Overview of the Ruspolia Plant Genus: Insights into Its Chemical Diversity and Biological Potential

The genus Ruspolia refers to a small group of plants in the Acanthaceae family, with two dominant species R. decurrens and R. hypocrateriformis essentially distributed in tropical parts of Africa. Decoctions from these plants are used in folk medicine for the treatment of a few human pathologies but the active ingredients at the origin of the bioactivities have been little studied. Here, we give an insight into the main phytochemicals of the Ruspolia species published in the literature so far and their pharmacological properties. The flavone glycosides justicialosides A-B likely serve as antioxidant agents and free radical scavengers. Several pyrrolidine alkaloids have been isolated from these Ruspolia species, notably (nor)ruspolinone and a few related products. These molecules have attracted the interest of medicinal chemists, with different synthetic routes leading to ruspolinone and analogues. There are versatile operating procedures to synthesize (nor)ruspolinone isomers. Despite these chemical efforts, the pharmacology of ruspolinone remains largely unknown. A few other Ruspolia alkaloids have been isolated, notably the rare bispyrrolidine benzodioxin alkaloid hypercratine, possibly acting as a ligand of β2-adrenergic receptors. A phytochemical survey of the Ruspolia species sheds light on the diversity of products in this family to promote further investigations into the mechanism of action of ruspolinone and related natural products.