Isolation and Structure Elucidation of Additional Alkaloids from the Tropical Rainforest Tree Galbulimima baccata

The structures of five new natural products (GB 27-GB 31, 1-5), isolated as minor components from the bark of Galbulimima baccata, have been determined by 2D NMR spectroscopy in combination with DFT calculations. Among the alkaloids, GB 31 (5) belongs to Class I, GB 27 (1) and 28 (2) belong to Class II, and GB 30 (4) belongs to Class III GB alkaloids. GB 31 is the first non-nitrogen-containing GB "alkaloid", being a biosynthetic oxidation product of himbacine, himandravine, or himbeline. GB 29 (3) has an entirely new natural product scaffold but belongs to Class IV (miscellaneous alkaloids). The isolation of a new Galbulimima scaffold has revealed a new pathway in the biosynthesis of the GB alkaloids. The new molecules isolated have shed further light on the biogenetic relationship among these structurally unique and complex groups of alkaloids. We present, for the first time, a unified biogenesis for the GB alkaloids that were first isolated in the 1950s and now number over 40 examples. This work also brings full circle the story of Galbulimima alkaloids. A life-long project of Wal Taylor involving one of his first students (Lew Mander) and one of his last students (Peter Karuso), a story stretching over six decades, has come to a final conclusion.

The Future of Retrosynthesis and Synthetic Planning: Algorithmic, Humanistic or the Interplay?

The practice of deploying and teaching retrosynthesis is on the cusp of considerable change, which in turn forces practitioners and educators to contemplate whether this impending change will advance or erode the efficiency and elegance of organic synthesis in the future. A short treatise is presented herein that covers the concept of retrosynthesis, along with exemplified methods and theories, and an attempt to comprehend the impact of artificial intelligence in an era when freely and commercially available retrosynthetic and forward synthesis planning programs are increasingly prevalent. Will the computer ever compete with human retrosynthetic design and the art of organic synthesis?

Structures of New Alkaloids from Rain Forest Trees Galbulimima belgraveana and Galbulimima baccata in Papua New Guinea, Indonesia, and Northern Australia

Following on our 60-year research on the chemical constituents of the rain forest trees Galbulimima belgraveana and Galbulimima baccata, we report the isolation of seven new alkaloids: GB14 (14), GB22 (15), GB25 (16), GB21 (17), GB23 (18), GB24 (19), and GB26 (20). Their structures were elucidated by a combination of spectroscopic analyses and single-crystal X-ray crystallography, as well as structure degradation and interconversion. The newly isolated alkaloids are precursors or derivatives of the known family members from our early studies and could be intermediates in the biosynthesis of the Galbulimima alkaloids. Therefore, the present study has expanded the range of structures in this family of alkaloids and provided some missing links in the biosynthetic sequences.

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.

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.

Tandem phenolic oxidative amidation-intramolecular Diels-Alder reaction: an approach to the himandrine core

An oxidative cyclization of dienic sulfonamides mediated by iodobenzene diacetate in TFA, followed by a tandem intramolecular Diels-Alder reaction, achieves desymmetrization of a "locally symmetrical" dienone with good levels of diastereoselectivity and leads to valuable synthetic intermediates for the himandrine alkaloids.

Approaches to the Synthesis of the Galbulimima Alkaloid Himandrine

The hexacyclic skeleton of himandrine (2), which is present in 15 of the more complex alkaloids obtained from the bark of the tropical rain forest tree Galbulimima belgraveana has been prepared by means of a 19-step synthesis beginning with the known [3.2.1]-benzobicyclooctene intermediate 9. An alternative approach is also described, thus far culminating in 34. Key transformations include Diels–Alder cycloadditions, ring contractions, a Curtius rearrangement, a Birch reduction, an intramolecular nucleophilic amination, and a palladium-mediated alkene amination.

Total synthesis of (-)-himandrine

We describe the first total synthesis of (-)-himandrine, a member of the class II galbulimima alkaloids. Noteworthy features of this chemistry include a diastereoselective Diels-Alder reaction in the rapid synthesis of the tricycle ABC-ring system in an enantiomerically enriched form, the use of a formal [3+3] annulation strategy to secure the CDE-ring system with complete diastereoselection, and successful implementation of our biogenetically inspired oxidative spirocyclization of an advanced intermediate. The successful and direct late-stage formation of the F-ring in the hexacyclic core of himandrine drew on the power of biogenetic considerations and fully utilized the inherent chemistry of a plausible biosynthetic intermediate.

Total synthesis of (-)-himgaline

The first total synthesis of (-)-himgaline and a highly enantioselective synthesis of its congener (-)-GB 13 are described. Decarboxylative aza-Michael reaction of the hexacyclic lactone precursor under acidic conditions, followed by basic workup, yielded (-)-GB 13 in 80% yield. Cyclization of (-)-GB 13 to oxohimgaline under acidic conditions, followed by internally coordinated sodium triacetoxyborohydride reduction, gave (-)-himgaline as the exclusive product.