Stable Tautomeric Forms Research Articles

Which tautomeric form of 5-methylcytosine predominates in vacuum?

An application of high-level ab initio quantum chemistry methods to study the tautomeric equilibria of 5-methylcytosine in the gas-phase is described. Calculations for the hydroxy-amino, oxo-amino and oxo-imino forms have been performed at the second-order level of the many body perturbation theory (MBPT(2)) with a double-zeta basis set augmented with polarization functions. The optimized geometry and vibrational modes were obtained with the SCF/3-21G method. The calculations clearly show that the most stable tautomeric form in vacuum is hydroxy-amino. The remaining two forms, i.e. oxo-amino and oxo-imino, are each estimated to be less stable than the main form by about 15 kJ mol −1.

ChemInform Abstract: The Tautomeric Equilibria of Thio Analogues of Nucleic Acid Bases. Part 2. AM1 and ab initio Calculations of 2‐Thiouracil and Its Methyl Derivatives.

Relative stabilities predicted by the AM1 method, and relative energies from 3-21 G* and 6-31 G ** calculations agree with qualitative and quantitative experimental vapour phase data for 2-thiouracil and for its four tautomeric monomethyl derivatives. The AM1 proton affinities for the more stable tautomeric forms are usually significantly lower than, but almost within experimental values of, the measured PA values. A plot of experimental vs. calculated PA gives a straight line of almost unit slope.

Structure of 1-amidino-3-(3-sulfamoylphenyl)urea hydrochloride.

1-(Diaminomethylene)-3-(3-sulfamoylphenyl)uronium chloride, C8H12N5O3S+.Cl, Mr = 293.71, monoclinic, P21/c, a = 10.148 (1), b = 7.881 (1), c = 15.286 (3) A, beta = 94.77 (1) degrees, V = 1218.2 A3, Z = 4, Dx = 1.601 g cm-3, lambda(Mo K alpha) = 0.71069 A, mu = 4.838 cm-1, F(000) = 608, T = 294 K, R = 0.069 for 1779 data. The molecular conformation of the protonated arylamidinourea is completely planar and all amidinourea N atoms have considerable sp2 hybridization. The stable tautomeric form has an intramolecular hydrogen bond between the ureido O atom and an amidino N atom. All N atoms in the molecular packing make contact with the Cl- ion.

Synthesis of 1‐hydroxy‐2H,5H‐dihydroisoxazolo[5,4‐c]quinoline. A novel heterocyclic ring system

AbstractThe synthesis of a new heterocyclic compound, 3a, combining a quinoline and a isoxazolol ring is described. The preparation involves cyclisation of the carbamate 14, followed by concomitant deprotection of the acidic and amino functions to give compound 3a, the stable tautomeric form of 3.

The tautomeric equilibria of thio analogues of nucleic acid bases. Part 2. AM1 and ab initio calculations of 2-thiouracil and its methyl derivatives

Relative stabilities predicted by the AM1 method, and relative energies from 3-21 G* and 6-31 G ** calculations agree with qualitative and quantitative experimental vapour phase data for 2-thiouracil and for its four tautomeric monomethyl derivatives. The AM1 proton affinities for the more stable tautomeric forms are usually significantly lower than, but almost within experimental values of, the measured PA values. A plot of experimental vs. calculated PA gives a straight line of almost unit slope.

Anomalous Reaction of Arylmalononitriles with Nitric Acid. Para-Directing Nature of Dicyanomethyl Group and a Through-Ring Nitro/aci-Nitro Tautomerism of 4-Nitrophenylmalononitrile

Abstract Phenylmalononitrile reacts with nitric acid in dichloromethane at room temperature to afford 1,2-bis(4-nitrophenyl)-1,1,2,2-tetracyanoethane as an initial product, which readily suffers oxidative cleavage to give 4-nitrobenzoyl cyanide. Contrary to common understanding, dicyanomethyl group exhibits no meta-directing property. 4-Nitrophenylmalononitrile is found to exist in two stable tautomeric forms, which undergo a facile nitro/aci-nitro interconversion across aromatic ring depending on the nature of solvent employed.

Tautomeric prototropic equilibrium in pentafluorophenyl-substituted leucoverdazyls

It was shown by NMR spectroscopy that unsymmetrically substituted pentafluoro-phenylleucoverdazyls exist in two stable tautomeric forms. The assignment of the signals in the PMR spectra to the individual tautomeric forms was made with the aid of a 15N-labeled leucoverdazyl. The free energy of activation of the proton transfer was determined.

Muscimol and some aza analogues: molecular orbital calculations relating to their potency as GABA mimetics

Ab initio molecular orbital calculations have been carried out on muscimol ( 1) and on two aza analogues, 5-aminomethylpyrazol-3-one ( 2) and 5-aminomethyl-1,2,4-triazol-3-one (3). Fully ionized species were studied in each case including the tautomeric forms of ( 2) and ( 3). The calculations reveal that the more stable tautomeric forms of ( 2) and ( 3) resemble muscimol in having a low energy barrier to rotation of the aminomethyl group (2–4 kcal mol −1) but differ substantially from muscimol in the electronic character of the heterocyclic ring. The change in π-electron distribution in the more stable tautomers of ( 2) and ( 3) compared with muscimol is summarized by the dipole moment decrease ( 1) 7·75 au, ( 2) 6·62 au, ( 3) 5·81 au. In addition, the frontier molecular orbitals show analogous differences. On the other hand, the less stable tautomers of ( 2) and ( 3) are similar to muscimol in π-electron distribution and frontier orbitals, but have a high energy barrier to rotation of the aminomethyl group (11–14 kcal mol −1). The calculated properties of the molecules are related to the ability of the three compounds to displace [ 3H]-GABA from the postsynaptic receptors of human cerebellum and it is suggested that the dominant influence on biochemical activity is the π-electron distribution in the heterocyclic ring.

Electronic structure of thiouracils and their interaction with adenine.

A calculation of the electronic structure and total energy of thiouracils has been performed by the CNDO/2 method. The most stable tautomeric forms have been determined. The electrostatic energy of the coplanar interaction of an adenine · thiouracil pair has been calculated. The stability of various geometric configurations has been discussed.

On 1,2-dihydrobenz[cd]indazole

Attempted preparation of 1,2-dihydrobenz[cd]indazole from dimethyl 1,2-dihydrobenz[cd]indazole-1,2-dicarboxylate shows that it exists in more stable tautomeric forms with the aromatic indazole rather than naphthalene nucleus.

Reaction of the lactone ring of diffugenin

Under the influence of caustic alkalies, diffugenin (14-anhydrostrophanthidin) undergoes a complex irreversible change with the opening of the butenolide ring. The main product is a γ-hydroxy-γ-lactone (lactol)-the stable tautomeric form of a γ-aldehydo acid. The existence of aldo-lactol tautomerism is confirmed by the preparation of derivatives of the α and Ω forms. The hypothesis has been put forward that the double bond at C14 in diffugenin and its derivatives readily undergoes autoxidation with the formation of epoxy compounds.