New Route to Pyrido[1,2-b]pyridazinium Inner Salts. Evidence of a 1,3-Dipolar Cycloaddition-Ring Expansion Process

The heterocyclic mesomeric betaines [1] 1 are well established versatile 1,3-dipoles, which allows them to take part in 1,3-dipolar cycloaddition reactions [2]. 2-Alkyl and 2-amino substituted structures 2 have the potential to function as 1,4-dinucleophiles via deprotonation and are capable of reacting with 1,2-diearbonyl compounds (Westphal reaction) [3] to give a variety of azonia derivatives possessing a quaternary bridgehead nitrogen [4-11]. By contrast, with the exception of a few examples [12-15] relatively less attention has been paid to the possibility of using ylides 3 as 1,4-nucleophile-electrophiles (Fig. 1).