Highly efficient and stereoselective N-vinylation of oxiranecarboxamides and unprecedented 8-endo-epoxy-arene cyclization: expedient and biomimetic synthesis of some Clausena alkaloids

Diastereocontrolled Construction of Spiroindolenines via Hexafluoroisopropanol-Promoted Dearomative Epoxide-Indole Cyclization

Herein, we report the discovery of the ipso-selective, dearomatizing spirocyclization of indole-tethered epoxides as a fundamentally new approach for constructing spiroindolenines equipped with three contiguous stereogenic centers under complete diastereocontrol (dr >99:1) and in high yields. Promoted by hexafluoroisopropanol, the protocol features a broad substrate scope, easy scale-up, and versatile transformations of the synthesized spiroindolenines.

Ligand- and Substrate-Controlled Chemodivergent Pd-Catalyzed Annulations of Cyclic β-Keto Esters with 3-Aryl-2H-azirines

Chemodivergent Pd-catalyzed annulations of cyclic β-keto esters with 3-aryl-2H-azirines have been developed to provide rapid access to eight-membered ring lactams, bicyclic 3,4-dihydro-2H-pyrrole derivatives, and (E)-methyl [2-(2-oxocyclohexylidene)-2-phenylethyl]carbamates with high efficiency. The chemoselectivity can be determined by tuning the mono- or bisphosphine ligands as well as the substrate structure of cyclic β-keto esters.

Diverse privileged N-polycyclic skeletons accessed from a metal-free cascade cyclization reaction

An exquisite metal-free cascade cyclization reaction of 2-acylbenzoic acids with amines was developed, which provided a powerful method for the one-pot synthesis of diverse isoindoloisoquinoline and benzoindolizinoindole derivatives. This protocol avoided the use of metal catalysts, proceeded with high efficiency and had broad substrate scope. These resulting products could be transformed into tertiary amines under the reduction of LiAlH4/AlCl3, followed by the Hofmann elimination offering lots of nitrogen-containing nine-membered ring compounds in excellent yields. All synthesized products containing fused N-polycyclic skeletons were difficult to be constructed using traditional methods and they have a wide range of applications in the pharmaceutical area.

Domino Reactions of Tertiary Enamides in Organic Synthesis

Studies in recent decade have shown that tertiary enamides, once thought of as unreactive and marginally useful enamine variants, are shelf-stable yet versatile nucleophilic intermediates in organic synthesis. Many compounds, especially N-heterocycles, have been synthesized by monofunctionalization reactions of tertiary enamides. By taking advantage of intramolecular and intermolecular interception of the putative acyliminium intermediates formed by the initial nucleophilic reactions of tertiary enamides, we have established several novel difunctionalization reaction methods for constructing diverse complex fused heterocyclic products. In this Account, we summarize our endeavors to develop difunctionalization reactions of tertiary enamides in a domino fashion.

1 Introduction

2 Strategic Considerations

3 Domino Reactions of Tertiary Enamides

3.1 Type I Domino Reactions

3.2 Type II Domino Reactions

3.3 Type III Domino Reactions

4 Conclusion and Perspectives

Catalytic enantioselective synthesis of indolizino[8,7-b]indole alkaloid derivatives based on the tandem reaction of tertiary enamides

An exquisite Pybox/Cu(OTf)₂-catalyzed asymmetric tandem reaction of tertiary enamides was developed, which enabled the expeditious synthesis of indolizino[8,7-b]indole derivatives in high yield, excellent enantioselectivity and diastereoselectivity.

Tandem Reaction of Tertiary Enamides as a Synthetic Strategy to Construct the Fused N-Pentacyclic Skeleton of Erythrina Alkaloid Derivatives

An exquisite protocol to the synthesis of erythrina-related structural derivatives was developed, which is composed of a nucleophilic addition of tertiary enamides to ketonic carbonyls and the trapping of acyliminium by an aromatic unit. This protocol afforded a powerful method for the construction of diverse fused N-pentacyclic skeletons in high efficiency and excellent diastereoselectivity by just using different acid catalysts.

