Pyrimidine mesomeric betaines, fused

Dipolar cycloadditions of dihydropyrimidine-fused mesomeric betaines 389, 391 and 394 with different dipolarophiles afforded 6-oxo-6H-pyrido[l,2-n]pyrimidine-3-carboxylates 390, 392, 393 and 396 (97JOC3109). 

The Diels-Alder cycloaddition potential of fused 4-aryldihydropyrimidine mesomeric betaines has been studied. The cross-conjugated thiazinium betaine 317 underwent 1,4-dipolar cycloaddition with electron-rich dipolaro-philes, and thus 1-diethylaminoprop-l-ine gave the pyrido[l,2-tf]pyrimidine 318 by loss of carbonyl sulfide (Equation 34). Reaction of 317b with 1,1-diethoxyethene resulted in the 8-ethoxy analogue of 318 (R = H) <1997JOC3109>. 

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