Pyrimidine, NMR spectra

The NMR spectra of pyrimidines bearing tautomeric groups (NH2, OH, SH) in the 2-, 4- or 6-position are discussed in connexion with the tautomerism of such pyrimidines (Section 2.13.1.8). 

There is an admirable summary of the stereochemistry of barbiturates and di- to hexahydropyrimidines Further information on reduced pyrimidines is collected <70HC 16-81)322) and some examples of the use of proton NMR spectra in elucidating the conformations of hydropyrimidines is given elsewhere (Section 2.13.1.3.1), based on the general principles of such work <65QR426). 

NMR spectra 31 were obtained from 6-butyltetrazolo[l,5-a]pyrimidine (/). What form does the heterocycle take ... 

In all types of NMR spectra H, C, N), 2-azidopyrimidine (2) can be distinguished by the symmetry of its pyrimidine ring (chemical equivalence of 4-H and 6-H, C-4 and C-6, N-1 and N-3) from tetrazolo[l,5-a]pyrimidine ( ) because the number of signals is reduced by one. Hence the prediction in Table 30.1 can be made about the number of resonances for the -butyl derivative. 

Nuclear magnetic resonance spectra of all four parent compounds have been measured and analyzed.The powerful potentialities of NMR as a tool in the study of covalent hydration, tautomerism, 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 N-bridgehead compounds such as pyrimido[l,2- ]pyrimidines and in several other structural assignments (cf. 74 and 75). 

Theoretical calculations (B3LYP/6-31G ) indicated that in their NMR spectra C(2) and C(9) carbons of n /2ydro-(2-hydroxy-4-oxo-4//-pyrido[l,2-n]pyrimidinium)hydroxide mesoionic forms appeared at significantly higher field (ca. 159-160 ppm and 115-116 ppm, respectively), than in the 2-hydroxy-4//-pyrido[l,2-n]pyrimidin-4-one tautomers (ca. 169-173 ppm and 130 ppm, respectively) (00JCS(P2)2096). C(8) carbon of mesoions (pyridine-y type carbon) appeared at lower field (144-146 ppm) than 6-C (140 ppm, pyridine-o type carbon), as is typical of pyridinium compounds. 

In the study on tetrasubstituted [l,2,4]triazolo[l,5- ]pyrimidines 45, H and 13C NMR spectra of 35 different compounds have been recorded <1995JHC407>. Substituents in position 2 were various amines and sulfano groups, whereas the other substituents (Rs, R6, and R7) were alkyl, phenyl, and acetyl groups. 

Attempts to detect the uncharged intermediates 6-amino-4-phenyl-l,6-dihydro-4-phenylpyrimidine (66), 2-amino-4-phenyl-l, 2-dihydropyrimidine (68), and the aminodiazahexatriene 67, by recording the NMR spectra of the solution during the stepwise addition of the ammonium chloride were not successful. The facile ring opening of the pyrimidine ring in 66 is not unexpected. It has also been observed in the reaction of A-methylpyrimi-dinium salts with liquid ammonia, which also involves as neutral covalent... 

The deoxyuracil derivatives-2-hydroxypyrimidine (69) (2-pymoH), 4-hydroxy-pyrimidine (70) (4-pymoH), 4,6-dimethyl-2-hydroxypyrimidine (71) (2-dmpymoH), and pyrimidine (72) (pym), shown here as the minor tautomer, have provided metallacalixarenes having closer resemblance to classical calixarenes, as compared with those obtained from uracil. The reactions of [(en)M(H20)2KN03)2 (M = Pd and Pt ) with 2-pymoH in water gave tetranuclear metallacalixarenes [(en)Pd(2-pymo-N2 N3)]4(N03)4 (73) and [ en)Ft 2-pjmo-Nl N3MN03)2 (74) (2000IC2301). As compared with uracil-based systems, the appearance of one set of resonances for each of pyrimidine PI, in their IT NMR spectra showed fast... 

In the NMR spectra, a push-pull effect of octahydropyrido[2,3- pyrimidine 28 was observed in the olefinic C-4a and G-8a due to the electronic behavior of their substituents. Thus, C a appears at low 6 values (88-93 ppm) and C-8a gives peaks at lower field (146.0-148.5 ppm) <1996JHC45>. 

Even though at present various obstacles prevent a more extensive application of NMR spectroscopy to the study of 77-electron distribution, it seems that it will be possible to make use of the interrelation between the magnitude of the chemical shift of a proton attached at a certain position and the 77-electron density at that position. Among the studies so far published attention should be drawn to the papers dealing with pyridine,111-112 substituted pyridines,113 and pyrimidine and its derivatives.114 Table VII presents data (taken from ref. 112) on experimental and theoretical 77-electron densities in pyridine and the pyridinium cation. The NMR spectra of some azines are briefly discussed by White.114"... 

The H NMR spectra of several thieno[2,3-d]pyrimidines (Figure 6) have been reported (76BSF761). Compared with thieno[3,2-d]pyrimidine (Figure 7) there is obviously no long-range coupling between the protons of the thiophene and pyrimidine rings. 

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