Asymmetric 1,3-Dipolar Cycloaddition Reactions

Copper(i)-catalyzed highly enantioselective [3 + 3]-cycloaddition of γ-alkyl enoldiazoacetates with nitrones

Chiral copper(i) catalysts are preferred over chiral dirhodium(ii) catalysts for [3 + 3]-cycloaddition reactions of γ-alkyl-substituted enoldiazoacetates compounds with nitrones.

Stoichiometric imbalance-promoted step-growth polymerization based on self-accelerating 1,3-dipolar cycloaddition click reactions

A stoichiometric imbalance-promoted step growth polymerization method was developed based on self-accelerating 1,3-dipolar cycloaddition click reactions of Sondheimer diyne and varied 1,3-dipoles, such as diazo, sydnone, and nitrone groups.

A [3 + 2] cycloaddition/C-arylation of isatin N,N′-cyclic azomethine imine 1,3-dipole with arynes

A [3 + 2] annulation/C-arylation reaction of 1,3-dipole 1 with arynes has been established for the synthesis of oxindole scaffolds.

Copper-based catalysts derived from salen-type ligands: synthesis of 5-substituted-1H-tetrazoles via [3+2] cycloaddition and propargylamines via A3-coupling reactions

Base-metal copper(ii) complexes derived from unsymmetrical salen-type ligands were designed and synthesized using ligands L¹H to L⁴H. These complexes were employed as catalysts for [3+2] cycloaddition and A³-coupling reactions.

Catalyst-controlled formal [4 + 1] annulation of N-vinyl fluorenone nitrones and allenoates to prepare spirofluorenylpyrrolines

A simple gimeracil-controlled formal [4 + 1] annulation was developed to prepare various spirofluorenylpyrrolines in good yields with high diastereoselectivity from N-vinyl fluorenone nitrones and allenoates.

Insights into C–O insertion in a carbene/alkyne metathesis cascade reaction catalyzed by Rh2(OAc)4: a DFT study

The mechanism and selectivities of C–O insertion in the carbene/alkyne metathesis of diazo compounds catalyzed by Rh₂(OAc)₄ are studied with DFT calculations.

Recent developments in cyclopropene chemistry

As readily accessible strained carbocycles, cyclopropenes show a diverse range of reactivities, and a lot of novel and useful transformations have been developed.

Asymmetric 1,3-dipolar cycloaddition reaction of chiral 1-alkyl-1,2-diphospholes with diphenyldiazomethane

The 1,3-dipolar cycloaddition of chiral 1-alkyl-1,2-diphosphacyclopenta-2,4-dienes ((1-(−)-menthyl)oxymethyl-1,2-diphosphole and 1-(+)-neomenthyl-1,2-diphosphole) with diphenyldiazomethane leads to novel P-chiral bicyclic phosphiranes having six chiral centers. The degree of diastereoselectivity depends on the substituent at phosphorus, and dramatically increases in the case of (+)-neomenthyl group (de up to 71%). DFT calculations indicate that the cycloaddition is thermodynamically controlled.

The 1,3-dipolar cycloaddition of chiral 1-alkyl-1,2-diphosphacyclopenta-2,4-dienes with diphenyldiazomethane leads to novel P-chiral bicyclic phosphiranes having six chiral centers.

Ambient access to a new family of pyrrole-fused pyrazine nitrones via 2-carbonyl-N-allenylpyrroles

The chemo-, regio- and stereoselective synthesis of pyrrole-fused pyrazine nitrones via the direct reaction of 2-carbonyl-N-allenylpyrroles (readily accessible from the corresponding NH-pyrroles) with hydroxyl amine hydrochloride has been developed.

Sc3N@Ih-C80 based donor–acceptor conjugate: role of thiophene spacer in promoting ultrafast excited state charge separation

Photoinduced charge separation and dark charge recombination occurring within picoseconds is observed in newly synthesized triphenylamine–thiophene-Sc₃N@I_h-C₈₀ and triphenylamine–thiophene-C₆₀ conjugates.

