Exploring the Complexity-Generating Features of the Pauson-Khand Reaction from a Synthetic Perspective

Asymmetric total synthesis and anti-hepatocellular carcinoma profile of enantiopure euphopilolide and jolkinolide E

A flexible asymmetric synthesis of both enantiomers of euphopilolide (1) and jolkinolide E (2) [(+)-and (-)-1, (+)-and (-)-2] has been accomplished. This synthesis features an intramolecular oxa-Pauson-Khand reaction (o-PKR) to expeditiously construct the challenging tetracyclic [6.6.6.5] abietane-type diterpene framework, elegantly showcasing the complexity-generating features of o-PKR synthetic methodology leveraging on a judiciously chosen suitable chiral pool scaffold. Furthermore, the anti-hepatocellular carcinoma (HCC) activity of synthetic (-)-euphopilolide (1), (-)-jolkinolide E (2) and their analogues was evaluated. We found that (-)-euphopilolide (1) and (-)-jolkinolide E (2) inhibited the proliferation and induced apoptosis in HCC cells. These findings lay a good foundation for further pharmacology studies of abietane lactone derivatives and provide valuable insight for the development of anti-HCC small molecule drug of natural product origin.

Total Synthesis of (±)- and (-)-Daphnillonin B

The first total synthesis of (±)- and (-)-daphnillonin B, a daphnicyclidin-type alkaloid with a new [7-6-5-7-5-5] A/B/C/D/E/F hexacyclic core, has been achieved. The [6-5-7] B/C/D ring system was efficiently and diastereoselectively constructed via a mild type I intramolecular [5+2] cycloaddition, followed by a Grubbs II catalyst-catalyzed radical cyclization. The [5-5] fused E/F ring system was synthesized via a diastereoselective intramolecular Pauson-Khand reaction. Notably, the synthetically challenging [7-6-5-7-5-5] hexacyclic core was reassembled by a unique Wagner-Meerwein-type rearrangement from the [6-6-5-7-5-5] hexacyclic framework found in calyciphylline A-type Daphniphyllum alkaloids.

Asymmetric Total Syntheses of Hypoestin A, Albolic Acid, and Ceroplastol II

The first asymmetric total synthesis of bioactive diterpenoid hypoestin A with an unprecedented [5-8-5-3] tetracyclic skeleton is accomplished in 15 steps from commercially available (R)-limonene. Furthermore, the second asymmetric total syntheses of sesterterpenoids albolic acid and ceroplastol II in 21 steps are also reported. The synthetically challenging and highly functionalized [X-8-5] (X = 5 or 7) tricarbocyclic ring systems found in hypoestin A, albolic acid, ceroplastol II, and schindilactone A, as well as other natural products, are efficiently and directly constructed via a unique intramolecular Pauson-Khand reaction of an allene-yne. This work represents the first reported use of the Pauson-Khand reaction to access synthetically challenging eight-membered-ring systems in natural product synthesis.

Navigating the Pauson-Khand Reaction in Total Syntheses of Complex Natural Products

"Total synthesis endeavors provide wonderful opportunities to discover and invent new synthetic reactions as a means to advance organic synthesis in general. Such discoveries and inventions can occur when the practitioner faces intransigent problems that cannot be solved by known methods and/or when method improvements are desired in terms of elegance, efficiency, cost-effectiveness, practicality, or environmental friendliness" (K. C. Nicolaou et al. from their review in CCS Chem. 2019, 1, 3-37). To date tens of thousands of bioactive compounds have been isolated from plants, microbes, marine invertebrates, and other sources. These chemical structures have been studied by chemists who scanned the breadth of natural diversity toward drug discovery efforts. Drug-likeness of natural products often possesses common features including molecular complexity, protein-binding ability, structural rigidity, and three-dimensionality. Considering certain biologically important natural products are scarce from natural supply, total synthesis may provide an alternative solution to generating these compounds and their derivatives for the purpose of probing their biological functions. Natural products bearing quaternary carbon stereocenters represent a group of biologically important natural entities that are lead compounds in the development of pharmacological agents and biological probes. However, the stereocontrolled introduction of quaternary carbons, with vicinal patterns that substantially expand the complexity of molecular architectures and chemical space in particular, presents distinct challenges because of the high steric repulsion between substituents. Though remarkable advance has been seen for quaternary carbon stereocenter generation, the process remains a daunting challenge given that the formation of highly congested stereocenters increases the difficulty in achieving orbital overlap.In the past two decades, our group has initiated a program to develop synthetic strategies and methods with the aim of advancing the frontiers of the total syntheses of biologically important complex natural products bearing all-carbon quaternary stereogenic centers. Typical endeavors have involved the use of a Pauson-Khand (PK) reaction as a key step in constructing core structures with all-carbon quaternary stereogenic center(s), with the aid of well-orchestrated thiourea-Co- and thiourea-Pd-catalyzed PK reactions. These methodological advances have enabled us to achieve total syntheses of a series of topologically complex natural products with diverse structural features. These methods will enable the assembly of molecules with improved biological functions and provide tool compounds for elucidation of mechanism of action or identification of potential cellular targets.

