Pyrimidinium salts, 1,2-dihydro-, reaction

Considerable rate enhancements are observed upon quaternization of the pyrimidine ring, due to its electron-deficiency. Heating of the l-ethyl-2-(pent-4-yn-l-yl)-5-phenylpyrimidinium tetrafluoroborate (X = CH2) at 180°C for 15 min ( ) gave in a more or less quantitative yield 1-ethyl-6,7-dihydro-3-phenyl-5//-l-pyridinium salt (X = CH2) (90T595) (Scheme 36). A similar reaction was also observed with the 2-(butynylthio)pyrimidinium salt (X = S). 

Certain types of pyrido[l,2-a]pyrimidines have aroused much interest owing to their valuable pharmacological properties. They are also used as synthetic intermediates or as additives to photographic materials and dyes. In the following sections the syntheses, reactions, physicochemical properties, and briefly the utilization of the pyrido[l,2-ci]pyrimidines are discussed. Within the individual sections the pyrido[l,2-ti]pyrimidinium salts, followed by the 2-oxo-2//-pyrido[l,2-a]pyrimidines, 4-oxo-4H-p3 rido[l,2-a]pyri-midines, 3,4-dihydro-2/7-pyrido[l,2- ]pyrimidines, 2-oxo-3,4-dihydro-2H-and 4-oxo-2,3-dihydro-47/-pyrido[l,2-a]pyrimidines, and finally miscellaneous pyrido[l,2- (]pyrimidines are dealt with. 

In the 2,3-dihydro-5-oxo-5Ff-oxazolo[3,2-c]pyrimidinium salt (207) there are three sites for reactions with nucleophilic reagents, viz. C-2, C-8a and C-7. Products resulting from attack at C-2 are observed with DMSO, water, alcohols, benzoate, chloride, diethylamine and pyridine. Products resulting from attack at C-8a are observed with water, hydroxide, alcohols, alkoxide and isopropylamine. Diethylamine also causes attack at C-7 of the cation, which results in cleavage of the pyrimidine ring (75JOC1713). 

---