Dihydropyridazine Derivatives with Cyclopenta‐, Benzo‐, Furo‐, Thiopyrano‐ and Pyrido‐Annulation

Abstract

Abstract Regioisomeric [ c ]annulated pyridazines were prepared from arylhydrazines and carbocyclic or heterocyclic β‐oxo esters with an α‐phenacetyl moiety. With AcOH/EtOH, the hydrazones were preferentially formed at the endocyclic ketone, which are further cyclized with trifluoroacetic acid (TFA)/CH 2 Cl 2 to give 2,4a‐dihydropyridazines. Use of TFA/CH 2 Cl 2 led hydrazones at the exocyclic benzoyl group, which reacted further to give 1,4‐dihydro‐4a H ‐pyridazines. In this investigation, examples of the rare or unknown heterocyclic systems furo[3,4‐ c ]‐, thiopyrano[4,3‐ c ]‐ and pyrido[4,3‐ c ]pyridazine were prepared.