Pyrimidines with acetylenic esters

The reaction of the pyrimidotriazine 4-oxide (315) with acetylenic esters results in ring contraction to give the pyrrolo [3,2-d] pyrimidine (318). 

In a more recent approach, 6-aminouracil or its 6-iminophosphorane is reacted with A -phenylbenzimidoyl chloride to give, in a Michael-type addition, a tautomeric pair of imines, A and B. The equilibrium is shifted in different solvents. Tautomer B is intercepted by an acetylenic ester to form a l,4-dihydropyrido[2,3-c/]-pyrimidine. The iminophosphorane of A and isocyanate results in an aza-Wittig reaction and polar 67r-electron cyclization reaction to give pyrimido[4,5-[Pg.159]

Adib has shown that pyridines undergo reaction with dialkyl acetylenedicarboxylates in the presence of isocyanates to produce functionalized 2-oxo-l,9a-dihydro-2//-pyrido[l,2-ajpyrimidines 43 in good yield <04TL1803>. The author postulates that the reaction proceeds through initial reaction of the pyridine 44 with the acetylenic ester 45, and the resulting anion then attacks the isocyanate 46 to yield a zwitterionic intermediate. The nitrogen of the zwitterionic intermediate adds to the pyridinium moiety thus generating the pyrido[l,2-a]pyrimidines 43. 

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