Total syntheses of hopeanol and hopeahainol A empowered by a chiral Brønsted acid induced pinacol rearrangement

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.

Leveraging the Persistent Radical Effect in the Synthesis of trans-2,3-Diaryl Dihydrobenzofurans

A simple method for accessing trans-2,3-diaryl dihydrobenzofurans is reported. This approach leverages the equilibrium between quinone methide dimers and their persistent radicals. This equilibrium is disrupted by phenols that yield comparatively transient phenoxyl radicals, leading to cross-coupling between the persistent and transient radicals. The resultant quinone methides with pendant phenols rapidly cyclize to form dihydrobenzofurans (DHBs). This putative biomimetic access to dihydrobenzofurans provides superb functional group tolerance and a unified approach for the synthesis of resveratrol-based natural products.

Catalyst-Assisted Selective Vinylation and Methylallylation of a Quaternary Carbon Center Using tert-Butyl Acetate

The In(OTf)₃-catalyzed α-vinylation of various hydroxy-functionalized quaternary carbon centers using in situ generated isobutylene from tert-butyl acetate is presented as a novel synthetic methodology. Moreover, tert-butyl acetate is a nonflammable feed stock and is a readily available source for the in situ production of vinyl substituents, as demonstrated by the vinylation reaction with quaternary hydroxy/methoxy compounds. Moreover, an excellent selectivity for methylallylation over vinylation was obtained with Ni(OTf)₂ as a catalyst. In the case of peroxyoxindole, methylallyl-functionalized 1,4-benzoxazin-3-one derivatives were formed through the sequential rearrangement of peroxyoxindole followed by the nucleophilic attack by isobutylene. The detailed mechanism for this reaction and rationalization for the selectivity are provided using kinetics and density functional theory studies.

Computation-Guided Total Synthesis of Vitisinol G

Computational chemistry is useful in synthetic organic chemistry, as it can be used not only to analyze reaction mechanisms, but also to calculate biosynthetic pathways and to plan and evaluate strategies for total syntheses. Here we report the computation-guided total synthesis of vitisinol G, a resveratrol dimer.

Overcoming O-H Insertion to Para-Selective C-H Functionalization of Free Phenols: Rh(II)/Xantphos Catalyzed Geminal Difunctionalization of Diazo Compounds

Para-selective C-H functionalization of free phenols by metal carbenoids is rather challenging due to the generally more favorable competing O-H insertion. Herein, with the use of the combination of Rh(II) and a Xantphos ligand as the catalyst, a novel multicomponent reaction of free phenols, diazoesters, and allylic carbonates was successfully developed, affording a wide variety of phenol derivatives, bearing an all-carbon quaternary center and a synthetically useful allylic unit. This reaction is likely to occur through a tandem process of carbene-induced para-selective C-H functionalization, followed by Rh(II)/Xantphos-catalyzed allylation. The distinctive reactivity of para-selective C-H rather than O-H insertion for the carbenoid intermediate, combined with features of excellent functional group compatibility, high atom and step economy, and ease in further diversification of the products, might render this protocol highly attractive in facile functionalization of unprotected phenols.

Computation-guided asymmetric total syntheses of resveratrol dimers

Although computational simulation-based natural product syntheses are in their initial stages of development, this concept can potentially become an indispensable resource in the field of organic synthesis. Herein we report the asymmetric total syntheses of several resveratrol dimers based on a comprehensive computational simulation of their biosynthetic pathways. Density functional theory (DFT) calculations suggested inconsistencies in the biosynthesis of vaticahainol A and B that predicted the requirement of structural corrections of these natural products. According to the computational predictions, total syntheses were examined and the correct structures of vaticahainol A and B were confirmed. The established synthetic route was applied to the asymmetric total synthesis of (−)-malibatol A, (−)-vaticahainol B, (+)-vaticahainol A, (+)-vaticahainol C, and (−)-albiraminol B, which provided new insight into the biosynthetic pathway of resveratrol dimers. This study demonstrated that computation-guided organic synthesis can be a powerful strategy to advance the chemical research of natural products.

Although computational simulation-based natural product syntheses are in their initial stages of development, this concept can potentially become an indispensable resource in the field of organic synthesis. Here the authors report asymmetric total syntheses of several resveratrol dimers based on a comprehensive computational simulation of their biosynthetic pathways.

