Double retro Diels-Alder reaction applied for preparation of a pyrimido[1,2-b]pyridazine

Boiling diendo- or diexo-2-aminobicyclo[2.2.1]hept-5-ene-3-carbohydrazides 1 with 3-aroylnorbornenecarboxylic acid 2 in toluene yielded the pyrimido[1 ,2-b]pyridazine 4 directly in a double retro Diels-Alder process. Similarly , the reaction of anthranilic hydrazide 5 and 2 furnished the tricyclic ben zo-fused analogue 7 via a single cycloreversion. The principle of the new m ethod applied: the reactants were built up on cyclopentadienes and the dien es were cleaved by heating after condensation to furnish hetero bicyclic co mpound.