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

Strategies for the Construction of Benzobicyclo[3.2.1]octane in Natural Product Synthesis

Benzobicyclo[3.2.1]octane is a cage-like unique motif containing a bicyclo[3.2.1]octane structure fused with at least one benzene ring. It is found in various natural products that exhibit structural complexities and important biological activities. The total synthesis of natural products possessing this challenging structure has received considerable attention, and great advances have been made in this field during the past 15 years. This review summarizes thus far achieved chemical syntheses and synthetic studies of natural compounds featuring the benzobicyclo[3.2.1]octane core. It focuses on strategic approaches constructing the bridged structure, aiming to provide a useful reference for inspiring further advancements in strategies and total syntheses of natural products with such a framework.

Synthesis and target annotation of the alkaloid GB18

Ingestion of alkaloid metabolites from the bark of Galbulimima (GB) sp. leads to psychotropic and excitatory effects in humans^1-4. Limited, variable supply of GB alkaloids⁵, however, has impeded their biological exploration and clinical development⁶. Here we report a solution to the supply of GB18, a structural outlier and putative psychotropic principle of Galbulimima bark. Efficient access to its challenging tetrahedral attached-ring motif required 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 enabled its assignment as a potent antagonist of κ- and μ-opioid receptors-the first new targets in 35 years-and lays the foundation to navigate and understand the biological activity of Galbulimima metabolites.

GB7 acetate, a galbulimima alkaloid from Galbulimima belgraveana, possesses anticancer effects in colorectal cancer cells

GB7 acetate is a galbulimima alkaloid obtained from Galbulimima belgraveana. However, information regarding its structure, biological activities, and related mechanisms is not entirely available. A series of spectroscopic analyses, structural degradation, interconversion, and crystallography were performed to identify the structure of GB7 acetate. The MTT assay was applied to measure cell proliferation on human colorectal cancer HCT 116 cells. The expressions of the related proteins were measured by Western blotting. Transmission electron microscopy (TEM), acridine orange (AO) and monodansylcadaverine (MDC) staining were used to detect the presence of autophagic vesicles and autolysosomes. A transwell assay was performed to demonstrate metastatic capabilities. Oxygen consumption rate (OCR) and extracellular acidification rate (ECAR) assays were performed to determine the mitochondrial oxidative phosphorylation (OXPHOS) and glycolysis activity of HCT 116 cells. The data showed that GB7 acetate suppressed the proliferation and colony-forming ability of HCT 116 cells. Pretreatment with GB7 acetate significantly induced the formation of autophagic vesicles and autolysosomes. GB7 acetate upregulated the expressions of LC3 and Thr172 phosphorylated adenosine 5'-monophosphate (AMP)-activated protein kinase α (p-AMPKα), which are key elements of autophagy. In addition, GB7 acetate suppressed the metastatic capabilities of HCT 116 cells. Additionally, the production of matrix metallo-proteinase-2 (MMP-2) and MMP-9 was reduced, whereas the expression of E-cadherin (E-cad) was upregulated. Furthermore, GB7 acetate significantly reduced mitochondrial OXPHOS and glycolysis. In conclusion, the structure of the novel Galbulimima alkaloid GB7 acetate was identified. GB7 acetate was shown to have anti-proliferative, pro-autophagic, anti-metastatic, and anti-metabolite capabilities in HCT 116 cells. This study might provide new insights into cancer treatment efficacy and cancer chemoprevention.

Concise syntheses of GB22, GB13, and himgaline by cross-coupling and complete reduction

Neuroactive metabolites from the bark of Galbulimima belgraveana occur in variable distributions among trees and are not easily accessible through chemical synthesis because of elaborate bond networks and dense stereochemistry. Previous syntheses of complex congeners such as himgaline have relied on iterative, stepwise installation of multiple methine stereocenters. We decreased the synthetic burden of himgaline chemical space to nearly one-third of the prior best (7 to 9 versus 19 to 31 steps) by cross-coupling high fraction aromatic building blocks (high Fsp2) followed by complete, stereoselective reduction to high fraction sp3 products (high Fsp3). This short entry into Galbulimima alkaloid space should facilitate extensive chemical exploration and biological interrogation.

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?

Enantioselective Synthesis, DFT Calculations, and Preliminary Antineoplastic Activity of Dibenzo 1-Azaspiro[4.5]decanes on Drug-Resistant Leukemias

The addition of 2-bromobenzylmagnesium bromide to chiral N- tert-butanesulfinyl imines derived from tetralone-type ketones proceeds with high levels of diastereocontrol. The resulting sulfinamide derivatives were transformed into dibenzoazaspiro compounds after a palladium-catalyzed intramolecular N-arylation. DFT calculations have been performed to rationalize the stereochemical course of the reaction. Similar results have been obtained considering either diethyl ether or toluene as a solvent, in both cases in an excellent agreement with experimental findings. NCI topological calculations have also been used to evidence crucial noncovalent interactions. In addition, the azaspiro compounds reduced the viability of chronic myeloid leukemia cells in the micromolar range. Notably, both the halogen-substituted ( R)- and ( S)-8g and -8h as well as ( R)-8j were at least two times more effective on a multidrug-resistant derivative than on the parental cell line, exerting a collateral sensitivity effect.