Breaking Cycles: Saponification-Enhanced NMR Fingerprint Matching for the Identification and Stereochemical evaluation of cyclic lipodepsipeptides from natural sources

We previously described NMR based fingerprint matching using peptide backbone (CHα) resonances as a fast and reliable way to perform structural dereplication of Pseudomonas cyclic lipodepsipeptides (CLiPs). In addition, its combination with total synthesis of a small library of CLiPs was shown to afford unambiguous determination of the stereochemistry, further opening these compounds for structure-activity relationship studies and three-dimensional structure determination. However, the need to include on-resin macrocycle formation in the synthetic workflow results in considerable burden and limits universal applicability to CLiPs. Here we show that this drawback can be removed by first converting the native CLiP of interest into its linearized analogue via controlled hydrolysis of the depsi bond through saponification. By avoiding the macrocycle formation altogether, the required synthesis effort is strongly reduced as it now only requires production of linear peptide analogues. The NMR fingerprints of the linear peptide analogues display a sufficiently distinctive chemical shift fingerprint to act as effective discriminators. The approach is developed using viscosin group CLiPs and subsequently demonstrated on putisolvin, leading to a structural revision, and tanniamide, a newly isolated compound defining a new group in the Pseudomonas CLiP portfolio. These examples demonstrate the effectiveness of the saponification- enhanced approach that broadens applicability of NMR fingerprint matching for the determination of the stereochemistry of CLiPs.