New tetracyclic and pentacyclic isomalabaricanes from the marine sponge Rhabdastrella globostellata (Carter, 1883)

Abstract

Three unprecedented tetracyclic and pentacyclic isomalabaricanes, named rhabdaglostelones A-C (1–3), were isolated from the marine sponge Rhabdastrella globostellata. Their chemical structures were determined by extensive analyses of HR-ESI-MS, 1D- and 2D-NMR spectra. Absolute configurations of them were identified by experimental and TD-DFT theoretical calculated ECD spectra. The tetracyclic and pentacyclic backbone are predicted biological generation from isomalabaricane triterpenoid, a unique compound class in the Rhabdastrella species. Compounds 1 and 2 exhibited cytotoxic activities toward LU1, HepG2, MCF7, and SKMel2 cell lines with their IC50 values ranging from 15.91 ± 1.21 to 23.21 ± 1.89 µM, while compound 3 was inactivity on these cancerous cell lines.