Isomalabaricane Chemical Composition of Vietnamese Marine Sponges Inspected by Metabolomic and Chemical Approaches

Reliable taxonomy of biological producers is essential for finding new natural substances. A recent study morphologically re-examined 21 accessed vouchers to confirm multiple reported misidentifications and suggested marine sponges from the genus <i>Rhabdastrella</i> as the only known source of the isomalabaricane triterpenoids. The present study aimed to find isomalabaricane-containing sponges among the samples collected during seven marine expeditions to the Vietnam waters of the South China Sea, accompanied with their identification confirmed using morphological and molecular (18S rRNA and 28S rRNA) analyses. As a result, nine sponges identified as <i>Rhabdastrella globostellata</i> were found to contain isomalabaricanes in their extracts. A chemical investigation of the <i>R. globostellata</i> (PIBOC O63-136) specimen led to the isolation of nine isomalabaricane triterpenoids including the new compound <b>1</b>, of which the chemical structure was elucidated based on HRESIMS and NMR data. Subsequently, a combination of LC-MS/MS, multivariate statistical analysis, and feature-based molecular networking was applied to detect, annotate, and characterize the isomalabaricane chemical diversity across the nine <i>R. globostellata</i> specimens. As a result, two primary chemotypes containing individual sets of annotated compounds were discovered within the Vietnamese population of this sponge. Moreover, obtained data showed a series of new extremely rare isomalabaricanes in <i>R. globostellata</i> extracts including nitrogen-containing metabolites and glycosides of this structural class.