Short Enantioselective Total Syntheses of Cheloviolenes A and B and Dendrillolide C via Convergent Fragment Coupling Using a Tertiary Carbon Radical

Photocatalytic systems: reactions, mechanism, and applications

The photocatalytic field revolves around the utilization of photon energy to initiate various chemical reactions using non-adsorbing substrates, through processes such as single electron transfer, energy transfer, or atom transfer. The efficiency of this field depends on the capacity of a light-absorbing metal complex, organic molecule, or substance (commonly referred to as photocatalysts or PCs) to execute these processes. Photoredox techniques utilize photocatalysts, which possess the essential characteristic of functioning as both an oxidizing and a reducing agent upon activation. In addition, it is commonly observed that photocatalysts exhibit optimal performance when irradiated with low-energy light sources, while still retaining their catalytic activity under ambient temperatures. The implementation of photoredox catalysis has resuscitated an array of synthesis realms, including but not limited to radical chemistry and photochemistry, ultimately affording prospects for the development of the reactions. Also, photoredox catalysis is utilized to resolve numerous challenges encountered in medicinal chemistry, as well as natural product synthesis. Moreover, its applications extend across diverse domains encompassing organic chemistry and catalysis. The significance of photoredox catalysts is rooted in their utilization across various fields, including biomedicine, environmental pollution management, and water purification. Of course, recently, research has evaluated photocatalysts in terms of cost, recyclability, and pollution of some photocatalysts and dyes from an environmental point of view. According to these new studies, there is a need for critical studies and reviews on photocatalysts and photocatalytic processes to provide a solution to reduce these limitations. As a future perspective for research on photocatalysts, it is necessary to put the goals of researchers on studies to overcome the limitations of the application and efficiency of photocatalysts to promote their use on a large scale for the development of industrial activities. Given the significant implications of the subject matter, this review seeks to delve into the fundamental tenets of the photocatalyst domain and its associated practical use cases. This review endeavors to demonstrate the prospective of a powerful tool known as photochemical catalysis and elucidate its underlying tenets. Additionally, another goal of this review is to expound upon the various applications of photocatalysts.

Pyridine-catalyzed ring-opening reaction of cyclopropenone with bromomethyl carbonyl compounds toward furan-2(5H)-ones

We developed a pyridine-catalyzed annulation of diaryl cyclopropenone with bromomethyl carbonyl compounds leading to 5-carbonyl furan-2(5H)-ones. Pyridinium, derived from the reaction of bromomethyl carbonyl and pyridine, triggered the reaction by the inter-molecular Michael addition to cyclopropenone. This procedure was sensitive neither to air nor moisture and proceeded at room temperature with broad substrate scopes and good functional group tolerance in moderate-to-good yields. As such, it represents a facile and practical pathway leading to 5-carbonyl furan-2(5H)-one derivatives.

Modern Photocatalytic Strategies in Natural Product Synthesis

Modern photocatalysis has proven its generality for the development and functionalization of native functionalities. To date, the field has found broad applications in diverse research areas, including the total synthesis of natural products. This contribution covers recent reports of total syntheses involving as a key step a photocatalytic reaction. Among the selected examples, the photocatalytic processes proceed in a highly chemo-, regio-, and stereoselective manner, thereby allowing the rapid access to structurally complex architectures under light-driven conditions.

Synergistic Effect of Cerium in Dual Photoinduced Ligand-to-Metal Charge Transfer and Lewis Acid Catalysis: Diastereoselective Alkylation of Coumarins

We report the dual role of cerium to promote the photoinduced ligand-to-metal charge transfer (LMCT) process for the generation of the alkyl radical and subsequent Lewis acid catalysis to construct stereodefined C-C bonds. This paradigm utilized ubiquitous carboxylic acids as alkyl radical surrogates and offers excellent diastereoselectivity for the formation of C-4 alkylated coumarins in good to excellent yield. UV-vis spectroscopy studies in combination with in situ Fourier transform infrared spectroscopy are consistent with the proposed mechanism, supporting the participation of the Ce^IV-carboxylate complex in photoinduced LMCT and its subsequent homolysis to generate the alkyl radial through the exclusion of CO₂. Finally, the oxophilicity of cerium enables a two-point complexation with the in situ generated enolate intermediate and facilitates the diastereoselective protonation to form the desired product. Furthermore, this mild and atom-economical catalytic manifolds allow the late-stage modification of pharmaceuticals.

