Total Synthesis and Biological Evaluation of Leptosphaerone B and Derivatives of Microketide A

Microketide A and B are fungal polyketides reported to display potent activity against Gram-negative pathogens, yet the lack of synthetic access has prevented detailed investigation of their mode of action and structure–activity relationship (SAR). Here, we report the first total synthesis of two close analogs of microketide A, dihydro-MikA and 11-deoxy-MikA, as well as of racemic leptosphaerone B, another member of this cyclohexenone-based natural product family. Our route features a modular assembly of highly functionalized fragments and enables divergent access to analogs through selective dihydroxylation and late-stage fragment fusion. Despite extensive exploration of multiple C–C bond-forming strategies, unfavorable sterics and competing eliminations prevented successful connection of the fragments required for microketide A. The synthesized compounds leptosphaerone B, dihydro-MikA, and 11-deoxy-MikA were evaluated for antibacterial activity and human cytotoxicity but showed no effects up to 200 μM. Competitive residue-specific chemoproteomics and in vitro nucleophile-trapping experiments further revealed no covalent protein engagement, indicating that these scaffolds are intrinsically weak electrophiles. Our findings suggest that microketide A possesses a restrictive SAR, in which even subtle modifications abolish biological function. The synthetic strategy described herein provides a robust platform for in-depth structure activity relationship studies assessing the biological potential of this natural product.