Recent Advances in the Area of Pyridodiazinium Systems Containing Bridgehead-Nitrogen Atom

A survey is provided on new synthetic pathways to pyrido[1,2-a]pyrimidinium, pyrido[1,2-a]pyrazinium, pyrido[1,2-c]pyrimidinium, and pyrido[1,2-b]pyridazinium systems and their partially saturated derivatives appeared during the recent years. Reaction mechanisms of these ring closure methodologies are also discussed. Some representative further transformations of these ring systems are also summarized. Thus, alkylation of olates and thiolates, ring opening reactions, photochemical transformations and flash vacuum pyrolytic reactions are reviewed.