Hydrogenation of β-isocinchonicine in mild conditions on Pt and Pd catalysts using HPLC-ESI-ion-trap MS: New results on the role of structure of cinchona alkaloids in the Orito reaction

The hydrogenation of β-isocinchonicine (β-ICNN) was studied on Pt- and Pd-alumina catalysts for the first time. Using mild experimental conditions (293 K, 1 bar hydrogen pressure) in toluene and in AcOH as solvents it was established that the enolo-ethereal CC double bond of β-ICNN is hydrogenated to the two DH-β-ICNN diastereomers. In toluene, hydrogenation proceeds at a significantly higher rate on Pd-alumina catalyst than on Pt-alumina. In AcOH, hydrogenation is fast on both catalysts. There was no significant difference between the selectivities of diastereomer formation on Pd and Pt catalysts. The presence of cinchonidine (CD) and cinchonine (CN) reduced the hydrogenation rate of β-ICNN and had no effect on the stereoselectivity of hydrogenation. Ethyl pyruvate (EtPy) hydrogenation on Pt-alumina catalyst modified with β-ICNN proceeded with low enantioselectivity, which is another piece of evidence for the essential role of the conformation of the chiral modifier in the Orito reaction.