The total synthesis of the Galbulimima alkaloid GB 13

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 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.

Samarium(ii) iodide-mediated reactions applied to natural product total synthesis

Natural product synthesis remains a field in which new synthetic methods and reagents are continually being evaluated. Due to the demanding structures and complex functionality of many natural products, only powerful and selective methods and reagents will be highlighted in this proceeding. Since its introduction by Henri Kagan, samarium(ii) iodide (SmI2, Kagan's reagent) has found increasing use in chemical synthesis. Over the years, many reviews have been published on the application of SmI2 in numerous reductive coupling procedures as well as in natural product total synthesis. This review highlights recent advances in SmI2-mediated synthetic strategies, as applied in the total synthesis of natural products since 2004.

Preparation and Synthetic Applications of Five-to-Seven-Membered Cyclic α-Diazo Monocarbonyl Compounds

The reactivity of cyclic α-diazo monocarbonyl compounds differs from that of their acyclic counterparts. In this review, we summarize the current literature available on the synthesis and synthetic applications of three major classes of cyclic α-diazo monocarbonyl compounds: α-diazo ketones, α-diazo lactones and α-diazo lactams.

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.

A Synthetic Route to The Core Structure of (-)-Retigeranic Acid A

Retigeranic acid A is a uniquely structured pentacyclic sesterterpene bearing eight stereogenic centers. We report a concise route to the core structure of (-)-retigeranic acid A. The stereochemistry of its six chiral centers and three quaternary carbon centers was well-controlled. This route features two intramolecular Pauson-Khand reactions (IMPKRs): the first forged the D and E rings to deliver the triquinane subunit, and the second constructed the A and B rings and diastereoselectively installed the quaternary C_6a center.

In Pursuit of Synthetic Efficiency: Convergent Approaches

The field of total synthesis has reached a stage in which emphasis has been increasingly focused on synthetic efficiency rather than merely achieving the synthesis of a target molecule. The pursuit of synthetic efficiency, typically represented by step count and overall yield, is a rich source of inspiration and motivation for synthetic chemists to invent innovative strategies and methods. Among them, convergent strategy has been well recognized as an effective approach to improve efficiency. This strategy generally involves coupling of fragments with similar complexity to furnish the target molecule via subsequent cyclization or late-stage functionalization. Thus, methodologies that enable effective connection of fragments are critical to devising a convergent plan. In our laboratory, convergent strategy has served as a long-standing principle for pursuing efficient synthesis during the course of planning and implementing synthetic projects. In this Account, we summarize our endeavors in the convergent synthesis of natural products over the last ten years. We show how we identify reasonable bond disconnections and employ enabling synthetic methodologies to maximize convergency, leading to the efficient syntheses of over two-dozen highly complex molecules from eight disparate families.In detail, we categorize our work into three parts based on the diverse reaction types for fragment assembly. First, we demonstrate the application of a powerful single-electron reducing agent, SmI₂, in a late-stage cyclization step, forging the polycyclic skeletons of structurally fascinating Galbulimima alkaloids and Leucosceptrum sesterterpenoids. Next, we showcase how three different types of cycloaddition reactions can simultaneously construct two challenging C-C bonds in a single step, providing concise entries to three distinct families, namely, spiroquinazoline alkaloids, gracilamine, and kaurane diterpenoids. In the third part, we describe convergent assembly of ent-kaurane diterpenoids, gelsedine-type alkaloids, and several drug molecules via employing some bifunctional synthons. To access highly oxidized ent-kaurane diterpenoids, we introduce the hallmark bicyclo[3.2.1]octane ring system at an early stage, and then execute coupling and cyclization by means of a Hoppe's homoaldol reaction and a Mukaiyama-Michael-type addition, respectively. Furthermore, we showcase how the orchestrated combination of an asymmetric Michael addition, a tandem oxidation-aldol reaction and a pinacol rearrangement can dramatically improve the efficiency in synthesizing gelsedine-type alkaloids, with nary a protecting group. Finally, to address the supply issue of several drugs, including anti-influenza drug zanamivir and antitumor agent Et-743, we exploit scalable and practical approaches to provide advantages over current routes in terms of cost, ease of execution, and efficiency.

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?

Alkynogenic fragmentation

Fragmentation reactions that generate alkynes in conjunction with C–C bond cleavage require special considerations but can produce high-value alkyne building blocks for organic synthesis.

Efficient New Protocols for Converting Primary Amides into Nitriles Initiated by P(NMe2)3, PCl3, or P(OPh)3

Three efficient and high-yielding procedures have been developed for the conversion of primary amides into nitriles, mediated by hitherto unexplored P(NMe2)3, PCl3, or P(OPh)3. The reactions were conducted under operationally simple and mild conditions and displayed broad substrate scope and good functional group tolerance.

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.

The reductive decyanation reaction: an overview and recent developments

This review presents an overview of the reductive decyanation reaction with a special interest for recent developments. This transformation allows synthetic chemists to take advantages of the nitrile functional group before its removal. Mechanistic details and applications to organic synthesis are provided.

