Kinetics and mechanism of racemization of Tic-hydantoins, potent sigma-1 agonists

The configurational stability of seven Tic-hydantoin sigma-1 agonists 1-7 was studied in organic and aqueous media to mimic conditions encountered during either their synthesis or their evaluation as a drug candidate (physico chemical property determination and pharmacological study). This study was performed from the enantiomers directly obtained by asymmetric synthesis (hydantoins 1, 3-7) or after semi-preparative separation of the racemic obtained according to the same asymmetric procedure (thiohydantoin 2), by chiral HPLC. The racemization phenomenon was followed using recently validated chiral HPLC and capillary electrophoresis separation methods using derivatized cellulose and amylose chiral stationary phases (Chiralcel OD-H and Chiralpak AD) and highly sulphated cyclodextrins highly S-β-CD in the background electrolyte (BGE)], respectively. The kinetic parameters (rate constant, half-life and apparent free energy barriers) of racemization were calculated using a first-order reaction model. The influence of the acid-base content, pH in aqueous medium, concentration of the buffer, and temperature were investigated. The fastest racemization rates were observed under basic conditions. All hydantoins were shown to present the same magnitude of configurational stability, whereas thiohydantoin 2 was characterized by a high chiral instability, especially in ethanolic and aqueous media: its high instability in ethanol explains that a racemic mixture was obtained after asymmetric synthesis; its instantaneous racemization observed from the neutral pH makes the preparation of the enantiomers of 2 not relevant. Finally, the mechanism of racemization was elucidated using nuclear magnetic resonance (NMR): the comparison of the kinetics of the deuteration to the kinetics of the racemization suggests the involvement of a common reaction mechanism of S_E1 type for hydantoin 1, while an S_E2 type reaction seems to be involved for thiohydantoin 2.