Adventures and Detours in the Synthesis of Hydropentalenes

Functionalized hydropentalenes (i.e., bicyclo[3.3.0]octanones) constitute important building blocks for natural products and for ligands for asymmetric catalysis. The assembly and tailored functionalization of this convex roof-shaped scaffold is challenging and has motivated a variety of synthetic approaches including our own contributions, which will be presented in this account.

1 Introduction

2 Biosynthesis of Hydropentalenes

3 Hydropentalenes through the Pauson–Khand Reaction

4 Hydropentalenes through Transannular Oxidative Cyclization of Cycloocta-1,4-diene

5 Functionalization of Bicyclo[3.3.0]octan-1,4-dione to Dodecahydrocyclopenta[a]indenes

6 Functionalization of Bicyclo[3.3.0]octan-1,4-diones to Crown Ether Hybrids

7 Functionalization of Bicyclo[3.3.0]octan-1,4-dione to Cylindramide

8 Tandem Ring-Opening Metathesis/Ring-Closing Metathesis/Cross-Metathesis of Bicyclo[2.2.1]heptanes

9 Functionalization of Bicyclo[3.3.0]octan-1,4-dione to Geodin A

10 Hydropentalenes through Enantioselective Desymmetrization of Weiss Diketones

11 Functionalization of Weiss Diketones by Carbonyl Ene Reactions

12 Functionalization of the Weiss Diketone to Cylindramide and Geodin A Core Units

13 Biological Properties of Bicyclo[3.3.0]octanes

14 Hydropentalenes through Vinylcyclopropane Cyclopentene Rearrangement

15 Functionalization of Bicyclo[3.3.0]octanes toward Chiral Dienes

16 Miscellaneous Syntheses of Hydropentalenes

17 Conclusion and Outlook

Evolution of Pauson-Khand Reaction: Strategic Applications in Total Syntheses of Architecturally Complex Natural Products (2016–2020)

Metal-mediated cyclizations are important transformations in a natural product total synthesis. The Pauson-Khand reaction, particularly powerful for establishing cyclopentenone-containing structures, is distinguished as one of the most attractive annulation processes routinely employed in synthesis campaigns. This review covers Co, Rh, and Pd catalyzed Pauson-Khand reaction and summarizes its strategic applications in total syntheses of structurally complex natural products in the last five years. Additionally, the hetero-Pauson-Khand reaction in the synthesis of heterocycles will also be discussed. Focusing on the panorama of organic synthesis, this review highlights the strategically developed Pauson-Khand reaction in fulfilling total synthetic tasks and its synthetic attractiveness is aimed to be illustrated.

Asymmetric Total Synthesis of Bufospirostenin A

The first and asymmetric total synthesis of bioactive bufospirostenin A, an unusual spirostanol with rearranged A/B rings, was accomplished. The synthetically challenging [5-7-6-5] tetracyclic ring system, found in bufospirostenin A and some other natural products, was efficiently constructed by the unique intramolecular rhodium-catalyzed Pauson-Khand reaction of an alkoxyallene-yne. The 11 stereocenters in the final product, including the 10 contiguous stereocenters, were installed diastereoselectively.

Access to Highly Functionalized Cyclopentenones via Diastereoselective Pauson-Khand Reaction of Siloxy-Tethered 1,7-Enynes

A diastereoselective Co₂(CO)₈-mediated Pauson-Khand reaction (PKR) of siloxy-tethered 1,7-enynes for the synthesis of cyclopentaoxasilinones has been developed. This transformation can be performed on a multigram scale and is characterized by a broad substrate scope, functional group compatibility, and high chemo- and diastereoselectivity. Oxidation of the resulting cyclopentaoxasilinones delivers stereoenriched β-alkylated cyclopentenones, which are inaccessible by intermolecular PKRs. This research provides a practical solution to the challenges associated with the classical intermolecular PKR.

Asymmetric Total Synthesis of Fawcettimine-Type Lycopodium Alkaloid, Lycopoclavamine-A

An asymmetric total synthesis of lycopoclavamine-A (1), a structurally unique fawcettimine-type Lycopodium alkaloid, was achieved via a stereoselective Pauson-Khand reaction and a stereoselective conjugate addition to construct a quaternary carbon center at C-12.

Total synthesis of sesterterpenoids

Total syntheses of biologically and structurally fascinating sesterterpenoids published between Jan. 2012 and Jan. 2018 are summarized and discussed here.

Natural product syntheses via carbonylative cyclizations

This review summarizes the application of various transition metal-catalyzed/mediated carbonylative cyclization reactions in natural product total synthesis.

Asymmetric total synthesis of (+)-astellatol and (−)-astellatene

Here we describe the full account of the total synthesis of (+)-astellatol, as well as the first total synthesis of (−)-astellatene.

