Total Synthesis and Bioactivity Evaluation of Hydrophobic Microcionamide‐Inspired Peptides

In this report, we describe the facile synthesis of four microcionamide‐inspired peptides where the atypical 2‐phenylethylenamine (2‐PEA) functional group in the marine natural product, microcionamide A, was replaced with a similarly‐aromatic but more easily incorporated tryptophan (Trp) residue. Compounds 1–4 were synthesized using a standard Fmoc‐based solid‐phase synthesis strategy followed by iodine‐mediated on‐resin cyclization for disulfide‐bridged compounds 1–3. Compound 1 showed antimicrobial activity against Staphylococcus aureus and Pseudomonas aeruginosa, with minimum inhibitory concentrations (MICs) of 9.1 μM and 15 μM, respectively. The inactivity of alanine analogs 2–4 against these pathogens suggests that the N‐terminal Val, the cyclic scaffold, the contiguous Ile residues, and consequently, the hydrophobicity of compound 1 are essential for antibacterial activity. Compound 1 also favorably exhibited minimal cytotoxicity against normal mammalian cell lines. In summary, we have synthesized an analog of microcionamide A where replacement of the 2‐PEA moiety with a Trp residue retained the antibacterial activity and with favorably low cytotoxicity.