Identification and synthesis of 4′-ortho-aminobenzoyl ascarosides as sex pheromones of gonochoristic Caenorhabditis nigoni†

Abstract

Using a combination of RP-C18 chromatography, MS and NMR techniques, a new class of homologous modular ascarosides carrying a 4′-ortho-aminobenzoyl moiety was identified from Caenorhabditis nigoni and Caenorhabditis tropicalis. These compounds could not be detected using targeted ascaroside screens based on precursor ion screening for m/z 73.0294 [C₃H₅O₂]⁻, which highlighted a limitation of the current protocols. Their structure assignment was established by total synthesis of AB-asc-C5 (SMID: abas#9) as a representative example in about 1% yield over 14 steps. To achieve this aim, a new method for the synthesis of orthogonally protected ascarosides has been developed which provides methyl 2-benzoyl-ascaroside as a highly versatile building block for regioselective ascaroside synthesis. Furthermore, a new synthesis for short chain C5 ascarosides was developed that employs selective reduction and Grubbs cross metathesis. The identity of synthetic AB-asc-C5 and the natural product isolated from C. nigoni was established by an NMR mixing experiment. Retention of C. nigoni males by the exclusively female produced AB-asc-C5 suggests a function as a sex pheromone component. Along with the indole ascarosides (icas), the new class of 4′-ortho-aminobenzoyl ascarosides (abas) represents a mechanism to translate bacterial food dependent L-tryptophan availability into species-specific signaling molecules.