Zeolite catalyzed highly selective synthesis of 2-methoxy-6-acetylnaphthalene by Friedel-Crafts acylation of 2-methoxynaphthalene in acetic acid reaction media

Zeolite catalyzed Friedel-Crafts acetylation of 2-methoxynaphthalene to produce 2-methoxy-6-acetylnaphthalene with high selectivity and conversion has been a challenging task, because the obtained compound is a key intermediate for an anti-inflammatory agent, Naproxen. However, no satisfactory results have been obtained with zeolite catalysts, and harmful solvents have been used to gain a high selectivity together with a high conversion. Here, we report the synthesis of 2-methoxy-6-acetylnaphthalene from 2-methoxynaphthalene with a high selectivity and a high conversion under an unprecedented simple reaction system; acetic anhydride as an acetylating agent, acetic acid as a solvent, and proton-type zeolite catalysts with low acidity. Among the examined zeolites, a proton-type H-MOR (SiO₂/Al₂O₃ = 200) with a low acid content shows a conversion of 82% and an 86% selectivity for 2-methoxy-6-acetylnaphthalene. Further, detailed control experiments using H-MOR catalyst in acetic acid solvent were carried out to propose a plausible reaction mechanism.