Search

An experimental and computational study (including DFT calculations and distortion/interaction analysis) was conducted to assess the effect of the nitrone structure in the outcome of dipolar 1,3‐cycloadditions with levoglucosenone, a biomass derived chiral enone. While B3LYP/6‐31G* (the most popular method for modeling these reactions according to our literature search) provides qualitatively good results, large outliers were found for some systems versus experimental data. An exhaustive exploration of other levels allowed us to determine the most appropriate ones to predict simultaneously reactivity and selectivity. The systematically predicted high exo selectivity by the majority of the levels led us to reconsider our initial assignment for the reaction with the nitrone derived from piperidine, which resulted in the discovery of an interesting case of conformational dynamics. [ABSTRACT FROM AUTHOR]

A class of novel chiral‐bridged biphenyl monophosphine ligands with a benzofuran moiety (He‐phos) has been successfully synthesized and reported. These ligands exhibited good performance in the gold‐catalyzed enantioselective cycloaddition reaction of 2‐(1‐alkynyl)‐2‐en‐1‐ones with nitrones, resulting in the successful preparation of a series of polyaryl‐substituted heterocyclic compounds with high enantioselectivities (up to 99% yield, 98% ee). The wide substrate adaptability (53 examples) and mild reaction conditions demonstrate the practicability of He‐phos in this reaction. [ABSTRACT FROM AUTHOR]

An asymmetric 1,3‐dipolar cycloaddition of β‐fluoroalkylated α,β‐unsaturated 2‐acyl imidazole with nitrones or N,N'‐cyclic azomethine imines to construct chiral α‐fluoroalkylated γ‐amino alcohols or chiral α‐fluoroalkylated 1,3‐diamino compounds has catalyzed by chiral‐at‐metal rhodium complexes is reported. The corresponding products with three contiguous tertiary stereocenters were obtained in 62%–98% yields with stereoselectivities up to >20:1 dr and 99% ee. Remarkably, as low as 0.06 mol% of chiral‐at‐metal rhodium complex can promote a gram‐scale reaction with nitrones or N,N'‐cyclic azomethine imines. [ABSTRACT FROM AUTHOR]

Owing to their distinctive 1,3‐dipolar structure, the catalytic asymmetric hydrogenation of nitrones to hydroxylamines has been a formidable and longstanding challenge, characterized by intricate enantiocontrol and susceptibility to N−O bond cleavage. In this study, the asymmetric hydrogenation and transfer hydrogenation of nitrones were accomplished with a tethered TsDPEN‐derived cyclopentadienyl rhodium(III) catalyst (TsDPEN: p‐toluenesulfonyl‐1,2‐diphenylethylene‐1,2‐diamine), the reaction proceeds via a novel 7‐membered cyclic transition state, producing chiral hydroxylamines with up to 99 % yield and >99 % ee. The practical viability of this methodology was underscored by gram‐scale catalytic reactions and subsequent transformations. Furthermore, mechanistic investigations and DFT calculations were also conducted to elucidate the origin of enantioselectivity. [ABSTRACT FROM AUTHOR]

Herein, we have synthesized multifunctionalized 2‐oxa‐3‐azabicyclo[3.1.1]heptanes, which are considered potential bioisosteres for meta‐substituted arenes, through Eu(OTf)3‐catalyzed formal dipolar [4π+2σ] cycloaddition of bicyclo[1.1.0]butanes with nitrones. This methodology represents the initial instance of fabricating bicyclo[3.1.1]heptanes adorned with multiple heteroatoms. The protocol exhibits both mild reaction conditions and a good tolerance for various functional groups. Computational density functional theory calculations support that the reaction mechanism likely involves a nucleophilic addition of nitrones to bicyclo[1.1.0]butanes, succeeded by an intramolecular cyclization. The synthetic utility of this novel protocol has been demonstrated in the concise synthesis of the analogue of Rupatadine. [ABSTRACT FROM AUTHOR]

