An Arylative Ring Expansion Cascade of Fused Cyclobutenes via Short-Lived Intermediates with Planar Chirality

Cascade ring expansion reactions for the synthesis of medium-sized rings and macrocycles

This Feature Article discusses recent advances in the development of cascade ring expansion reactions for the synthesis of medium-sized rings and macrocycles. Cascade ring expansion reactions have much potential for use in the synthesis of biologically important medium-sized rings and macrocycles, most notably as they don't require high dilution conditions, which are commonly used in established end-to-end macrocyclisation methods. Operation by cascade ring expansion method can allow large ring products to be accessed via rearrangements that proceed exclusively by normal-sized ring cyclisation steps. Ensuring that there is adequate thermodynamic driving force for ring expansion is a key challenge when designing such methods, especially for the expansion of normal-sized rings into medium-sized rings. This Article is predominantly focused on methods developed in our own laboratory, with selected works by other groups also discussed. Thermodynamic considerations, mechanism, reaction design, route planning and future perspective for this field are all covered.

Entry into Lithium Ynolates from α,α,α-Tribromomethyl Ketones: Synthesis of Cyclobutenes via the [2 + 2] Cycloaddition with α,β-Unsaturated Carbonyls

This study reports the synthesis of cyclobutene derivatives in good yields via the [2 + 2] cycloaddition between lithium ynolates and α,β-unsaturated carbonyls. The ynolates are generated from α,α,α-tribromomethyl ketones and tert-butyl lithium via a simple and novel method, which does not produce any harmful byproducts, such as lithium alkoxide, which induces a polymerization reaction with α,β-unsaturated carbonyls.

Catalytic σ-Bond Annulation with Ambiphilic Organohalides Enabled by β-X Elimination

We describe a catalytic cascade sequence involving directed C(sp3 )-H activation followed by β-heteroatom elimination to generate a PdII (π-alkene) intermediate that then undergoes redox-neutral annulation with an ambiphilic aryl halide to access 5- and 6-membered (hetero)cycles. Various alkyl C(sp3 )-oxygen, nitrogen, and sulfur bonds can be selectively activated, and the annulation proceeds with high diastereoselectivity. The method enables modification of amino acids with good retention of enantiomeric excess, as well as σ-bond ring-opening/ring-closing transfiguration of low-strain heterocycles. Despite its mechanistic complexity, the method employs simple conditions and is operationally straightforward to perform.

Palladium-Catalyzed Synthesis of Fused Carbo- and Heterocycles: Recent Advances

The use of palladium catalysts in fused ring synthesis has been increasingly noteworthy in recent years in organic synthesis. It has a lot of potential compared to other transition metal catalysts, because of its one-of-a-kind feature that makes them more widely applicable in a variety of disciplines application. Palladium is important in a variety of Heck processes, including intramolecular, intermolecular, and reductive Heck reactions, which produce diverse carbocycles and heterocycles of biological importance. Under optimal reaction conditions, carbocyclization or heterocyclization occurs, resulting in the production of numerous structural building blocks of naturally occurring compounds. Beside intramolecular Heck-type reactions, cycloaddition, cycloalkylation, oxidative coupling, C-H functionalization, cross-coupling reactions, and carboamidation reactions have also been employed extensively to access fused carbo- and heterocycles of immense biological importance. This review article provides a well-summarized discussion (since 2001) on fused carbo- and heterocycle ring synthesis using palladium catalysts, overviewing their applications, and mechanistic insights.

Acid-promoted intra- and intermolecular [2+2] cycloaddition of indoles to aryl alkynes to access cyclobutene-fused indolines

Herein, we have reported the first example of both intra- and intermolecular [2+2] cycloaddition of the electron-rich indoles and unactivated aryl alkynes promoted by the combination of Fe(NO₃)₃ and HNO₃, which highlights efficient and selective access to several different types of functionalized cyclobutene-fused indolines from readily available starting materials with cheap catalysts and simple operations.

