Abstract
A new and convenient synthesis of 6-fluoro-4(1H)-oxopyrido[2, 3-c]pyridazine-3-carboxylate derizatives was achieved. One-pot reactions of ethyl 2-diazo-2-(6-chloro- and 6-tolylthio-5-fluoro-2-halonicotinoyl)acetates (9a and 9b, c) with tri-n-butylphosphine or tricyclohexylphosphine gave ethyl 7-chloro- and 7-tolylthio-6-fluoro-4(1H)-oxopyrido[2, 3-c]pyridazine-3-carboxylates (12a and 12b), respectively. The reaction of 9a-c with triphenylphosphine gave {[1-ethoxycarbonyl-1-(6-chloro- and 6-tolylthio-5-fluoro-2-halonicotinoyl)methylene]hydrazono}triphenylphosphoranes (10a-c, R=Ph), which were hydrolyzed to the corresponding hydrazones 11a-c. Intramolecular cyclization of the hydrazones 11b and 11c furnished an alternative and efficient synthesis of 12b. Possible mechanisms for the reaction of 9 leading to 12 are discussed.
