Ring-opening, cycloaddition and rearrangement reactions of nitrogen-substituted cyclopropane derivatives

Iron(II)-Catalyzed Radical [3 + 2] Cyclization of N-Aryl Cyclopropylamines for the Synthesis of Polyfunctionalized Cyclopentylamines

A facile iron(II)-catalyzed radical [3 + 2] cyclization of N-aryl cyclopropylamines with various alkenes to access the structurally polyfunctionalized cyclopentylamine scaffolds has been developed. Using low-cost FeCl2·4H2O as catalyst, N-aryl cyclopropylamines could be utilized to react with a wide range of alkenes including exocyclic/acyclic terminal alkenes, cycloalkenes, alkenes from the natural-occurring compounds (Alantolactone, Costunolide), and known drugs (Ibuprofen, l-phenylalanine, Flurbiprofen) to obtain a variety of cyclopentylamines fused with different useful motifs in generally good yields and diastereoselectivities. The highlight of this protocol is also featured by no extra oxidant, no base, EtOH as the solvent, gram-scale synthesis, and further diverse transformations of the synthetic products. More importantly, an iron(II)-mediated hydrogen radical dissociation pathway was proposed based on the mechanism research experiments.

Bicyclic N-dihalocyclopropylamide derivatives as precursors of nitrogen-containing fused polycyclic systems

Examples of carbon-carbon bond-forming cyclisation reactions, involving allyl cations generated by the thermal ring-opening of halocyclopropanes, have been scarcely reported. In this contribution, we are describing the results of a study conducted with N-dihalocyclopropylamide substrates, designed as precursors of cyclic iminium intermediates that were aimed at participating in intramolecular reactions with electron-rich aromatic groups. Competitive side-reactions were identified, and access to the desired polycyclic products was carefully evaluated. The results were found to be strongly dependent on the substitution pattern of the nucleophilic aromatic moieties, as well as on the sizes of the rings of the target products. In spite of the rather moderate yields generally obtained, this approach represents a particularly short and inexpensive route to various interesting nitrogen-containing polycyclic systems, namely benzoindolizidine, benzoquinolizidine, piperidinobenzoazepane and azepanoisoquinoline compounds.

Activation of aminocyclopropanes via radical intermediates

Aminocyclopropanes are versatile building blocks for accessing high value-added nitrogen-containing products. To control ring-opening promoted by ring strain, the Lewis acid activation of donor-acceptor substituted systems is now well established. Over the last decade, alternative approaches have emerged proceeding via the formation of radical intermediates, alleviating the need for double activation of the cyclopropanes. This tutorial review summarizes key concepts and recent progress in ring-opening transformations of aminocyclopropanes via radical intermediates, divided into formal cycloadditions and 1,3-difunctionalizations.

Visible-Light-Accelerated Copper-Catalyzed [3 + 2] Cycloaddition of N-Tosylcyclopropylamines with Alkynes/Alkenes

Copper-catalyzed [3 + 2] cycloadditions of N-tosylcyclopropylamine with alkynes and alkenes have been accomplished under visible light irradiation. The developed approach is compatible with a range of functionalities and allows the synthesis of diversified aminated cyclopentene and cyclopentane derivatives being relevant for drug synthesis. The protocol is operationally simple and economically affordable as it does not require any ligand, base, or additives. As the key step, the one-electron oxidation of the N-tosyl moiety by visible light-induced homolysis of a transient Cu(II)-tosylamide complex is proposed, providing a facile entry for N-centered radicals.

Diastereoselective Synthesis of Tetrahydrobenzo[b]azocines by Lu(OTf)3 Catalyzed [4 + 4] Cycloaddition of Donor–Acceptor Cyclobutanes with Anthranils

The construction of N-heterocyclic eight-membered rings remains a challenging due to unfavorable transannular strain. Herein, we report a Lu(OTf)3 catalyzed formal [4 +4] cycloaddition reaction of cyclobutane 1,1-diesters with anthranils...

