Total Synthesis and Evaluation of [Ψ[CH2NH]Tpg4]Vancomycin Aglycon: Reengineering Vancomycin for Dual d-Ala-d-Ala and d-Ala-d-Lac Binding

Abstract

An effective synthesis of [Ψ[CH₂NH]Tpg⁴]vancomycin aglycon (5) is detailed in which the residue 4 amide carbonyl of vancomycin aglycon has been replaced with a methylene. This removal of a single atom was conducted to enhance binding to d-Ala-d-Lac, countering resistance endowed to bacteria that remodel their d-Ala-d-Ala peptidoglycan cell wall precursor by a similar single atom change (ester O for amide NH). Key elements of the approach include a synthesis of the modified vancomycin ABCD ring system featuring a reductive amination coupling of residues 4 and 5 for installation of the deep-seated amide modification, the first of two diaryl ether closures for formation of the modified CD ring system (76%, 2.5−3:1 kinetic atropodiastereoselectivity), a Suzuki coupling for installation of the hindered AB biaryl bond (90%) on which the atropisomer stereochemistry could be thermally adjusted, and a macrolactamization closure of the AB ring system (70%). Subsequent DE ring system introduction enlisted a room-temperature aromatic nucleophilic substitution reaction for formation of the remaining diaryl ether (86%, 6−7:1 kinetic atropodiastereoselectivity), completing the carbon skeleton of 5. Consistent with expectations and relative to the vancomycin aglycon, 5 exhibited a 40-fold increase in affinity for d-Ala-d-Lac (K_a = 5.2 × 10³ M^-1) and a 35-fold reduction in affinity for d-Ala-d-Ala (K_a = 4.8 × 10³ M^-1), providing a glycopeptide analogue with balanced, dual binding characteristics. Beautifully, 5 exhibited antimicrobial activity (MIC = 31 μg/mL) against a VanA-resistant organism that remodels its d-Ala-d-Ala cell wall precursor to d-Ala-d-Lac upon glycopeptide antibiotic challenge, displaying a potency that reflects these binding characteristics.