A Sc(OTf)3‐catalyzed tandem (3 + 3)‐annulation/lactonization of donor‐acceptor cyclopropane 1,1‐dieters with salicylaldehyde nitrones has been reported, affording a wide range of tetrahydrochromeno[4,3‐c][1,2]oxazin‐5(1H)‐ones in moderate yields with excellent diastereoselectivities. Besides, the gram‐scale reaction has also been explored to demonstrate the utility of this protocol. Moreover, the mechanism was investigated through control experiments, which show the reaction proceeds via a tandem (3 + 3)‐annulation/lactonization reaction. [ABSTRACT FROM AUTHOR]

N‐Selective carbamoylation reaction of oximes with isocyanates generates nitrones, which undergo 1,3‐dipolar cycloaddition with various dipolarophiles to afford diverse isoxazolidines. Notably, combinations of highly electron‐rich oxime and highly electron‐deficient dipolarophile exhibited high reactivity, with product yields of up to 94 %. The substituent on the isoxazolidine‐nitrogen atom could be successfully removed without loss of the cyclic structure. Computational studies have also elucidated the mechanism of the reaction and origin of stereoselectivity. [ABSTRACT FROM AUTHOR]

The influence of ethylene substitution and the tether length between the two reacting counterparts on the selectivity and reactivity of the intramolecular [3 + 2] cycloaddition (IM32CA) reactions of cyclic nitrones leading to tricyclic isoxazolidines have been studied within the Molecular Electron Density theory at the MPWB1K/6‐311G(d,p) computational level. These zw‐type IM32CA reactions follow one‐step mechanism, and the activation barrier decreases with the introduction of electron withdrawing (EW) substituent at the alkene moiety in both the intramolecular and intermolecular versions. The IM32CA reactions involving unsubstituted alkene have non‐polar character with minimal electron density flux classified as null electron density flux type, while that involving the EW nitro substituted ethylene is more facile with a strong electron density flux from the nitrone to the ethylene moiety, classified as forward electron density flux type. The increase in the polar character of the IM32CA reaction decreases the activation Gibbs free energies associated with these intramolecular processes, while the highly polar IM32CA reactions are disfavored with respect to the intermolecular ones. Interestingly, the preferred regioselectivity observed in low polar IM32CA reactions having three methylene units between the nitrone and ethylene frameworks is reversed to that in nitrones separated with four methylene units, in conformity with the experimental outcome. Finally, electron localization function and quantum theory of atoms‐in‐molecules studies reveal that, in general, these IM32CA reactions involve early transition state structures in which the formation of new C‐C and C‐O single bonds have not yet started. [ABSTRACT FROM AUTHOR]

Complex spirocyclic isoindolines and pyrrolidines are formed in a tandem process involving Co(III)‐catalyzed dienylation of cyclic C‐aryl nitrones with 2,3‐butadien‐1‐ol carbonates, followed by intramolecular 1,3‐dipolar cycloaddition. The nitrone moiety serves both as a directing group for the C(aryl)−H activation and the dipole in the cycloaddition step. High regioselectivity of the fused vs. bridged product can be obtained by adjusting the reaction temperature. Reactions with substituted allenic substrates provide heterocycles with additional stereocenters usually with complete diastereoselectivity. The reaction products were readily transformed further into other complex nitrogen‐containing spirocyclic systems. [ABSTRACT FROM AUTHOR]

δ,ϵ‐Unsaturated oximes, on heating with O‐alkyl S‐(pyridin‐2‐yl)carbonothioates (PySCO2R) in refluxing toluene, undergo intramolecular cycloaddition to give N‐alkoxycarbonylated multi‐cyclic compounds. Mechanistic studies indicate that the reaction involves direct generation of N‐alkoxycarbonyl nitrones followed by cycloaddition, instead of intramolecular oxime‐olefin cycloaddition (IOOC) followed by alkoxycarbonylation. DFT calculation indicates that N‐alkoxycarbonylation of oxime with PySCO2R is a concerted reaction. The reaction of oxime with a chiral alkoxycarbonylation reagent exhibits a reasonably high diastereo‐face selectivity. [ABSTRACT FROM AUTHOR]

