A semiempirical study of the conformational behavior of cinchonidine and its interaction with methyl pyruvate

Hydrogenation of methyl pyruvate on a palladium or platinum surface in the presence of cinchona alkaloids leads to a high degree of enantiodifferentiation. In the present study, the semiempirical AMI and PM3 methods are employed to perform a detailed analysis of the conformational behavior of cinchonidine and to study its interaction with methyl pyruvate. Nine different minima were located on the potential energy surface for cinchonidine by both the AM I and the PM3 methods. Some barriers to interconversion between them are relatively hi gh; however, it is always possible to connect two minima through barriers lower than 3.0 kcal/mol so most of the minima can interact with the substrate. The interaction between cinchonidine and methyl pyruvate was calculated by placing methyl pyruvate near the cinchonidine molecule in different orientations and optimizing the final complex. The calculated interaction energy is lower than 3.5 kcal/mol and is predominantly due to van der Waals noncovalent interactions. An analysis of the structure of possible pro-R and pro-S complexes indicates that interaction between cinchonidine and methyl pyruvate alone is not enough to induce enantiodifferentiation.