Progress in 1,3-dipolar cycloadditions in the recent decade: an update to strategic development towards the arsenal of organic synthesis

(R)-DM-SEGPHOS-Ag(I)-Catalyzed Enantioselective Synthesis of Pyrrolidines and Pyrrolizidines via (1,3)- and Double (1,3)-Dipolar Cycloaddition Reactions

An efficient diastereo- and enantioselective route to access a wide range of highly substituted pyrrolidine and pyrrolizidine derivatives has been described via (1,3)- and double (1,3)-dipolar cycloaddition reactions catalyzed by the (R)-DM-SEGPHOS-Ag(I) complex. The reactions proceed smoothly at ambient temperature, affording a variety of pyrrolidines and pyrrolizidines in high yields (up to 93%) with up to 99:1 dr and excellent enantioselectivities (up to 98% ee) without any additives. The newly synthesized pyrrolidine and pyrrolizidine derivatives contain four and seven contiguous stereogenic centers, respectively. Moreover, the synthetic utility of enantioenriched products has been demonstrated by transforming them into various synthetically useful advanced intermediates.

A Unique (3+2) Annulation Reaction between Meldrum's Acid and Nitrones: Mechanistic Insight by ESI-IMS-MS and DFT Studies

The fragile intermediates of the domino process leading to an isoxazolidin-5-one, triggered by unique reactivity between Meldrum's acid and an N-benzyl nitrone in the presence of a Brønsted base, were determined thanks to the softness and accuracy of electrospray ionization mass spectrometry coupled to ion mobility spectrometry (ESI-IMS-MS). The combined DFT study shed light on the overall organocatalytic sequence that starts with a stepwise (3+2) annulation reaction that is followed by a decarboxylative protonation sequence encompassing a stereoselective pathway issue.

Switchable Synthesis of 3-Substituted 1H-Indazoles and 3,3-Disubstituted 3H-Indazole-3-phosphonates Tuned by Phosphoryl Groups

3-Alkyl/aryl-1H-indazoles and 3-alkyl/aryl-3H-indazole-3-phosphonates were synthesized efficiently through a 1,3-dipolar cycloaddition reaction between α-substituted α-diazomethylphosphonates and arynes under simple reaction conditions. The product distribution was controlled by the phosphoryl group, which acted both as a tuning group and a traceless group in the reaction.

Cycloaddition/annulation strategies for the construction of multisubstituted pyrrolidines and their applications in natural product synthesis

Pyrrolidines are privileged substructures of numerous bioactive natural products and drugs.

Diversity-oriented synthesis of chromenopyrrolidines from azomethine ylides and 2-hydroxybenzylidene indandiones via base-controlled regiodivergent (3+2) cycloaddition

An organobase-controlled, regiodivergent (3+2) cycloaddition resulting in two different cascade reactions is demonstrated from azomethine ylides and 2-hydroxybenzylidineindandiones to achieve diverse chromanopyrrolidines.

Metal-catalyzed 1,3-dipolar cycloaddition reactions of nitrile oxides

In this review recent advances in the metal-catalyzed 1,3-dipolar cycloaddition reactions of nitrile oxides are highlighted, covering references from the period 2000 until August 2018.

Heterogeneous graphene oxide as recyclable catalyst for azomethine ylide mediated 1,3 dipolar cycloaddition reaction in aqueous medium

Graphene oxide (GO) catalyzed regio and diastereoselective synthesis of spiro-indenoquinoxaline pyrrolizidines and spiro-oxindoles pyrrolizidines is described with good substrate scope and yield using azomethine ylide under aq. EtOH condition at RT.

A Diastereoselective Synthesis of Dispiro[oxindole-cyclohexanone]pyrrolidines by 1,3-Dipolar Cycloaddition

For the first time, arylmethylidene cyclohexanones that are non-symmetrical due to the presence of peripheral substituents were studied in 1,3-dipolar cycloaddition reactions. It is shown that the interaction with the azomethine ylide generated from sarcosine proceeds regio- and diastereoselectively, with the participation of two non-equivalent parts of the dipolarophile. Also for the first time, β-amino ketones (Mannich bases) were used as dipolarophile equivalents of unsaturated ketones. It was found that cycloaddition occurs diastereoselectively at the generated center.

