Bicyclic Systems with Two Bridgehead (Ring Junction) Nitrogen Atoms

Abstract

CHEC-II(1996) first covered bicyclic systems with two ring junction nitrogen atoms as a separate chapter. Only a small number of new ring systems that come under the title of this chapter have been discovered since. Also for most ring systems covered in CHEC-II(1996), the newly published papers deal mostly with an extension of the science known. However, there are also some important exceptions, which are stressed in the text. The improved retrieving capability of SciFinder also revealed several ring systems overlooked during the CHEC-II(1996) preparation and the relevant ones are included into this edition. This chapter deals with three main types of bicyclic heterocycles containing two ring junction nitrogen atoms, namely bicyclic 5-5, 5-6, and 6-6 ring fused systems. Analogously as in CHEC-II(1996), only structures containing additional nitrogen, oxygen, and sulfur have been reviewed. All of these basic heterocyclic types could be present in several degrees of saturation. The most unsaturated forms, that are fully conjugated, are their monocationic (6-5 bicyclic systems), or bicationic systems (5-5 and 6-6 bicyclic systems). Mesomeric betaines, which cannot be described by a single structure and their real structure is a resonance hybrid of several dipolar structures, represent another degree of saturation. Of course, most examples discussed in the text are either partially saturated or fully saturated ring systems. Some practical applications, namely in fields of drug discovery, agrochemical research, analytical chemistry, and energetic materials, are also mentioned.