We herein disclose a simple and general method for the synthesis of chromendiones through Rh(III)‐catalyzed C−H functionalization/annulation reactions of nitrones with cyclic 2‐diazo‐1,3‐diones by using nitrone group as a traceless directing group. This method exhibits high efficiency and broad functional group compatibility. The derivatization of these chromendiones has been also demonstrated to further strengthen the synthetic utility of the transformation. [ABSTRACT FROM AUTHOR]

By resorting to the principle of remote activation, we herein demonstrate the first photoredox catalyzed (3+3) dipolar cycloaddition of nitrones with aryl cyclopropanes. Key to the fidelity of the reaction resides in a facile manner of substrate activation by single‐electron transfer (SET) oxidation with photoredox catalysis, and the reaction takes place through a stepwise cascade encompassing a three‐electron‐type nucleophilic substitution triggered cyclopropane ring‐opening and a diastereoselective 6‐endo‐trig radical cyclization manifold. The reaction proceeds under mild conditions with excellent regio‐ and stereoselectivity, nicely complementing the well‐developed Lewis acid catalyzed cycloaddition of donor‐acceptor cyclopropanes. Other merits of the protocol include wide scope of aryl cyclopropanes with diversified substitution patterns and good functional‐group compatibility. A mechanism involving an aryl radical cation promoted remote activation mode was also proposed and supported by mechanistic experiments. [ABSTRACT FROM AUTHOR]

Gold‐catalyzed cascade reactions between 1,5‐diyn‐3‐ols and nitrones to deliver carbazole derivatives are described. Such cascade reactions are applicable to facile synthesis of polyaromatic compounds containing carbazole subunits. Notably, the reaction mechanism involves unexpected oxoarylations, rather than oxidative Mannich reactions as known for but‐1‐yn‐4‐ols. Our control experiments indicate that the presence of a second alkyne as in 1,5‐diyn‐3‐ols enables such oxoarylations due to a weak bonding between gold and this alkyne, rending the tethered alcohol less conformationally flexible. [ABSTRACT FROM AUTHOR]

A variety of functionalized 2‐pyrrolidinones bearing N‐trifluoromethylphenyl moieties were prepared in moderate to good yields with high diastereoselectivity through copper(II)‐catalyst and base co‐catalyzed formal [4+1] cycloaddition of cinnamaldehyde nitrones with 1‐ethynylnaphthalen‐2‐ols in the presence of m‐CPBA as the oxidant. The triple bond of 1‐ethynylnaphthalen‐2‐ol served as one‐carbon synthon for the pyrrolidinone ring formation. Mechanistic studies revealed that m‐CPBA played an important role to interrupt the intramolecular O‐cyclization to afford the 2‐pyrrolidinones. Moreover, the reaction could be easily performed at gram scales and the obtained products were selectively converted into diverse pyrroline scaffolds. [ABSTRACT FROM AUTHOR]

The electro‐reductive coupling of nitro and carbonyl compounds enables a facile, environmentally friendly and energy benign transformation toward value‐added nitrones or imines, but the selectivity is still challenging. Here, the surface roughness of Cu electrodes is introduced for the first time as the determinant to switch products from nitrones to imines owing to the controllable reduction of nitroarenes to hydroxylamines or amines on tailored CuI/Cu0 interfaces. The roughness‐dependent selectivity, that is the decrease of nitrones and the increase of imines with enhanced roughness, is visible in the electro‐reductive coupling of nitrobenzene and furfural. Thus, the high selectivity of nitrone (98 %) and imine (80 %) can be achieved on a surface smooth Cu foil and the one electrochemically roughened in the presence of I−, respectively. Such roughness‐dependence of nitrone/imine selectivity on Cu electrodes is further verified in a wide substrate scope, highlighting the promise of surface/interfacial engineering for electrochemical synthesis. [ABSTRACT FROM AUTHOR]