Metal-free oxidative [2+2+1] heteroannulation of 1,7-enynes with thiocyanates toward thieno[3,4-c]quinolin-4(5H)-ones

A new metal-free oxidative [2+2+1] heteroannulation of 1,7-enynes with thiocyanates for producing thieno[3,4-c]quinolin-4(5H)-ones is presented. This reaction employs benzoylperoxide (BPO) as the oxidant and sodium thiocyanate as the sulfur source to enable the formation of three chemical bonds, two C-S bonds and one C-C bond, in a single reaction, and represents a new, practical access to S-heterocycles with the avoidance of the use of metal catalysts and excess bases.

Asymmetric Total Synthesis of Lancifodilactone G Acetate. 2. Final Phase and Completion of the Total Synthesis

The asymmetric total synthesis of lancifodilactone G acetate was accomplished in 28 steps. The key steps in this synthesis include (i) an asymmetric Diels-Alder reaction for formation of the scaffold of the BC ring; (ii) an intramolecular ring-closing metathesis reaction for the formation of the trisubstituted cyclooctene using a Hoveyda-Grubbs II catalyst; (iii) an intramolecular Pauson-Khand reaction for construction of the sterically congested F ring; (iv) sequential cross-metathesis, hydrogenation, and lactonization reactions for installation of the anomerically stabilized bis-spiro ketal fragment of lancifodilactone G; and (v) a Dieckmann-type condensation reaction for installation of the A ring. The strategy and chemistry developed for the total synthesis will be useful in the synthesis of other natural products and complex molecules.

Total Synthesis of Astellatol

A nearly-30-year-old unanswered synthetic puzzle, astellatol, has been solved in an enantiospecific manner. The highly congested pentacyclic skeleton of this rare sesterterpenoid, which possesses a unique bicyclo[4.1.1]octane motif, ten stereocenters, a cyclobutane that contains two quaternary centers, an exo-methylene group, and a sterically encumbered isopropyl trans-hydrindane motif, makes astellatol arguably one of the most challenging targets for sesterterpenoid synthesis. An intramolecular Pauson-Khand reaction was exploited to construct the right-hand side scaffold of this sesterterpenoid. An unprecedented reductive radical 1,6-addition, mediated by SmI₂ , forged the cyclobutane motif. Last, a strategic oxidation/reduction step provided not only the decisive solution for the remarkably challenging late-stage transformations, but also a highly valuable unravelling of the notorious issue of trans-hydrindane synthesis. Importantly, the synthesis of astellatol showcases a rapid, scalable strategy to access diverse complex isopropyl trans-hydrindane sesterterpenoids.

Cycloaddition Reactions of Cobalt-Complexed Macrocyclic Alkynes: The Transannular Pauson-Khand Reaction

The Pauson-Khand reaction is a powerful tool for the synthesis of cyclopentenones through the efficient [2 + 2 + 1] cycloaddition of dicobalt alkyne complexes with alkenes. While intermolecular and intramolecular variants are widely known, transannular versions of this reaction are unknown and the basis of this study. Macrocyclic enyne and dienyne complexes were readily synthesized by palladium(II)-catalyzed oxidative macrocyclizations of bis(vinyl boronate esters) or ring-closing metathesis reactions followed by complexation with dicobalt octacarbonyl. Several reaction modalities of these macrocyclic complexes were uncovered. In addition to the first successful transannular Pauson-Khand reactions, other intermolecular and transannular cycloaddition reactions included intermolecular Pauson-Khand reactions, transannular [4 + 2] cycloaddition reactions, intermolecular [2 + 2 + 2] cycloaddition reactions, and intermolecular [2 + 2 + 1 + 1] cycloaddition reactions. The structural and reaction requirements for each process are presented.

Palladium-catalyzed dearomatizative [2 + 2 + 1] carboannulation of 1,7-enynes with aryl diazonium salts and H2O: facile synthesis of spirocyclohexadienone-fused cyclopenta[c]quinolin-4(5H)-ones

The first 1,7-enyne dearomatizative [2 + 2 + 1] carboannulation using aryl diazonium salts as the one-carbon units is described.

Mechanism of the Palladium-Catalyzed Synthesis of Münchnones: The Role of Ligands in N-Acyl Iminium Salt Carbonylation

The palladium-catalyzed carbonylative coupling of imines, acid chlorides, and dipolarophiles can provide efficient routes to prepare nitrogen-containing heterocycles. One challenge in developing this reaction, and in the creation of more active catalyst systems, is the lack of data on how this complex transformation proceeds. To address this, we report here the results of our mechanistic studies on this system, and in particular the formation of mesoionic münchnones. This includes the synthesis of key catalytic intermediates, model reactions, and kinetic studies that support the role of these compounds in catalysis. Together, these studies provide a clear picture of the impact of catalyst structure, ligands, and palladium nanoparticles on facilitating the carbonylation of in situ generated iminium salts, and suggest an avenue for the creation of more active catalyst systems.