Modelling the enantiomerization and atroposelectivity in thiohydantoin derivatives

Abstract

Thiohydantoins have a wide range of pharmacological and biological properties. In the previous studies, nonracemic axially chiral thiohydantoins were synthesized and computational investigations have rationalized the synthesis mechanism of the prod ucts. Two kinds of isomerism are studied; atropisomerism due to hindered rotation around a chiral axis and central chirality. Selective synthesis of one enantiomeric form is important during drug synthesis because of the different pharmacological properties of enantiomers and diastereomers. Enantiopure products have less complex and more selective pharmacodynamic profile compared to racemic mixtures. In order to shed light on the factors determining the selectivity on the reactions of thiohydantoin molecules, DFT methods have been used to investigate the substituent and solvent effect on the kinetics of the reactions and the thermal stabilities of thiohydantoin stereoisomers SP, SM, RP and RM. This study consists of four parts, with different reaction conditions and mechanisms. In the first part, the substituent effect on the rotational barriers has been investigated and the results have been compared with the experimental data. In the second part, the solvent assisted racemization and rotation reactions have been modelled and their activation energies were calculated to establish the most plausable mechanism. After determination of the racemization mechanism in ethanol, the sub stituent effect on racemization has been modelled and the results have been compared with the experimental data. In the third part, we proposed an approach to calculate the product distributions of thiohydantoin stereoisomers by the Boltzmann distribu tion. In the last part, we have modeled the aldol formation reactions and explored the face selectivity of the enolate due to the bulky ortho-aryl substituent.