Intramolecular reactions of N-acyliminium intermediates

Nitrogen Heterocycle Synthesis through Hydride Abstraction of Acyclic Carbamates and Related Species: Scope, Mechanism, Stereoselectivity, and Product Conformation Studies

Acyliminium ions and related species are potent electrophiles that can be quite valuable in the synthesis of nitrogen-containing molecules. This manuscript describes a protocol to form these intermediates through hydride abstractions of easily accessible allylic carbamates, amides, and sulfonamides that avoids the reversibility that is possible in classical condensation-based routes. These intermediates are used in the preparation of a range of nitrogen-containing heterocycles, and in many cases high levels of stereocontrol are observed. Specifically areas of investigation include the impact of chemical structure on oxidation efficiency, the geometry of the intermediate iminium ions, the impact of a substrate stereocenter on stereocontrol, and an examination of transition state geometry.

Mannich-type addition of cyclic 1,3-diketones to N-acyliminium ions: Access to aza-sugars

A novel three-component strategy has been developed for the synthesis of iminosugars in good to excellent yields. This is the first report on the Mannich type addition of cyclic 1,3-diketones to aza-acetal derived from hydroxy-γ-lactone and arylamine to produce a novel series of aza-sugars with high selectivity.

α-Hydroxylactams as Efficient Entries to Diversely Functionalized Ferrociphenols: Synthesis and Antiproliferative Activity Studies

The [ferrocene-ene-phenol] motif has been identified as the pharmacophore responsible for the anticancer activity of the family of ferrocene-based molecules coined ferrocifens, owing to its unique redox properties. The addition of imide entities to the historical ferrociphenol scaffold tremendously enhanced the cytotoxic activity of a large panel of cancer cell cultures and preliminary studies showed that the reduction of one of the carbonyl groups of the imide groups to the corresponding α-hydroxylactams only slightly affected the antiproliferative activity. As a continuation to these studies, we took advantage of the facile conversion of α-hydroxylactams to highly electrophilic N-acyliminium ions to graft various substituents to the imide motif of phthalimido ferrocidiphenol. Cell viability studies showed that the newly synthesized compounds showed diverse cytotoxic activities on two breast cancer cell lines, while only one compound was significantly less active on the non-tumorigenic cell line hTERT-RPE1.

One-Pot Transformation of Furans into 1-Azaspirocyclic Alkaloid Frameworks Induced by Visible Light

High-value 1-azaspirocyclic scaffolds have been made from simple and readily accessible furan precursors in a single operation. The protocol is a one-pot sequence using highly sustainable conditions (oxygen, visible light, and a favored green solvent) that leads to a dramatic increase in molecular complexity. The initial substrates can include functionalities that are suitable for further elaboration; in this way, the pruned polycyclic skeletons of the stemonamine, cylindricine, and lepadiformine natural products were rapidly accessed.

Amide-Type Substrates in the Synthesis of N-Protected 1-Aminomethylphosphonium Salts

Herein we describe the development and optimization of a two-step procedure for the synthesis of N-protected 1-aminomethylphosphonium salts from imides, amides, carbamates, or lactams. Our “step-by-step” methodology involves the transformation of amide-type substrates to the corresponding hydroxymethyl derivatives, followed by the substitution of the hydroxyl group with a phosphonium moiety. The first step of the described synthesis was conducted based on well-known protocols for hydroxymethylation with formaldehyde or paraformaldehyde. In turn, the second (substitution) stage required optimization studies. In general, reactions of amide, carbamate, and lactam derivatives occurred at a temperature of 70 °C in a relatively short time (1 h). On the other hand, N-hydroxymethylimides reacted with triarylphosphonium salts at a much higher temperature (135 °C) and over longer reaction times (as much as 30 h). However, the proposed strategy is very efficient, especially when NaBr is used as a catalyst. Moreover, a simple work-up procedure involving only crystallization afforded good to excellent yields (up to 99%).

Synthesis of spirocyclic heterocycles from α,β-unsaturated N-acyliminium ions

The reactions of α,β-unsaturated N-acyliminium ions, generated in situ from 4(S)-O-substitutedhydroxy-5-hydroxy-5-vinyl-N-alkylpyrrolidin-2-ones, with allylsilanes and indoles leading to the formation of spirocyclic heterocycles, are reported.

A Versatile Approach to Dynamic Amide Bond Formation with Imine Nucleophiles

Dynamic covalent chemistry has rapidly become an important approach to access supramolecular structures. While the products generated in these reactions are held together by covalent bonds, the reversible nature of the transformations can limit the utility of many these systems in creating robust materials. We describe herein a method to form stable and commonly employed amide bonds by exploiting the reversible coupling of imines and acyl chlorides. The reaction employs easily accessible reagents, is dynamic under ambient conditions, without catalysts, and can be trapped with simple hydrolysis. This offers an approach to create broad families of amide products under thermodynamic control, including the selective formation of amide macrocycles or polymers.

