A simple one-pot and efficient method is described for the synthesis of pyrimido [5,4-c]pyridazines 5 and of thiino [2,3-d] pyrimidines 15 by a domino process involving an aza-Wittig reaction/heterocyclization. The iminophosphorane 2 reacted with phenylisocyanate, followed by heterocyclization on addition of amines to give the corresponding guanidine intermediates 4. The guanidine intermediates were cyclized in the presence of catalytic amount of sodium ethoxide to pyrimido[5,4-c]pyridazines 5. Similarly, iminophosphorane 12 reacted with phenylisocyanate and amines to give thiino[2,3-d]pyrimidines 15 in moderate yields. Furthermore, pyridazino[4,3-d]oxazines 10 and thiino [2,3-d]oxazines 19 were synthesized by the intremolecular aza-Wittig reaction of phosphazenes 2 and 12, respectively, with acid chlorides followed by heterocylization via imidoyl chloride intermediates.

Pyrimidopyridazines (VI) and thiinopyrimidines (XI) are synthesized by a domino aza‐Wittig—heterocyclization reaction of phosphazenes (III) and (X) with phenylisocyanate (IV) and amines (V).