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.