Transition-Metal-Free Functionalization of Saturated and Unsaturated Amines to Bioactive Alkaloids Mediated by Sodium Chlorite

New approaches to the synthesis of alkaloids through the straightforward functionalization of C(sp3)–H and C(sp2)=C(sp2) bonds of simple five- and six-membered-ring N-heterocycles are highlighted. The direct functionalization of pre-existing N-heterocycles to advanced alkaloids intermediates is a chemical operation that commonly requires the intervention of transition or precious metals. Regardless the inherent unwanted waste production, the high economical cost of many transition-metal catalysts limits their use globally. Here, we account our efforts directed toward the synthesis of bioactive alkaloids under an economic and ecological fashion by using NaClO2 as the key activating or oxidizing reagent that substitutes the use of transition-metal catalysts. While undesired metal wastes are collected during the extraction process of a transition-metal-catalyzed reaction, innocuous NaCl is the commonly product waste when NaClO2 is employed in our chemical transformations. Beginning with the synthesis of 2,3-epoxyamides from allyl amines, we concluded with the functionalization of multiple and remote C(sp3)–H and C(sp3)–C(sp3) bonds in piperidine rings that enabled the preparation of important bioactive alkaloids. For the latter functionalization, a precise amount of co-oxidant reagent (NaOCl) and radical 2,2,6,6-tetramethylpiperidinyloxy (TEMPO) were needed.

1 Introduction

2 Direct Chemical Method for Preparing 2,3-Epoxyamides

3 Dual C(sp3)–H Oxidation of Cyclic Amines to 3‑Alkoxyamine Lac­tams

4 Electrochemical Deamination of 3-Alkoxyamine Lactams

5 Direct C–H Oxidation of Piperazines and Morpholines to 2,3-Diketopiperazines and 3-Morpholinones, Respectively

6 Transition-Metal-Free Triple C–H Oxidation

7 Deconstructive Lactamization of Piperidines

8 Conclusion

Construction of the Erythrinane Core Skeleton via Asymmetric Catalytic Cascade Reaction of Tertiary Enamides

We report herein an efficient cascade strategy for the rapid construction of a highly enantioenriched erythrinane core skeleton. Under the sequential catalysis of a chiral Cr(III)(salen)Cl and InCl₃, cyclohexanone-derived tertiary enamides undergo an intramolecular enantioselective nucleophilic addition followed by diastereoselective Pictet-Spengler cyclization. This method is highly enantio- and diastereoselective, leading to diverse erythrina alkaloid derivatives as the sole diastereoisomer with up to 98% ee.

Tertiary Enamides: Versatile and Available Substrates in Synthetic Chemistry

BACKGROUND

Enamines and their variant enamides as powerful and versatile synthons have attracted great attention in synthetic chemistry. Enamides display unique stability and reduce enaminic reactivity in view of the electron-withdrawing effect of N-acyl group. A great deal of satisfactory achievements in the synthesis and application of enamides has been made in recent years. Especially, tertiary enamides without N-H bond regarded as low reactivity of compounds in the past can act as excellent nucleophiles to react with electrophiles for the construction of various nitrous molecules.

OBJECTIVE

This review focuses on recent advances on tertiary enamides in the synthetic strategies and applications including addition, coupling reaction, functionalization and electro- or photo-chemical reaction.

CONCLUSION

Tertiary enamides as electron-deficient nucleophiles display a satisfactory balance between stability and reactivity to offer multiple opportunities for the construction of various functionalized nitrogencontaining compounds. Further exploration of the reactive mechanisms involved tertiary enamides and the development of novel and efficient transformations to generate ever more complex building blocks starting from tertiary enamides are particularly worth pursuing.

Stereoselective synthesis of medium lactams enabled by metal-free hydroalkoxylation/stereospecific [1,3]-rearrangement

Rearrangement reactions have attracted considerable interest over the past decades due to their high bond-forming efficiency and atom economy in the construction of complex organic architectures. In contrast to the well-established [3,3]-rearrangement, [1,3] O-to-C rearrangement has been far less vigorously investigated, and stereospecific [1,3]-rearrangement is extremely rare. Here, we report a metal-free intramolecular hydroalkoxylation/[1,3]-rearrangement, leading to the practical and atom-economical assembly of various valuable medium-sized lactams with wide substrate scope and excellent diastereoselectivity. Moreover, such an asymmetric cascade cyclization has also been realized by chiral Brønsted acid-catalyzed kinetic resolution. In addition, biological tests reveal that some of these medium-sized lactams displayed their bioactivity as antitumor agents against melanoma cells, esophageal cancer cells and breast cancer cells. A mechanistic rationale for the reaction is further supported by control experiments and theoretical calculations.

