Rapid Total Synthesis of Cyclic Lipodepsipeptides as a Premise to Investigate their Self‐Assembly and Biological Activity

Abstract

A rapid and efficient total synthesis is reported for the cyclic lipodepsipeptide pseudodesmin A. This member of the Pseudomonas viscosin group is active against Gram-positive bacteria and features self-assembling properties. A conserved serine residue within the lactone macrocycle is exploited for initial immobilization on 2-chlorotrityl chloride resin through ether formation with the side-chain alcohol. Subsequent elongation proceeds through Fmoc solid-phase peptide synthesis, including automated incorporation of the enantioselectively synthesized (R)-3-hydroxydecanoic acid lipid tail. Following esterification to generate the incipient lactone bond, the macrocycle is formed by on-resin head-to-tail macrolactamization and cleaved from the resin to give the desired compound in good purity. The short and efficient synthesis route allows rapid generation of analogues by facile variation of both the peptide and lipid moieties with good control of epimerization while maximizing automation. Synthesis of the pseudodesmin A enantiomer yields identical self-assembly and biological activity to that observed for the natural compound, showing that activity is not mediated by chiral interactions. A D-Asn8 analogue developed en route retains self-assembly, but loses activity. The synthesis strategy should be generally applicable for the rapid generation of analogues from various cyclic lipodepsipeptide groups, allowing an investigation of their self-assembling properties and structure-activity relationships.

First total synthesis of WLIP: on the importance of correct protecting group choice

“…These extended reaction times were found to cause considerable epimerization. However, optimized conditions were found resulting in complete conversion to 7 with a limited amount (15%) of epimerization (Figure (A)) . Although these conditions were applied for the synthesis of more than 30 pseudodesmin analogues, comprising both modifications of amino acids (besides the 2nd residue) as well as the lipid moiety (unpublished data), we here observed that the synthesis of 8 turned out to be extremely cumbersome (Figure (B)), when applying identical conditions.…”

“…This residue thus appears to be a key residue in determining the properties of the CLP. As part of a larger project aiming to gain more insight in the structure–activity relationship within the viscosin group of CLPs, the earlier developed total synthesis strategy for pseudodesmin A was exploited to carry out a synthetic Ala scan, resulting among others in a so‐called pseudodesmin Q2A analogue . It occurred to us that this synthetic route could be equally expanded towards WLIP, essentially being a pseudodesmin Q2E analogue (Scheme ).…”

“…In summary, the first ever synthesis of the naturally occurring cyclic lipodepsipeptide WLIP has been achieved. It was shown that the synthesis strategy previously developed for Gln2‐containing CLPs such as pseudodesmin A is less robust for the its Glu2 containing counterparts like WLIP . Especially the outcome of the esterification reaction is adversely affected resulting in increased reaction times and the need for repeated couplings.…”

“…All necessary building blocks were obtained according to recently published procedures . A more detailed discussion about the design and development of the strategy can be found there.…”

“…Among the hydrophilic positions, only the second residue is found to vary, being either a Glu residue or a Gln residue. Close inspection of Figure (C), reveals that WLIP can be considered an acid analogue of pseudodesmin A ( 2 ), for which our laboratory recently developed a rapid and efficient synthesis . This strategy is highly automated and allows modifications of the amino acid residues as well as in the lipid moiety.…”