Lewis acid-catalyzed [4 + 2] cycloaddition of donor–acceptor cyclobutanes with iminooxindoles: access to spiro[piperidine-3,2′-oxindoles]

A Lewis acid-catalyzed [4 + 2] cycloaddition reaction from D–A cyclobutanes and iminooxindoles, providing the corresponding spiro[piperidine-3,2′-oxindoles] under mild conditions.

Synthesis of 1,4-Dihydropyridines and Related Heterocycles by Iodine-Mediated Annulation Reactions of N-Cyclopropyl Enamines

The annulation of N-cyclopropyl enamines to produce 1,4-dihydropyridine (1,4-DHP) derivatives is described. In the presence of molecular iodine (I₂), an N-cyclopropyl enamine substrate undergoes iodination, opening of the cyclopropyl ring, and annulation with a second molecule of the substrate to form the 1,4-DHP product. This reaction is amenable to gram-scale operations under mild reaction conditions with no transition metals being required. Further transformations of the 1,4-DHPs leads to related pyridine and bicyclic frameworks.

Diamine Synthesis via the Nitrogen-Directed Azidation of σ- and π-C-C Bonds

Diamines are essential building blocks for the synthesis of agrochemicals, drugs, and organic materials, yet their synthesis remains challenging, as both nitrogens need to be differentiated and diverse substitution patterns (1,2, 1,3, or 1,4) are required. We report herein a new strategy giving access to 1,2, 1,3, and 1,4 amido azides as orthogonally protected diamines based on the nitrogen-directed diazidation of alkenes, cyclopropanes, and cyclobutanes. Commercially available copper thiophene-2-carboxylate (CuTc, 2 mol %) as catalyst promoted the diazidation of both π and σ C-C bonds within 10 min in the presence of readily available oxidants and trimethylsilyl azide. Selective substitution of the formed α-amino azide by carbon nucleophiles (electron-rich aromatic, malonate, organosilicon, organoboron, organozinc, and organomagnesium compounds) was then achieved in a one-pot fashion, leading to the formation of 1,2-, 1,3-, and 1,4-diamines with the amino groups protected orthogonally as an amide/carbamate and an azide.

Recent advances in ring-opening of donor acceptor cyclopropanes using C-nucleophiles

Ring-opening transformations of donor-acceptor cyclopropanes (DAC) with carbon-centered nucleophiles is a simple, straight-forward approach to 1,3-bifunctional compounds that has witnessed remarkable progress over the past several years. To date, different reactivity patterns of DACs have been successfully exploited in racemic/stereoselective syntheses of various acyclic compounds or carbocycles with an impressive structural diversity. The thriving strategies have been successfully utilized in multistep synthesis of complex target molecules. Herein, the recent advances (2015-present) in the ring-opening of DAC involving electron rich arenes and indoles, active methylene compounds, various dipolarophiles, organoborates/boronates, vinyl ethers etc. following Friedel-Crafts alkylation, annulation/formal cycloaddition reaction, organocatalytic reaction, Nazarov cyclisation etc. are presented.

Homologation of the Fischer Indolization: A Quinoline Synthesis via Homo-Diaza-Cope Rearrangement

We disclose a new Brønsted acid promoted quinoline synthesis, proceeding via homo-diaza-Cope rearrangement of N-aryl-N'-cyclopropyl hydrazines. Our strategy can be considered a homologation of Fischer's classical indole synthesis and delivers 6-membered N-heterocycles, including previously inaccessible pyridine derivatives. This approach can also be used as a pyridannulation methodology toward constructing polycyclic polyheteroaromatics. A computational analysis has been employed to probe plausible activation modes and to interrogate the role of the catalyst.

Ring Opening of Donor-Acceptor Cyclopropanes with Acyclic 1,3-Diketones for the Synthesis of 1,6-Dicarbonyl Compounds

1,6-Dicarbonyl compounds, representing the formal addition products of the α-position of acetophenone derivatives to donor-acceptor cyclopropanes, were synthesized in two steps via first ring opening of donor-acceptor cyclopropanes with acyclic 1,3-diketones followed by DBU catalyzed retro-Claisen-type C-C bond cleavage reactions. In the first step, acyclic 1,3-diketones selectively worked as C-nucleophiles to add to donor-acceptor cyclopropanes. In the second step, the alkyl ketone part of the ring-opening products resulting from unsymmetrical 1,3-diketones was selectively cleaved in the presence of DBU in methanol.

