Synthesis of 6-hydroxy-2-naphthoic acid from 2,6-diisopropylnaphthalene using NHPI as a key catalyst

A new strategy to 6-hydroxy-2-naphthoic acid (HNPA) and 4-hydroxybenzoic acid from 2,6-diisopropylnaphthalene and p-cymene, respectively, was developed using the NHPI-catalyzed aerobic oxidation as a principal reaction. 2,6-Diisopropylnaphthalene was oxidized by the oxidation with O2 (1 atm) by NHPI (10 mol %) combined with Co(OAc)2 (0.5 mol %) to give 6-acetyl-2-isopropylnaphthalene, which then was converted to 6-isopropyl-2-naphthoic acid under O2 (1 atm) in the presence of Co(OAc)2 (0.5 mol %) and Mn(OAc)2 (0.5 mol %). Esterification of the resulting acid followed by the aerobic oxidation produced methyl 6-hydroxy-2-naphthoate whose hydrolysis led to the desired HNPA. An alternative route involves the oxidation of 6-acetyl-2-isopropylnaphthalene to 6-acetyl-2-naphthol on which subsequent oxidation and deacetylation gave HNPA. This method was successfully extended to the synthesis of 4-hydroxybenzoic acid from p-cymene.

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