Total Synthesis of Resiniferatoxin Enabled by Photocatalytic Decarboxylative Radical Cyclization

Resiniferatoxin (1) is a complex daphnane diterpenoid with a highly oxygenated 5/7/6-membered ABC-ring system. Here we report a new synthetic route to 1 that requires 27 steps from a starting d-ribose derivative. The carbon spacer and A-ring are sequentially attached to the C-ring by radical allylation and Stille coupling reactions, respectively. An Ir(III)-catalyzed photoinduced decarboxylative radical reaction then forged the sterically hindered bond between the tetra- and trisubstituted carbons to cyclize the central seven-membered B-ring.

Diastereoselective Alkene Hydroesterification Enabling the Synthesis of Chiral Fused Bicyclic Lactones

Palladium-catalysed diastereoselective hydroesterification of alkenes assisted by the coordinative hydroxyl group in the substrate afforded a variety of chiral γ-butyrolactones bearing two stereocenters. Employing the carbonylation-lactonization products as the key intermediates, the route from the alkenes with single chiral center to chiral THF-fused bicyclic γ-lactones containing three stereocenters was developed.

Catalytic Asymmetric Hydroalkoxylation of C-C Multiple Bonds

Asymmetric hydroalkoxylation of alkenes constitutes a redox-neutral and 100% atom-economical strategy toward enantioenriched oxygenated building blocks from readily available starting materials. Despite their great potential, catalytic enantioselective additions of alcohols across a C-C multiple bond are particularly underdeveloped, especially compared to other hydrofunctionalization methods such as hydroamination. However, driven by some recent innovations, e.g., asymmetric MHAT methods, asymmetric photocatalytic methods, and the development of extremely strong chiral Brønsted acids, there has been a gratifying surge of reports in this burgeoning field. The goal of this review is to survey the growing landscape of asymmetric hydroalkoxylation by highlighting exciting new advances, deconstructing mechanistic underpinnings, and drawing insight from related asymmetric hydroacyloxylation and hydration. A deep appreciation of the underlying principles informs an understanding of the various selectivity parameters and activation modes in the realm of asymmetric alkene hydrofunctionalization while simultaneously evoking the outstanding challenges to the field moving forward. Overall, we aim to lay a foundation for cross-fertilization among various catalytic fields and spur further innovation in asymmetric hydroalkoxylations of C-C multiple bonds.

Concise Large-Scale Synthesis of Tomatidine, A Potent Antibiotic Natural Product

Tomatidine has recently generated a lot of interest amongst the pharmacology, medicine, and biology fields of study, especially for its newfound activity as an antibiotic agent capable of targeting multiple strains of bacteria. In the light of its low natural abundance and high cost, an efficient and scalable multi-gram synthesis of tomatidine has been developed. This synthesis uses a Suzuki-Miyaura-type coupling reaction as a key step to graft an enantiopure F-ring side chain to the steroidal scaffold of the natural product, which was accessible from low-cost and commercially available diosgenin. A Lewis acid-mediated spiroketal opening followed by an azide substitution and reduction sequence is employed to generate the spiroaminoketal motif of the natural product. Overall, this synthesis produced 5.2 g in a single pass in 15 total steps and 15.2% yield using a methodology that is atom economical, scalable, and requires no flash chromatography purifications.

Evolution towards green radical generation in total synthesis

The use of radicals as intermediates in total synthesis has evolved since their initial use in the latter half of the twentieth century. Radical generation from metal hydride methodologies has shifted to "greener" techniques including catalytic metal-mediated systems, electrochemical and photoredox-mediated processes. This review will focus on these classical and contemporary methods for radical generation and their applications in recent total syntheses.

Recent Advances on Photoinduced Cascade Strategies for the Synthesis of N-Heterocycles

Over the last decade, visible-light photocatalysis has proved to be a powerful tool for the construction of N-heterocyclic frameworks, important constituents of natural products, insecticides, pharmacologically relevant therapeutic agents and catalysts. This account highlights recent developments and established methods towards the photocatalytic cascades for preparation of different classes of N-heterocycles, giving emphasis on our contribution to the field.

Strategic Use of Visible-Light Photoredox Catalysis in Natural Product Synthesis

Recent progress in the development of photocatalytic reactions promoted by visible light is leading to a renaissance in the use of photochemistry in the construction of structurally elaborate organic molecules. Because of the rich functionality found in natural products, studies in natural product total synthesis provide useful insights into functional group compatibility of these new photocatalytic methods as well as their impact on synthetic strategy. In this review, we examine total syntheses published through the end of 2020 that employ a visible-light photoredox catalytic step. To assist someone interested in employing the photocatalytic steps discussed, the review is organized largely by the nature of the bond formed in the photocatalytic step.

Marine natural products

This review covers the literature published between January and December in 2018 for marine natural products (MNPs), with 717 citations (706 for the period January to December 2018) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1554 in 469 papers for 2018), together with the relevant biological activities, source organisms and country of origin. Reviews, biosynthetic studies, first syntheses, and syntheses that led to the revision of structures or stereochemistries, have been included. The proportion of MNPs assigned absolute configuration over the last decade is also surveyed.

