Recent progress in the glycosylation of lupanes

Betulin, betulinic acid, and lupeol – pentacyclic triterpenoids belonging to the lupane family and easily accessible from the bark of Betula species – are interesting bioactive natural compounds due to their diverse pharmacological and biological properties. Nevertheless, poor hydrosolubility, as well as other pharmacokinetic parameters, limits their clinical applications. These problems can be addressed by the introduction of polar moieties at the C-3 and/or C-28 positions, e.g., by glycosylation to form saponins. The incorporation of a sugar moiety to the triterpene scaffold improves the pharmacological properties and water solubility by adding a hydrophilic carbohydrate fragment to the lupane core. Synthesis of natural and unnatural lupane-carbohydrate conjugates (saponins), as well as unwanted side reactions frequently observed during the glycosylation reaction, including acyl migration and the Wagner–Meerwein rearrangement, are discussed.