Phase-transfer-catalyzed methylation of hydroxyaromatic acids, hydroxyaromatic aldehydes, and aromatic polycarboxylic acids

The phase-transfer-catalyzed methylation of hydroxyaromatic acids, hydroxyaromatic aldehydes, and aromatic polycarboxylic acids with iodomethane or dimethyl sulfate in dichloromethane-water solvent system at room temperature using tetra-n-butylammonium hydroxide provided moderate (68%) to near quantitative yields of O-methylation products. It was observed that iodomethane tends to give more ring methylation products than dimethyl sulfate under identical conditions. The methylation products were identified by their spectral characteristics. The 13C nuclear magnetic resonance specta of the methyl derivatives are discussed in detail.