The roles of substituent and solvent effects in promoting the 4π electrocyclization of N‐alkenylnitrones to give azetidine nitrones have been investigated by experimental examination of relative rates, activation energies, and linear free energy relationships. These transformations are synthetically important because they favor the formation of a strained heterocyclic ring with imbedded functionality and stereochemical information for versatile derivatization. Mechanistic investigations, including Hammett studies, solvent‐dependent Eyring studies, and solvent isotope effects, provide insight into the steric and electronic factors that control these electrocyclizations and identify trends that can be used to advance this approach towards the rapid synthesis of complex azetidines. [ABSTRACT FROM AUTHOR]

A variety of 4‐phosphinylpyrrolidin‐3‐ones was prepared via a [3+2] cycloaddition between aryl aldonitrones and phosphinylallenes. The products were isolated as unique 4,5‐trans diastereomers, in yields between 47% and 80%, over 23 examples. In the case of chiral racemic allenes, a 2:1 to 4:1 moderate 2,5‐diastereoselectivity was observed. Under the reaction conditions, the cycloadducts directly undergo a rearrangement to afford selectively the corresponding pyrrolidin‐3‐ones. DFT calculations provide some insights into the mechanism, involving the homolytic cleavage of the N−O bond of the cycloadduct. [ABSTRACT FROM AUTHOR]

We described a copper(II)‐catalyzed [3+2] cycloaddition of N‐aryl‐α,β‐unsaturated nitrones with disubstituted allenoates bifurcated to prepare various [1,3]oxazino[3,2‐a]indolines and dihydropyrido[1,2‐a]indolines in moderate to excellent yields. Mechanistic studies revealed that [1,3]oxazino[3,2‐a]indoline was a kinetically favored product and dihydropyrido[1,2‐a]indoline was thermodynamically favored product. Moreover, the reaction was easily performed at gram scales and chiral dihydropyrido[1,2‐a]indoline could be obtained by using chiral auxiliary. The present method highlights broad substrate scope, good functional group tolerance, and diversity of indoline scaffolds with high diastereoselectivity. [ABSTRACT FROM AUTHOR]

Herein we report the first redox‐neutral visible‐light photocatalytic intermolecular dipolar cycloaddition for the diastereoselective synthesis of isoxazolidines. We have found that vinyl nitrones with a diverse substitution pattern undergo visible‐light‐promoted cycloadditions with conjugated carbonyls in the presence of catalytic amounts of Ru(bpy)3Cl2, with the reaction proceeding with high yields and good selectivities. A redox‐neutral pathway is proposed as the primary photoredox mechanism for this transformation. [ABSTRACT FROM AUTHOR]

Comprehensive Summary: An efficient and practical synthesis of heterocyclic nucleosides is developed by a catalyst‐free highly regioselective and diastereoselective [3+2] annulation of α‐purine‐substituted acrylates with nitrones. The reaction operates with excellent functional group tolerance, very mild reaction conditions, and with the green, sustainable, and eco‐friendly 2‐methyltetrahydrofuran (2‐MeTHF) as solvent. Compared with other reactions of electron‐deficient olefin dipolarophiles with nitrones, different regioselective cycloaddition products were observed in this work. This 1,3‐dipolar cycloaddition reaction gives a series of isoxazolidinyl nucleosides in good to excellent yields with promising applications in biochemistry and medicinal chemistry. [ABSTRACT FROM AUTHOR]

Comprehensive Summary: An efficient and practical synthesis of heterocyclic nucleosides is developed by a catalyst‐free highly regioselective and diastereoselective [3+2] annulation of α‐purine‐substituted acrylates with nitrones. The reaction operates with excellent functional group tolerance, very mild reaction conditions, and with the green, sustainable, and eco‐friendly 2‐methyltetrahydrofuran (2‐MeTHF) as solvent. Compared with other reactions of electron‐deficient olefin dipolarophiles with nitrones, different regioselective cycloaddition products were observed in this work. This 1,3‐dipolar cycloaddition reaction gives a series of isoxazolidinyl nucleosides in good to excellent yields with promising applications in biochemistry and medicinal chemistry. [ABSTRACT FROM AUTHOR]

