Asperustins K-P, Chlorinated Austocystins from Aspergillus ustus NRRL 5856.

Asperustins K-P (1-6), six unreported chlorinated austocystins, were isolated from Aspergillus ustus NRRL 5856. Their structures, including absolute configurations, were elucidated by extensive spectroscopic analyses, electronic circular dichroism (ECD) calculations, and single-crystal X-ray diffraction. Asperustin K (1) features an unprecedented carbon skeleton with a 6/6/6/5/5/6/6/6 fused octacyclic ring system, while asperustin L (2) represents a distinctive carbon skeleton with a 6/6/6/5/5/5 hexacyclic ring system. Compounds 3-6 demonstrated the first examples of C-4'-O-C-5' bridge linked asperustin heterodimers. Compounds 1, 3-6 showed potent immunosuppressive activities against concanavalin A (ConA)-induced T cell proliferation and lipopolysaccharide (LPS)-induced B cell proliferation, with EC50 values ranging from 5.7 to 12 μM, and from 4.6 to 19 μM, respectively.