Total Synthesis of (±)-Phomoidride D

Catalytic asymmetric construction of bridged bicyclo[m.3.1] rings using an intramolecular Diels-Alder reaction

Herein, we presented an enantioselective intramolecular Diels-Alder (IMDA) reaction with vinyl branched vinylidene ortho-quinone methide (VQM). The control of site selectivity in the IMDA reaction led to both chiral bridged bicyclo[4.3.1] and [5.3.1] architectures with high isolated yields (up to 85%) and excellent enantioselectivities (up to 97% ee).

Diastereoselective Dearomatizing Cyclizations of 5-Arylpentan-2-ones by Samarium Diiodide - A Computational Analysis

This study analyzes the samarium diiodide-promoted cyclizations of 5-arylpentan-2-ones to dearomatized bicyclic products utilizing density functional theory. The reaction involves a single electron transfer to the carbonyl group, which occurs synchronously with the rate determining cyclization event, and a second subsequent proton-coupled electron transfer. These redox reactions are accurately computed employing small core pseudo potentials explicitly involving all f-electrons of samarium. Comparison of the energies of the possible final products rules out thermodynamic control of the observed regio- and diastereoselectivities. Kinetic control via appropriate transition states is correctly predicted, but to obtain reasonable energy levels the influence of the co-solvent hexamethylphosphortriamide has to be estimated by using a correction term. The steric effect of the bulky samarium ligands is decisive for the observed stereoselectivity. Carbonyl groups in para-position of the aryl group change the regioselectivity of the cyclization and lead to spiro compounds. The computations suggest again kinetic control of this deviating outcome. However, the standard mechanism has to be modified and the involvement of a complex activated by two SmI2 moieties is proposed in which two electrons are transferred simultaneously to form the new C-C bond. Computation of model intermediates show the feasibility of this alternative+ mechanism.

Retro-[4+2]/Intramolecular Diels–Alder Cascade Allows a Concise Total Synthesis of Lucidumone

Lucidumone is a recently isolated meroterpenoid displaying interesting biological activity. This natural product possesses a complex structure, including a bicyclo[2.2.2]octane possessing 6 contiguous stereogenic centers. Herein, we discuss strategies to solve this synthetic challenge. In particular, we developed a new method for the inverse electron-demand Diels–Alder cycloaddition between 2-pyrones and acyclic enol ethers, as a mean to obtain a ‘masked’ cyclohexadiene. This method allowed an expeditious enantioselective synthesis of (+)-lucidumone through a retro-[4+2]/intramolecular Diels–Alder reaction cascade.

1 Introduction

2 Retrosynthetic Considerations on the Bicyclo[2.2.2]octane

3 Development of a Methodology for Enantioselective IEDDA Cycloadditions

4 Enantioselective Total Synthesis of (+)-Lucidumone

5 Conclusion

Maleidride biosynthesis - construction of dimeric anhydrides - more than just heads or tails

Covering: up to early 2022Maleidrides are a family of polyketide-based dimeric natural products isolated from fungi. Many maleidrides possess significant bioactivities, making them attractive pharmaceutical or agrochemical lead compounds. Their unusual biosynthetic pathways have fascinated scientists for decades, with recent advances in our bioinformatic and enzymatic understanding providing further insights into their construction. However, many intriguing questions remain, including exactly how the enzymatic dimerisation, which creates the diverse core structure of the maleidrides, is controlled. This review will explore the literature from the initial isolation of maleidride compounds in the 1930s, through the first full structural elucidation in the 1960s, to the most recent in vivo, in vitro, and in silico analyses.

Samarium Diiodide Acting on Acetone-Modeling Single Electron Transfer Energetics in Solution

Samarium diiodide is a versatile single electron transfer (SET) agent with various applications in organic chemistry. Lewis structures regularly insinuate the existence of a ketyl radical when samarium diiodide binds a carbonyl group. The study presented here investigates this electron transfer by the means of computational chemistry. All electron CASPT2 calculations with the inclusion of scalar relativistic effects predict an endotherm electron transfer from samarium diiodide to acetone. Energies calculated with the PBE0-D3(BJ) functional and a small core pseudopotential are in good agreement with CASPT2. The calculations confirm the experimentally measured increase of the samarium diiodide reduction potential through the addition of hexamethylphosphoramide also known as HMPA.

Gram-Scale Enantioselective Synthesis of (+)-Lucidumone

The first enantioselective total synthesis of (+)-lucidumone is described through a 13-step synthetic pathway (longest linear sequence). The key steps involve the formation of a bridged bicyclic lactone by an enantioselective inverse-electron-demand Diels-Alder cycloaddition, C-O bond formation to assemble two fragments, and a one-pot retro-[4 + 2]/[4 + 2] cycloaddition cascade. The synthesis is scalable, and more than one gram of natural product was synthesized in one batch.

