Mechanistic study of the intramolecular hemiacetalization of warfarin: transition states and reaction pathways from DFT calculations.

Warfarin is one of the most widely prescribed oral anticoagulants and exhibits a complex structural equilibrium involving an open keto-enol form and two cyclic hemiacetal diastereomers. In a previous study carried out by our research group, multinuclear NMR spectroscopy combined with DFT calculations and DP4+ statistical analysis demonstrated that, in DMSO solution, warfarin exists predominantly as the (R,R)- and (R,S)-hemiacetal diastereomers, while the open keto-enol tautomer is present only in minor proportion. Although the structures of the species present in solution have been established, the transition states and reaction pathways associated with their intramolecular cyclization have not been yet characterized.

In this work, the intramolecular hemiacetalization mechanism of warfarin was investigated using density functional theory calculations. Geometry optimizations identified the most stable open conformations capable of undergoing ring closure. Transition states associated with the formation of each hemiacetal diastereomer were located and characterized by vibrational frequency analysis. Intrinsic Reaction Coordinate (IRC) calculations confirmed the connection between each transition state and the corresponding reactant and product minima, establishing the reaction pathways leading to the experimentally observed cyclic species.

This study provides a mechanistic description of the intramolecular cyclization of warfarin in solution, complementing previous experimental findings and extending the structural characterization to the elementary steps involved in hemiacetal formation. The results provide a computational basis for future studies on the energetic factors governing the equilibrium between the different tautomeric and diastereomeric forms of this clinically relevant anticoagulant.