Recent Advances on Benzofuranones: Synthesis and Transformation via C–H Functionalization

The benzofuranone structure is important in many fields, such as natural products, pharmaceuticals, building blocks, antioxidants, and dyes. The efficient synthesis and transformation of benzofuranones have attracted great attention in organic synthesis. They can be synthesized by the Friedel–Crafts reaction and intramolecular dehydration ring-closing and transition-metal-catalyzed reactions, among others. Their direct utilization in the preparation of other functional molecules further enhance their application. Due to their low pK a value and easy enolization, the transformation of benzofuranones via C(3)–H bond functionalization has been a hot issue since 2010. Herein, we highlight advances in the synthesis of benzofuranones and their transformation via C–H functionalization. Other transformations related to benzofuranones are also discussed.

1 Introduction

2 Synthesis of Benzofuranones

3 C–H Functionalization of Benzofuranones

4 Other Types of Reactions of Benzofuranones

5 Conclusion and Outlook

Enantioselective Total Syntheses of Manginoids A and C and Guignardones A and C

An enantioselective synthetic approach for preparing manginoids and guignardones, two types of biogenetically related meroterpenoids, is reported. This bioinspired and divergent synthesis employs an oxidative 1,3-dicarbonyl radical-initiated cyclization and cyclodehydration of the common precursor to forge the central ring of the manginoids and guignardones, respectively, at a late stage. Key synthetic steps include silica-gel-promoted semipinacol rearrangement to form the 6-oxabicyclo[3.2.1]octane skeleton and the Suzuki-Miyaura reaction of vinyl bromide to achieve fragment coupling. This synthesis protocol enables the asymmetric syntheses of four fungal meroterpenoids from commercially available materials.

The Development of Reaction Cascades to Synthesize Dimeric Coccinellid Alkaloids

Over the course of the past decade, our group has been intensely interested in achieving the laboratory synthesis of varied members of the coccinellid alkaloid family of natural products. These compounds, produced by varied species of ladybugs throughout the world as defensive agents, include several polycyclic members that can formally be considered as either monomeric or dimeric with architectures that contain between 3 and 7 ring systems along with an array of stereocenters. As a result of their fascinating structures, many groups have achieved syntheses of varied monomeric members using a variety of synthetic strategies and tactics. However, no efforts to synthesize any of the dimeric structures had been reported at the time we began our studies, and only a modest amount of study had been performed as relates to their biosynthesis, with little knowledge of how the larger structures might actually arise in Nature. In this Account, we provide an overview of our general synthetic considerations to achieve a global synthesis of the collection, efforts that have led to date to the formal and total synthesis of 12 different members, 4 at the dimer level. Critical was (1) the identification of a key, common intermediate to enable access to a large number of monomeric substructures in short order, (2) careful thinking as to how the larger structures might arise biosynthetically to fuel building block design, and (3) the development of several reaction cascades that rapidly assembled the majority of their molecular complexity in single-pot operations. Key discoveries in the program include the finding that when efforts to achieve intermolecular dimerizations fail with advanced intermediates, attempts to couple more functionalized fragments earlier and then fold them into the desired structure can be an effective strategy. We also highlight suggestive evidence that a non-natural isomer we originally prepared from one of those cascades may, in fact, be a natural product. And, in particular, we will focus on how two key cascades were developed, as a result of synthetic challenges at varied points in our explorations, which proved capable of forging multiple bonds, rings, and stereocenters in the target structures. One of these includes a designed event that combined 9 different chemical reactions in a single pot and may prove useful for the synthesis of other targets.

Recent advances in total syntheses of natural products containing the benzocycloheptane motif

Covering: 2010 to 2020Benzocycloheptane is a fundamental and unique structural motif found in pharmaceuticals and natural products. The total syntheses of natural products bearing the benzocycloheptane subunit are challenging and there are only a few efficient approaches to access benzocycloheptane. Thus, new methods and innovative strategies for preparing such natural products need to be developed. In this review, recent progress in the total syntheses of natural products bearing the benzocycloheptane motif is presented, and key transformations for the construction of benzocycloheptane are highlighted. This review provides a useful guide for those engaged in the syntheses of natural products containing the benzocycloheptane motif.

Cu-Catalyzed radical-triggered spirotricyclization of enediynes and enyne-nitriles for the synthesis of pentacyclic spiroindenes

A new copper-catalyzed radical-triggered fluoromethylation-spirotricyclization of enediyne- and enyne-nitrile-containing para-quinone methides (p-QMs) was reported for the first time, and used to produce a series of hitherto unreported pentacyclic spiroindenes.