A new iron‐catalyzed hydroxyamination of alkenyl ketoximes using TBHP as the hydroxy source to selectively assemble cyclic nitrones is described. Using inexpensive FeCl3 as the catalyst and TBHP as both the oxidant and hydroxy source, this reaction enables the formation of two new bonds, one C−N bond and one C−O bond, in a single reaction initiating from the oxime side through the nitrogen‐center radical geneation, radical addition across the C=C bond and radical hydroxylation cascades. This reaction has advantages of exquisite selectvity and excellent functional group tolerance, thus providing an economical and efficient strategy for the construction of complex heterocyclic compounds. [ABSTRACT FROM AUTHOR]

A general and flexible approach toward the development of α‐l‐rhamnosidase (α‐l‐Rha‐ase) inhibitors is described. Five enantiopure poly‐substituted pyrrolidine‐based scaffolds bearing the C1‐aminomethyl moiety were designed and synthesized from five‐membered cyclic nitrones. Each structurally diversified amide library of these scaffolds was rapidly generated via combinatorial parallel synthesis and applied for in‐situ inhibition study against α‐l‐Rha‐ase, allowing us to efficiently identify new inhibition hits. Surprisingly, all promising inhibitors are derived from the same scaffold 3. Among them, the most potent and selective inhibitor is pyrrolidine 19 with Ki=0.24 μM, approximately 24‐fold more potent than the reference compound DAA (Ki=5.7 μM). It is the first study to comprehensively prepare pyrrolidine‐based scaffolds and libraries for inhibition study against α‐l‐Rha‐ase. [ABSTRACT FROM AUTHOR]

A self‐enabled electrooxidative protocol for the synthesis of nitrones from bench stable amines has been developed under constant current electrolysis at room temperature, rendering metal and external oxidant‐free protocol in an alkaline medium. The electrocatalysis steps of reported strategy involve anodic oxidation of benzyl amines followed by cathodic reduction of nitro‐functional group in an undivided cell affording functional nitrones. The robustness of the electrocatalysis was reflected by tolerant substrate scope, oxidant‐free conditions, and an operationally friendly procedure. Moreover, the cyclic voltammetry and control experiment studies also support the electrochemical process in alkaline medium. [ABSTRACT FROM AUTHOR]

Comprehensive Summary: A highly enantioselective gold‐catalyzed intermolecular [3 + 2] cycloaddition of N‐allenamides with nitrones was realized by the application of Ming‐Phos M6 as a chiral ligand. Both enantiomers of the cycloadducts with opposite configuration could be obtained in high yields with high regio‐ and enantioselectivity by the employment of either diastereomer of the chiral ligand. The acidic N—H bond (hydrogen bonding) of sulfinamide moiety of Ming‐Phos and pentaflurophenyl substituent (fluorine effect) may play important roles in enhancement of enantioselectivity, that is, Ming‐Phos is a multifunctional ligand in this transformation. [ABSTRACT FROM AUTHOR]

We describe a DBU‐controlled 6π‐azaelectrocyclization of dibenzylideneacetone‐derived N‐vinyl‐α,β‐unsaturated nitrones to prepare 6‐alkyl pyridine N‐oxides in 24–82% yields. Mechanistic studies revealed that DBU had a great impact on the formation of 6‐alkyl pyridine N‐oxides and served as a base and the carrier of hydrogen sources to in situ reduce C=C bonds. The 6‐alkyl pyridine N‐oxide was prepared at gram‐scales and converted to estrone‐derived pyridine and pyrido[2,3‐c]carbazole 4‐oxide in 46% and 52% yields in two steps, respectively. [ABSTRACT FROM AUTHOR]

