Total Synthesis and Stereochemical Assignment of Turnercyclamycin C

Abstract

Turnercyclamycins (TCMs) are a new family of cyclic lipopeptide antibiotics (cLPAs) isolated from Teredinibacter turnerae, a symbiont of shipworms. They contain thirteen amino acids, eight of which are contained in a macrocycle closed by an ester bond. Attached to the macrocycle is a linear pentapeptide that is lipidated at the N-terminus. The four members of this family, TCM A to D, only differ from one another in their lipid tail composition. Seven of the thirteen amino acids are non-proteinogenic: D-ornithine, D-threo-β-hydroxyaspartic acid (D-t-HOAsp), D-threo-β-hydroxydiaminobutyric acid (D-t-HODab), L-β-hydroxyisoleucine (L-HOIle), two D-allo-isoleucines (D-allo-Ile) and L-homoserine (L-hSer). Their mechanism of action (MoA) is unknown. The TCMs exhibit good activity against Gram-negative pathogens Escherichia coli, Klebsiella pneumoniae and Acinetobacter baumannii (A. baumannii) including colistin-resistant A. baumannii suggesting that the TCMs act by a MoA that is different from colistin. These findings and the favourable pharmacokinetic properties displayed by the TCMs suggest that they may be good leads for the development of a novel antibiotic for treating colistin-resistant A. baumannii via a unique MoA. The total chemical synthesis of the TCMs has not been achieved and the configuration at the -carbon of L-HOIle is unknown. Here, our efforts to develop a total chemical synthesis of one of the TCMs, TCM C, which has a dodecanoyl fatty acyl tail, are described. These efforts began with the synthesis of D-t-HOAsp, D-t-HODab, and both stereoisomers of L-HOIle, suitably protected for Fmoc solid phase peptide synthesis (SPPS) using both literature and novel routes. Before synthesizing TCM C, an analogue of TCM C, called TCM C-CA, in which we substituted D-t-HODab for D-Dab, D-t-HOAsp for D-Asp and L-HOIle for L-Ile was prepared. This was achieved using Fmoc SPPS in an overall 5% yield. This synthesis provided valuable information on how to best perform some of the key steps such as ester bond formation and macrocycle formation without using up the valuable non-canonical amino acids prepared by chemical synthesis. The synthesis of two versions of TCM C, each one containing one of the two stereoisomers of L-HO-Ile, is in progress.