Enantioselective Synthesis of Medium-Sized-Ring Lactones via Iridium-Catalyzed Z-Retentive Asymmetric Allylic Substitution Reaction

Medium-sized rings are important structural units, but their synthesis, especially in a highly enantioselective manner, has been a great challenge. Herein we report an enantioselective synthesis of medium-sized-ring lactones by an iridium-catalyzed Z-retentive asymmetric allylic substitution reaction. The reaction features mild conditions and a broad substrate scope. Various eight- to 11-membered-ring lactones can be afforded in moderate to excellent yields (up to 88%) and excellent enantioselectivity (up to 99% ee). The utilization of both Z-allyl precursors and an Ir catalyst is critical for the medium-sized-ring formation.

New-Generation Ligand Design for the Gold-Catalyzed Asymmetric Activation of Alkynes

Gold(I) catalysts are ideal for the activation of alkynes under very mild conditions. However, unlike allenes or alkenes, the triple bond of alkynes cannot be prochiral. In addition, the linear coordination displayed by gold(I) complexes places the chiral ligand far away from the substrate resulting in an inefficient transfer of chiral information. This poses a significant challenge for the achievement of high enantiocontrol in gold(I)-catalyzed reactions of alkynes. Although considerable progress on enantioselective gold(I)-catalyzed transformations has recently been achieved, the asymmetric activation of non-prochiral alkyne-containing small molecules still represents a great challenge. Herein we summarize recent advances in intra- and intermolecular enantioselective gold(I)-catalyzed reactions involving alkynes, discussing new chiral ligand designs that lie at the basis of these developments. We also focus on the mode of action of these catalysts, their possible limitations towards a next-generation of more efficient ligand designs. Finally, square planar chiral gold(III) complexes, which offer an alternative to chiral gold(I) complexes, are also discussed.

Recent advances in palladium-catalyzed (hetero)annulation of C[double bond, length as m-dash]C bonds with ambiphilic organo(pseudo)halides

Palladium has proven to be effective in catalyzing the (hetero)annulation of C[double bond, length as m-dash]C bonds with ambiphilic organo(pseudo)halides. Through the employment of appropriate ambiphilic coupling partners, efficient annulation of a variety of allenes, 1,3-dienes, strained alkenes, styrenes, and other C[double bond, length as m-dash]C bond variants can be achieved to provide direct access to numerous useful hetero- and carbocyclic scaffolds. In this Feature Article, we summarize palladium-catalyzed (hetero)annulation methods reported since 2005 (spanning just over 15 years) and discuss outstanding challenges in this area of study.

Small rings in the bigger picture: ring expansion of three- and four-membered rings to access larger all-carbon cyclic systems

The release of the inherent ring strain of cyclobutane and cyclopropane derivatives allows a rapid build-up of molecular complexity. This review highlights the state-of-the-art of the ring expansions of three- and four-membered cycles and is organised by types of reactions with emphasis on the reaction mechanisms. Selected examples are discussed to illustrate the synthetic potential of this elegant synthetic tool.

Formal [4 + 4]-, [4 + 3]-, and [4 + 2]-cycloaddition reactions of donor-acceptor cyclobutenes, cyclopropenes and siloxyalkynes induced by Brønsted acid catalysis

Brønsted acid catalyzed formal [4 + 4]-, [4 + 3]-, and [4 + 2]-cycloadditions of donor-acceptor cyclobutenes, cyclopropenes, and siloxyalkynes with benzopyrylium ions are reported. [4 + 2]-cyclization/deMayo-type ring-extension cascade processes produce highly functionalized benzocyclooctatrienes, benzocycloheptatrienes, and 2-naphthols in good to excellent yields and selectivities. Moreover, the optical purity of reactant donor-acceptor cyclobutenes is fully retained during the cascade. The 1,3-dicarbonyl product framework of the reaction products provides opportunities for salen-type ligand syntheses and the construction of fused pyrazoles and isoxazoles that reveal a novel rotamer-diastereoisomerism.

