New route to pyrido[1,2-b]pyridazinium inner salts. Evidence of a 1,3-dipolar cycloaddition-ring expansion process

2-Alkoxycarbonylpyridinium N-aminides behave as 1,3-dipoles when reacted with Michael accepters, giving rise to the corresponding cycloadducts which, depending on their regioisomeric nature, subsequently undergo a ring expansion to give pyrido[1,2-b]pyridazinium inner salts.We wish to express our thanks to the Comisión Interministerial de Ciencia y Tecnología (CICYT, project PM97-004) and to the Ministerio de Educación y Ciencia for a grant to one of\ud us (J.V.M)