Bicyclic 5-6 Systems with One Bridgehead (Ring Junction) Nitrogen Atom: Two Extra Heteroatoms 2:0

Abstract

Bicyclic 5-6 heterocyclic ring system incorporating one ring junction nitrogen atom and two extra heteroatoms (mainly N, O, S, B, P, and Si) are reviewed in this chapter, from 1996 to 2006, covering various aspects such as theoretical methods, structural determination (mainly NMR and X-ray), thermodynamic properties, reactivity of the ring atoms, reactivity of the substituents attached to the ring, ring synthesis classified by number of ring heteroatoms, ring synthesis from cyclic starting material, and applications. Many reactions which were performed with these molecules are aimed at incorporating these synthons as potential pharmacophores in view of studying their structure–activity relationships. Among the 20 different ring systems covered by this chapter, by far the most represented are the triazolopyridines. This bicyclic system gives rise to three possible isomers which differ by the position of the nitrogen atoms in the fused triazole ring. Triazolopyridines were extensively studied for their useful biological properties. Many of these compounds were prepared as enzyme inhibitors, antimicrobial agents, anionic sugar mimics, or potential helicating agents. Those properties have attracted much attention for the preparation of such compounds, extending the scope of their syntheses toward mild and versatile experimental protocols compatible with the presence of other functional groups.