Antimicrobial Chlorinated Carbazole Alkaloids from the Sponge‐Associated Actinomycete Streptomyces diacarniLHW51701

Main observation and conclusion Four new chlorinated carbazole alkaloids, chlocarbazomycins (CCBs) A—D, were isolated from sponge associated bacterium Streptomyces diacarni LHW51701. Their structures were elucidated via spectroscopic techniques (including HRESIMS, 1D & 2D NMR), and X‐ray crystallographic analysis. Structurally, the 4‐chloro coupled with 3‐methoxyl substituents in the tricycle nucleus of CCBs A—D and the N ‐methoxyl group of CCB‐D are rare in naturally occurred tricyclic carbazole alkaloids. Moreover, CCBs A—D do not bear the typical C1 para ‐alkyl and C2 meta ‐methyl side chains of bacterial tricycle carbazoles, suggesting a novel mechanism of aromatic ring formation in biosynthesis. The biological evaluation showed that CCB‐C possessed inhibitory activities against pathogenic microorganisms including methicillin‐resistant Staphylococcus aureus (MRSA), Mycobacterium smegmatis , Bacillus mycoides , and Candida albicans .