Synthetic studies on complex cyclic peptide antibiotics

Abstract

With the use and abuse of antibiotics for a long period of time, the resistance of bacteria becomes severe. Being afraid that no drugs can be used, numerous efforts have been devoted to inhibiting the occurrence of the drug resistance of bacteria and to developing novel antibiotics. Therefore, a lot of old antibiotics are being re-evaluated and modified. Among these drugs, cyclic peptide antibiotics are becoming prevalent due to their features of lower toxicity, rapid degradation and targeting selectively as peptides. Moreover, they are superior since they are more resistant to hydrolysis of hydrolases, more receptor-selective and sometimes more membrane permeable. Daptomycin, a cyclic lipodepsipeptide which was isolated from the fermentation of a strain of Strptomyces roseosporus, is the first member of a new class of antibiotics and highly effective against Gram-positive bacteria, including Methicillin-Resistant Staphylococcus aureus (MRSA) and Vancomycin-Resistant Enterococci (VRE). Daptomycin contains a 10-amino acid cyclic core tailing with an exocyclic 3-amino-acid long tail which is further modified by an n-decanoyl lipid chain. Due to its good potency, various researches have been carried out to study the mechanism of daptomycin and its structure-activity relationship (SAR). Over the years, L-3-methylglutamic acid, one of the two nonproteinogenic amino acids in the daptomycin cyclic core, was proved to be crucial for daptomycin antibacterial activities. However, it is still not fully understood. Thus, I have applied the total synthesis approach for daptomycin, first reported by our group, to synthesize three daptomycin analogues with (2S, 3R) methylglutamic acid (mGlu) substituted by (2S, 3R) methylglutamine (mGln), (2S, 3R) ethylglutamic acid (eGlu) and dimethylglutamic acid (dimethyl Glu) and test their activities. My studies showed that even a subtle change to mGlu caused a dramatic loss of the activity. According to the proposed mode of action of daptomycin that it binds to Ca2+ ions prior to aggregating on the membrane, the carboxylic acids in daptomycin are responsible for the sequestering calcium and thus their orientation in space is important. My findings suggest that the acidic groups of daptomycin should be in a network of interactions rather than placed randomly in space. Mannopeptimycins, a cyclic glycopeptide family isolated from Streptomyces hygroscopicus LL-AC98 in 1958, contain a common cyclic hexapeptide core and are effective to current clinically important Gram-positive bacteria, including Vancomycin-Resistant Enterococci (VRE) and Methicillin-Resistant Staphylococcus aureus (MRSA). Even though the potencies of them are not very promising, mannopeptimycins are believed to be a good lead compound due to their novel mode of action, although not fully understood. The complex structures of mannopeptimycins, with the presence of four non-proteinogenic amino acids and the hexapeptide ring render mannopeptimycin a challenging target for the total synthesis. I have finished the synthesis of four non-proteinogenic amino acids of mannopeptimycin ε, namely L-β-hydroxyenduracididine (L-βhEnd), N-Mannosyl D-β-hydroxyenduracididine (N-Man-D-βhEnd), O-dimannosyl tyrosine (O-diMan-Tyr) and β-methyl phenalanine (βmPhe) in scalable approaches. Subsequently, I have tried to apply Fmoc solid phase peptide synthesis (SPPS) to assemble the linear peptide, but I failed. Therefore, I have re-designed a new 3+3 strategy towards the synthesis of mannopeptimycin ε, which will be further tested in the future.