Resumen de Isolation of Betulin and Rearrangement to Allobetulin. A Biomimetic Natural Product Synthesis.
Brian Green, Michael D. Bentley, Bong Y. Chung, Nicholas G. Lynch, Bruce L. Jensen
The triterpenes are a diverse class of widely distributed natural products derived from squalene. Various cyclization and subsequent rearrangement reactions produce many complex structural types. These compounds frequently display a wide divergence of biological properties. For example the pentacyclic triterpene, betulin, is isolated from white birch species and its carboxylic acid derivative, betulinic acid, is presently being studied as a selective inhibitor of human melanoma, brain cancer, and HIV. The betulin required for this microscale experiment is obtained by extraction from birch bark. The isolated material is then purified and converted to allobetulin by heating with p-toluenesulfonic acid for sixty minutes. Allobetulin is a colorless solid produced by a carbocation rearrangement and subsequent E-ring expansion. Betulin and allobetulin display 1H NMR spectra that include axial�equatorial coupling characteristic of a C3-alcohol, conformational analysis by the use of dihedral angles and the Karplus equation, coupling patterns caused by diastereotopic protons and long-range interactions, and chemical shift values that are influenced by electronegativity and stereochemistry. All data are easily interpreted and provide a valuable educational platform for organic chemistry students to study a wide variety of reaction and NMR topics.
