Indoleamine 2,3-dioxygenase inhibitors isolated from the sponge Xestospongia vansoesti: structure elucidation, analogue synthesis, and biological activity.

Two new IDO inhibitory meroterpenoids, xestolactone A (1) and xestosaprol O (2), have been isolated from the sponge Xestospongia vansoesti. Xestolactone A (1) has an unprecedented degraded meroterpenoid carbon skeleton. A short synthesis of the xestosaprol O (2) analogues 3 and 4 features the application of a rarely used photochemical coupling reaction. Synthetic analogue 3 is similar to 40 times more potent than the inspirational natural product 2.