Synthesis of spiro[indoline-3,3'-pyrrolizines] by 1,3-dipolar reactions between isatins, L-proline and electron-deficient alkenes

Two spiro[indoline-3,3'-pyrrolizine] derivatives have been synthesized in good yield with high regio- and stereospecificity using one-pot reactions between readily available starting materials, namely L-proline, substituted 1H-indole-2,3-diones and electron-deficient alkenes. The products have been fully characterized by elemental analysis, IR and NMR spectroscopy, mass spectrometry and crystal structure analysis. In (1'RS,2'RS,3SR,7a'SR)-2'-benzoyl-1-hexyl-2-oxo-1',2',5',6',7',7a'-hexahydrospiro[indoline-3,3'-pyrrolizine]-1'-carboxylic acid, C₂₈H₃₂N₂O₄, (I), the unsubstituted pyrrole ring and the reduced spiro-fused pyrrole ring adopt half-chair and envelope conformations, respectively, while in (1'RS,2'RS,3SR,7a'SR)-1',2'-bis(4-chlorobenzoyl)-5,7-dichloro-2-oxo-1',2',5',6',7',7a'-hexahydrospiro[indoline-3,3'-pyrrolizine], which crystallizes as a partial dichloromethane solvate, C₂₈H₂₀Cl₄N₂O₃·0.981CH₂Cl₂, (II), where the solvent component is disordered over three sets of atomic sites, these two rings adopt envelope and half-chair conformations, respectively. Molecules of (I) are linked by an O-H...·O hydrogen bond to form cyclic R₆⁶(48) hexamers of \overline{3} (S₆) symmetry, which are further linked by two C-H...O hydrogen bonds to form a three-dimensional framework structure. In compound (II), inversion-related pairs of N-H...O hydrogen bonds link the spiro[indoline-3,3'-pyrrolizine] molecules into simple R₂²(8) dimers.

Diastereoselective [3 + 2] vs [4 + 2] Cycloadditions of Nitroprolinates with α,β-Unsaturated Aldehydes and Electrophilic Alkenes: An Example of Total Periselectivity

Diastereoselective multicomponent reactions of enantioenriched 4-nitroprolinates, obtained by enantiocatalyzed 1,3-dipolar cycloaddition (1,3-DC) of imino esters and nitroalkenes, with α,β-unsaturated aldehydes and electrophilic alkenes proceed with total periselectivity depending on the structure of the aldehyde employed. This process evolves through a [3 + 2] 1,3-DC when cinnamaldehyde is used in the presence of an azomethine ylide, giving the corresponding highly substituted pyrrolizidines with endo selectivity. However, in the case of the α,β-unsaturated aldehyde, which contains a hydrogen atom at the γ position, an amine-aldehyde-dienophile (AAD) [4 + 2] cycloaddition takes place by formation of an intermediate 1-amino-1,3-diene, affording highly functionalized cyclohexenes with high endo diastereoselectivity. This AAD process only occurred when a nitro group is bonded to the 4-position of the initial enantiomerically enriched pyrrolidine ring. DFT calculations have been carried out with the aim of explaining this different behavior between pyrrolidines with or without a nitro group, demonstrating the strong nitro-group-dependent periselectivity. The results of these computational studies also support the experimentally obtained absolute configuration of the final adducts.

Radical Nitration-Debromination of α-Bromo-α-fluoroalkenes as a Stereoselective Route to Aromatic α-Fluoronitroalkenes-Functionalized Fluorinated Building Blocks for Organic Synthesis

A new highly efficient method for the synthesis of 2-fluoro-2-nitrostyrenes was described. Radical nitration of readily available 2-bromo-2-fluorostyrenes with Fe(NO₃)₃·9H₂O resulted in the formation of the corresponding α-fluoro-nitroalkenes in isolated yields up to 92%. The reaction proceeded as a nitration-debromination sequence to highly stereoselectively give α-fluoro-nitroalkenes as Z-isomers only. The broad scope of this method was demonstrated. Prepared monofluorinated alkenes were shown to be versatile building blocks for the synthesis of various fluorinated products.

Synthesis of Pyrrole via a Silver-Catalyzed 1,3-Dipolar Cycloaddition/Oxidative Dehydrogenative Aromatization Tandem Reaction

Pyrroles are an important group of heterocyclic compounds with a wide range of interesting properties, which have resulted in numerous applications in a variety of fields. Despite the importance of these compounds, there have been few reports in the literature pertaining to the synthesis of pyrroles from simple alkenes using a one-pot sequential 1,3-dipolar cycloaddition/aromatization reaction sequence. Herein, we report the development of a benzoyl peroxide-mediated oxidative dehydrogenative aromatization reaction for the construction of pyrroles. We subsequently developed a one-pot tandem reaction that combined this new method with a well-defined silver-catalyzed 1,3-dipolar cycloaddition reaction, thereby providing a practical method for the synthesis of multisubstituted pyrroles from easy available alkenes. The mechanism of this oxidative dehydrogenative aromatization reaction was also examined in detail.

On the outside looking in: rethinking the molecular mechanism of 1,3-dipolar cycloadditions from the perspective of bonding evolution theory. The reaction between cyclic nitrones and ethyl acrylate

Understanding the molecular mechanism of 1,3-dipolar cycloadditions using the bonding evolution theory.

