
Procedure works well on the large scale, several grams of pure salicyclic acid being available in a day or two. Each step has crystalline end products, no purification is needed.
Compounds made: 3-iso-propyl-2-hydroxybenzoic acid, 3,5-di-tert-butyl-2-hydroxybenzoic acid, 3,5-di-methyl-2-hydroxybenzoic acid.
Depends on the starting phenol.
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.
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