Lobosteroids A-F: Six New Highly Oxidized Steroids from the Chinese Soft Coral <i>Lobophytum</i> sp.

Abstract

To explore the steroidal constituents of the soft coral <i>Lobophytum</i> sp. at the coast of Xuwen County, Guangdong Province, China, a chemical investigation of the above-mentioned soft coral was carried out. After repeated column chromatography over silica gel, Sephadex LH-20, and reversed-phase HPLC, six new steroids, namely lobosteroids A-F (<b>1</b>-<b>6</b>), along with four known compounds <b>7</b>-<b>10</b>, were obtained. Their structures were determined by extensive spectroscopic analysis and comparison with the spectral data reported in the literature. Among them, the absolute configuration of <b>1</b> was determined by X-ray diffraction analysis using Cu K<i>α</i> radiation. These steroids were characterized by either the presence of an <i>α</i>,<i>β</i>-<i>α</i>',<i>β</i>'-unsaturated carbonyl, or an <i>α</i>,<i>β</i>-unsaturated carbonyl moiety in ring A, or the existence of a 5<i>α</i>,8<i>α</i>-epidioxy system in ring B, as well as diverse oxidation of side chains. The antibacterial bioassays showed that all isolated steroids exhibited significant inhibitory activities against the fish pathogenic bacteria <i>Streptococcus parauberis</i> FP KSP28, <i>Phoyobacterium damselae</i> FP2244, and <i>Streptococcus parauberis</i> SPOF3K, with IC₉₀ values ranging from 0.1 to 11.0 µM. Meanwhile, compounds <b>2</b> and <b>6</b>-<b>10</b> displayed potent inhibitory effects against the vancomycin-resistant <i>Enterococcus faecium</i> bacterium G7 with IC₉₀ values ranging from 4.4 to 18.3 µM. Therefore, ten highly oxidized steroids with strong antibacterial activities were isolated from the Chinese soft coral <i>Lobophytum</i> sp., which could be developed as new chemotypes of antibacterial drug leads.