Assembly of polysubstituted chiral cyclopropylamines via highly enantioselective Cu-catalyzed three-component cyclopropene alkenylamination

Enabled by a commercial bisphosphine ligand, the Cu-catalyzed three-component cyclopropene alkenylamination with alkenyl organoboron reagent and hyroxyamine esters proceeds with exceptionally high enantioselectivity to deliver poly-substituted cis-1,2-alkenylcyclopropylamines that contain up to all three stereogenic centers on the cyclopropane.

Desymmetrization of 1-Symmetrical Donor-Acceptor (D-A) Cyclopropanes via Reactions with 1,3-Cyclodiones

A new type of 1-unsymmetrical D-A cyclopropanes containing a cyclic enone motif was obtained by the desymmetrization of 1-symmetrical D-A cyclopropanes via first the Lewis acid-catalyzed O-nucleophilic ring-opening reaction with 1,3-cyclodiones followed by an organobase-promoted unexpected multistep intramolecular transformation.

Synthesis and hetero-Diels–Alder reactions of enantiomerically pure dihydro-1H-azepines

Thermolysis of enantiomerically pure 3-substituted 7,7-dihalo-2-azabicyclo[4.1.0]heptanes in the presence of K₂CO₃gives in good yields 2-alkyl-6-halo-1-tosyl-2,3-dihydro-1H-azepines.

Iron catalysts with N-ligands for carbene transfer of diazo reagents

This review provides an overview of the catalytic activity of iron complexes of nitrogen ligands in driving carbene transfers towards CC, C–H and X–H bonds. The reactivity of diazo reagents is discussed as well as the proposed reaction mechanisms.

Ring-opening reactions of donor–acceptor cyclopropanes with cyclic ketals and thiol ketals

Cyclic (thiol) ketals were used as hetero-nucleophiles for the ring opening of donor–acceptor cyclopropanes to afford functionalized diethers.

Ring-Opening 1,3-Aminochalcogenation of Donor-Acceptor Cyclopropanes: A Three-Component Approach

A 1,3‐aminothiolation was realized by reacting 2‐substituted cyclopropane 1,1‐dicarboxylates with sulfonamides and N‐(arylthio)succinimides. Under Sn(OTf)₂ catalysis the transformation proceeded smoothly to the corresponding ring‐opened products bearing the sulfonamide in the 1‐position next to the donor and the arylthio residue in the 3‐position next to the acceptor. The procedure was extended to the corresponding selenium analogues by employing N‐(phenylseleno)succinimides as an electrophilic selenium source.

Diversity-Oriented Synthesis toward Fused and Bridged Benzobicyclic Piperidin(on)es

An efficient diverse synthesis of cis-fused indenopiperidines and bridged benzomorphanones, starting from cyclopropane-fused benzomorphanothiones and benzomorphanones, respectively, using NaBH₄/NiCl₂·6H₂O/EtOH as a reducing system, is described. High rigidity of substrates allowed axially controlled syntheses of their trans-mono-alkylated derivatives, subsequently enabling access to both trans-alkyl-functionalized benzobicyclic piperidin(on)es. Diversity-oriented synthesis of naphthalene ring-containing fused naphthoindenopiperidines and bridged naphthomorphanone directly from 2-pyridones was also performed.

The Degradation Chemistry of GSK2879552: Salt Selection and Microenvironmental pH Modulation to Stabilize a Cyclopropyl Amine

The cyclopropyl amine moiety in GSK2879552 (1) degrades hydrolytically in high pH conditions. This degradation pathway was observed during long-term stability studies and impacted the shelf life of the drug product. This article describes the work to identify the degradation impurities, elucidate the degradation mechanism, and design a stable drug product. It was found that salt selection and control of the microenvironmental pH of the drug product formulation blend significantly improved the chemical stability of the molecule in the solid state.

