Tulongicin, an Antibacterial Tri-Indole Alkaloid from a Deep-Water Topsentia sp Sponge

Antibacterial-guided fractionation of an extract of a deep-water Topsentia sp. marine sponge led to the isolation of two new indole alkaloids, tulongicin A (1) and dihydrospongotine C (2), along with two known analogues, spongotine C (3) and dibromodeoxytopsentin (4). Their planar structures were determined by NMR spectroscopy. Their absolute configurations were determined through a combination of experimental and computational analyses. Tulongicin (1) is the first natural product to contain a di(6-Br-1H-indol-3-yl)methyl group linked to an imidazole core. The coexistence of tri-indole 1 and bis-indole alcohol 2 suggests a possible route to 1. All of the compounds showed strong antimicrobial activity against Staphylococcus aureus.