Tautomerism of azine derivatives. 11.14N-NMR and17O-NMR investigation of intrachelate tautomerism of acylmethylpyridines

Intrachelate [1, 5]-sigmatropic tautomerism in a series of acylmethylpyridines has been studied by¹⁴N- and¹⁷O-NMR spectroscopy. Principles of tautomer modelling or simulation have been proposed and examined, nitrogen and oxygen chemical shift spectra have been determined, and the accuracy of this method for the determination of tautomer composition has been evaluated. The presence of acceptor (electron withdrawing) substituents in the acylmethyl side-chain fragment has been found to stabilize the NH-tautomer.For communication 10, see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 802–809, June, 1986.     

Tautomerism of derivatives of azines. 17. Effect of solvents on the position of the azinyl-ylidene tautomeric equilibrium of substitued azinylmethanes

The effect of solvents on the position of the azinyl-ylidene tautomeric equilibrium in series of azinylmethanes was studied. It was shown that an increase in the polarity of the solvent leads to stabilization of the ylidene tautomer; the sensitivity of the tautomeric equilibrium to the effects of the solvent depends on the form of the side fragment that undergoes tautomerization. It was concluded that stabilization of the ylidene tautomeric form by polar solvents is a general tendency in series of prototropic equilibrium of the azinyl-ylidene type as a whole.See [1] for Communication 16.Deceased.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1668–1672, December, 1987.     

Tautomerism of derivatives of azines. 16. Tautomerism of acylmethylpyrazines and -quinoxalines

The tautomeric equilibria of acylmethylpyrazines and -quinoxalines in chloroform were studied by ¹H, ¹⁴N, and ¹⁷O NMR spectroscopy. It was shown that keto-enol tautomerism is realized in the acylmethylpyrazine series. Annelation leads to the development of an ylidene tautomer in the acylmethylquinoxaline series. A marked dependence of the position of the intrachelate equilibrium on the character of the solvent was observed.See [1] for Communication 15.Deceased.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1663–1667, December, 1987.     

Tautomerism of azine derivatives. 4. Keto-enol tautomerism of β-keto esters of the azine series

The structures of pyrazinoyl-, 3-pyridazinoyl-, 4-pyrimidoyl-, and 2-, 3-, and 4-pyridoylacetic esters were studied by means of IR, NMR, and¹H and¹³C NMR spectroscopy and quantum-chemical calculations (Pariser-Parr-Pople and CNDO/2). The effect of solvents (including strongly and weakly basic solvents) on the position of the tautomeric equilibria of these β-keto esters was studied. The o⁺ constants for the keto and enol fragments were estimated by means of quantum-chemical calculations and¹³C NMR spectroscopy. See [1] for communication No. 3. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 822–826, June, 1980.     

Tautomerism of azine derivatives. 5. Effect of the heteroatom on the keto-enol tautomerism of β-keto esters of the azine series

The effect of a nitrogen atom in various positions of the heteroaromatic ring on the ketol-enol equilibrium of 2-, 3-, and 4-pyridoyl-, 2- and 4-pyrimidoyl-, pyrazinoyl-, and 3- and 4-pyrazinoylacetic esters is examined. The anomalous effect of a nitrogen atom on the position relative to the tautomeric fragment is noted and is explained by interaction of the unshared pair of the nitrogen atom with the orbitals of the carbonyl group, which may lead to a decrease in the acceptor character of the azine substituent.See [1] for Communication 4. In Communication 4 [Khim. Geterotsikl. Soedin., No. 6, p. 823 (1980)] there is a misprint in the scheme that illustrates the tautomeric equilibrium -keto esters with the participation of protonated forms. See [16] for the corrected scheme.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 530–535, April, 1981.     

Imine-enamine tautomerism of dihydroazolopyrimidines. 6. Synthesis and tautomerism of thienyl derivatives of dihydro-1,2,4-triazolo-[1,5-a]pyrimidines

The cyclocondensation of thiophene analogs of chalcone with 3-amino-1,2,4-triazole gives 5- and 7-(α-thienyl)dihydro-1,2,4-triazolo[1,5-a]pyrimidines. The tautomer composition of synthesized compounds was analyzed. For communication 5, see [1]. Kharkov State University, 310077 Kharkov, Ukraine, Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 678–682, May, 1999.     

Tautomerism of derivatives of azines. 19. Effect of solvents on the o,p-quinoid equilibria of the ylidene forms of 4-pyrimidinylcyanoacetic acid esters

The effect of solvents on the tautomeric equilibria of the ylidene forms of 4-pyrimidinylcyanoacetic acid esters with o- and p-quinoid orientations of the double bonds in the heteroring was determined. The relative stability of the p-quinoid tautomer increased markedly on passing to polar solvents due to nonspecific solvation and the formation of hydrogen bonds.See [1] for Communication 18.Deceased.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 521–524, April, 1988.     