The reticent tautomer: exploiting the interesting multisite and multitype reactivity of 4-pyrrolin-2-ones

Multisite and multitype reactivities of the highly versatile and valuable synthetic building block 4-pyrrolin-2-one are covered in this review.

Nickel-Catalyzed Amidoalkylation Reaction of γ-Hydroxy Lactams: An Access to 3-Substituted Isoindolinones

An efficient Ni(ClO₄)₂·6H₂O-promoted amidoalkylation reaction for the synthesis of 3-substituted isoindolinones involving various γ-hydroxy lactams and nucleophiles has been successfully developed. The transformation proceeds with both carbon (ketones and arenes) and heteroatom (alcohols, thiols, and amines) nucleophiles and in both intermolecular and intramolecular manners. The prominent features of the present strategy are wide substrate scope, excellent group tolerability, and moderate to good yields (up to 96% yield). The present strategy is also characterized by remarkable superiority over the current synthetic methods. Furthermore, the reaction could be scaled up to the multigram scale.

Iron-Catalyzed Meerwein Carbooxygenation of Electron-Rich Olefins: Studies with Styrenes, Vinyl Pyrrolidinone, and Vinyl Oxazolidinone

The arylative oxygenation of the electron-rich olefins styrene, α-methylstyrene, vinyl pyrrolidinone, and vinyl oxazolidinone was accomplished using arenediazonium salts and catalytic amounts of FeSO₄ in an effective single electron transfer radical process. A broad range of aryldiazonium salts was tolerated using water, methanol, or their combination with acetonitrile to furnish the corresponding carbohydroxylated and carbomethoxylated products (42 examples), including functionalized dihydroisocoumarin and dihydrobenzofuran systems in good to excellent yields (up to 88%). The protocols developed for the Fe(II)-catalyzed carbohydroxylation were also compared to Ru(II) and Ir(III) photoredox carbooxygenations of these electron-rich olefins. The Fe(II)-catalyzed process proved to be highly competitive compared to the photoredox and the uncatalyzed processes. The proposed mechanism for the Fe(II)-catalyzed reactions involves the synergic combination with an effective Fe⁺²/Fe⁺³ redox system and a radical polar crossover mechanism featuring an unprecedented capture of the reactive N-acyliminium in the case of vinyl pyrrolidinone and vinyl oxazolidinone.

A Modular Construction of Epidithiodiketopiperazines

Epidithiodiketopiperazines (ETPs) possess remarkably diverse biological activities and have attracted significant synthetic attention. The preparation of analogues is actively pursued; however, they are structurally challenging, and more direct and modular methods for their synthesis are desirable. To this end, the utility of a bifunctional triketopiperazine building block for the straightforward synthesis of ETPs is reported. A modular strategy consisting of enolate alkylation followed by site-selective nucleophile addition enables the concise synthesis of (±)-hyalodendrin and a range of analogues.

1,2-Addition versus homoconjugate addition reactions of indoles and electron-rich arenes to α-cyclopropyl N-acyliminium ions: synthetic and computational studies

α-Cyclopropyl-N-acyliminium ions towards reaction with indoles to give 5-(3-indoyl)-5-cyclopropylpyrrolidin-2-ones and, in the case of highly electron deficient indoles and electron rich arenes, spiroheterocycles.

1-(N-Acylamino)alkyltriarylphosphonium Salts with Weakened Cα-P⁺ Bond Strength-Synthetic Applications

The α-amidoalkylating properties of 1-(N-acylamino)alkyltriarylphosphonium salts with weakened Cα-P⁺ bond strength are discussed and examined. It is demonstrated that such type of phosphonium salts reacts smoothly with a diverse array of carbon- and heteroatom-based nucleophiles, including 1-morpholinocyclohexene, 1,3-dicarbonyl compounds, benzotriazole sodium salt, p-toluenesulfinate sodium salt, benzylamine, triarylphosphines, and other P-nucleophiles. Reactions are conducted at room temperature, in a short time (5⁻15 min) and mostly without catalysts. Simple work-up procedures result in good or very good yields of products. The structures of known compounds were established by spectroscopic methods and all new compounds have been fully characterized using ¹H-, 13C-, 31P-NMR, IR spectroscopy, and high-resolution mass spectrometry. Mechanistic aspects of described transformations are also performed and discussed. It was demonstrated that unique properties make 1-(N-acylamino)alkyl-triarylphosphonium salts with weakened Cα-P⁺ bond strength interesting building blocks with great potential, especially in α-amidoalkylation reactions.