Various spirooxindole‐benzo[d]oxazoles and dihydrobenzofurans were prepared in good to excellent yields by [3+2] cycloaddition and selective rearrangement of N‐vinyl oxindole nitrones and arynes under transition metal‐free conditions. Experimental results showed that the substituent on the nitrone N‐vinyl group controlled the [1,3]‐ or [3,3]‐rearrangement of their cycloadduct owing to its steric effect. The present method features broad substrate scope, good functional group tolerance, controllable [1,3]‐ or [3,3]‐rearrangement, and diverse oxindole scaffolds. [ABSTRACT FROM AUTHOR]

A variety of chiral nine‐membered N‐heterocycles were prepared in moderate to good yields with high diastereoselectivity through a silver(I)‐catalyzed [3+2] cycloaddition and [3,3]‐rearrangement of N‐vinyl‐α, β‐unsaturated nitrones and chiral 3‐propioloyloxazolidin‐2‐ones. Experimental studies showed that silver catalyst promoted the cycloaddition and rearrangement process, and the stereochemistry of the nine‐membered N‐heterocycles was controlled via [3,3]‐rearrangement by chiral oxazolidinone‐auxiliary through a boat‐like transition state. Moreover, the obtained nine‐membered N‐heterocycle diastereomers were converted to chiral pyrrolizines with high diastereoselectivity and pyrrolizine carboxylate was obtained in 54% yield with 90% ee by the removal of chiral auxiliary. [ABSTRACT FROM AUTHOR]

The BF3 Lewis acid (LA) catalyzed [3+2] cycloaddition (32CA) reaction of 1‐pyrroline‐1‐oxide (PNO) with 2,3‐dihydrofuran (DHF) has been studied within the molecular electron density theory (MEDT) at the ωB97X‐D/6‐311G(d,p) computational level. Electron localization function (ELF) characterizes PNO and the corresponding PNO : BF3 complex as zwitterionic species. Conceptual DFT indices allow classifying PNO and DHF as marginal electrophiles and strong nucleophiles, while the PNO : BF3 complex is a strong electrophile. Consequently, while the non‐catalyzed 32CA reaction is non‐polar, the BF3 catalyzed one has a high polar character, revealed by the analysis of the global electron density transfer (GEDT) at the transition state structures. The BF3 catalyzed 32CA reaction, which is classified as of reverse electron density flux (REDF), presents high exo stereoselectivity, and total ortho regioselectivity. The presence of the BF3 catalyst decreases the activation enthalpy of the 32CA reaction in dichloromethane by 7.5 kcal mol−1, as a consequence of the increase of the polar character of the reaction, in complete agreement with the experimentally observed acceleration. [ABSTRACT FROM AUTHOR]

Ynamide, a unique species with inherited polarization of nitrogen lone pair electron to triple bond, has been largely used for the developement of novel synthetic methods and the construction of unusual N‐bearing heterocycles. The reaction versatility of ynamide on umpolung reactivity, radical reactions and asymmetric synthesis have been recently reviewed. This review provides an overall scenic view into the gold catalyzed transformation of ynamides. The ynamides reactivity towards nitrogen‐transfer reagents, such as azides, nitrogen ylides, isoxazoles, and anthranils; oxygen atom‐transfer reagents, like nitrones, sulfoxides, and pyridine N‐oxides; and carbon nucleophiles under gold catalysis are herein uncovered. The scope as well the mechanistic insights of each reaction is also briefed. [ABSTRACT FROM AUTHOR]

β‐Aryl/alkyl vinyl diazoacetates were investigated in metallo‐vinylcarbene reactions with nitrones, revealing a Rh2(OAc)4‐catalyzed cyclopropene dimerization reaction and a copper(I) catalyzed [3+3]‐cycloaddition of nitrones. The chiral cyclopropyl‐In‐SaBox ligand with copper(I) catalysis could realize the asymmetric version of the cycloaddition reaction, delivering various 3,6‐dihydro‐1,2‐oxazine derivatives in good yield and with excellent enantioselectivity under mild conditions. [ABSTRACT FROM AUTHOR]

