Stereocalpin B, a New Cyclic Depsipeptide from the Antarctic Lichen Ramalina terebrata .

Stereocalpin B, a new cyclic depsipeptide ( 1 ), and a new dibenzofuran derivative ( 3 ), were isolated from the Antarctic lichen, Ramalina terebrata (Ramalinaceae), along with a known cyclic depsipeptide ( 2 ). The structures of new compounds were characterized by comprehensive spectrometric analyses; high-resolution fast atom bombardment mass spectrometry (HR-FABMS) and liquid chromatography-tandem mass spectrometry (LC-MS/MS). Stereocalpin B ( 1 ) existed in a rotameric equilibrium, which was confirmed using nuclear Overhauser effect spectroscopy (NOESY)/exchange spectroscopy (EXSY) spectrum. Absolute configurations of the amino acid units in 1 were assigned using the advanced Marfey's method and subsequent NOESY analysis of the 5-hydroxy-2,4-dimethyl-3-oxo-decanoic acid residue confirmed the complete stereochemistry of 1 . Compounds 1-3 exhibited moderate antimicrobial activities against E. coli , with the IC 50 values ranging from 18-30 μg/mL. Compound 2 exhibited cell growth inhibition against HCT116 cell lines, with the IC 50 value of 20 ± 1.20 μM, and compounds 1 and 2 also showed potent anti-inflammatory activities against lipopolysaccharide (LPS)-induced RAW264.7 macrophages with the IC 50 values ranging from 5-7 μM.