2- Oxo-3,4-dihydro-277-pyrido

Oxo-3,4-dihydro-2//-pyrido[l,2-ci]pyrimidines (142) and their hydrogen halide salts can be prepared by reacting 2-aminopyridine or methyl-substituted 2-aminopyridines with alkyl 3-halopropionates,20,196-200 3-halopropionic acids,200-203 or alkyl acrylates18,20 197,204 205... 

Oxo-3,4-dihydro-2//-pyrido[l,2-a]pyrimidines (142) may easily be converted (e.g., by heating in water) to the betaines (143)200,204 The latter can also be prepared from 2-aminopyridines with propiolactone,206 acrylic acid,18 3-substituted propionitriles, and other acrylic acid derivatives.207,208 The betaines (143) revert to the bicyclic compounds (142) upon heating in acid 200 204-206-207... 

Baltrusis and Beresnevicius205 found that a mixture of 2-amino-5-chloropyridine and methyl acrylate in acetic acid in a sealed tube at 100 C gives 4-oxo-2,3-dihydro-4H-pyrido[l,2-a]pyrimidine (147 X = Cl) and the propionic acid (144 R = 5-0, R1 = H), but at room temperature the mixture instead gives 2-oxo-3,4-dihydro-2//-pyrido[l,2-a]pyrimidine (142 R = 7-C1) and the propionate (144 R = 5-0, R1 = Me). Compound (147 X = Cl)... 

On the action of water200-204-205 or ammonia,200 2-oxo-3,4-dihydro-2//-pyrido[l,2-a]pyrimidines (142) undergo a facile ring opening. The betaine-type products (143) can be recyclized to the parent pyridopyrimidines by heating with acids. Treatment with alkalis leads to 2-aminopyri-dines.200,205,208... 

The pyrimidine ring of 2-oxo-3,4-dihydro-2//-pyrido[l,2-a]pyrimidines is also susceptible to ring opening on the action of bases.205,215... 

Oxo-3,4-dihydro-2H-pyrido[l,2-fl]pyrimidines are neutral or slightly basic acid binders in the preparation of esters, amides, lactones, ethers, and... 

Fluor-l-methyl-pyridinium-tosylat/2-Oxo-3,4-dihydro-2H-128 2-Chlor-l-methyl-pyridinium-jodid/2-Oxo-3,4-dihydro-2H-128... 

Irradiation of 9-substituted 6-oxo-3,4-dihydro-2//,6//-pyrido[l,2-Z)]-[1,3]thiazine-4-carboxylate 93 in benzene afforded tricyclic derivatives 94, sometimes as a diastereomeric mixture (00JCS(P1)4373). 

Dihydro-2//-pyrido[l,2-n]pyrimidin-2-one was used in the synthesis of antiallergic tricyclic triazolobenzazepine derivatives (99MIP3). 8-[2-(4-f-Propyl-2-thienyl)ethenyl]- and 8-[(4-/-propryl-2-thienyl)methoxy]-4-oxo-4//-pyrido[l,2-n]pyrimidine-3-carboxylic acids were patented for the treatment of preventing and/or treating microbial infectious diseases (OlMIPl). 

Thermolysis of 3-alkyloxy- and 3-aroyloxy-5-phenyl-l,2,4-pyrrolo[l,2- ]quinoxaline-l,2,4-triones afforded 5-phenyl-9-(4-phenyl-3-oxo-3,4-dihydro-2-quinoxalinyl)-6,8,10-trioxo-5,6,9,10-tetrahydro-87/-pyrido[l,2-tf ]quinoxaline-7,9-dicarboxylates <2000CHE615, 2004CHE1295> and 7-aroyl-8-aroyloxy-5-phenyl-9-(4-phenyl-3-oxo-3,4-dihydro-2-quinoxalinyl)-5,6-dihydro-1077-pyrido[l,2- ]quinoxaline-6,10-diones <2002CHE498>, respectively. Thermolysis of 3-aryl-2-(5-aryl-2,3-dioxo-2,3-dioxo-4-furanyl)quinoxalines gave 7-aroyl-8-aroyloxy-6-aryl-9-(3-aryl-2-quinoxalyl)-10/7-pyrido[l,2- ]quinoxalin-10-ones <2001CHE1314, 2002RCB850>. 

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. 

Methoxyphenyl)-3,4-dihydro-2/7-pyrimido[2,1 - z]isoquinoline exhibited good inhibition of melanocortin-4 receptors (01MIP5). A-Methyl-ll-oxo-ll/7-pyrido[2,l-Z7]quinazolin-7-carboxamide did not exhibit any MAO A and B inhibitory activities (97JMC2466). 

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