Sesquiterpene Derivatives and Steroids from the Sponge Dactylospongia elegans Collected from the South China Sea

Abstract

Sponges of the genus Dactylospongia have been widely investigated for their biologically active compounds. The majority of the reported metabolites are sesquiterpene quinones [1, 2], as well as a few macrolides [3]. For example, smenospongine, a sesquiterpene aminoquinone isolated from the Indonesian sponge D. elegans, showed multifaceted antitumor activity on leukemia cells [4]. Sponge Material. The sponge of Dactylospongia elegans was collected in April 2006 from Meishan coral reef, Sanya, China, and was authenticated (RMNH POR. 5235) by Dr. Nicole J. de Voogd, Netherlands Centre for Biodiversity Naturalis, The Netherlands. A voucher specimen (HN-SY-2006WMS-18) was deposited in the Key Laboratory of Marine Drugs, Ministry of Education, Ocean University of China, Qingdao, China. Extraction and Isolation. The sponge Dactylospongia elegans (0.7 kg, dry weight) was extracted exhaustively with 95% ethanol (2000 mL 5) at room temperature. The solvent-free EtOAc extract (1.0 g) was subjected to column chromatography on silica gel and eluted with EtOAc in petroleum (0–100%, gradient) to yield six fractions. The subfractions were then isolated by successive column chromatography on silica gel and Sephadex LH-20 and purified by semi-preparative HPLC, leading to the isolation of compounds (1/2) (50.6 mg), (3) (2.4 mg), (4) (4.2 mg), (5) (1.8 mg), and (6) (8.5 mg). On the basis of extensive spectroscopic analyses and by comparison with those reported in the literature, the structures of compounds 1–6 were established as dactyltronic acids (1/2) [5, 6], dictyoceratin C (3) [7], polyfibrospongol A (4) [8], 3 -hydroxycholesta-5,8-dien-7-one (5) [9], and pregna-1,20-dien-3-one (6) [10]. Compounds 3, 5, and 6 were obtained from the genus Dactylospongia for the first time. The antibacterial activity in vitro of the isolated compounds was evaluated against a panel of pathogenic bacterial strains, Staphylococcus aureus, S. albus, Bacillus subtilis, B. cereus, Micrococcus tetragenus, Kocuria rhizophila, Vibrio parahemolyticus, V. anguillarum, and Pseudomonas putida, by using a standard screening protocol [11]. The results indicated that dactyltronic acids 1/2 showed pronounced antibacterial activity against V. parahemolyticus with a MIC value of 3.45 M (the positive control ciprofloxacin MIC = 1.25 M). Pregna-1,20-dien-3-one (6) also exhibited activity toward B. cereus with a MIC value of 4.19 M (ciprofloxacin MIC = 1.25 M). However, compounds 3, 4, and 5 only showed weak antibacterial activity.