Asperusmides A–D, Immunosuppressive and Anti-Inflammatory Indole Diketopiperazines from <i>Aspergillus ustus</i> NRRL 5856

Asperusmides A-D (<b>1</b>-<b>4</b>), four austamide-type indole diketopiperazines, together with two known analogues (<b>5</b>-<b>6</b>), were isolated from <i>Aspergillus ustus</i> NRRL 5856. Their structures, including absolute configurations, were determined by extensive spectroscopic analysis and single-crystal X-ray diffraction. Compound <b>1</b> possesses an unprecedented 6/5/7/6/5/5/5/6 fused octacyclic ring system, representing the first austamide-isobenzofuran carbon skeleton, which is linked by a tetrahydrofuran bridge of C-20-C-32 and C-21-C-25 between the austamide and isobenzofuran units. Compounds <b>2</b>-<b>4</b> are diastereomers and constitute the first instances of austamides with a C-20 oxygenated substitution. Compound <b>1</b> exhibited potent immunosuppressive activities against concanavalin A-induced T cell proliferation and lipopolysaccharide (LPS)-induced B cell proliferation with EC₅₀ values of 3.9 and 4.5 μM, respectively. Additionally, compounds <b>1</b>-<b>2</b> and <b>4</b>-<b>6</b> inhibit nitric oxide production in LPS-induced Raw264.7 cells, with IC₅₀ values of 6.8, 8.0, 17.9, 26, and 14.8 μM, respectively.