Stereospecific [1,3]-rearrangements are rarely reported method to efficiently build complex organic architectures. Here, the authors describe a metal-free intramolecular hydroalkoxylation/[1,3]-rearrangement sequence affording medium-sized lactams with wide scope, also in an asymmetric fashion.

Tertiary Enamide-Triggered SEAr: Domino Allylation and Enamine-Type Addition

Two unprecedented domino reactions are described, starting from ketospiro-enesulfonamides. By treatment with ZrCl₄ and allylsilane, an intramolecular electrophilic aromatic substitution and subsequent allylation is observed. By treatment with TiCl₄ and allylsilane, a double enamine-type reaction takes place, thus creating simultaneously four contiguous stereogenic centers diastereoselectively.

Moschamine-Related Indole Alkaloids

Moschamine-related alkaloids originate mainly from feruloylserotonin by cyclization or dimerization. This review provides a comprehensive overview on the achievements in the field of moschamine-related alkaloids. In the isolation part, a detailed structural characterization of moschamine-related alkaloids is followed by their spectral data. Besides the well-known antioxidative activities, other screened biological properties are also outlined. Since their isolation, different protocols have been developed to synthesize these alkaloids. The synthetic part is organized according to the different synthetic methodologies for the formation of key structural elements.

Lewis acid catalyst-steered divergent synthesis of functionalized vicinal amino alcohols and pyrroles from tertiary enamides

We report herein two Lewis acid catalyst-steered distinct reaction pathways of N-formylmethyl tertiary enamides.

Total syntheses of pyrroloazocine indole alkaloids: challenges and reaction discovery

Total syntheses of pyrroloazocine indole alkaloids (lapidilectines, grandilodines, lundurines and tenuisines) are discussed in terms of existing retrosynthetic solutions and novel reaction discoveries.

Synthesis of cananodine by intramolecular epoxide opening

Cananodine is a guaipyridine alkaloid with activity against liver cancer. Cananodine was synthesized using a remarkable intramolecular opening of a trisubstituted epoxide as the key step in construction of the seven-membered carbocycle of the target. The epoxide opening strategy allows all four stereoisomers of cananodine to be prepared.

Synthesis of 8-azaprotosappanin A derivatives via intramolecular palladium-catalyzed ortho C–H activation/C–C cyclization and their antibacterial activity

A novel synthetic protocol for the construction of eight-membered heterocycles by intramolecular palladium-catalyzed ortho C–H activation/C–C cyclization was proposed.

Constituents from the leaves of Clausena lansium and their anti-inflammatory activity

Five new acyclic amides, clausenalansamides C-G (1-5), clausenaline G (6) and (±)-5-(4-methylphenyl)-γ-valerolactone (7) reported from the natural source for the first time, were characterized from the leaves of Clausena lansium. Their structures were established by spectroscopic and spectrometric methods, and the absolute configurations of new compounds 1-5 were determined by electronic circular dichroism and single-crystal X-ray diffraction analyses. Eighteen compounds were evaluated for their anti-inflammatory activity and only imperatorin (11) and wampetin (12) displayed significant inhibition of fMLP/CB-induced superoxide anion generation with IC₅₀ values of 1.7 ± 0.3 and 6.8 ± 1.1 μM, respectively.

Exploring tertiary enamides as versatile synthons in organic synthesis

Tertiary enamides have long been thought of as stable and marginally valuable enamine variants in synthesis. This notion has been challenged, however, in recent years. Enabling the regulation of the cross-conjugation system of the tertiary enamides has been successfully shown to enhance delocalization of the nitrogen lone-pair electrons into a carbon-carbon double, thereby reinvigorating the enaminic reactivity of the tertiary enamides. In this article, I summarize the recent advances in the exploration of the nucleophilic reactions of tertiary enamides and their applications in the synthesis of natural products and heterocyclic compounds of biological and pharmaceutical relevance, with a primary focus on our own work.

