Chem. Pharm. Bull. 54(12) 1653—1658 (2006)

cellent s-donors. They readily form complexes with transition metals. Since 1990s, the use of NHCs as ligands has lead to significant advancements in the area of Pd-catalyzed carbon–carbon bond-forming reactions, Ru-catalyzed olefin metathesis, and Rh-catalyzed hydrosilylations. NHCs have also attracted significant attention as organocatalysts. Several reactions have been catalyzed by NHCs, for e.g., benzoin condensation, Stetter reaction, transesterification/acylation reaction, and cyanosilylation. We have previously reported NHC-catalyzed nucleophilic substitution on aromatic heterocyclic rings. It was considered impossible to introduce aroyl groups directly into the electron-deficient positions of aromatic rings in the conventional electrophilic acylation reaction—Friedel–Crafts reaction. In this NHC-catalyzed reaction, the aroyl groups (derived from aromatic aldehydes) are directly introduced into heteroarenes by nucleophilic aromatic substitution under the catalysis of 1,3-dimethylimidazolidenyl carbene 2 that is obtained from 1,3-dimethylimidazolium iodide (1) (Chart 1). In other words, aromatic ketones can be synthesized from aldehydes and heterocyclic compounds by NHC catalysis in a single step. We have recently broadened the scope of reaction substrates to include benzene rings by using fluoride ion as a leaving group. Aroylation was presented as a new method to synthesize benzophenone derivatives from fluorobenzenes and benzaldehydes. The reaction mechanism is shown in Chart 2. In order to develop a new synthetic route to heterocyclic compounds, we examined NHC-catalyzed aroylation as a tool for the syntheses of heterocycles. In this study, we report the syntheses of xanthones 6, acridones 7, and other heterocyclic compounds via NHC-catalyzed aroylation (Chart 3). Results and Discussion Among the xanthone and acridone families, there are quite a few compounds that are reported to exhibit bioactivities such as antitumor, antibacteria, and antivirus. From both chemical and pharmaceutical viewpoints, it would be significant to provide a new synthetic route to these heterocyclic compounds. There are two major routes to synthesize xanthones. The first one is based on the cyclization of 2-hydroxybenzophenones, and the second involves the cyclodehydration of o-phenoxybenzoic acids. Our method is based on the cyclization of 2,2 -difluorobenzophenones 5 prepared by NHC-catalyzed aroylation of 3,4-difluoronitrobenzene (3) December 2006 1653 Chem. Pharm. Bull. 54(12) 1653—1658 (2006)