Acylation of 2-methoxynaphthalene with acyl chlorides in the presence of a catalytic amount of Lewis acids

Abstract

The regiochemistry of the reaction of 2-methoxynaphthalene 1 with benzoyl chloride 2a using a catalytic amount of a Lewis acid is strongly influenced by the identity of the acid catalyst employed as well as by the reaction temperature. By using InCl₃, FeCl₃, SnCl₄ or ZnCl₂ and heating at 160 °C, 2-benzoyl-6-methoxynaphthalene 4a is selectively produced along with 1-benzoyl-7-methoxynaphthalene 5a, while in the case of AlCl₃, SbCl₅ or TiCl₄, 1-benzoyl-2-methoxynaphthalene 3a is the major product. 2-Acyl-6-methoxynaphthalenes 4b–e can be selectively obtained using InCl₃ and the corresponding acyl chlorides 2b–e in place of 2a. In the presence of a stoichiometric amount of InCl₃, the reaction of 1 with 2a also gives 4a as the predominant product along with 5a even at 50 °C. This reaction appears to involve isomerisation of 3a to 4a and 5a.