9- tetrahydro-47/-pyrido tautomerism

Oxo-2,3,6,7-tetrahydro-5//-pyrido[l,2,3- /e]-l,4-benzoxazine-2-carbox-ylates (291, X = H2) were obtained by hydrogenation of 4-substituted 3, 4-dihydro-2//-1,4-benzoxazine-2-carboxylates (290, X = H2) over 10% Pd/C catalyst, then by the treatment of free acids with (CF3C0)20 (99EUP894796). 5,7-Dioxo-2,3,6,7-tetrahydro derivatives 291 (X = 0) were prepared similarly from 290 (X = O). The products 291 (X = O) exist in 7-hydroxy-5-oxo-2,3-dihydro-5// tautomeric form. 

Ethyl 4-oxo-6,7,8,9-tetrahydro-4//-pyrido[l,2-n]pyrimidine-3-carboxyl-ate was alkylated at position 9 with isocyanates and isothiocyanates, occasionally in the presence of sodium hydride.306,320 The 9-carbamoyl compounds exhibited triple tautomerism between forms 251 and 253.320... 

The UV spectra of 9-(phenylamino)tetrahydro-4f/-pyrido[ 1,2-a] pyrimidin-4-ones 23 indicated the presence of an imine-enamine type of tautomerism in solution. A carboxyl or ester group in position 3 (R1 = COOR3 R3 = H, Et) and a methyl group in position 6 (R = Me) shifted the equilibrium toward the enamine form in a polar solvent [85JCS(P1) 1015]. For the imine tautomer, UV maxima at 235-250 nm and 280-300 nm and for the enamine absorbances at 260-265 nm and 355— 365 nm are characteristic. 

The 9-benzoyl- and 9-[(ethoxycarbonyl)carbonyl]tetrahydro-4//-pyrido[l,2-a]pyrimidin-4-ones also exhibit predominantly the 1,6,7,8-tetrahydro tautomeric forms in solution [85JHC593 89JCS(P2)1613]. 

Dioxo-6,7,8,9-tetrahydro-4//-pyrido[l, 2-a]pyrimidines 35 exhibit an oxo-enol tautomerism, which was studied by UV, IR, and NMR methods (85JHC1253). In the solid phase enol tautomers are present, while in a polar solvent an electron withdrawing substituent at position 3 shifts the equilibrium toward the enol form. The analogous 9-amino derivatives exist exclusively as 9-amino-6,7-dihydropyrido[l,2-a]pyrimidin-4-ones. 

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