Diastereoselective Radical 1,4-Ester Migration: Radical Cyclizations of Acyclic Esters with SmI2

Reductive cyclizations of carbonyl compounds, mediated by samarium(II) diiodide (SmI₂, Kagan’s reagent), represent an invaluable platform to generate molecular complexity in a stereocontrolled manner. In addition to classical ketone and aldehyde substrates, recent advances in radical chemistry allow the cyclization of lactone and lactam-type substrates using SmI₂. In contrast, acyclic esters are considered to be unreactive to SmI₂ and their participation in reductive cyclizations is unprecedented. Here, we report a diastereoselective radical 1,4-ester migration process, mediated by SmI₂, that delivers stereodefined alkene hydrocarboxylation products via radical cyclization of acyclic ester groups in α-carbomethoxy δ-lactones. Isotopic labeling experiments and computational studies have been used to probe the mechanism of the migration. We propose that a switch in conformation redirects single electron transfer from SmI₂ to the acyclic ester group, rather than the “more reactive” lactone carbonyl. Our study paves the way for the use of elusive ketyl radicals, derived from acyclic esters, in SmI₂-mediated reductive cyclizations.

Mander’s reagent for the deoxycyanation of β-diketones: a direct synthesis of oxoalkenenitriles

Ethyl and methyl cyanoformate (Mander’s reagent) are reagents to routinely perform C-selective ketone alcoxycarbonylations. Interestingly, both reagents were found to yield oxoalkenenitriles through an unprecedented deoxycyanation on 1,3-dicarbonyl compounds (e. g. 2-methyl-1,3-cyclohexanedione). Although this method was not general, this is the first time both Mander’s reagent and ethyl cyanoformate were used for the deoxycyanation of 1,3-dicarbonyl compounds for the preparation of synthetically useful oxoalkenenitriles. Limitations on the substrate scope for the present methodology are discussed.

Recent Applications of the Diels–Alder Reaction in the Synthesis of Natural Products (2017–2020)

The Diels–Alder reaction has long been established as an extremely useful procedure in the toolbox of natural product chemists. It tolerates a wide spectrum of building blocks of different complexity and degrees of derivatization, and enables the formation of six-membered rings with well-defined stereochemistry. In recent years, many total syntheses of natural products have been reported that rely, at some point, on the use of a [4+2]-cycloaddition step. Among classic approaches, several modifications of the Diels–Alder reaction, such as hetero-Diels–Alder reactions, dehydro-Diels–Alder reactions and domino-Diels–Alder reactions, have been employed to extend the scope of this process in the synthesis of natural products. Our short review covers applications of the Diels–Alder reaction in natural product syntheses between 2017 and 2020, as well as selected methodologies which are inspired by, or that can be used to access natural products.

1 Introduction

2 Syntheses from 2017

3 Syntheses from 2018

4 Syntheses from 2019

5 Syntheses from 2020

6 Conclusion

Application of Multi-component Reaction in the Synthesis of Heterocyclic [3.3.3] Propellane Derivatives

Abstract:

Propellanes and derivatives have attractive properties due to their unique structure. Therefore, [3.3.3] propellanes, containing tricyclic structures with one of the carbon-carbon bonds common in three rings, were used in natural products, pharmaceutical compounds, and heterocyclic compounds, which were biologically important. The various multi-component reactions were applied in the synthesis of propellanes, which were highlighted throughout this review

Evolution of a Strategy for the Total Synthesis of (+)-Cornexistin

Herein is given a full account of the evolution of the first total synthesis of (+)-cornexistin. Initial efforts were based on masking the reactive maleic anhydride moiety as a 3,4-substituted furan and on forming the nine-membered carbocycle in an intramolecular Conia-ene or Nozaki-Hiyama-Kishi (NHK) reaction. Those strategies suffered from low yields and were jeopardized by a late-stage installation of the Z-alkene, as well as the stereocenters along the eastern periphery. These issues were addressed by employing a chiral-pool strategy that involved construction of the crucial stereocenters at C2, C3 and C8 at an early stage with installation of the maleic anhydride as late as possible. The successful approach featured an intermolecular NHK coupling to install the Z-alkene, a syn-Evans-aldol reaction to forge the stereocenters along the eastern periphery, an intramolecular allylic alkylation to close the nine-membered carbocycle, and a challenging stepwise hydrolysis of a β-keto nitrile to furnish the maleic anhydride.