Site-Selective and Stereoselective C-H Functionalization of N-Cyclopropylamides via a Directed Remote Metalation Strategy

A new methodology for site-selective and stereoselective C-H functionalization of aminocyclopropanes via directed remote lithiation has been developed. Treatment of N-directing group (DG = pivaloyl, tetramethylsuccinimidoyl) arylcyclopropanes with t-BuLi results in a clean β-lithiation and, following quench with electrophiles, leads to a range of cyclopropane derivatives. Sequential double lithiation-methylation to give a dimethylated cyclopropane has been achieved. X-ray, NMR, and computational studies allow rationalization of syn-DG β-deprotonation selectivity via a DG-lithium base coordinated complex.

Providing a New Aniline Bioisostere through the Photochemical Production of 1-Aminonorbornanes

This report describes the photochemical conversion of aminocyclopropanes into 1-aminonorbornanes via formal [3+2] cycloadditions initiated by homolytic fragmentation of amine radical cation intermediates. Aligning with the modern movement toward sp ³ -rich motifs in drug discovery, this strategy provides access to a diverse array of substitution patterns on this saturated carbocyclic framework while offering the robust functional group tolerance (e.g. -OH, -NHBoc) necessary for further derivatization. Evaluating the metabolic stability of selected morpholine-based 1-aminonorbornanes demonstrated a low propensity for oxidative processing and no proclivity toward reactive metabolite formation, suggesting a potential bioisosteric role for 1-aminonorbornanes. Continuous flow processing allowed for efficient operation on gram-scale, providing promise for translation to industrially-relevant scales. This methodology only requires low loadings of a commercially-available, visible light-active photocatalyst and a simple salt, thus it stays true to sustainability goals while readily delivering saturated building blocks that can reduce metabolic susceptibility within drug development programs.

Detection of Fleeting Amine Radical Cations and Elucidation of Chain Processes in Visible-Light-Mediated [3 + 2] Annulation by Online Mass Spectrometric Techniques

Visible-light-mediated photoredox reactions have recently emerged as a powerful means for organic synthesis and thus have generated significant interest from the organic chemistry community. Although the mechanisms of these reactions have been probed by a number of techniques such as NMR, fluorescence quenching, and laser flash photolysis and various degrees of success has been achieved, mechanistic ambiguity still exists (for instance, the involvement of the chain mechanism is still under debate) because of the lack of structural information about the proposed and short-lived intermediates. Herein, we present the detection of transient amine radical cations involved in the intermolecular [3 + 2] annulation reaction of N-cyclopropylaniline (CPA, 1) and styrene 2 by electrospray ionization mass spectrometry (ESI-MS) in combination with online laser irradiation of the reaction mixture. In particular, the reactive CPA radical cation 1^+•, the reduced photocatalyst Ru(I)(bpz)₃⁺, and the [3 + 2] annulation product radical cation 3^+• are all successfully detected and confirmed by high-resolution MS. More importantly, the post-irradiation reaction with an additional substrate, isotope-labeled CPA, following photolysis of 1, 2, and Ru catalyst provides strong evidence to support the chain mechanism in the [3 + 2] annulation reaction. Furthermore, the key step of the proposed chain reaction, the oxidation of CPA 1 to amine radical cation 1^+• by product radical cation 3^+• (generated using online electrochemical oxidation of 3), is successfully established. Additionally, the coupling of ESI-MS with online laser irradiation has been successfully applied to probe the photostability of photocatalysts.

Iridium-catalysed hydrosilylation of cyclopropanes via regioselective carbon-carbon bond cleavage

While cyclopropanes have been explored as synthetically valuable building blocks, their transformation without conjugated substituents or directly substituted heteroatoms remains challenging. The current study describes the iridium-catalysed ring-opening hydrosilylation of cyclopropanes. A nitrogen-based directing group was found to control the reactivity of iridium active species as well as the regiochemistry of carbon-carbon bond cleavage and hydrosilylation.

Isolation of a Reactive Cyclopropane Intermediate via a Unique Catalyst-Free Spontaneous Cyclopropanation Step at 0 °C

A remarkably mild, catalyst-free cyclopropanation reaction is confirmed by low-temperature isolation of a fused bicyclic cyclopropane intermediate. This intermediate is one of two that were isolated and fully characterised in a three-step temperature-controllable reaction cascade.