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Calculated Values fob the Wavelength Maxima (m^i) in the Ultraviolet Spectra of Pyridopyrimidines0
77 -> 77* transition band
Compound 1 2 3
Pyrido[2,3-djpyrimidine 300 264 212
Pyridof 3,2 -d]pyrimidine 300 261 218
Pyrido[3,4-d]pyrimidine 307 269 219
Pyrido[4,3-iZJpyrimidine 295 279 220
a G. Favini, I. Vandoni, and M. Simonetti, Theoret. Chim. Acta 3, 418 (1965).
These results were in fair agreement with the experimentally deter-mined values33 for the parent compounds in nonionic solvents. Additional values for the n -> 77* transitions have been determined for pyrido[3,2-d]pyrimidine (345 m/x loge 2.07) and for pyrido[4,3-d]-pyrimidine (330 ̣ö, loge 2.49).33
Substituted pyridopyrimidines show the same three principal (7r 77*) absorption bands as the parent compounds but with batho-
chromic shifts which may obliterate bands due to the n > 77* transitions.
107 G. Favini, I. Vandoni, and M. Simonetti, Theoret. Chim. Acta 3, 45 (1965).
108 G. Favini, I. Vandoni, and M. Simonetti, Theoret. Chim. Acta 3, 418 (1965).
The prediction of the values of such shifts has been considered in closely related iV-heteroaromatic systems.100 Ultraviolet absorption maxima have been determined for pyrido| 2,3-eZ]pyrimidines,9¦1(>! 16-33-41-44, 5â, no-112 pyrido[3,2-ci]pyrimidines,10•le' 33’79, 81 pyrido[3,4-d] pyrimidines,33'91 andpyrido[4,3-cZ]pyrimidines.83 The experimentally determined values have been used for studies of covalent hydration, structural assignments,9-10’41> 44 and tautomerism.10,118
C. Infrared Spectra
Armarego et al.n4 have determined the infrared spectra of the four parent pyridopyrimidines (148) in the solid phase as KBr discs, and have compared them with other di-, tri-, and tetraazanaphthalenes. Thirteen in-plane skeletal vibrations and ten CH bending vibrations
are theoretically possible in the 1700-650 cm'1 region. Slightly less than this number of bands were actually observed and the results did not provide a simple criterion for distinguishing between predominantly skeletal and predominantly CH vibrations. CH out-of-plane bending vibrations were thought to account for most of the intense bands found in the 1000-650 cm-1 region. CH stretching bands in the range 3100-3000 cm-1, and overtone and combination bands in the range 2000-1750 cm-1, were also observed.
109 S. E\ Mason, in “Physical Methods in Heterocyclic Chemistry” (A. R. Katritzky, ed.), Vol. II, Chapt. 7. Academic Press, New York (1963).
110 R. K. Robins and G. H. Hitchings, J. Am. Chem. Soc. 78, 973 (1956).
111 Y. Inoue and D. Ë. Perrin, J. Chem. Soc. p. 5166 (1963).
112 G. H. Hitchings, Drugs, Parasites Hosts, Symp. Middlesex Hosp. Med. School p. 196 (1962).
113 A. R. Katritzky and J. M. Lagowski, Advan. Heterocyclic Chem. 1, 341—436
114 \\' l ó Armarego, G. B. Barlin, and E. Spinner, Spectrochim. Acta 22, 117
Mason115 has determined the infrared spectrum of pyrido[3,2-d]-pyrimidin-4(3#)-one (149, N in position 5) in chloroform solution and as a KBr disc and has suggested that the low frequency of the NH band (3389 cm-1) and high frequency of the C=0 band (1746 cm-1) in the solution spectra are indicative of a quasi o-quinonoid form. The infrared spectra of the four pyridopyrimidin-4(3.H’)-ones (149), the four 2,4(l-Hr,31/)-diones (150), and a number of substituted derivatives, have been determined, asNujol mulls, in these laboratories.23-8e> 91,103' 104, no The presence of NH stretching absorption in the range 3200-3060 cm-1 in these compounds, and of C=0 stretching absorption in the range 1730-1690 cm-1, both in the pyridopyrimidinones themselves and in their 1- and 3-methylated derivatives suggest that the oxo structures depicted (149 and 150) are the predominant forms in the solid phase. In the absence of solvent shift studies it remains possible that some of the bands also observed in the 1626-1510 cm-1 region may also have been at least partly “ carbonyl ” in character as is the case with pyridin-4-ones113 and quinolin-4-ones.113
D. Nuclear Magnetic Resonance Spectra
Nuclear magnetic resonance spectra of all four parent compounds have been measured and analyzed.117 The powerful potentialities of NMR as a tool in the study of covalent hydration,2 tautomerism,118 or protonation have, however, as yet received no consideration for the pyridopyrimidines. NMR spectra have been used to distinguish between pyrido[3,2-d]pyrimidines and isomeric iV-bridgehead compounds such as pyrimido[l,2-a]pyrimidines44 and in several other structural assignments 44, 66, 60> 125 (cf. 74 and 75).
The NMR spectral parameters of a number of pyridopjrrimidines are shown in Table II.
The signal from H-4 in the parent compounds underwent a greater downfield shift than that from H-2 when the solvent polarity was increased (CDC1S —> Me2CO - > DMSO).117 This was ascribed to the counterbalancing effect of preferential solvation at the N-l position.