Sulfoxide-induced stereoselection in [1,5]-sigmatropic hydrogen shifts of vinylallenes. A computational study

The sulfoxide-induced preference for a migrating trajectory in the vinylallene [1,5]-H sigmatropic shift (resulting in stereodefined trienes in the conceptual equivalent of torquoselectivity in electrocyclizations), originally reported by Okamura, has been computationally studied at the B3LYP/6-311++G(3df,2p)//B3LYP/6-31++G(d,p) level. The face selectivity this group induces in the [1,5]-H shift is enhanced by bulky geminal substituents and is not reproduced by any of the other (more than 20) substituents tested. Analysis of transition-state geometries or charges and evaluation of steric effects did not show any correlation with the preferences. The origin of this selectivity is thought to lie in a secondary orbital interaction (SOI) involving the termini of the pericyclic array and the sulfinyl group which is only observed for this substituent. This secondary orbital interaction, arising from the favorable energies of the orbitals involved, is enhanced in the transition structure due to a better orbital overlap (piC2-C3-->sigma*C1-S), which correlates with a piC2-C3-->sigma*C6-H SOI, which is more important in the transition structure, that weakens the C-H bond, thus lowering the energy of the corresponding transition structure.     

Research in the imidazole series. 95. synthesis of derivatives of imidazo[1,5-a]imidazole

The interaction of 1-acylinethyl-2-methyl-4-nitro-5-bromoimidazoles with ammonia or primary amines, followed by cyclization of the 1-acylmethyl-2-methyl-4-nitro-S-alkyl(aryl)aminoimidazoles in a medium of lower organic acids, leads to the formation of derivatives of imidazo(1,5-ajimidazole.Center for the Chemistry of Medicinal Substances, All-Russian Scientific-Research Chemical-Pharmaceutical Institute, Moscow 119815. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4. pp. 490–494. April, 1994. Original article submitted January 21, 1994.     

Tautomerism of some imidazo[4,5-b]pyridine derivatives

It was shown by means of UV and IR spectroscopy and the ionization constants that the tautomeric equilibrium of 2-amino- and 2-mercapto derivatives of 3-methyl-3H-imidazo[4,5-b]pyridine is shifted to favor the amino and thione forms, respectively.     

Unusual ring-chain tautomerism in bicyclo[4.2.0]octane derivatives

A new type of ring-chain tautomerism, which consists of the reversible conversion of bicyclo[4.2.0]octane derivatives into trisubstituted enamines was found and studied by ¹H NMR spectroscopy. The starting materials were prepared by the stereoselective reaction of (E)-3,3,3-trichloro-1-nitropropene with cyclohexanone enamines.     

1,3-Dipolar cycloadditions of diazoalkanes to pyridazine derivatives. Thermal 1,5-sigmatropic rearrangement of methyl groups in 3,3-dimethyl-3H-pyrazolo[3,4-d]pyridazine derivatives

Thermal 1,5-sigmatropic rearrangements of one of the methyl group attached at position 3 of 3,3-dimethyl-3H-pyrazolo[3,4-d]pyridazin-4(5H)-ones 1–3 taking place either in a clock-wise or anti-clockwise direction gave N₂-methylated products 4–6 and C_3a-methylated products 7– 9 . The -7(6)-one derivative 10 and -4,7(5H,6H)-dione derivative 12 gave only N₂-methylated products 11 and 13 respectively, and 1,2-dihydro derivative 14 produced after elimination of methane, 15 .     

Nitro azines. 21. Reactivities and electron structures of 6-nitroazolo[1,5-a]pyrimidines

6-Nitroazolopyrimidines react with the acetonyl anion to give 6-nitro-7-acetonyl-4,7-dihydroazolo[1,5-a]pyrimidines. The reactivity of this class of compounds with respect to charged and uncharged nucleophiles is determined by their aromatic character and the deficit of electron density in the pyrimidine ring, while the direction of attack is determined by the overall charge on fragments of the valence-bonded atoms.For Communication 20 see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 807–813, June, 1993.     

Tautomers of azine derivatives. 21. Tautomerism in 2-(2-hydroxyaryl)azines

Tautomerism in 2-(2-hyroxyaryl)azines has been studied using ¹⁷O NMR and UV spectroscopy. Consideration has been given to the influence of solvent and structural factors which facilitate the establishment of a tautomeric equilibrium in these compounds.Communication 20, see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 75–80, January, 1990.     

Asymmetric version of P-S to P-C [1,3]-sigmatropic rearrangement in the ferrocene series

The synthesis of new 2-sulfanylferrocenylphosphonates and derivatives has been achieved through a P-S to P-C [1,3]-sigmatropic rearrangement. The asymmetric version of this reaction has been successfully performed starting from a ferrocenylphosphorodiamidothioate and afforded, as a single diastereoisomer, enantiomerically pure planar chiral ortho-thio-substituted ferrocenyl phosphorodiamidate.     

Synthesis of 4-substituted 1,5-diaryl-3-diphenylmethoxy-3-pyrrolin-2-ones and their [1,5]-sigmatropic rearrangement

Reactions of 1,4,5-trisubstituted 3-hydroxy-3-pyrrolin-2-ones with diphenyldiazomethane yield the O-alkylation products. Thermolysis of 1,5-diaryl-4-heteroyl-3-hydroxy-3-pyrrolin-2-ones is accompanied with suprafacial [1,5]-sigmatropic rearrangement.