Nine New Antibacterial Diterpenes and Steroids from the South China Sea Soft Coral <i>Lobophytum catalai</i> Tixier-Durivault.

Abstract

Five new cembrane-type diterpenes, lobocalines A-E (<b>1</b>-<b>5</b>), and four new steroids, lobocaloids A-D (<b>9</b>-<b>12</b>), along with six known related compounds (<b>6</b>-<b>8</b> and <b>13</b>-<b>15</b>) were isolated from the Yalong Bay soft coral <i>Lobophytum catalai</i> Tixier-Durivault. The structures of the new compounds were elucidated by extensive spectroscopic analysis, NMR calculation with DP4+ analysis, time-dependent density functional theory-electronic circular dichroism (TDDFT-ECD) calculations, X-ray diffraction analyses and comparison with the reported spectroscopic data of known compounds. Further, with the aid of X-ray diffraction analysis, the structure of lobocrasol B (<b>15</b>) was firmly revised as <b>15a</b>. In in vitro bioassays, compound <b>2</b> showed moderate antibacterial activities against fish pathogenic bacteria <i>Streptococcus parauberis</i> KSP28 and <i>Phoyobacterium damselae</i> FP2244 with minimum inhibitory concentration (MIC) values of 8.7 and 17.3 µg/mL, respectively. All the steroids exhibited antibacterial activities against the <i>S. parauberis</i> KSP28 with MIC values ranging from 12.3 to 53.6 µg/mL. Compounds <b>2</b>, <b>7</b> and <b>14</b> have remarkable inhibitory effects on the hemolysin production of <i>Staphylococcus aureus</i>, while compounds <b>8</b>-<b>12</b> have medium inhibitory effects on the pyocyanin production in <i>Pseudomonas aeruginosa</i>.