Total synthesis of [β-HIle]2-nodupetide: effect of ester to amide substitution on its antimicrobial activity

Abstract

The increasing prevalence of deaths due to multidrug-resistant bacteria (MDRB) in infectious disease therapy has become a global health concern. This led to the development of new antimicrobial therapeutic agents that can combat resistance to pathogenic bacteria. The utilization of natural peptide compounds as potential antimicrobial agents is very promising. Nodupetide, a cyclodepsipeptide with very strong antimicrobial activity against Pseudomonas aeruginosa was isolated from the fermentation of Nodulisporium sp. Unfortunately, one of its residues (3S,4S)-3-hydroxy-4-methylhexanoic acid (HMHA) is not commercially available and the synthesis strategies applied have not been successful. Hence, we synthesized its cyclopeptide analogue [β-HIle]²-nodupetide by replacing HMHA with isoleucine homologue. A combination of solid- and solution-phase peptide synthesis was successfully carried out to synthesize [β-HIle]²-nodupetide with an overall yield of 10.4%. The substitution of HMHA with β-homoisoleucine (β-HIle) changed the ester bond into an amide bond in nodupetide's backbone. The analogue was considerably inactive against Pseudomonas aeruginosa. It can be concluded that the ester bond is crucial for the antimicrobial activity of nodupetide.

Fig. 1. Structure of nodupetida 1 and its analogue [β-HIle]²-nodupetide 2.

Fig. 2. General procedure of Fmoc-strategy SPPS and solution-phase macrocyclization, ((a)–(g) in solid phase) (a) (1) Fmoc-Gly-OH, DIPEA, DCM, 4 h, rt; (2) MeOH/DCM/DIPEA (15 : 80 : 5); (3) 20% piperidine in DMF, (b) (1) Fmoc-l-β-HIle-OH, DIC, OxymaPure, DMF, 2 h, rt; (2) 20% piperidine in DMF, (c) (1) Fmoc-l-Val-OH, DIC, OxymaPure, DMF, 2 h, rt; (2) 20% piperidine in DMF, (d) (1) Fmoc-l-Ala-OH, DIC, OxymaPure, DMF, 2 h, rt; (2) 20% piperidine in DMF, (e) (1) Fmoc-d-Leu-OH, DIC, OxymaPure, DMF, 2 h, rt; (2) 20% piperidine in DMF, (f) (1) Fmoc-l-Val-OH, DIC, OxymaPure, DMF, 2 h; (2) 20% piperidine in DMF, (g) 20% TFA in DCM, 2 × 10 min, rt, (h) (solution phase) HATU, DIPEA, DCM, 7 days, rt.