The galbulimima alkaloids—a new frontier in alkaloid synthesis

Synthesis of psychotropic alkaloids from Galbulimima

Efficient syntheses of valuable natural products open gateways from kind learning environments to wicked worlds, where long-term, interdisciplinary research questions can be asked and answered. In this Perspective, we discuss the Galbulimima (GB) alkaloids, metabolites of a rainforest canopy tree that exhibit potent but poorly understood effects in humans, including accounts of hallucination. Recent syntheses from our group have opened up GB alkaloid chemical space for investigation by way of new cross-coupling reactions and gram-scale target production. Although natural product synthesis can be challenging, its objective is obvious. Realization of long-term, enabling goals will be a circuitous journey at the interface of chemistry, pharmacology and neuroscience-a potent mix to foster discovery in the coming century.

Intramolecular (4+3) Cycloadditions of Pyrroles and Application to the Synthesis of the Core of Class II Galbulimima Alkaloids

The first intramolecular (4+3) cycloaddition of pyrroles with epoxy enolsilanes as the electrophiles was developed and used to generate optically-enriched cycloadducts containing the nortropane substructure in good yields. Using this pyrrole cycloaddition as the key step, we achieved the asymmetric synthesis of a nortropane compound bearing the BCDEF ring structure common to the Class II galbulimima alkaloids.

Galbulimima Alkaloids

This chapter provides a comprehensive overview of recent achievements in the area of Galbulimima alkaloids. Following a discussion of the isolation of Galbulimima alkaloids and structural features of members of this fascinating family of secondary metabolites, biological properties of selected compounds are briefly discussed. Furthermore, the proposed biosynthetic routes toward Galbulimima alkaloids are outlined. The main section of the chapter is devoted to a detailed discussion and comparison of all total syntheses of Galbulimima alkaloids published to date.

Total Synthesis of the Galbulimima Alkaloids Himandravine and GB17 Using Biomimetic Diels-Alder Reactions of Double Diene Precursors

The enantioselective total syntheses of himandravine and GB17 were completed through a common biomimetic strategy involving Diels-Alder reactions of unusual double diene containing linear precursors. The double diene precursors, containing or lacking a C12 substituent as required to produce GB17 or himandravine, respectively, were found to undergo Diels-Alder reactions to afford mixtures of regioisomeric cycloadducts that map onto the alternative carbocyclic frameworks of both himandravine and GB17. Computational investigations revealed that these Diels-Alder reactions proceed via transition state structures of similar energy that have a high degree of bispericyclic character and that the low levels of regioselectivity observed in the reactions are a consequence of competing orbital interaction and distortion energies. The combined experimental and computational results provide valuable insights into the biosynthesis of the Galbulimima alkaloids.

Further studies toward himandrine via sequential oxidative amidation – intramolecular Diels–Alder reactions

Exploratory work toward the alkaloid himandrine evaluated the directing ability of a pyrrolidine C-3 substituent in a diastereotopic group elective intramolecular Diels–Alder reaction of a spirodienone obtained by the oxidative amidation of a phenol. The study also defined a technique for the construction of ring D of the alkaloid.