Total Synthesis and Structural Revision of Keenamide A

Area of Science:

• Natural Product Chemistry
• Organic Synthesis
• Structural Biology

Background:

• Cyanobactins are ribosomally synthesized and post-translationally modified peptides.
• Stereochemical assignments of thiazoline-containing cyanobactins are complex due to epimerization.
• Keenamide A is a cyanobactin-like cyclopeptide with a leucine-thiazoline moiety.

Purpose of the Study:

• To re-evaluate the proposed configuration of the leucine-thiazoline moiety of keenamide A.
• To demonstrate the utility of synthetic methods in natural product structure elucidation.
• To revise the stereochemical assignment of keenamide A.

Main Methods:

• Developed a rapid and versatile strategy for synthesizing thiazoline-containing cyclopeptides.
• Synthesized all four possible stereoisomers of keenamide A.
• Oxidized one stereoisomer to mollamide C, the thiazole analog.
• Utilized Nuclear Magnetic Resonance (NMR) spectroscopy and X-ray crystallography.

Main Results:

• The originally proposed (R)-configuration of keenamide A's thiazoline ring was revised to (S)-configuration.
• NMR spectra of synthetic stereoisomers matched natural keenamide A, confirming the revision.
• X-ray crystallographic analysis provided definitive structural evidence.

Conclusions:

• Synthetic approaches are powerful tools for natural product structure elucidation and revision.
• The stereochemistry of keenamide A has been definitively assigned as (S).
• This study resolves ambiguity in the structural assignment of a complex natural product.