Synthesis and target annotation of GB18

Ingestion of alkaloid metabolites from the bark of Galbulimima (GB) sp. leads to psychotropic and excitatory effects in humans. Only a single target, the muscarinic acetylcholine receptor, has been assigned. Limited, variable supply of GB alkaloids has im-peded their biological exploration and clinical development. Here we report a solution to the supply of GB18, a structural outlier and putative psychotropic principle of Galbulimima bark. Efficient access to the challenging tetrahedral attached-ring motif re-quired the development of a ligand-controlled endo-selective cross-electrophile coupling and a diastereoselective hydrogenation of a rotationally-dynamic pyridine. Reliable, gram-scale access to GB18 allowed its assignment as a potent antagonist of kappa- and mu- opioid receptors and lay the foundation to navigate and understand the biological activity of Galbulimima metabolites.