Cyanomethyl pyridinium and isoquinolinium salts: a versatile chemical reagent for the synthesis of annulated heterocycles

Over the years, numerous advancements have been in synthesizing various heterocyclic compounds using pyridinium ylides. Imidazo pyridine derivatives are highly valued heterocyclic motifs that are commonly present in various natural and pharmaceutical compounds. These can be conveniently synthesized using cyanomethyl salts of pyridine and analogous isoquinolines. In this mini-review, we will discuss the latest developments regarding the use of these salts in the creation of annulated heterocycles. Various annulated heterocycles such as chromeno-imidazo pyridines, isoquinolines, imidazothiazine, chromenoimidazocarbolines, imidazo pyridines, chromeno azepines, pyrido indolizine carbonitriles, pyridoindolizines, cyanoindolizinyl acetamides, tetrahydroindolizines, indolizinoindol amines, pyridobenzimidazoles, cyclo azines, pyrroloisoquinolines have been synthesized under mild reaction conditions. These salts are also used for the synthesis of spirocyclic isoxazolo pyrrole isoquinolines and optically active pyrroloisoquinolines, pyrrolophthalazine, and tetrahydropyrrolophthalazinyl pentadienoate derivatives.