Pd-Catalyzed Domino Reactions Involving Alkenes To Access Substituted Indole Derivatives

Palladium-catalyzed domino reactions are advanced tools in achieving various nitrogen-containing heterocycles in an efficient and economical manner due to the reduced number of steps in the process. This review highlights recent advances in domino processes aimed at the synthesis of indole derivatives and polycyclic systems containing the indole nucleus in intra/intra- or intra/intermolecular reactions. In particular, we consider domino processes that involve a double bond in a step of the sequence, which allow the issue of regioselectivity in the cyclization to be faced and overcome. The different sections in this review focus on the synthesis of the indole nucleus and functionalization of the scaffold starting from different substrates that have been identified as activated starting materials, which involve a halogenated moiety or unactivated unsaturated systems. In the former case, the reaction is under Pd(0) catalysis, and in the second case a Pd(II) catalytic species is required and then an oxidant is necessary to reconvert the Pd(0) into the active Pd(II) species. On the other hand, the second method has the advantage that it uses easy available and inexpensive substrates.

1 Introduction

2 Indole Scaffold Synthesis

2.1 Activated Substrates

2.2 Unactivated Substrates

3 Functionalization of Indole Scaffold

3.1 Activated Substrates

3.2 Unactivated Substrates

4 Conclusions

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.

Synthesis and Target Identification of Benzoxepane Derivatives as Potential Anti-Neuroinflammatory Agents for Ischemic Stroke

Benzoxepane derivatives were designed and synthesized, and one hit compound emerged as being effective in vitro with low toxicity. In vivo, this hit compound ameliorated both sickness behavior through anti-inflammation in LPS-induced neuroinflammatory mice model and cerebral ischemic injury through anti-neuroinflammation in rats subjected to transient middle cerebral artery occlusion. Target fishing for the hit compound using photoaffinity probes led to identification of PKM2 as the target protein responsible for anti-inflammatory effect of the hit compound. Furthermore, the hit exhibited an anti-neuroinflammatory effect in vitro and in vivo by inhibiting PKM2-mediated glycolysis and NLRP3 activation, indicating PKM2 as a novel target for neuroinflammation and its related brain disorders. This hit compound has a better safety profile compared to shikonin, a reported PKM2 inhibitor, identifying it as a lead compound in targeting PKM2 for the treatment of inflammation-related diseases.

Large-Scale Flow Photochemical Synthesis of Functionalized trans-Cyclooctenes Using Sulfonated Silica Gel

Functionalized trans-cyclooctenes are useful bioorthogonal reagents that are typically prepared using a flow photoisomerization method where the product is captured by AgNO3 on silica gel. While this method is effective, the leaching of silver can be problematic when scaling up syntheses. It is shown here that Ag(I) immobilized on tosic silica gel can be used to capture trans-cyclooctene products at higher loadings without leaching. It is demonstrated that the sulfonated silica gel can be regenerated and reused with similar yields over multiple runs. Nine different trans-cyclooctenes were synthesized, including those commonly utilized in bioorthogonal chemistry as well as new amine and carboxylic acid derivatives.

Wolff/Cope Approach to the AB Ring of the Sesterterpenoid Variecolin

A stereoselective synthesis of the AB ring of the complex sesterterpenoid variecolin is presented. Our strategy features the development of a tandem Wolff/Cope rearrangement of α-diazo cyclobutyl ketones for the construction of fused, 8-membered carbocycles. Preliminary studies revealed a facile Wolff rearrangement but a difficult vinyl ketene cyclobutane Cope rearrangement. We have leveraged an efficient microwave-promoted tandem rearrangement to prepare the desired functionalized cyclooctadienones that we envision as potential key intermediates in the convergent synthesis of variecolin.

Selective Synthesis of Cyclooctanoids by Radical Cyclization of Seven-Membered Lactones: Neutron Diffraction Study of the Stereoselective Deuteration of a Chiral Organosamarium Intermediate

Seven-membered lactones undergo selective SmI2 -H2 O-promoted radical cyclization to form substituted cyclooctanols. The products arise from an exo-mode of cyclization rather than the usual endo-attack employed in the few radical syntheses of cyclooctanes. The process is terminated by the quenching of a chiral benzylic samarium. A labeling experiment and neutron diffraction study have been used for the first time to probe the configuration and highly diastereoselective deuteration of a chiral organosamarium intermediate.