Rhabdastrellins A-F, Isomalabaricane Triterpenes from the Marine Sponge Rhabdastrella aff. distincta Fang Lv, †,⊥ Minjuan Xu, † Zhiwei Deng, ‡ Nicole J. de Voogd, § Rob W. M. van Soest, § Peter Proksch, | and Wenhan Lin* ,† State Key Laboratory of Natural and Biomimetic Drugs, Peking UniVersity, Beijing 100083, People’s Republic of China, Analytical and Testing Center, Beijing Normal UniVersity, Beijing, 100875, People’s Republic of China, National Museum of Natural History Naturalis, 2300 RA Leiden, The Netherlands, Institute of Pharmaceutical Biology and Biotechnology, Heinrich-Heine UniVersity, 40225 Duesseldorf, Germany, and College of Life Science and Technology, Beijing Institute of Technology, Beijing 100081, People’s Republic of China ReceiVed June 1, 2008 Chemical examination of the marine sponge Rhabdastrella aff. distincta resulted in the isolation of six new isomalabaricane triterpenes, rhabdastrellins A-F(1-6), which were present as minor components, along with stellettins L and M. Their structures were elucidated on the basis of extensive spectroscopic data analyses and comparison with spectroscopic data of known analogues. The cytotoxicity of compounds 1-6 against a small panel of human tumor cell lines was also evaluated. Sponges of the genus Rhabdastrella (Ancorinidae) are widely distributed in tropical oceans, with about 19 species being docu- mented. 1 Previous chemical investigations mainly focused on R. globostellata, from which about 30 isomalabaricane triterpenoids were isolated. 2-8 Isomalabaricane is a trivial name representing a group of compounds possessing a trans-syn-trans tricyclic nucleus conjugated to a polyene side chain. This rare class of triterpenoids is mostly found in marine sponges from the genera Stelletta, Jaspis, Geodia, and Rhabdastrella, all of which belong to the order Astrophorida. 9-14 Isomalabaricanes were previously reported to significantly inhibit the growth of tumor cells. In the South China Sea, two species of these yellow sponges (Rhabdastrella globos- tellata and R. aff. distincta) were previously examined chemically, from which 12 isomalabaricane analogues were isolated. 7,8 The present work reports on the isolation and structural elucidation of six new isomalabaricane triterpenoids (1-6) that were isolated as minor components. Results and Discussion Bioassay-guided fractionation of a MeOH extract of R. aff. distincta revealed that the CHCl 3 -soluble fraction exhibited inhibi- tory activity against human tumor cell lines HL-60, BGC-823, and MDA-MB-423. This cytotoxic fraction was then subjected to repeated chromatographic separation followed by semipreparative HPLC purification to obtain rhabdastrellins A-F (1-6), together with stellettins L and M. The latter compounds were recently reported from the sponge Stelletta tenuis. 11 The molecular formula C 30 H 40 O 5 of rhabdastrellin A (1) was determined through HREIMS (m/z 487.3408 [M + 1] + , calcd 487.3418) and NMR data, implying 11 degrees of unsaturation. The UV absorption maxima at 312 and 395 nm in MeOH suggested the presence of a highly conjugated polyene chromophore, while the IR absorptions at 3265 and 1688 cm -1 implied the presence of carbonyl and hydroxyl functionalities. The 13 C NMR spectrum exhibited 30 carbon resonances, including 10 olefinic carbons, six methyl groups, a ketone (δ C 206.2, qC, C-12), and an ester-type carbonyl carbon (δ C 161.8, qC), which was attributed to an unsaturated δ-lactone as indicated in stellettins A-C. 15 The 1 H NMR spectrum of 1 showed three tertiary methyl resonances at δ H 0.75 (3H, s, H-19), 1.46 (3H, s, H-28), and 1.19 (3H, s, H-30) and three olefinic methyl resonances at δ H 2.68 (3H, s, H-18), 1.71 (3H, s, H-21), and 1.98 (3H, s, H-27). In addition, five olefinic protons represented an ABX spin system at δ H 6.99 (1H, d, J ) 15.2 Hz, H-15), 7.06 (1H, dd, J ) 10.4, 15.2 Hz, H-16), and 7.58 (1H, d, J ) 10.4 Hz, H-17) and an AB spin system at δ H 5.62 (1H, d, J ) 6.8 Hz, H-23) and 6.40 (1H, d, J ) 6.8 Hz, H-24). These NMR spectroscopic features were characteristic of an isomalabaricane triterpene and were very similar to those of jaspolide B. 14 The only difference was that a methyl group at C-4 of the latter was replaced by a hydroxymethylene unit (δ H 3.22, 4.32, δ C 63.9) in 1, which was confirmed from the HMBC correlations of the hydroxymeth- ylene protons to C-3 (δ C 80.4, CH), C-4 (δ C 43.6, qC), and C-5 (δ C 47.1, CH) and a long-range W-coupling to C-28 (δ C 23.8, CH 3 ). Complete assignments of protons and carbons of 1 (Tables 1 and 2) were established through 2D NMR experiments ( 1 H- 1 H COSY, HMQC, and HMBC). The geometry of the tricycle core was determined as trans-syn-trans based on NOE interactions between H 3 -19/H-9 (δ H 1.52, dd) and H 3 -30/H-5 (δ H 1.62, d) and the comparable NMR data between 1 and jaspolide B. 14 The double doublet of H-3 (δ H 3.24, dd, J ) 5.7, 12.1 Hz) indicated an axial orientation of H-3 and consequently a 3-hydroxy group, in contrast to a broad singlet for H-3 of the known analogues. 4 The -orientation of the hydroxymethylene group at C-4 was evident from the NOESY correlations between H 3 -19/H-29a (δ H 4.32, d) and H-3/H 3 -28. The significant downfield shift of H 3 -18 (δ H 2.68, s) due to its location in the deshielding zone of the ketone group at * To whom correspondence should be addressed. Tel: +86 10 82806188. E-mail: whlin@bjmu.edu.cn. † Peking University. ‡ Beijing Normal University. § National Museum of Natural History Naturalis. | Heinrich-Heine University. ⊥ Beijing Institute of Technology. J. Nat. Prod. 2008, 71, 1738–1741 1738 10.1021/np800324v CCC: $40.75  2008 American Chemical Society and American Society of Pharmacognosy Published on Web 10/01/2008