Allenamides as a Powerful Tool to Incorporate Diversity: Thia-Michael Lipidation of Semisynthetic Peptides and Access to ß-Keto Amides.

Lipopeptides are an important class of biomolecules for drug development. Compared with conventional acylation, a chemoselective lipidation strategy offers a more efficient strategy for late-stage structural derivatisation of a peptide scaffold. It provides access to chemically diverse compounds possessing intriguing and non-native moieties. Utilising an allenamide, we report the first semisynthesis of antimicrobial lipopeptides leveraging a highly efficient thia-Michael addition of chemically diverse lipophilic thiols. Using chemoenzymatically prepared polymyxin B nonapeptide (PMBN) as a model scaffold, an optimised allenamide-mediated thia-Michael addition effected rapid and near quantitative lipidation, affording vinyl sulfide-linked lipopeptide derivatives. Harnessing the utility of this new methodology, 22 lipophilic thiols of unprecedented chemical diversity were introduced to the PMBN framework. These included alkyl thiols, substituted aromatic thiols, heterocyclic thiols and those bearing additional functional groups (e.g., amines), ultimately yielding analogues with potent Gram-negative antimicrobial activity and substantially attenuated nephrotoxicity. Furthermore, we report facile routes to transform the allenamide into a ß-keto amide on unprotected peptides, offering a powerful "jack-of-all-trades" synthetic intermediate to enable further peptide modification.