Pyrido pyrimidines, mesomeric

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>. 

Mesomeric betaines (116) and isocyanates gave rise to 4-oxo-4//-pyrido-[l,2- ]pyrimidines (117) in 36-64°/, yield.175 177... 

The structure of the reaction product of 2-aminopyridine and diethyl malonate, described by Chichibabin as 2,4-dioxo-3,4-dihydro-2//-pyrido-[l,2-<7]pyrimidine,96 was first questioned by Snyder and Robison253 on the basis of the high melting point and poor solubility of the compound. They suggested the tautomeric 2-hydroxy-4-oxo-4H-pyrido[l,2-a]pyrimidine structure. The problem was solved by Katritzky and Waring273 who compared the UV spectrum of the product with that of fixed tautomers and found that the product may best be described as anhydro- 2-hydroxy-4-oxo-4/f-pyrido[l,2- ]pyrimidinium)hydroxide (63). Because of the chemical behavior of these compounds, however, the contribution of other mesomeric forms to the structure has also been considered.122 Thus, PPP-SCF quantum chemical calculations suggest that 1,4-dipolar cycloadditions to the C-3 and C-9a atoms are to be expected.352 This type of reaction does in fact occur (see Section III,C,10). Katritzky and Waring273 estimated the ratio of the mesomeric betaine (63 R = H) and the 2-hydroxy-4-oxo tautomers to be about 20 1. 

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