First example of an organocatalytic asymmetric Mannich reaction between aldimines of glycinates and sulphonyl imines

The catalytic enantioselective Mannich-type reaction between glycinate Schiff base and imines has been one of the most efficient routes for accessing α,β-diamino acids. However, the glycinate Schiff bases used in the references were almost ketimines. Only several examples of aldimines were used in the presence of metal catalyst. We developed the first example of an asymmetric direct Mannich reaction using aldimines of glycinates instead of ketimines of glycinates. The reaction was well catalyzed by chiral guanidine with high yield (up to 92%) and moderate stereoselectivity (up to 65%).

Efficient Synthesis of Fully Substituted Pyrrolidine-Fused 3-Spirooxindoles via 1,3-Dipolar Cycloaddition of Aziridine and 3-Ylideneoxindole

Drug-like spirocyclic scaffolds have been prepared by fusing fully functionalized pyrrolidine with oxindoles in an approach based on 1,3-dipolar cycloaddition. Reaction between aziridine and 3-ylideneoxindole generated diverse spirooxindole-pyrrolidines in good yield (up to 95%) with high diastereoselectivity (up to >20:1). The reaction also proceeded smoothly with several other synthetically useful activated trisubstituted olefins. The mild reaction conditions, short reaction times, and high tolerance for various substitutions make this approach attractive for constructing pharmacologically interesting spiro-architectures.

One-Pot Reductive 1,3-Dipolar Cycloaddition of Secondary Amides: A Two-Step Transformation of Primary Amides

The one-pot reductive 1,3-dipolar cycloaddition of secondary aromatic N-(trimethylsilylmethyl)amides with reactive dipolarophiles is reported. The method relies on the in situ generation of nonstabilized NH azomethine ylide dipoles via amide activation with triflic anhydride, partial reduction with 1,1,3,3-tetramethyldisiloxane (TMDS), and desilylation with cesium fluoride (CsF). Running under mild conditions, the reaction tolerated several sensitive functional groups and provided cycloadducts in 71-93% yields. The use of less reactive dipolarophile methyl acrylate led to the cycloadduct in only 40% yield. A (Z) geometric intermediate of NH-azomethine 1,3-dipole was postulated to account for the observed higher yields and higher cis diastereoselectivity for the substrates bearing an electron-withdrawing group. This model features an unconventional cyclic transition state via carbanion-aryl ring interaction. Because the starting secondary amides can be prepared from common primary amides, the current method also constitutes a two-step transformation of primary amides.

Synthesis of Enantiomeric Polyhydroxyalkylpyrrolidines from 1,3-Dipolar Cycloadducts. Evaluation as Inhibitors of a β-Galactofuranosidase

Enantiomeric 2,3,4-tris(hydroxyalkyl)-5-phenylpyrrolidines have been synthesized from the major cycloadducts obtained by the 1,3-dipolar cycloaddition of sugar enones with azomethine ylides derived from natural amino acids. Reduction of the ketone carbonyl group of the cycloadducts, which possess a basic structure of bicyclic 6-(menthyloxy)hexahydropyrano[4,3-c]pyrrol-7(6H)one, afforded a number of pyrrolidine-based bicyclic systems. A sequence of reactions, which involved hydrolysis of the menthyloxy substituent, reduction, N-protection, and degradative oxidation, afforded varied pyrrolidine structures having diverse configurations and patterns of substitution; in particular, polyhydroxylated derivatives have been obtained. The unprotected products were isolated as pyrrolidinium trifluoroacetates. Because of the furanose-like nature of the target trihydroxyalkyl pyrrolidines, these molecules have been evaluated as inhibitors of the β-galactofuranosidase from Penicillium fellutanum. The compounds showed practically no inhibitory activity for concentration of pyrrolidines in the range of 0.1-1.6 mM.

Diastereoselective synthesis of novel tetra-and pentacyclic annulated coumarino-δ-sultam pyrrolidine, pyrrolizidine, pyrrolothiazole and isoxazolidine derivatives via intramolecular 1,3-dipolar cycloadditions

A facile and efficient strategy towards the novel tetra- and pentacyclic annulated coumarino-δ-sultam pyrrolidine, pyrrolizidine, pyrrolothiazole and isoxazolidine via intramolecular 1,3-dipolar cycloadditions of is described.

Ag-catalyzed diastereoselective [6 + 3] cycloaddition of tropone with homoserine lactone-derived azomethine ylides: synthesis of tricyclic spiropiperidines

Ag-catalyzed [6 + 3] cycloaddition of tropone with homoserine lactone derived azomethine ylides has been developed.