Bicyclic 5-5 Systems with One Bridgehead (Ring Junction) Nitrogen Atom: Four Extra Heteroatoms 3:1

Abstract

This review compiles the recent advances concerning the synthesis and the chemistry of fused bicyclic 5-5 ring systems with one bridgehead nitrogen atom and four additional heteroatoms. The majority of compounds derive from the class of pyrazolo[1,5-d]tetrazoles while some tri- and polycyclic derivatives are also covered. Besides theoretical considerations describing the equilibrium between monocyclic azide derivatives and bicyclic tetrazole derivatives, and the factors that determine the equilibrium, also the reaction pathways for the cyclization reaction leading to the bicyclic systems are summarized. Furthermore, selected analytical data are presented, such as the chemical shift of the bridgehead carbon atom in 13C NMR spectroscopy, as well as IR and UV data. The newest synthetic approaches for the general synthesis of 5-5 bicyclic ring systems incorporating photochemical reactions, synthetic approaches via condensation, reactions of sulfenyl chlorides, as well as intramolecular [3+2] cycloaddition reactions of azides with heteroatom-bonded nitriles or nucleophilic aromatic substitutions are covered in detail. Some 5-5 bicyclic ring systems have been tested for biological activity, and the industrial applications of 5-5 bicyclic compounds as imaging materials, solar cell materials, hair dyes, or inkjet colorants are covered.