The synthesis of 1‐pyrrolines from N‐alkenylnitrones and alkynes has been explored as a retrosynthetic alternative to traditional approaches. These cascade reactions are formal [4+1] cycloadditions that proceed through a proposed dipolar cycloaddition and N‐alkenylisoxazoline [3,3′]‐sigmatropic rearrangement. A variety of cyclic alkynes and terminal alkynes have been shown to undergo the transformation with N‐alkenylnitrones under mild conditions to provide the corresponding spirocyclic and densely substituted 1‐pyrrolines with high regio‐ and diastereoselectivity. Mechanistic studies provide insight into the balance of steric and electronic effects that promote the cascade process and control the diastereo‐ and regioisomeric preferences of the 1‐pyrroline products. Diastereoselective derivatization of the 1‐pyrrolines prepared by the cascade reaction demonstrate the divergent synthetic utility of the new method. [ABSTRACT FROM AUTHOR]

The enantio‐ and chemoselective iridium‐catalyzed N‐allylation of oximes is described for the first time. Intramolecular kinetic resolution provides access to cyclic nitrones and enantioenriched aliphatic allylic alcohols. Salient features of this transformation are its ability to employ E/Z‐isomeric mixtures of oxime starting materials convergently and high functional group tolerance. The implementation of N‐allylation/1,3‐dipolar cycloaddition reaction sequences furnishes tricyclic isoxazolidines in highly enantio‐ and diastereoselective fashion. The synthetic utility of the approach is demonstrated by the efficient, formal synthesis of the marine natural product (+)‐halichlorine. [ABSTRACT FROM AUTHOR]

A silver‐catalyzed protocol for [3+1+1] annulation of nitrones and isocyanoacetates is reported. The reaction is proposed to proceed through sequential [3+2] cycloaddition, ring‐opening, cyclization‐oxidation, and dealkylation, allowing access to a broad scope of valuable polysubstituted imidazoles in high yields. [ABSTRACT FROM AUTHOR]

The regio‐ and enantioselective (3+3) cycloaddition of nitrones with 2‐indolylmethanols was accomplished by the cooperative catalysis of hexafluoroisopropanol (HFIP) and chiral phosphoric acid (CPA). Using this approach, a series of indole‐fused six‐membered heterocycles were synthesized in high yields (up to 98 %), with excellent enantioselectivities (up to 96 % ee) and exclusive regiospecificity. This approach enabled not only the first organocatalytic asymmetric (3+3) cycloaddition of nitrones but also the first C3‐nucleophilic asymmetric (3+3) cycloaddition of 2‐indolylmethanols. More importantly, theoretical calculations elucidated the role of the cocatalyst HFIP in helping CPA control the reactivity and enantioselectivity of the reaction, demonstrating a new mode of cooperative catalysis. [ABSTRACT FROM AUTHOR]

A gold‐catalyzed synthesis of 1,3‐dihydrooxazolo[3,4‐a]indoles from 1‐oxo‐3‐yn‐4‐ols and nitrones is described; this new bicyclic annulation presents the first examples that internal alkynes can react with nitrones to undergo an oxoarylation route. DFT calculations indicate a [3,3]‐sigmatropic shift of initial alkenylgold intermediates to elude the intermediacy of gold carbenes. We also developed new [3+2]‐annulations of the same 1‐oxo‐3‐yn‐4‐ols with imines, yielding oxazolidin‐4‐ylidene derivatives efficiently. The tethered alcohols of these 1‐oxo‐3‐ynes allow trapping of their metastable 2‐azadienium intermediates to enable a novel annulation. Our mechanistic analysis indicates that the two products, despite their structural relevance, are produced from two independent systems. [ABSTRACT FROM AUTHOR]

