Chapter 6.1: Six-Membered Ring Systems: Pyridine and Benzo Derivatives

Abstract

This chapter focuses on developing the chemistry of pyridines and their benzo derivatives because of their unique chemical, biological, and electronic properties. In particular, efforts have been directed towards establishing more efficient, practical, and environmentally friendly procedures involving these molecules. Cyclocondensations and related polar cyclizations are the most widely used methods for preparing pyridines, dihydropyridines, and pyridinones. The reactions are routinely employed in the synthesis of biologically important pyridines and those of interest in material sciences. In general, these reactions are catalyzed by base as well as by Brønsted and Lewis acids. Efforts have been directed towards utilizing multicomponent cyclocondensation reactions as one-pot procedures to conveniently synthesize a variety of pyridine derivatives in an efficient manner. The Hantzsch synthesis of pyridine is a cyclocondensation method of considerable importance. This route classically involves the condensation of four components, including two molecules of β-carbonyl compounds, an aldehyde, and ammonia (or an equivalent) to form 1,4-dihydropyridines, which can be aromatized into pyridines.