Metal-Catalyzed Spiroannulation-Fluoromethylfunctionaliztions of 1,5-Enynes for the Synthesis of Stereodefined (Z)-Spiroindenes

Two classes of new catalytic spiroannulation-fluormethylfunctionaliztions of para-quinone methide (p-QM)-containing 1,5-enynes have been established under redox-neutral conditions. Palladium-catalyzed spiroannulation-iododifluoromethylation with ethyl difluoroiodoacetate oriented completely stereoselective access to (Z)-spiroindenes and the latter included copper-catalyzed three-component spiroannulation-cyanotrifluoromethylation starting from Togni's reagent and trimethylsilanecarbonitrile (TMSCN). Both reaction pathways involve fluoroalkyl radical-triggered 1,6-addition/5-exo-dig annulation/metal radical cross-coupling/reductive elimination sequence, providing practical and stereoselective protocols for rapidly constructing cyclohexadienone-containing spiroindenes with generally good yields.

Synthesis of Vitisins A and D Enabled by a Persistent Radical Equilibrium

The first total synthesis of the resveratrol tetramers vitisin A and vitisin D is reported. Electrochemical generation and selective dimerization of persistent radicals is followed by thermal isomerization of the symmetric C8b-C8c dimer to the C3c-C8b isomer, providing rapid entry into the vitisin core. Computational results suggest that this synthetic approach mimics Nature's strategy for constructing these complex molecules. Sequential acid-mediated rearrangements consistent with the proposed biogenesis of these compounds afford vitisin A and vitisin D. The rapid synthesis of these complex molecules will enable further study of their pharmacological potential.

Catalytic Enantioselective House-Meinwald Rearrangement: Efficient Construction of All-Carbon Quaternary Stereocenters

A catalytic asymmetric House-Meinwald rearrangement for the synthesis of both cyclic and acyclic ketones is disclosed. From readily accessible racemic tetrasubstituted epoxides, this approach provides efficient access to chiral ketones bearing α all-carbon quaternary stereocenters with high enantiocontrol. The observation of positive nonlinear effects and nontrivial kinetic feature provided important insights into the mechanism.

Electrochemical Dimerization of Phenylpropenoids and the Surprising Antioxidant Activity of the Resultant Quinone Methide Dimers

A simple method for the dimerization of phenylpropenoid derivatives is reported. It leverages electrochemical oxidation of p-unsaturated phenols to access the dimeric materials in a biomimetic fashion. The mild nature of the transformation provides excellent functional group tolerance, resulting in a unified approach for the synthesis of a range of natural products and related analogues with excellent regiocontrol. The operational simplicity of the method allows for greater efficiency in the synthesis of complex natural products. Interestingly, the quinone methide dimer intermediates are potent radical-trapping antioxidants; more so than the phenols from which they are derived-or transformed to-despite the fact that they do not possess a labile H-atom for transfer to the peroxyl radicals that propagate autoxidation.

Recent applications of the 1,2-carbon atom migration strategy in complex natural product total synthesis

1,2-Carbon atom rearrangement has been broadly applied as a guiding strategy in complex molecule assembly. As it entails the carbon-carbon or carbon-heteroatom bond migration between two vicinal atoms, this type of reaction is capable of generating structural complexity through a molecular skeletal reorganization. This review will focus on recent employment of this strategy in the total synthesis of natural products, highlighting the exceptional utility of such synthetic methodologies in the construction of intricate carbocycles, heterocycles or structurally complex motifs from synthetically more accessible precursors.

Resveratrol Exerts Antioxidant Effects by Activating SIRT2 To Deacetylate Prx1

Resveratrol is a promising chemical agent that treats multiple aging-related diseases and improves life span. While reactive oxygen species undoubtedly play ubiquitous roles in the aging process and resveratrol has been shown to be an effective antioxidant, the mechanism through which resveratrol acts against oxidative stress remains unknown. Here we show that resveratrol activates SIRT2 to deacetylate Prx1, leading to an increased H₂O₂ reduction activity and a decreased cellular H₂O₂ concentration. Knockdown of SIRT2 or Prx1 by RNA interference abrogates resveratrol's ability to reduce the H₂O₂ level in HepG2 cells. Using purified SIRT2 and a Prx1 mutant harboring acetyllysine at position 27 (Prx1-27AcK), we show that resveratrol enhances SIRT2's activity to deacetylate Prx1-27AcK, resulting in a significantly increased H₂O₂ reducing activity. Thus, SIRT2 and Prx1 are targets for modulating intracellular redox status in the therapeutic strategies for the treatment of aging-related disorders.

