Globostelletins J–S, isomalabaricanes with unusual cyclopentane sidechains from the marine sponge Rhabdastrella globostellata

Nine new isomalabaricane-type triterpenoids namely globostelletins J–R (1–9) were isolated from the marine sponge Rhabdastrella globostellata. Their structures were established on the basis of extensive spectroscopic data analysis including IR, MS, 1D and 2D NMR, and CD spectra. Globostelletins J–R are characterized by the presence of an unusual cyclopentane unit linked at different positions of sidechains. These unprecedented structures enriched a number of new skeletons in isomalabaricane family. The biogenetic relationship of the isolated compounds is postulated. All compounds were tested for the inhibitory activities against human tumor-related protein kinases.