Total Synthesis of Paenibacterin and Its Analogues

Paenibacterin is a recently discovered cyclic lipodepsipeptide antibiotic produced by the soil bacterium Paenibacillus thiaminolyticus. It is produced as a mixture of three compounds with isomeric 15-carbon acyl lipids, designated P-A1 (linear lipid), P-A2 (anteiso lipid), and P-A3 (iso lipid). Here, we report the total synthesis of P-A1 and P-A2, as well as two analogues of P-A1 in which the threonine residue in P-A1 was replaced with l-2,3-diaminopropionic acid (P-A1-Dapa) and (2 S,3 R)-2,3-diaminobutyric acid (P-A1-Daba), converting the ring-closing ester bond to an amide bond. Solid phase peptide chemistry was used to prepare branched precursors which were cyclized off-resin to obtain the target peptides in good to excellent overall yields. The use of a pseudoproline dipeptide building block was found to be important for obtaining good yields. The antibacterial activity of the peptides was determined against Escherichia coli K-12 (G^-) and Bacillus subtilis 1046 (G⁺). The minimum inhibitory concentrations of P-A1 and P-A2 were the same despite the variation in the structure of the acyl tail. Replacing the ring-closing ester bond with an amide bond had little or no effect on activity. The synthetic routes developed here should prove to be useful for creating new antibiotics based on the structure of paenibacterin.