Thioxo form

P2j Z = 2 D = 1.57 R = 0.048 for 931 intensities. The base exists in the thioxo form, with C-8=S and N-7 protonated. The 8-thio substituent causes the base to assume the syn (—102.6°) orientation. The o-ribosyl group is 2T3 (174.8 °, 44.1 °). The exocyclic, C-4 -C-5 bond orientation is trans (—173.2°). This does not favor intramolecular hydrogen-bonding of 0-5 to N-3 of the syn base. The C=S distance is 166.8 pm. The S atom is involved in a weak, acceptor hydrogen-bond to a water molecule, S H-O(w) = 361 pm. The bases are stacked head-to-tail, with overlap of the C=S bonds and the purine ring, in contrast to the known, related structure l-/ -D-ribofuranosyl-2-thioxo-3ff-benzimidazole,197 where similar head-to-tail stacking of the bases involves overlap of the base rings only. 

A strictly dehned region of chemical shifts of C2, C4, and C5 atoms in A-oxides of 4A-imidazoles allows to dehne clearly the position of the A-oxide oxygen atom (102). Chemical shifts of the a-C nitrone group in a-N-, O-, and S-substituted nitrones are located in the region of 137 to 150 ppm (388, 413). On the basis of 13C NMR analysis of 3-imidazoline-3-oxide derivatives, the position of tautomeric equilibria in amino-, hydroxy-, and mercapto- nitrones has been estimated. It is shown that tautomeric equilibria in OH- and SH-derivatives are shifted toward the oxo and thioxo forms (approximately 95%), while amino derivatives remain as amino nitrones (413). In the compounds with an intracyclic amino group, an aminonitrone (A) - A-hydroxyaminoimino (B) tautomeric equilibrium was observed (Scheme 2.76), depending on both, the nature of the solvent and the character of the substituent in position 2 of the heterocycle (414). 

Hydroxy, thiol, and amino groups in pyrimidine exist in tautomeric equilibria with their oxo, thioxo, and imino forms. An amino group in an electrophilic position exists predominantly as such, and the compound is named as an amine. Pyrimidines with a hydroxy or thiol group in an electrophilic position are dominated by the oxo or thioxo forms and are named as such, or with -one or -thione suffixes, if these are the principal groups. In the benzenoid 5-position, these derivatives are mainly present in the hydroxy, thiol, or amino forms and are named as such. Similar considerations apply to the nomenclature of quinazolines and perimidines <1996CHEC-II(6)93>. 

IR and UV/Vis [65a], mass spectrometric [65b], photoelectron [65c], microwave [65d], as well as low-temperature matrix-isolation IR spectroscopic measurements [65e] reveal that 2- and 4-hydroxypyridine (as well as 2- and 4-mercaptopyridine [65f]) exist in the gas phase and in inert matrices (N2, Ar) under equilibrium conditions mainly in the lactim (hydroxy or mercapto) form, in contrast to the situation in solution. While in nonpolar solvents such as cyclohexane and chloroform both tautomers exist in comparable amounts, the tautomeric equilibrium is shifted entirely in favour of the lactam (0x0 or thioxo) form in polar solvents such as water, as well as in the crystalUne state [66, 67, 141-145, 251-255], Supercritical-fluid 1,1-difluoroethane can be used to adjust the tautomeric constant Ki = [(llb)]l[(lla)] iso thermally over a continuum from gas-phase values to those measured in polar solvents, simply by increasing the pressure [254]. The gas-phase and solution equilibrium constants of 2- and 4-hydroxypyridine are given in Table 4-4. 

Substituted thiazoles at C-2 or C-4 with hydroxy, thio, and amino groups can be represented by five tautomeric forms (26a-e) (Scheme 1). However, form (26e) has never been observed <85JPR25l>. The aminothiazoles exist predominantly in the amino form when the substituent is in position two (26b, Z = NH). On the other hand, when the substituent is in position four the amino form is preferred, although there are exceptions to this rule (for instance, in the case Y = Z = NHCOMe). The hydroxy and thio derivatives predominantly exist in the oxo and thioxo forms (26a, Y, Z = O, S). The main evidence of these arguments came from the C NMR spectra of the thioxo derivatives and from the IR spectra of the oxo and amino derivatives. 

Photoelectron spectra of 2-, 3- and 4-mercaptopyridines and fixed structure models have been measured for evaluation of tautomeric populations. Whereas for 3- and 4-mercaptopyridine a great predominance of the mercapto form in vapor phase is demonstrated, 2-mercaptopyridine shows the existence of a prototropic equilibrium between the mercapto and thioxo forms. From the integration of band areas it follows that there is about 10% of the thioxo form in the equilibrium (77JCS(P2)1652, 79TL2585). 3-Formyl-2-mercaptopyridine was found to exist in the gas phase as a tautomeric mixture containing about 16% of the thione form (81T2663). 

The substitution of the hydroxy group by a thiol group (or transformation of the oxo form into the thioxo form) is achieved by treatment with phosphorus pentasulfide (P4S10) in pyridine at reflux temperature. In the case of pyrimido[4,5-c/]pyridazine-2,4-diol (3) it is possible to exchange only the 4-hydroxy group or both hydroxy groups57. 

In this section, the prineipal results are summarized. It was found that in general for oxygen (X = O) or sulfur (X = S) substituted 1,2,4-triazines the oxo- or thioxo form (36a-39a) predominates while in nitrogen substituted 1,2,4-triazines (X = NH) the amino form (36l -39b) predominates. 

On the other hand, a more detailed study into the tautomerism of thioformic acid with solvent interactions suggested that the thioxo form was favored [23]. For monomeric thioformic acid complexed with dimethyl ether, the thioxo isomer 14b becomes 5 kcal/mol more stable than the thiol isomer 14a (Scheme 8). The reason for the stabilization of the thioxo isomer is the stronger hydrogen bond interactions (0-H---0 type) in complex 14b compared to those in 14a (S-H O type). These results are in agreement with experimental observations, as described infra [16]. 

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