High-resolution hyaluronidase inhibition profiling combined with HPLC-HRMS-SPE-NMR for identification of anti-necrosis constituents in Chinese plants used to treat snakebite

Inhibition of the necrotizing hyaluronidase, phospholipase A2 and protease enzymes in four snake venoms by crude water and ethanol extracts of 88 plant species used against snakebites in traditional Chinese medicine was measured. High-resolution hyaluronidase inhibition profiles were constructed for the 22 plants showing highest hyaluronidase inhibition, and the results were used to guide subsequent structural analysis towards specific hyaluronidase inhibitors. Structural analysis was performed by high-performance liquid chromatography, high-resolution mass spectrometry, solid-phase extraction and nuclear magnetic resonance spectroscopy, i.e., HPLC-HRMS-SPE-NMR. This allowed identification of four non-tannin inhibitors, i.e., lansiumamide B (6) from Clausena excavata Burm.f., myricetin 3-O-β-D-glucopyranoside (7) from Androsace umbellata (Lour.) Merr., and vitexin (8) and 4',7-dihydroxy-5-methoxyflavone-8-C-β-D-glucopyranoside (9) from Oxalis corniculata L. Absolute configuration of 2,3-dihydroxy-N-methyl-3-phenyl-N-[(Z)-styryl]propanamide (1) was determined using the Mosher method, which revealed two enantiomers, i.e., (2S,3R)-2,3-dihydroxy-N-methyl-3-phenyl-N-[(Z)-styryl]propanamide and (2R,3S)-2,3-dihydroxy-N-methyl-3-phenyl-N-[(Z)-styryl]propanamide with a ratio of 7:3.

Enantioselective biotransformations of nitriles in organic synthesis

The hydration and hydrolysis of nitriles are valuable synthetic methods used to prepare carboxamides and carboxylic acids. However, chemical hydration and hydrolysis of nitriles involve harsh reaction conditions, have low selectivity, and generate large amounts of waste. Therefore, researchers have confined the scope of these reactions to simple nitrile substrates. However, biological transformations of nitriles are highly efficient, chemoselective, and environmentally benign, which has led synthetic organic chemists and biotechologists to study these reactions in detail over the last two decades. In nature, biological systems degrade nitriles via two distinct pathways: nitrilases catalyze the direct hydrolysis of nitriles to afford carboxylic acids with release of ammonia, and nitrile hydratases catalyze the conversion of nitriles into carboxamides, which then furnish carboxylic acids via hydrolysis in the presence of amidases. Researchers have subsequently developed biocatalytic methods into useful industrial processes for the manufacture of commodity chemicals, including acrylamide. Since the late 1990s, research by my group and others has led to enormous progress in the understanding and application of enantioselective biotransformations of nitriles in organic synthesis. In this Account, I summarize the important advances in enantioselective biotransformations of nitriles and amides, with a primary focus on research from my laboratory. I describe microbial whole-cell-catalyzed kinetic resolution of various functionalized nitriles, amino- and hydroxynitriles, and nitriles that contain small rings and the desymmetrization of prochiral and meso dinitriles and diamides. I also demonstrate how we can apply the biocatalytic protocol to synthesize natural products and bioactive compounds. These nitrile biotransformations offer an attractive and unique protocol for the enantioselective synthesis of polyfunctionalized organic compounds that are not readily obtainable by other methods. Nitrile substrates are readily available, and the mild reaction conditions are specific toward cyano and amido functional groups without interfering with other reactive functional groups. I anticipate that further advances in this field will lead to new and engineered nitrile-hydrolyzing enzymes or catalytic systems with improved activity and altered selectivity. These advances will broaden the scope of these transformations and their applications in organic synthesis.

Functionalized imidazoliniums from the three-component domino reaction of N-formylmethylcarboxamides with amines and isocyanides

Functionalized imidazoliniums were synthesized efficiently from the three-component domino reaction of N-formylmethyl-substituted tertiary amides and enamides with amines and isocyanides.

Brønsted acid promoted benzylic C-H bond functionalization of azaarenes: nucleophilic addition to aldehydes

A practical Brønsted acid promoted benzylic C-H functionalization of 2-methylazaarenes and nucleophilic addition to aldehydes was developed in good to excellent yields. A six-membered hydrogen-bonding transition state is proposed to be crucial for the reaction. Ready availability of the two starting materials, the use of acetic acid as the catalyst and the facile reaction conditions will guarantee this synthetic method attractive to the synthesis of bioactive pyridine and quinoline derivatives.