Triphenylphosphonium Analogues of Betulin and Betulinic Acid with Biological Activity: A Comprehensive Review

Naturally occurring pentacyclic lupane triterpenoids such as betulin (1) or betulinic acid (2) and their synthetic derivatives display a broad spectrum of biological activities and, therefore, have been the subject of great interest. However, the use of these compounds as potential therapeutic agents is limited by their low bioavailability, high hydrophobicity, and insufficient intracellular accumulation. In this context, research on modifications of the parent structures that will improve their pharmacokinetic properties is particularly important. In the past few years, methods of synthesis as well as cytotoxic and antiparasitic properties of a series of lupane triterpenoids modified by introducing one or two triphenylphosphonium moieties at the C-2, C-3, C-28, or C-30 positions by carbon carbon or ester bonds have been described. The presence of triphenylphosphonium groups affects not only physical properties but also the mechanism of action of a potential drug. This review summarizes published findings on synthetic methods and biological properties of the triphenylphosphonium derivatives of betulin and betulinic acid.