Recent advances in the chemistry of ketyl radicals

Ketyl radicals are valuable reactive intermediates for synthesis and are used extensively to construct complex, functionalized products from carbonyl substrates. Single electron transfer (SET) reduction of the C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O bond of aldehydes and ketones is the classical approach for the formation of ketyl radicals and metal reductants are the archetypal reagents employed. The past decade has, however, witnessed significant advances in the generation and harnessing of ketyl radicals. This tutorial review highlights recent, exciting developments in the chemistry of ketyl radicals by comparing the varied contemporary – for example, using photoredox catalysts – and more classical approaches for the generation and use of ketyl radicals. The review will focus on different strategies for ketyl radical generation, their creative use in new synthetic protocols, strategies for the control of enantioselectivity, and detailed mechanisms where appropriate.

Ketyl radicals are valuable reactive intermediates for synthesis. This review highlights exciting recent developments in the chemistry of ketyl radicals by comparing contemporary and more classical approaches for their generation and use.

SmI2-Catalyzed Intermolecular Coupling of Cyclopropyl Ketones and Alkynes: A Link between Ketone Conformation and Reactivity

The archetypal single electron transfer reductant, samarium(II) diiodide (SmI₂, Kagan's reagent), remains one of the most important reducing agents and mediators of radical chemistry after four decades of widespread use in synthesis. While the chemistry of SmI₂ is very often unique, and thus the reagent is indispensable, it is almost invariably used in superstoichiometric amounts, thus raising issues of cost and waste. Of the few reports of the use of catalytic SmI₂, all require the use of superstoichiometric amounts of a metal coreductant to regenerate Sm(II). Here, we describe a SmI₂-catalyzed intermolecular radical coupling of aryl cyclopropyl ketones and alkynes. The process shows broad substrate scope and delivers a library of decorated cyclopentenes with loadings of SmI₂ as low as 15 mol %. The radical relay strategy negates the need for a superstoichiometric coreductant and additives to regenerate SmI₂. Crucially, our study uncovers an intriguing link between ketone conformation and efficient cross-coupling and thus provides an insight into the mechanism of radical relays involving SmI₂. The study lays further groundwork for the future use of the classical reagent SmI₂ in contemporary radical catalysis.

Phomoidrides E–G, three dimeric anhydrides from the fungus Pleosporales sp. give new insight to the biosynthesis of phomoidrides

Three novel dimeric anhydrides with unprecedented skeletons, phomoidrides E–G, were isolated from the fungus Pleosporales sp. A new biosynthetic strategy for dimerization with a key Claisen-like intermediate M1 is proposed.

Synthetic applications of type II intramolecular cycloadditions

Type II intramolecular cycloadditions ([4+2], [4+3], [4+4] and [5+2]) have emerged recently as an efficient and powerful strategy for the construction of bridged ring systems. In general, type II cycloadditions provide access to a wide range of bridged bicyclo[m.n.1] ring systems with high regio- and diastereoselectivity in an easy and straightforward manner. In each section of this review, an overview of the corresponding type II cycloadditions is presented, which is followed by highlights of method development and synthetic applications in natural product synthesis. The goal of this review is to provide a survey of recent advances in the field covering literature up to 2020. The review will serve as a useful reference for organic chemists engaged in the total synthesis of natural products containing bridged bicyclo[m.n.1] ring systems and provide strong stimulus for invention and further advances in this exciting research field.

Synthetic studies toward longeracemine: a SmI2-mediated spirocyclization and rearrangement cascade to construct the 2-azabicyclo[2.2.1]heptane framework

Longeracemine, a member of the Daphniphyllum family of alkaloids contains a novel carbon framework featuring a highly functionalized 2-azabicyclo[2.2.1]heptane core as part of an overall 5/6/5/5/6/5 skeleton. A synthetic intermediate containing the core of longeracemine has been efficiently prepared by employing a stereoselective SmI₂-mediated cascade reaction to advance a 7-azabicyclo[2.2.1]heptadiene to a 2-azabicyclo[2.2.1]heptene that is functionally poised for conversion to the natural product.

Recent Advances in the Total Synthesis of Natural Products Containing Eight-Membered Carbocycles (2009-2019)

Natural products containing eight-membered carbocycles constitute a class of structurally intriguing and biologically important molecules such as the famous diterpenes taxol and vinigrol. Such natural products are being increasingly investigated because of their fascinating architectural features and potent medicinal properties. However, synthesis of natural products with cyclooctane moieties has proved to be highly challenging. This review highlights the recently completed total syntheses of natural products with eight-membered carbocycles with a focus on strategic considerations. A collection of 27 representative studies from the literature covering the decade from 2009 to 2019 is described in chronological order with relevant studies grouped together, including syntheses of the same natural product by different research groups using different strategies. Finally, a summary and outlook including a discussion of the major features of each strategy used in the syntheses are presented. This review illustrates the diversity and creativity in the elegant synthetic designs of eight-membered carbocycles. We hope this review will provide timely illumination and beneficial guidance for future synthetic efforts for organic chemists who are interested in this area.