Biomimetic Total Syntheses of Ergot Alkaloids via Decarboxylative Giese Coupling

Biomimetic total syntheses of Festuclavine and Pyroclavine were achieved by a sequential radical coupling. The key steps include intramolecular decarboxylative Giese reaction to form the central C ring and 4-nitrobenzenesulfonyl (Ns)-directed indole C4-H olefination to introduce the indole C4 component. In addition, D-ring formation was completed by decarboxylative alkenylation and intramolecular S_N2 reaction.

General Access to Concave-Substituted cis-Dioxabicyclo[3.3.0]octanones: Enantioselective Total Syntheses of Macfarlandin C and Dendrillolide A

The evolution of a strategy to access the family of rearranged spongian diterpenoids harboring a concave-substituted cis-2,8-dioxabicyclo[3.3.0]octan-3-one fragment is described. The approach involves late-stage fragment coupling of a tertiary-carbon radical and an electron-deficient double bond to form vicinal quaternary and tertiary stereocenters with high fidelity. A stereoselective Mukaiyama hydration is the key step in the subsequent elaboration of the cis-2,8-dioxabicyclo[3.3.0]octan-3-one moiety. This strategy was utilized in enantioselective total syntheses of (-)-macfarlandin C and (+)-dendrillolide A. An efficient construction of enantiopure tetramethyloctahydronaphthalenes was developed during the construction of (-)-macfarlandin C.

Catalytic Hydroxycyclopropanol Ring-Opening Carbonylative Lactonization to Fused Bicyclic Lactones

A novel palladium-catalyzed ring opening carbonylative lactonization of readily available hydroxycyclopropanols was developed to efficiently synthesize tetrahydrofuran (THF) or tetrahydropyran (THP)-fused bicyclic γ-lactones, two privileged scaffolds often found in natural products. The reaction features mild reaction conditions, good functional group tolerability, and scalability. Its application was demonstrated in a short total synthesis of (±)-paeonilide. The fused bicyclic γ-lactone products can be easily diversified to other medicinally important scaffolds, which further broadens the application of this new carbonylation method.

Total Syntheses of Norrisolide-Type Spongian Diterpenes Cheloviolene C, Seconorrisolide B, and Seconorrisolide C

The first total syntheses of three unusual norrisolide-type rearranged spongian diterpenes, cheloviolene C, seconorrisolide B, and seconorrisolide C, have been accomplished via a common intermediate through late-stage ring-scissoring. The synthesis features a Wolff ring contraction for the synthesis of the trans-hydrindane system, and a crucial retro Diels-Alder reaction/intramolecular ene cyclization for the rapid stereoselective construction of the furo[2,3-b]furan system, which is commonly seen in rearranged spongian diterpenes.

Dynamic NMR and Computational Studies Inform the Conformational Description of Dendrillane Terpenes from the Nudibranch Goniobranchus coi

Two new oxygenated terpenes (1 and 2) have been characterized from the Australian nudibranch Goniobranchus coi. Broadened ¹H NMR signals, together with the absence of individual carbon NMR signals, complicated analysis of 5,9-epoxydendrillolide A (1); increasing the temperature to 323 K revealed the missing NMR signals. Low-temperature ¹H NMR experiments provided an activation barrier of ∼15 kcal mol^-1 and, together with DFT calculations, supported interconversion of a twist chair conformer with two different chair conformers. X-ray crystallographic analysis coupled with biosynthetic reasoning suggested a (5R, 8S, 9R, 13R, 14R, 15R, 16R) configuration. Ketone 2 demonstrated similar dynamic conformational processes to 1.

The photoredox alkylarylation of styrenes with alkyl N-hydroxyphthalimide esters and arenes involving C-H functionalization

The In(OTf)₃-promoted three-component photoredox alkylarylation of styrenes with alkyl NHP esters and arenes to access alkylated arene derivatives through C-C bond cleavage and C-H functionalization is reported. By utilizing visible-light photoredox catalysis, alkyl N-hydroxyphthalimide esters serving as alkyl carbon-centered radicals and a wide range of arenes (e.g., indoles, pyrrole, and electron-rich arenes) as nucleophiles were used to enable the introduction of various alkyl groups and aryl groups across the C[double bond, length as m-dash]C bonds with excellent selectivity and functional group tolerance.

Photoredox-catalyzed stereoselective alkylation of enamides with N-hydroxyphthalimide esters via decarboxylative cross-coupling reactions

Stereoselective β-C(sp2)-H alkylation of enamides with redox-active N-hydroxyphthalimide esters via a photoredox-catalyzed decarboxylative cross-coupling reaction is demonstrated. This methodology features operational simplicity, broad substrate scopes, and excellent stereoselectivities and functional group tolerance, affording a diverse array of geometrically defined and synthetically valuable enamides bearing primary, secondary or tertiary alkyl groups in satisfactory yields.

