Functionalized in Triplicate: A Ring-By-Ring Approach to Tailored Prodiginine Derivatives for Site-Specific Conjugation Through Click Chemistry.

The tripyrrole prototype alkaloid prodigiosin and members of the prodigiosin family are structurally diverse bacterial secondary metabolites. The privileged scaffold accounts for versatile biological activities, for example, antimicrobial, antitumoral, and immunosuppressive. Its Lewis-basic lipophilic tripyrrole core and the aliphatic side chains allow for passive membrane diffusion, thereby trespassing the natural permeability barrier. However, diffusion-controlled uptake is accompanied by low target specificity, hampering the development of tailored prodigiosin therapeutics and their selective delivery to target sites. To address this downside, this work focuses on providing the chemical methodology to synthesize prodiginines that are amenable to click chemistry on each of the three pyrrole moieties and facilitate the development of prodigiosin conjugates. Installing reactive azides and maleimides in the A-, B-, and C-ring of the cytotoxic scaffold enables further functionalization per azide-alkyne cycloaddition (CuAAC and SPAAC) and thiol-maleimide addition, giving rise to protein-conjugable prodiginines for the first time. The presented synthetic routes comprise yields of 3.24.7% over 1215 steps and grant access to valuable synthetic elements for expanding the toolbox of click chemistry to other pyrrole-, pyrrolidinone-, and tetramic acid-containing natural products. Together, the devised methodology for prodiginine derivatization will collectively advance the development of alkaloid-based conjugate therapeutics, eligible for target-selective delivery.