Tautomerism of pyrimidines

The present volume encompasses a wide range of heterocyclic chemistry. Syntheses of heterocycles from thioureas are reviewed by T. S. Criffin, T. S. Woods, and 1). L. Klayman, while S. W. Schneller describes the chemistry of benzothiins and their derivatives (thiochromans, thiochromones, and thio-chromanones). Developments in chrom-3-ene chemistry are reviewed by L. Merlini. F. D. Popp contributes a chapter on the isatins. A discussion of theoretical aspects of the tautomerism of pyrimidines, by J. S. Kwiatkowski and B. Pullman, follows up a corresponding earlier contribution (Vol. 13) on tautomeric purines. In the final chapter P. and D. Cagniant describe the natural occurrence and synthesis of the benzofurans. 

The tautomerism of pyrimidines was discussed in Chapter 2, Section A,6. The tautomeric forms shown in Fig. 5-1 predominate. However, it is possible that minor tautomers such as the following are sometimes preferentially bound into active sites of enzymes where the dielectric... 

Valence tautomerization of pyrimidine-alkyne photoadducts is mediated by treatment with trimethylsilyl chloride/hexamethyldisilazane, followed by silica gel, affording diazocin-2,4-dione derivatives (8) in good yield (vide supra) <82TL4207,83JOC2337). Oxidative cleavage of a bicyclic diaza compound by rose Bengal-sensitized addition of oxygen affords the benzo-3,4-dihydro-1,3-diazocin-2,6(17/,5//)-dione derivative (38) (Equation (15)) <87H(26)2477>. 

D. Shngar and K. Szczepaniak, Tautomerism of pyrimidines and purines in the gas phase and in low-temperature matrices, and some biological implications, Int. J. Quantum Chem. 20 (1981), pp. 573-581. 

The structure of pyrimidines with two or more tautomeric groups... 

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). 

Since IR spectra are essentially due to vibrational transitions, many substituents with single bonds or isolated double bonds give rise to characteristic absorption bands within a limited frequency range in contrast, the absorption due to conjugated multiple bonds is usually not characteristic and cannot be ascribed to any particular grouping. Thus IR spectra afford reference data for identification of pyrimidines, for the identification of certain attached groups and as an aid in studying qualitatively the tautomerism (if any) of pyrimidinones, pyrimidinethiones and pyrimidinamines in the solid state or in non-protic solvents (see Section 2.13.1.8). 

Hydroxypyrimidine (89a) can tautomerize to two alternative 0X0 forms, 90 (R = H) and 91 (R = H). The infrared solution spectra of pyrimidin-2- and -4-one clearly indicate the presence of both C=0 and NH groups, and by using these spectra Mason demonstrated that pyrimidin-4-one existed predominantly in the o-quinonoid... 

No data on tautomerism of dihydropyiimidines were available at the time of the early summary (76AHCS1), but much has been done since then. The results of tautomeric studies carried out during the period between 1976 and 1984 were reviewed comprehensively in [85AHC(38)l,pp. 63-77]. Later, Weis and vanderPlas published an excellent review on the synthesis, structure, and tautomerism of dihydropyrimidines [86H(24)1433], where the tautomeric interconversions of these compounds were discussed in detail. In a more recent review on dihydropyrimidines (94MI1), the question of tautomerism in partially hydrogenated pyrimidines was also included. 

Cycloaddition of azirines 5 to 1.2,4,5-tetrazines 6 is followed by loss of nitrogen and ring enlargement to yield 5//-1,2,4-triazepines 7, which tautomerize spontaneously by a [1,51-hydrogen shift to the 2/7-1,2,4-triazepines 8. The triazepinesare accompanied by variable amounts of pyrimidines and pyrazoles.335 - 338... 

An important method for the synthesis of 1,3-diazocines consists of the reaction of pyrimidines 1 with ynamines to give [2 + 2] cycloadducts, which rearrange to the eight-membered-ring compounds 2 by valence tautomerization.2-3... 

Figure 33-2. Tautomerism of the oxo and amino functional groups of purines and pyrimidines.

An El report deals with the tautomerism of unsubstituted and the differentiation of substituted pyrimidin-4(3//)- and -4(l//)-ones (90OMS115). Another publication on 2-alkyl (Et, Pr) or 2-arylmethyl-substituted derivatives shows that the substituent decisively influences their fragmentation and that, especially in the case of 2-arylmethyl substituents, an intramolecular cyclization in the formation of the ion again becomes very... 

The substituent tautomerism of six-membered ring heterocycles, including pyrimidines, quinazolines, and perimidines, has recently been reviewed in detail <2006AHC(91)1>. 

Carbon-13 magnetic resonance spectra of the naturally occurring cytidines have been described in several papers.79-82 The electronic structure of the compounds is reflected in the carbon-13 shifts. For instance, the observed chemical shifts for these and other pyrimidine and purine nucleosides were correlated, at least qualitatively, with the calculated charge densities (see Section VIII) and with the known reactivity of these molecules. It is difficult to draw conclusions from the carbon-13 spectra about the tautomerism of cytosine. 

Daniels352 has shown that the excitation spectra for triplet state formation and fluorescence emission from uracil and thymine in neutral aqueous solution at room temperature were anomalous when compared with the UV absorption spectra of these pyrimidines. These experimental facts have been critically examined with respect to three molecular models, of which the model based on tautomerism of uracil in aqueous solution is, in the opinion of Daniels, the best. The data suggested that the fluorescing tautomer contained an enol group, and the UV data favored the 4-hydroxy structure, i.e., form 28. The second tautomer, from which the triplet originated, was expected to be the predominant diketo form (32). 

B. Tautomerism of Minor Pyrimidine Bases of the Nucleic Acids in Solution... 

The X-ray crystallographic studies show that thiouracils and 2-thiocytosine exists in the crystalline state in forms 51 and 52, respectively, and that isocytosine exists as a mixture of two amine-lactam forms, 42 and 43. Several experimental studies on these pyrimidines in solution confirm the conclusions from the crystal. However, studies on the tautomerism of the minor pyrimidine bases are few. In a number of cases the conclusions about the dominant structures are intuitive rather than proved. We present the essential experimental data on the structure of the minor pyrimidine bases. 

The 15N magnetic resonance studies of the pyrimidine bases or their derivatives are scarce. Roberts et al.BB have measured the 1H and 15N magnetic resonance spectra of a number of pyrimidines including uracil and 1-methylcytosine. The most important result of this study was the elucidation of the dominant tautomeric structures of uracil and proto-nated 1-methylcytosine as the diketo, 32, and keto-amine form, 7, respectively (cf. Sections II and IV and the spectrum of 1-methylcytosine hydrochloride labeled only in the amino group62). In the case of uracil,85 the two 15N-bonded protons gave two doublets centered at 10.78 and 10.96 ppm (measured downfield from internal tetramethyl-... 

---