Recent Developments on 1,3-Dipolar Cycloaddition Reactions by Catalysis in Green Solvents

The use of eco-compatible synthetic procedures in organic reactions and, in particular, in 1,3-dipolar cycloaddition reactions, has recently received a great deal of attention and considerable progress has been achieved in this area in the last years. This review summarizes the approaches currently employed to synthesize heterocyclic compounds by catalyzed 1,3-dipolar cycloadditions in green solvents in the last six years. Our choice to do a selection of the literature from 2014 to 2019 was made considering the absence of a recent review about this period, to our knowledge. Several examples to construct heterocycles by 1,3-dipolar cycloadditions will be discussed in this work subdivided in function of the most important class of non-conventional and green solvents, i.e., ionic liquids (ILs), deep eutectic solvents (DES), and water.

Regiodivergent Intramolecular Nucleophilic Addition of Ketimines for the Diverse Synthesis of Azacycles

Azacycles such as indoles and tetrahydroquinolines are privileged structures in drug development. Reported here is an unprecedented regiodivergent intramolecular nucleophilic addition reaction of imines as a flexible approach to access N-functionalized indoles and tetrahydroquinolines, by the control of reaction at the N-terminus and C-terminus, respectively. Using ketimines derived from 2-(2-nitroethyl)anilines with isatins or α-ketoesters, the regioselective N-attack reaction gives N-functionalized indoles, while the catalytic enantioselective C-attack reaction affords chiral tetrahydroquinolines featuring an α-tetrasubstituted stereocenter. Mechanistic studies reveal that hydrogen-bonding interactions may greatly facilitate such unusual N-attack reactions of imines. The utility of this protocol is highlighted by the catalytic enantioselective formal synthesis of (-)-psychotrimine, and the construction of various fused aza-heterocycles.

Chiral ScIII-N,N'-Dioxide-Catalyzed 1,3-Dipolar Cycloaddition of Diaziridines with Chalcones

A highly enantioselective 1,3-dipolar cycloaddition of meso-diaziridines with chalcones was realized by utilizing the Sc^III-N,N'-dioxide complex as the catalyst. In this transformation, the 1,3-dipole intermediates generated from the C-N bond cleavage of diaziridine were trapped by chiral N,N'-dioxide/scandium(III) complex activated chalcones to undergo enantioselective 1,3-dipolar cycloaddition. A range of chiral 1,5-diazabicylo[3.3.0]octane derivatives were readily synthesized in good yields with high diastereo- and enantioselectivities.

(3 + 1) Annulation/Rearrangement Cascade of C,N-Cyclic Azomethine Imines and 3-Chlorooxindoles: Construction of Hexahydroindeno[2,1-c]pyrazole Spirooxindole Frameworks

The Brønsted base promoted (3 + 1) annulation reaction of C,N-cyclic azomethine imines and 3-chlorooxindoles was investigated, finally delivering complex hexahydroindeno[2,1-c] pyrazoles incorporating a spirocyclic oxindole motif after an unexpected rearrangement process. The asymmetric version of this new transformation could be accomplished, but slow racemization of the chiral product was observed at ambient temperature. Experiments and DFT calculations were further conducted to elucidate the reaction process and racemization mechanism.

Silver-mediated three-component cycloaddition reaction for direct synthesis of 1-N-vinyl-substituted 1,2,3-triazoles

Herein, we report direct synthesis of 1-N-vinyl-1,2,3-triazoles via silver-mediated three-component cycloaddition reaction of phenylacetylenes, trimethylsilylazide, and 1,3-dicarbonyl compounds. The synthetic protocol proceeds with operational simplicity, good substrate and functional group compatibility, and easily available feedstocks, and without the need for pre-installation of vinylazide precursors, and offers a practical method for the efficient elaboration of triazole derivatives.

Gold-Catalyzed Aromatizations of 3-Ene-5-siloxy-1,6-diynes with Nitrosoarenes To Enable 1,4-N,O-Functionalizations: One-Pot Construction of 4-Hydroxy-3-aminobenzaldehyde Cores

This work describes gold-catalyzed aromatizations of 3-ene-5-siloxy-1,6-diynes with nitrosoarenes to form 4-hydroxy-3-aminobenzaldehyde derivatives, manifesting the use of nitrosoarenes as 1,4-N,O-functionalization sources. Various 3-ene-5-siloxy-1,6-diynes in benzoid and nonbenzoid types are applicable substrates. A series of ¹⁸O- and ²H-labeling experiments have been conducted to exclude gold-π-alkyne intermediates. We postulate a mechanism of dual gold catalysis involving initial formation of gold-π-alkynylgold species that activates a 1,5-hydrogen shift to form reactive 1,6-dipoles, thus furnishing intramolecular Michael-type reactions with nitrosonium electrophiles.