Total Synthesis of Natural Products with Bridged Bicyclo[m.n.1] Ring Systems via Type II [5 + 2] Cycloaddition

Natural products containing bridged ring systems are widely identified and show significant biological activity. The development of efficient synthesis reactions and strategies to construct bridged ring systems is a long-standing but very significant challenge in organic chemistry. In 2014, our group developed a unique type II [5 + 2] cycloaddition reaction that provides a facile and direct methodology for constructing highly functionalized bridged bicyclo[4.3.1], bicyclo[4.4.1], bicyclo[5.4.1], bicyclo[6.4.1], and other bicyclo[m.n.1] systems containing a strained bridgehead double bond. In this Account, we summarize the methodology development and report the results of application of our unique strategy for the total synthesis of several natural products with bridged ring systems (i.e., cyclocitrinol, cerorubenic acid-III, and vinigrol) during the past 5 years in our laboratory. In the first part, we introduce the logic behind the design and discovery of type II [5 + 2] cycloadditions. The substrates can be easily synthesized by a modular approach, followed by base-promoted group elimination under heat to form an oxidopyrylium ylide, which can undergo cycloaddition under relatively mild conditions with a variety of double bonds to generate bridged bicyclo[m.n.1] frameworks in high yield. The diastereocontrol and unique endo selectivity of this methodology are favorable for further application to the synthesis of complex natural products. In the second part, we highlight our endeavors in the total synthesis of several different types of molecules bearing bridged ring systems using our methodology. The bridged bicyclo[4.4.1] system is the core structure of two different types of natural products, cyclocitrinol and cerorubenic acid-III, that can be efficiently constructed by type II [5 + 2] cycloadditions. The development of suitable strategies and methods for site-selective cleavage of the C-O bond of the oxa-[3.2.1] ring system in the products of type II [5 + 2] cycloadditions is also discussed and highlighted during the syntheses. Moreover, the bridged bicyclo[5.3.1] system is the core structure of vinigrol, which can be constructed through a novel ring contraction sequence of the bicyclo[5.4.1] system formed by a type II [5 + 2] cycloaddition. By combining with a ring contraction cascade, we believe that type II [5 + 2] cycloadditions have the potential to be used as a unified approach to constructing natural products containing bridged bicyclo[m.n.1] frameworks.

Nonadride and Spirocyclic Anhydride Derivatives from the Plant Endophytic Fungus Talaromyces purpurogenus

Six new nonadride derivatives (1-6) and three new spirocyclic anhydride derivatives (7-9) were isolated from the endophytic fungus Talaromyces purpurogenus obtained from fresh leaves of the toxic medicinal plant Tylophora ovata. The structures of these compounds were determined by spectroscopic analyses including 1D and 2D NMR, HRESIMS, and ECD techniques. Maleic anhydride derivatives 1-9 were evaluated for their in vitro anti-inflammatory activities. Compound 1 showed significant inhibitory activity against NO production in LPS-induced RAW264.7 cells with an IC₅₀ value of 1.9 μM. Compounds 2 and 6 showed moderate inhibitory activities toward XOD and PTP1b, respectively, at 10 μM with inhibition rates of 67% and 76%.

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.

New Strategy To Access Enantioenriched Cyclohexadienones: Kinetic Resolution of para-Quinols by Organocatalytic Thiol-Michael Addition Reactions

Existing stereoselective routes to 2,5-cyclohexadienones involve either desymmetrization of an achiral substrate or have attempted to perform an asymmetric dearomatization of a phenol. Herein, we report proof-of-principle experiments aimed at developing a kinetic resolution as an alternative method for accessing enantioenriched 2,5-cyclohexadienones. More specifically, chiral bifunctional thiourea catalysts were used to promote the addition of 2-thionapthalene into unsymmetric para-quinols. The selectivity of the kinetic resolution was found to be quite sensitive to substitution around the substrate.

Scalable Synthesis of Cyclocitrinol

A 10-step synthesis of the C25 steroid natural product cyclocitrinol from inexpensive, commercially available pregnenolone is reported. This synthesis features a biomimetic cascade rearrangement to efficiently construct the challenging bicyclo[4.4.1] A/B ring system, which enabled a gram-scale synthesis of the bicyclo[4.4.1] enone intermediate 18 in only nine steps. This work also provides experimental support for the biosynthetic origin of cyclocitrinol.

Iridium(iii) complexes as reaction based chemosensors for medical diagnostics

This frontier article introduces recent developments and applications of iridium(iii) complexes as luminescent probes for ions and biomolecules.