Synthesis of resveratrol tetramers via a stereoconvergent radical equilibrium

Persistent free radicals have become indispensable in the synthesis of organic materials through living radical polymerization. However, examples of their use in the synthesis of small molecules are rare. Here, we report the application of persistent radical and quinone methide intermediates to the synthesis of the resveratrol tetramers nepalensinol B and vateriaphenol C. The spontaneous cleavage and reconstitution of exceptionally weak carbon-carbon bonds has enabled a stereoconvergent oxidative dimerization of racemic materials in a transformation that likely coincides with the biogenesis of these natural products. The efficient synthesis of higher-order oligomers of resveratrol will facilitate the biological studies necessary to elucidate their mechanism(s) of action.

Development of resveratrol-curcumin hybrids as potential therapeutic agents for inflammatory lung diseases

Acute lung injury (ALI) is a major cause of acute respiratory failure in critically-ill patients. Resveratrol and curcumin are proven to have potent anti-inflammatory efficacy, but their clinical application is limited by their metabolic instability. Here, a series of resveratrol and the Mono-carbonyl analogs of curcumin (MCAs) hybrids were designed and synthesized by efficient aldol construction strategy, and then screened for anti-inflammatory activities in vitro and in vivo. The results showed that the majority of analogs effectively inhibited the LPS-induced production of IL-6 and TNF-α. Five analogs, a9, a18, a19, a20 and a24 exhibited excellent anti-inflammatory activity in a dose-dependent manner along with low toxicity in vitro. Structure activity relationship study revealed that the electron-withdrawing groups at meta-position and methoxyl group (OCH₃) at the para position of the phenyl ring were important for anti-inflammatory activities. The most promising compound a18 decreased LPS induced TNF-α, IL-6, IL-12, and IL-33 mRNA expression. Additionally, a18 significantly protected against LPS-induced acute lung injury in the in vivo mouse model. The research of resveratrol and MCAs hybrids could bring insight into the treatment of inflammatory diseases and compound a18 may serve as a lead compound for the development of anti-ALI agents.

Hopeahainol A binds reversibly at the acetylcholinesterase (AChE) peripheral site and inhibits enzyme activity with a novel higher order concentration dependence

Natural product inhibitors of AChE are of interest both because they offer promise as inexpensive drugs for symptomatic relief in Alzheimer's disease and because they may provide insights into the structural features of the AChE catalytic site. Hopeahainol A is an uncharged polyphenol AChE inhibitor from the stem bark of Hopea hainanensis with a constrained, partially dearomatized bicyclic core. Molecular modeling indicates that hopeahainol A binds at the entrance of the long but narrow AChE active site gorge because it is too bulky to be accommodated within the gorge without severe distortion of the gorge as depicted in AChE crystal structures. We conducted inhibitor competition experiments in which AChE inhibition was measured with hopeahainol A together with either edrophonium (which binds at the base of the gorge) or thioflavin T (which binds to the peripheral or P-site near the gorge mouth). The results agreed with the molecular modeling and indicated that hopeahainol A at lower concentrations (<200 μM) bound only to the P-site, as hopeahainol A and thioflavin T were unable to form a ternary complex with AChE while hopeahainol A and edrophonium did form a ternary complex with essentially no competition between them. Inhibition increased to a striking extent at higher concentrations of hopeahainol A, with plots analogous to classic Dixon plots showing a dependence on hopeahainol A concentrations to the third- or fourth order. The inhibition at higher hopeahainol A concentrations was completely reversed on dilution and blocked by bound edrophonium. We hypothesize that bound hopeahainol A induces conformational changes in the AChE active site that allow binding of additional hopeahainol A molecules, a phenomenon that would be unprecedented for a reversible inhibitor that apparently forms no covalent bonds with AChE.

Comparison of antidiabetic potential of (+) and (−)-hopeaphenol, a pair of enantiomers isolated from Ampelocissus indica (L.) and Vateria indica Linn., with respect to inhibition of digestive enzymes and induction of glucose uptake in L6 myotubes

The remarkable α-glucosidase inhibition exhibited by the acetone extract of the rhizome of Ampelocissus indica (L.) and stem bark of Vateria indica Linn. (IC₅₀ 23.2 and 1.47 μg mL⁻¹) encouraged us to isolate the phytochemicals from these plants.