The crystal structures and Hirshfeld surface analysis of 6-(naphthalen-1-yl)-6a-nitro-6,6a,6b,7,9,11a-hexa-hydro-spiro-[chromeno[3',4':3,4]pyrrolo-[1,2-c]thia-zole-11,11'-indeno-[1,2-b]quinoxaline] and 6'-(naphthalen-1-yl)-6a'-nitro-6',6a',6b',7',8',9',10',12a'-octa-hydro-2H-spiro-[ace-naphthyl-ene-1,12'-chromeno[3,4-a]indolizin]-2-one

The title compounds, 6-(naphthalen-1-yl)-6a-nitro-6,6a,6 b,7,9,11a-hexa-hydro-spiro-[chromeno[3',4':3,4]pyrrolo-[1,2-c]thia-zole-11,11'-indeno-[1,2-b]quinoxaline], C37H26N4O3S, (I), and 6'-(naphthalen-1-yl)-6a'-nitro-6',6a',6b',7',8',9',10',12a'-octa-hydro-2H-spiro-[ace-naphthyl-ene-1,12'-chromeno[3,4-a]indolizin]-2-one, C36H28N2O4, (II), are new spiro derivatives, in which both the pyrrolidine rings adopt twisted conformations. In (I), the five-membered thia-zole ring adopts an envelope conformation, while the eight-membered pyrrolidine-thia-zole ring adopts a boat conformation. An intra-molecular C-H⋯N hydrogen bond occurs, involving a C atom of the pyran ring and an N atom of the pyrazine ring. In (II), the six-membered piperidine ring adopts a chair conformation. An intra-molecular C-H⋯O hydrogen bond occurs, involving a C atom of the pyrrolidine ring and the keto O atom. For both compounds, the crystal structure is stabilized by inter-molecular C-H⋯O hydrogen bonds. In (I), the C-H⋯O hydrogen bonds link adjacent mol-ecules, forming R 2 2(16) loops propagating along the b-axis direction, while in (II) they form zigzag chains along the b-axis direction. In both compounds, C-H⋯π inter-actions help to consolidate the structure, but no significant π-π inter-actions with centroid-centroid distances of less than 4 Å are observed.

Synthesis of 7'-Arylidenespiro[indoline-3,1'-pyrrolizines] and 7'-Arylidenespiro[indene-2,1'-pyrrolizines] via [3 + 2] Cycloaddition and β-C-H Functionalized Pyrrolidine

The acetic acid catalyzed three-component reaction of pyrrolidine, aromatic aldehydes, and 3-arylideneoxindolin-2-ones in refluxing toluene afforded functionalized 7'-arylidenespiro[indoline-3,1'-pyrrolizines] in good yields and with high diastereoselectivity. The similar three-component reaction with 2-arylidene-1,3-indanediones also gave 7'-arylidenespiro[indene-2,1'-pyrrolizines] in good yields. However, the reaction with 3-phenacylideneoxindoles resulted in a mixture of spiro[indoline-3,1'-pyrrolizines] and 7'-arylidene-substituted spirooxindoles in moderate yields. The reaction mechanism included generation of azomethine ylides, β-C-H functionalization of pyrrolidine, and sequential [3 + 2] cycloaddition. The obtained spirooxindole derivatives were investigated by in vitro evaluation against mouse colon cancer cells CT26 and human liver cancer cells HepG2 by MTT assay.

DABCO-mediated [3+3] cycloaddition of azomethine imines with in situ generated nitrile oxides from hydroximoyl chlorides

A novel cross 1,3-dipolar cycloaddition between azomethine imines with in situ generated nitrile oxides has been developed. This is the first example of employing a reaction partner containing two heteroatoms in the [3+3] cycloaddition involving azomethine imines. This strategy not only provides structurally diverse N,O-heterocycles but also greatly enriches the chemistry of azomethine imines and nitrile oxides.