Strategy towards the enantioselective synthesis of schiglautone A

The enantioselective synthesis of a functionalized intermediate comprising 6 of the 7 stereocenters of schiglautone A is reported and features a lithiation–borylation enzymatic resolution sequence.

Addition of silylated nucleophiles to α-oxoketenes

A general evaluation of silylated nucleophiles to intercept transient α-oxoketenes generated by microwave-assisted Wolff rearrangement of 2-diazo-1,3-dicarbonyl compounds is presented.

2,6-Diiminopiperidin-1-ol: an overlooked motif relevant to uranyl and transition metal binding on poly(amidoxime) adsorbents

We report a new cyclic diimino ligand resulting from glutardiamidoxime cyclization and associated multinuclear complexes with U(vi), Cu(ii), and Ni(ii).

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.

A biomimetic strategy to access the silybins: total synthesis of (-)-isosilybin A

We report the first asymmetric, total synthesis of (-)-isosilybin A. A late-stage catalytic biomimetic cyclization of a highly functionalized chalcone is employed to form the characteristic benzopyranone ring. A robust and flexible approach to this chalcone provides an entry to the preparation of the entire isomeric family of silybin natural products.

Acid promoted synthesis of cyclic 1,3-dione fused symmetrical 2,8-dioxabicyclo[3.3.1]nonanes

Synthesis of symmetrical 2,8-dioxabicyclo[3.3.1]nonanes (4) is accomplished by the reaction of β-enamino ketones (1) and 1,3-cyclohexanediones (2) in AcOH. However, the presence of TFA in toluene gave xanthenes (5).

Synthesis of nitriles via palladium-catalyzed water shuffling from amides to acetonitrile

Palladium-catalyzed synthesis of nitriles from amides has been described. Two similar, but complementary reaction conditions have been identified to convert various amides including α,β,γ,δ-unsaturated amides, cinnamides, aromatic amides and alkyl amides to the corresponding nitriles in good to excellent yield.

Catalyst-free one-pot domino reactions for selective synthesis of functionalized 2,8-oxazabicyclo[3.3.1]-nonanes and 5H-indeno[1,2-b]pyridin-5-ones

A simple and efficient method for one-pot selective synthesis of functionalized 2,8-oxazaxabicyclo[3.3.1]nonanes and hydroxy-containing 5H-indeno[1,2-b]pyridin-5-ones under catalyst-free conditions has been developed.

C-C fragmentation: origins and recent applications

It has been 60 years since Eschenmoser and Frey disclosed the archetypal CC fragmentation reaction. New fragmentations and several variants of the original quickly followed. Many of these variations, which include the Beckmann, Grob, Wharton, Marshall, and Eschenmoser-Tanabe fragmentations, have been reviewed over the intervening years. A close examination of the origins of fragmentation has not been described. Recently, useful new methods have flourished, particularly fragmentations that give alkynes and allenes, and such reactions have been applied to a range of complex motifs and natural products. This Review traces the origins of fragmentation reactions and provides a summary of the methods, applications, and new insights of heterolytic CC fragmentation reactions advanced over the last 20 years.

Total synthesis of alkaloid (+/-)-G. B. 13 using a Rh(I)-catalyzed ketone hydroarylation and late-stage pyridine reduction

Total synthesis of the Galbulimima alkaloid G. B. 13 was achieved utilizing a functionalized pyridine moiety as a piperidine surrogate. Key to the success of the synthesis was the development of an unprecedented rhodium-catalyzed 1,2-addition of an arylboronic ester into an unactivated ketone.

Total Synthesis of the Galbulimima Alkaloid (±)-GB 13

The synthesis of alkaloid GB 13 (4), isolated from the North Australian rain forest tree Galbulimima belgraveana is described. Birch reductive alkylation of 2,5-dimethoxybenzoic acid by 3-methoxybenzyl bromide, followed by an acid-catalyzed cyclization was used to synthesize the [3.3.1]bicyclononane 12. Ring contraction performed on the diazoketone 19 followed by a Diels–Alder reaction generated a pentacyclic intermediate 34 with a carbon skeleton closely resembling the target alkaloid. The surplus nitrile substituent, required for activation and regioselectivity in the Diels–Alder reaction, was removed by treatment with lithium and liquid ammonia. Birch reduction of the aromatic ring could be performed at the same time to give diene 38 and thence enone 41, which was cleaved by means of an Eschenmoser fragmentation. The piperidine ring found in the natural product was formed by reductive cyclization of bis-oxime 49 derived from the alkynyl ketone 48 and the resulting material further elaborated to GB 13.

Origins of the regioselectivities in the Diels-Alder reactions of vinylindenes with 1,4-quinone monoketal and acrolein dienophiles

Computations with density functional theory (B3LYP/6-31G(d)) have elucidated the origins of regioselectivities in the Diels-Alder reaction of vinylindene with a 1,4-quinone monoketal reaction that was employed as the key step in the synthesis of fluostatin C. Frontier Molecular Orbital theory and an electrostatic model are applied to the reactions of alkyl-substituted and vinylindene dienes with 1,4-quinone monoketal and acrolein dienophiles. Regiochemical results that deviate from expectation are explained by a progression from electronic to steric control upon the addition of a Lewis acid catalyst.