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Nature’s Chiral Catalyst and Anti-Malarial Agent: Isolation and Structure Elucidation of Cinchonine and Quinine from Cinchona calisaya

Nature is a well-recognized source of compounds of interest, but access is often an issue. One pertinent example is the cinchona alkaloids from the bark of Cinchona calisaya. In this experiment, students at the third-year undergraduate level undertake the selective isolation and characterization of two of the four main alkaloids present in the bark. Beginning with powdered bark, acid-base extraction, followed by selective crystallization, serves to yield cinchonine, nature’s chiral catalyst, from the complex mixture. Slow crystallization provides suitable quality crystals for X-ray analysis. Students readily appreciate the three-dimensional nature of this chiral catalyst, which aids subsequent NMR spectroscopic analysis. Manipulation of the mother liquor by thin-layer and flash column chromatographic techniques proves a simple but elegant method to furnish quinine, nature’s anti-malarial agent. Both alkaloids are treated to an intensive structure elucidation workshop comprising 1- and 2- dimensional NMR, infrared, and mass spectrometry. The method is economical, polished, and robust, bringing the student on a journey from crude plant material to medicinally important natural products in three, 2-h laboratory sessions. Moreover, questions in the student handout and model answers in instructor’s notes, respectively, require that students engage further in topics associated with the context of this practical.

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