Ilknur Dogan

Determination of energy barriers and racemization mechanisms for thermally interconvertable barbituric and thiobarbituric acid enantiomers

Abstract The enantiomers of the 5,5-dimethyl-1-( o -aryl)barbituric and 2-thiobarbituric acid derivatives have been separated by micropreparative liquid chromatography on the Chiralcel OD-H column. The activation barriers for the conversion of one enantiomer to its counterpart (M⇄P) have been determined upon thermal racemization of the separated enantiomers by following the intensity changes in the HPLC chromatograms with time. The activation barrier of the 1-( o -tolyl)barbituric acid has been determined by temperature-dependent NMR. The racemization mechanisms are discussed with reference to the determined barriers.

Barriers to internal rotation around the C–N bond in 3-(o-aryl)-5-methyl-rhodanines using NMR spectroscopy and computational studies. Electron density topological analysis of the transition states

We have investigated the pairs of rotational isomers for six 3-(o-aryl)-5-methyl-rhodanines (Z = H, F, Cl, Br, OH, and CH3) using NMR spectroscopy and density functional theory (DFT) calculations. Electron density topological and NBO analysis has demonstrated the importance of non-covalent interactions, characterised by (3, -1) bond critical points (BCPs), between the oxygen and sulfur atoms on the thiazolidine ring with the aryl substitutents in stabilizing the transition states. The energetic activation barriers to rotation have also been determined using computational results; rotational barriers for 3-(o-chlorophenyl)-5-methyl-rhodanine (3S) and 3-(o-tolyl)-5-methyl-rhodanine (6S) were determined experimentally based on NMR separation of the diastereoisomeric pairs, and the first-order rate constants used to derive the value of the rotational barrier from the Eyring equation.

Determination of barriers to rotation of axially chiral 5-methyl-2-(o-aryl)imino-3-(o-aryl)thiazolidine-4-ones

Thermally interconvertible axially chiral 5-methyl-2-(o-aryl)imino-3-(o-aryl)-thiazolidine-4-ones have been synthesized diastereoselectively, and conformations of the major and minor enantiomeric pairs have been determined by (1)H nuclear magnetic resonance. Chromatographic resolutions of each compound have been performed by enantioselective high-performance liquid chromatography, and the barriers to rotation about the N(3)-C(aryl) bond have been determined by following the thermal interconversion process of the major to minor isomers until equilibrium. The rotational barriers range from 96.2 to 115.2 kJ/mol, depending on the size of ortho substituent on N(3)-aryl ring.

Conformational and Spectral Investigations of Thiazolidinone Derivatives by 1H and 13C NMR Spectroscopy

Abstract The magnetic non-equivalence of isomeric proton and 13C nuclei has been investigated on rotational isomers of 3-aryl-2,4-thiazolidinediones and 3-aryl-2-arylimino-4-thiazolidinones. The detection of magnetic non-equivalence for proton nuclei was better in quinoline solutions. Free energies of activation have been estimated. The varying hetero atom in the hetero ring, in comparision to earlier data1–3, is seen to effect the additive shift parameters and the shielding of distant aryl carbon bonded to hetero ring.

Reactions of Barbituric and 2-Thiobarbituric Acid Derivatives with Acetone

Abstract Barbituric and 2-thiobarbituric acid derivatives carrying a 1-N-aryl substituent were found to react with acetone at the 5-position of the heteroring. Acetone adducts were identified by their 13C NMR and mass spectra.

Atroposelective Synthesis of Axially Chiral Thiohydantoin Derivatives

Nonracemic axially chiral thiohydantoins were synthesized atroposelectively by the reaction of o-aryl isothiocyanates with amino acid ester salts in the presence of triethylamine (TEA). The synthesis of the nonaxially chiral derivatives, however, gave thiohydantoins racemized at C-5 of the heterocyclic ring. The micropreparatively resolved enantiomers of the nonaxially chiral derivatives from the racemic products were found to be optically stable under neutral conditions. On formation of the 5-methyl-3-arylthiohydantoin ring, bulky o-aryl substituents at N3 were found to suppress the C-5 racemization and in this way enabled the transfer of chirality from the α-amino acid to the products. The corresponding 5-isopropylthiohydantoins turned out to be more prone to racemization at C-5 during the ring formation. The isomer compositions of the synthesized axially chiral thiohydantoins have been determined through HPLC analyses with chiral stationary phases. In most cases a high prevalence of the P isomers over the M isomers has been obtained. The barriers to rotation determined around the Nsp(2)-Caryl chiral axis were found to be dependent upon the size of the o-halo aryl substituents.

Stereochemical assignments of aldol products of 2-arylimino-3-aryl-thiazolidine-4-ones by 1H NMR

The aldol reactions of 2‐arylimino‐3‐aryl‐thiazolidine‐4‐ones with benzaldehyde carried out at −78 °C were found to produce sec‐carbinols. Intramolecular hydrogen bonding within the aldol products forming a six‐membered ring enabled the assignment of stereochemistries of the major and minor diastereomers via analysis of the syn and anti 3JH,H 1H NMR coupling constants. Copyright

Solvent Dependence of Tautomeric Equilibria of 1‐(o‐Substituted Phenyl)Barbituric and ‐2‐Thiobarbituric Acid Derivatives

Abstract The enolisation tendencies of 1‐(o‐substituted phenyl)barbituric and ‐2‐thiobarbituric acid derivatives have been studied by observing the behaviour of the compounds in different solvents by 1H and 13C NMR. It has been found that the enolisation tendencies of the thiobarbituric acid derivatives observed in polar solvents are greater than those of the barbituric acid derivatives. The ratio of keto–enol tautomers of thiobarbituric acid derivatives in DMSO and in DMF has been calculated.

Radical Cations from Imidazole Derivatives

Abstract Heteroaromatic radical cations are obtained from the oxidaton of N-methyl substituted imidazoles by sulphate anion radical in aqueous solution. The optical absorbtion spectra of the radical cations in acidic medium are reported. In basic solution the radical cations are found to react with hydroxyl anion to produce allylic, OH-substituted radicals.

THE REACTION OF 5-METHYL-3-(o-TOLYL)RHODANINE WITH ACETONE-d6 AND METHANOL

5-Methyl-3-(o-tolyl)rhodanine was found to react with acetone-d6 and methanol at the 5-position of the heterocyclic ring. The reactions have been followed and the products have been identified by 1H NMR.