From d- to l-Monosaccharide Derivatives via Photodecarboxylation-Alkylation

Photodecarboxylation-alkylation of conformationally locked monosaccharides leads to inversion of stereochemistry at C5. This allows the synthesis of l-sugars from their readily available d-counterparts. Via this strategy, methyl l-guloside was synthesized from methyl d-mannoside in 21% yield over six steps.

Total Synthesis and Structure Revision of (-)-Illisimonin A, a Neuroprotective Sesquiterpenoid from the Fruits of Illicium simonsii

Illisimonin A was isolated from Illicium simonsii and has a previously unreported tricyclic carbon framework. It displayed neuroprotective effects against oxygen-glucose deprivation-induced cell injury in SH-SY5Y cells. It incorporates a highly strained trans-pentalene ring system. We report the first synthesis of (±)-illisimonin A. Notable steps in the route include a 1,3-dioxa-2-silacyclohexene templated Diels-Alder cycloaddition and type-3 semipinacol rearrangement to generate the trans-pentalene. The final step is an iron-catalyzed C-H oxidation. The synthetic route is robust, with 94 mg of racemic material prepared in a single pass. Resolving an intermediate enabled the synthesis of natural (-)-illisimonin A. The absolute configuration of (-)-illisimonin A was revised to 1S,4S,5S,6S,7R,9R,10R based on the X-ray structure of a heavy-atom analogue.

Forging C(sp3)-C(sp3) Bonds with Carbon-Centered Radicals in the Synthesis of Complex Molecules

Radical fragment coupling reactions that unite intricate subunits have become an important class of transformations within the arena of complex molecule synthesis. This Perspective highlights some of the early contributions in this area, as well as more modern applications of radical fragment couplings in the preparation of natural products. Additionally, emphasis is placed on contemporary advances that allow for radical generation under mild conditions as a driving force for the implementation of radical fragment couplings in total synthesis.

Progress towards the synthesis of aconitine: construction of the AE fragment and attempts to access the pentacyclic core

We reported the first successful preparation of fully functionalized aconitine AE fragment and attempts to access the pentacyclic skeleton of aconitine via radical cascade.

Second-Generation Synthesis of (+)-Fastigiatine Inspired by Conformational Studies

(+)-Fastigiatine is a complex alkaloid isolated from the alpine club moss Lycopodium fastigatum, most commonly found in New Zealand. It has been the subject of two successful synthetic campaigns. A second-generation route toward fastigiatine was developed to resolve two problematic steps from our initial synthesis. Selective reduction and protection of the C13 ketone improved the yield and reliability of the dibromocarbene ring expansion step. In the prior synthesis, cuprate addition to the C10 enone generated a 1:1 mixture of isomers in an advanced intermediate. Protection of the C13 alcohol with a large silyl group changed the conformational preference of the enone and led to a more selective conjugate addition to produce the desired β-epimer at C10. MacMillan's decarboxylative photoredox addition method proved to be more practical than the prior aminomethyl cuprate addition chemistry. The second-generation synthesis is longer than the original but improves the selectivity and reproducibility of the overall route.

Total Synthesis of (-)-Chromodorolide B By a Computationally-Guided Radical Addition/Cyclization/Fragmentation Cascade

The first total synthesis of a chromodorolide marine diterpenoid is described. The core of the diterpenoid is constructed by a bimolecular radical addition/cyclization/fragmentation cascade that unites two complex fragments and forms two C-C bonds and four contiguous stereogenic centers of (-)-chromodorolide B in a single step. This coupling step is initiated by visible-light photocatalytic fragmentation of a redox-active ester, which can be accomplished in the presence of an iridium or a less-precious electron-rich dicyanobenzene photocatalyst, and employs equimolar amounts of the two addends. Computational studies guided the development of this central step of the synthesis and provide insight into the origin of the observed stereoselectivity.

Chromolactol, an Oxygenated Diterpene from the Indo-Pacific Nudibranch Goniobranchus coi: Spectroscopic and Computational Studies

A rearranged spongian diterpene chromolactol was obtained from the mantle extract of the Indo-Pacific nudibranch Goniobranchus coi. The structure of chromolactol, either 1a or 1b, which was investigated by extensive NMR experiments and by data comparison as well as by molecular modelling studies and density functional calculations, has a different relative configuration of the 2,8-dioxabicyclo-[3.3.0]-octane ring compared with the co-metabolite norrisolide (2). A biosynthetic pathway leading to the preferred diastereomer of chromolactol (1a) is presented.