Natural product cercosporin as a bioinspired photocatalyst for the synthesis of peptides containing kynurenine via an energy transfer mechanism

Cercosporin-catalyzed photo-mediated oxidation for the rapid and efficient synthesis of kynurenine (Kyn) derivatives as well as Kyn-containing peptides under mild conditions was developed, exhibiting great functional group tolerance and broad substrate scope to produce various nonproteinogenic peptides. This environmentally friendly process utilized the natural product cercosporin (CP) as a metal-free photocatalyst, visible light as a sustainable energy source, and oxygen as a green oxidant. Moreover, the synthetic utility of this photooxidation reaction was exemplified by the gram-scale synthesis employing crude CP conveniently obtained from the fermentation broth of Cercospora, and the successive transformation to kynurenine. Finally, preliminary mechanistic studies revealed that an energy transfer pathway was involved in the reaction.