Bicyclic 5-6 systems: five heteroatoms 2:3 or 3:2

Bicyclic 5-6 heterocyclic systems with five heteroatoms distributed 2:3 or 3:2 among two fused rings and with no heteroatoms at ring junctions are reviewed. Reactions of azoloazines include at the nitrogen in which the coupling with sugars gave isosteres of purines. Nucleophilic substitutions at the fused trazinone, pyridazinone ring and substitution of various leaving groups are further azoloazine reactions. Structure elucidation of pyrazolo[4,3-e][1,2,4]triazine natural products is described. Reactions of imidazo[4,5-c]thiadiazines afforded SO2-analogs of acyclovir. [1,3]Thiazolo[4,5-e][1,2,4]triazines were formed by oxidative aromatization, and electrophilic attack at sulfur occurred with thioamides of pyrazolo[3,4-d][1,3,2]diazaphosphinine. Reactions of substituents include condensation of hydrazine derivatives giving triazoles. The synthesis of the fused six-membered ring was achieved by diazotization, hydrazine condensation, and condensation of imine derivatives giving [1,2,3]triazines, pyridazines, and pyrimidines, respectively. Six-membered ring annulations are an important route to oxazoloazines and azolooxazines. Lawesson’s reagent was used to fuse a [1,3,2]diazaphosphinine ring, and thionyl chloride introduced the sulfonyl group into [1,2,5]thiadiazolo[3,4-c][1,2,6]thiadiazines and [1,2,5]selenadiazolo[3,4-c][1,2,6]thiadiazines. The synthesis of the fused imidazole, pyrazole, [1,2,3]triazole, and [1,2,5]oxadiazole rings was used to give imidazo[4,5-e][1,2,4]triazines, pyrazolo[4,3-e][1,2,4]triazines, [1,2,3]triazolo[4,5-d]pyrimidines, and [1,2,5]oxadiazolo[3,4-d]pyrimidines, respectively. Five-membered condensation reactions were used to give thiazoloazines, azolothiazines, and selenium analogs. Various rings were formed by transforming another ring. Important applications of these heterocycles are described.