A molecular electron density theory study on the [3+2] cycloaddition reaction of 5,5-dimethyl-1-pyrroline N-oxide with 2-cyclopentenone

In the present work, the [3 + 2] cycloaddition reaction of 5,5-dimethyl-1-pyrroline N-oxide (Nit-5) and 2-cyclopentenone (CPN-6), experimentally reported by Tamura et al., was theoretically studied using the newly introduced molecular electron density theory (MEDT). Based on the experimental findings, this reaction takes place in an O3-C4 regio- and an exo-stereospecific fashion to give corresponding [3 + 2] exo cycloadduct as the sole product. The results of the potential energy surface analysis indicated that the experimentally reported product is more favorable both thermodynamically and kinetically relative to other possible adducts. In complete agreement with the experimental outcomes, the conceptual density functional theory reactivity indices explained the reactivity and regioselectivity of the reaction. Calculation of global electron density transfer of the energetically most preferred transition state indicated that the electron density fluxes from Nit-5 as a nucleophilic species toward CPN-6 as an electrophilic species. Analysis of the molecular electrostatic potential map of the most favorable transition state showed that approach of Nit-5 and CPN-6 locates the oppositely charged regions over each other leading to attractive forces between two reagents rationalizing the exo stereoselectivity predominance. The molecular mechanism of the reactions was specified using electron localization function analysis over some relevant points along the intrinsic reaction coordinate profile of the most favorable transition state and the results indicated that this zwitterionic-type [3 + 2] cycloaddition reaction proceeds through a two-stage one-step mechanism. In fact, while the O3-C4 single bond is initialy formed between two fragments through donation of some electron density from the O3 oxygen lone electron-pairs of Nit-5 toward the C4 carbon atom of CPN-6, the delayed C1-C5 single bond begins to form via C1- to -C5 coupling of pseudodiracal centers created on theses atoms over the course of reaction.

A Rh(III)-Catalyzed Formal [4+1] Approach to Pyrrolidines from Unactivated Terminal Alkenes and Nitrene Sources

We have developed a formal [4+1] approach to pyrrolidines from readily available unactivated terminal alkenes as 4-carbon partners. The reaction provides a rapid construction of various pyrrolidine containing structures, especially for the diastereoselective synthesis of spiro-pyrrolidines. Mechanistic investigation suggests a Rh(III)-catalyzed intermolecular aziridination of the alkene and subsequent acid-promoted ring expansion for the pyrrolidine formation.

[3 + 2]-Cycloaddition of Azaoxyallyl Cations with 1,2-Benzisoxazoles: A Rapid Entry to Oxazolines

A novel and efficient [3 + 2] cycloaddition reaction of azaoxyallyl cations and 1,2-benzisoxazoles to give oxazoline derivatives has been developed. The transformation provides a rapid entry to functionalized oxazoline scaffolds under mild and transition-metal-free conditions, which will greatly expand the reaction types of heterocycle chemistry and pave the way for syntheses of bioactive compounds.

Elucidating the origin of selectivity of [3 + 2]-cycloaddition reactions between thioketone and carbohydrate-derived nitrones by the DFT

The mechanism and origin of selectivity for [3 + 2]-cycloaddition (32CA) reactions between thioketone and carbohydrate-derived nitrones in THF were investigated by using the density functional theory (DFT) at the M06-2X/6-311+G(d,p)//M06-2X/6-31+G(d,p) level of theory combined with the solvation SMD model. The calculated results revealed that the 32CA reactions proceed through the asynchronous one-step manner. For the chemoselectivity in thioketone, the C=S bond as a dipolarophile attacking three-atom-component (TAC) nitrone in reactivity was more preferential than the C=O bond. The theoretical results also confirmed the stereoselectivity of two 32CA reactions of thioketone with carbohydrate-derived nitrones with the anti-form product being more favored than the syn-form product, and the predicted anti/syn product ratios are in agreement with the experimental ones in literature. Furthermore, the analysis of the conceptual density functional theory reactivity indices showed that the 32CA reactions have polar character. Weak noncovalent interaction and Parr function analyses are used to reveal the origin of the stereoselectivity. Graphical abstract [3 + 2]-cycloaddition reactions between thioketone and carbohydrate-derived nitrones.