Regioselective direct ortho C-acylation of phenols catalyzed by ZnCl2 supported on multi-walled carbon nanotubes as catalyst under solvent-free and microwave conditions

Abstract

Acylated phenols and naphthols are an important structural motif found in a diversity of bioactive molecules, especially in the synthesis of natural compounds (some examples in Figure 1). Also they serve as protecting groups [1], polyfunctional molecules such as nucleosides, carbohydrates, peptides [2] or modified peptideand bond such as; isoesters [3] steroids [4]. ortho-Hydroxyacetophenone is a key intermediate for producing 4-hydroxycoumarin and warfarin, which are both used as anticoagulant drugs [5], and it has been also employed for obtaining flavones [6]. The goal of the present work is to study and compare various processes of the ortho-acylation of hydroxyaryl compounds with organic acids, catalyzed by ZnCl2 supported on MWCNTs as new catalyst, and to evaluate the influence of these treatments on the catalytic performances of ZnCl2@MWCNTs catalysts for selective acylation. The direct ortho C-acylation of phenol formed the phenylacetate intermediate, which is consecutively transformed via intermolecular phenol/phenylacetate C-acylation. Reactions were accomplished in the absence of solvent, to afford the corresponding ortho-acylated hydroxyaryl compounds, in high yields (scheme 1).