Structure Revision of Protoaculeine B, a Post-translationally Modified N-Terminal Residue in the Peptide Toxin Aculeine B.

The structure of protoaculeine B, the N-terminal residue of the marine peptide toxin aculeine B, is revised to the cis-1,3-disubstituted tetrahydro-β-carboline framework. We prepared two truncated model compounds that lack a long-chain polyamine using the one-step Pictet-Spengler reaction of tryptophan and compared their NMR, mass spectra, and chemical reactivity with those of the natural protoaculeine B. The synthetic models reproduced the profiles of the natural product well, which confirmed the appropriateness of the structure revision.