Transformations of Ferric Chloride-Generated Stilbene Cation Radicals. The Effect of Aromatic Substitution and a Comparison with Anodic Oxidation

A systematic study on the FeCl3-induced oxidation of 1,2-diarylalkenes was carried out with the focus on the variation of product type as a function of aromatic substitution, as well as to compare the reactivity of stilbene cation radicals generated via Fe(III) oxidation with those generated by anodic oxidation. The aromatic substituents were found to fall into three main categories, namely those that give rise to tetralins and/or dehydrotetralins, those that give products possessing pallidol and ampelopsin F-type carbon skeletons, and last, those that give rise to trimeric products, indanes, and dehydrotetralins/tetralins. The latter are those stilbenes with a para-methoxy substituent in one ring and a para- or meta-EWG (CF3, NO2, Cl, F) in the other, and represent the most prominent departure when compared with the behavior of the same stilbenes under the conditions of anodic oxidation. Reaction pathways to rationalize the formation of the different products are presented.

Gold-catalyzed carboalkoxylations of 2-ethynylbenzyl ethers to form 1- and 3-substituted 2-methoxy-1-H-indenes: Brønsted acids versus gold catalysis

Selective synthesis of 1- and 3-substituted 2-methoxyindenes from the carboalkoxylations of 2-ethynylbenzyl ethers is described; the former is obtained efficiently with P(t-Bu)2(o-biphenyl)AuCl/NaBARF in DCM/MS 4 Å whereas the latter is produced preferably with P(t-Bu)2(o-biphenyl)AuCl/AgNTf2 in pre-dried DCM. Both 1- and 3-substituted 2-indenyl ethers are subjected to ozone oxidations to afford two distinct carbonyl products. Our new data indicate that 1-substituted 2-indenyl ethers are generated from gold catalysts whereas their 3-substituted analogues arise from Brønsted acids.

A scalable biomimetic synthesis of resveratrol dimers and systematic evaluation of their antioxidant activities

An efficient synthetic route to the resveratrol oligomers quadrangularin A and pallidol is reported. It features a scalable biomimetic oxidative dimerization that proceeds in excellent yield and with complete regioselectivity. A systematic evaluation of the natural products and their synthetic precursors as radical-trapping antioxidants has revealed that, contrary to popular belief, this mode of action is unlikely to account for their observed biological activity.

Enantioselective intramolecular C-H insertion reactions of donor-donor metal carbenoids

The first asymmetric insertion reactions of donor–donor carbenoids, i.e., those with no pendant electron-withdrawing groups, are reported. This process enables the synthesis of densely substituted benzodihydrofurans with high levels of enantio- and diastereoselectivity. Preliminary results show similar efficiency in the preparation of indanes. This new method is used in the first enantioselective synthesis of an oligoresveratrol natural product (E-δ-viniferin).

Catalytic asymmetric α-Iminol rearrangement: new chiral platforms

A series of 19 different asymmetric catalysts were screened in an effort to identify the first chiral catalyst for the rearrangement of α-hydroxy imines to α-amino ketones involving a 1,2-carbon shift. Although aluminate complexes of VAPOL, VANOL, and 7,7'-(t)Bu2VANOL were quite effective catalysts giving up to 88% ee, the ne plus ultra catalyst for this reaction was found to be a zirconium complex of VANOL which gives 97 to >99% ee for the majority of the substrates examined. An X-ray diffraction study of the catalyst reveals that the zirconium exists as a homoleptic complex with three VANOL ligands and two protonated N-methyl imidazoles.

Enantioselective Black rearrangement catalyzed by chiral bicyclic imidazole

A newly developed chiral imidazole nucleophilic catalyst, , was readily prepared and successfully applied to the enantioselective Black rearrangement with up to 88% ee for a wide range of substrates possessing different substituted groups. A plausible mechanism for the high enantioselectivity was proposed.

A strategy for complex dimer formation when biomimicry fails: total synthesis of ten coccinellid alkaloids

Although dimeric natural products can often be synthesized in the laboratory by directly merging advanced monomers, these approaches sometimes fail, leading instead to non-natural architectures via incorrect unions. Such a situation arose during our studies of the coccinellid alkaloids, when attempts to directly dimerize Nature's presumed monomeric precursors in a putative biomimetic sequence afforded only a non-natural analogue through improper regiocontrol. Herein, we outline a unique strategy for dimer formation that obviates these difficulties, one which rapidly constructs the coccinellid dimers psylloborine A and isopsylloborine A through a terminating sequence of two reaction cascades that generate five bonds, five rings, and four stereocenters. In addition, a common synthetic intermediate is identified which allows for the rapid, asymmetric formal or complete total syntheses of eight monomeric members of the class.