Electrogenerated Cationic Reactive Intermediates: The Pool Method and Further Advances

Electrochemistry serves as a powerful method for generating reactive intermediates, such as organic cations. In general, there are two ways to use reactive intermediates for chemical reactions: (1) generation in the presence of a reaction partner and (2) generation in the absence of a reaction partner with accumulation in solution as a "pool" followed by reaction with a subsequently added reaction partner. The former approach is more popular because reactive intermediates are usually short-lived transient species, but the latter method is more flexible and versatile. This review focuses on the latter approach and provides a concise overview of the current methods for the generation and accumulation of cationic reactive intermediates as a pool using modern techniques of electrochemistry and their reactions with subsequently added nucleophilic reaction partners.

Phenolic Activation in Chiral Brønsted Acid-Catalyzed Intramolecular α-Amidoalkylation Reactions for the Synthesis of Fused Isoquinolines

An organolithium addition-intramolecular α-amidoalkylation sequence on N-phenethylimides has been developed for the synthesis of fused tetrahydroisoquinoline systems using 1,1'-bi-2-naphthol (binol)-derived Brønsted acids. This transformation is the first in which activated benzene derivatives are used as internal nucleophiles, instead of electron-rich heteroaromatics, generating a quaternary stereocenter. Phenolic substitution on the aromatic ring of the phenethylamino moiety and the use of binol-derived N-triflylphosphoramides as catalysts are determinants to achieve reasonable levels of enantioselection, that is, up to 75% enantiomeric excess, in the α-amidoalkylation step. The procedure is complementary to the intermolecular α-amidoalkylation process, as opposite enantiomers are formed, and to the Pictet-Spengler cyclization, which allows the formation of tertiary stereocenters.

Selective synthesis of three product classes from imine and carboxylic acid precursors via direct imine acylation

Three divergent procedures allow the selective generation of δ-lactams, β-lactams and tetrahydropyrimidinones from common imine and carboxylic acid precursors.

Chiral Brønsted Acid-Catalyzed Enantioselective α-Amidoalkylation Reactions: A Joint Experimental and Predictive Study

Enamides with a free NH group have been evaluated as nucleophiles in chiral Brønsted acid-catalyzed enantioselective α-amidoalkylation reactions of bicyclic hydroxylactams for the generation of quaternary stereocenters. A quantitative structure-reactivity relationship (QSRR) method has been developed to find a useful tool to rationalize the enantioselectivity in this and related processes and to orient the catalyst choice. This correlative perturbation theory (PT)-QSRR approach has been used to predict the effect of the structure of the substrate, nucleophile, and catalyst, as well as the experimental conditions, on the enantioselectivity. In this way, trends to improve the experimental results could be found without engaging in a long-term empirical investigation.

An efficient lactamisation/N-acyliminium Pictet–Spengler domino strategy for the diasteroselective synthesis of polyhydroxylated quinoxalinone, β-carboline and quinazolinone derivatives

A novel Pictet–Spengler cascade sequence has been developed. It involves a lactamisation/N-acyliminium key step and gives a diverse set of sugar-derived fused polycyclic heterocycles diastereoselectively and efficiently.

A unified approach to pyrrole-embedded aza-heterocyclic scaffolds based on the RCM/isomerization/cyclization cascade catalyzed by a Ru/B-H binary catalyst system

An easy and straightforward preparation of pyrrole-embedded aza-heterocyclic scaffolds employing a Ru/B-H binary catalyst system has been developed.

Stereoselective synthesis of 3-substituted tetrahydropyrazinoisoquinolines via intramolecular cyclization of enantiomerically enriched dihydro-2H-pyrazines

The preparation of 3-substituted tetrahydropyrazinoisoquinolines using the tributyltin hydride mediated intramolecular radical cyclization of suitably protected 2-substituted 3,4-dihydropyrazines is reported. The compounds are obtained as single enantiomers, as the relative configuration of the new generated stereogenic center is driven by the stereochemistry of the 2-substituted carbon in the starting materials, which is in turn derived from naturally occurring amino acids.

The Shono-type electroorganic oxidation of unfunctionalised amides. Carbon-carbon bond formation via electrogenerated N-acyliminium ions

N-acyliminium ions are useful reactive synthetic intermediates in a variety of important carbon–carbon bond forming and cyclisation strategies in organic chemistry. The advent of an electrochemical anodic oxidation of unfunctionalised amides, more commonly known as the Shono oxidation, has provided a complementary route to the C–H activation of low reactivity intermediates. In this article, containing over 100 references, we highlight the development of the Shono-type oxidations from the original direct electrolysis methods, to the use of electroauxiliaries before arriving at indirect electrolysis methodologies. We also highlight new technologies and techniques applied to this area of electrosynthesis. We conclude with the use of this electrosynthetic approach to challenging syntheses of natural products and other complex structures for biological evaluation discussing recent technological developments in electroorganic techniques and future directions.