Chlororesistoflavins A and B, Chlorinated Benzopyrene Antibiotics Produced by the Marine-Derived Actinomycete sp. Strain EG32.

As part of a collaborative biomedical investigation of actinomycete bacteria isolated from sediments collected along the northern coast of Egypt (Mediterranean Sea), we explored the antibacterial metabolites from a bacterium identified as a sp., strain EG32. HPLC analysis and antibacterial testing against methicillin-resistant (MRSA) resulted in the identification of six compounds related to the resistoflavin and resistomycin class. Two of these metabolites were the chlorine-containing analogues chlororesistoflavins A () and B (). The absolute configurations of the lone stereogenic center (C-11b) in these metabolites were assigned by analysis of their ECD spectra. Interestingly, the ECD spectrum of chlororesistoflavin A () shows a Cotton effect of the n-π* transition antipodal to that of the parent natural product, a consequence of 1,3-allylic strain induced by the adjacent bulky chlorine atom that distorts the coplanarity of the carbonyl group with the π-system. The chiroptical analysis thus resolves the paradox and uniformly aligns the configuration of all analogues as identical to that reported for natural resistoflavin. Chlororesistoflavins A () and B () exhibited antibacterial activity against MRSA with a minimum inhibitory concentration of 0.25 and 2.0 μg/mL, respectively.