Herein we report a mild and diastereoselective access to ketonitrones by reacting easily available aryl acetic acid esters and other active methylene compounds, with nitrosoarenes under catalytic loading of 2‐tert‐butylimino‐2‐diethylamino‐1,3‐dimethylperhydro‐1,3,2‐diazaphosphorine (BEMP) at room temperature. Depending on the substitution pattern and nature of the aryl moiety, a switch toward the formation of imines can be observed. The mechanistic framework is put to scrutiny by experimental and theoretical studies, pointing to the formation of a nitroso aldol intermediate, whose fate toward one of the competing pathways, namely hydride transfer or elimination, would depend upon the NOH/CHα relative acidities. [ABSTRACT FROM AUTHOR]

Nitrones are key intermediates in organic synthesis. Herein, we report the first photo‐redox synthesis of methylene nitrone intermediates from nitroarenes and arylamines. The highly reactive methylene nitrones are in situ trapped by alkenes to afford various isoxazolidines. This three‐component reaction features the use of N,N‐dimethylanilines or N‐aryl glycines as C1 building blocks, which allow for the one‐pot formal [1+2+2] cycloaddition from simple starting materials. A wide range of useful isoxazolidines can be obtained under mild conditions with moderate to good yields. Mechanistic investigations support the formation of methylene nitrone via selective N−CH3 bond cleavage and methylene transfer. [ABSTRACT FROM AUTHOR]

An efficient two‐step strategy for the synthesis of constrained C‐glycosyl amino acid derivatives from C‐vinylglycosides involving a 1,3‐dipolar cycloaddition using l‐(–)‐menthone‐derived nitrone as the key step is described. After optimization of 1,3‐dipolar cycloaddition conditions, various C‐vinylglycosides were tested leading exclusively to one diastereoisomer of the corresponding cycloadduct in good to excellent yields. The total facial selectivity observed was also studied by DFT calculations. Original conformationally restricted C‐glycosyl amino acid derivatives (8 examples) were isolated after simple cleavage of the chiral auxiliary. [ABSTRACT FROM AUTHOR]

Herein we report the synthesis of the new statin derivative MC609 in 80 % overall yield by sequential transformation of simvastatin including dehydration, and Michael addition of N‐benzyl hydroxylamine followed by cyclization. The preparation of nitronestatins MC649 and MC645 is also described from (2S,4R)‐4‐((tert‐butyldimethylsilyl)oxy)‐6‐oxotetrahydro‐2H‐pyran‐2‐carbaldehyde by reaction with N‐benzyl hydroxylamine, and N‐hydroxylamine followed by reduction with sodium cyanoborohydride and coupling with commercially available 2‐chloro‐6‐methylquinoline‐3‐carbaldehyde, in 77 % and 24 % overall yield, respectively. [ABSTRACT FROM AUTHOR]

The synthesis of a library of 6‐alkylidene‐β‐lactams, derived from 6‐aminopenicillanic acid is reported. The 1,3‐dipolar cycloaddition of these 6‐alkylidenepenicillanates with nitrones was explored as an approach to synthesize novel chiral spiroisoxazolidine‐β‐lactams. Quantum chemical calculations, at the DFT level of theory, were carried out to elucidate the 3D structure of the synthesized compounds. The reported methodology, which involves the generation of three new consecutive stereogenic centers, proved to be regio‐ and stereoselective, leading to novel chiral spiroisoxazolidine‐penicillanates efficiently. [ABSTRACT FROM AUTHOR]

The 1,3‐dipolar cycloadditions of azomethine‐ylides and ‐imines, nitrones, nitrilimines, and azides catalysed by inorganic nanoparticles are described. Emphasis is given to the nanometric catalysts involved, their structure, characterisation, and recyclability and, when remarkable, their preparation. Whenever possible, the improvements with respect to cycloadditions performed with the usual (non‐nano) catalysts are discussed. [ABSTRACT FROM AUTHOR]

New five‐membered cyclic nitrones with gem‐difluoroalkyl groups in γ‐position have been prepared by a 3‐step sequence starting from propargylic alcohols. This domino process involves a base‐mediated isomerization reaction to enones, which are trapped in situ by nitroalkane anions. In a final step, starting from these key precursors, a reduction‐cyclization process affords the target molecules. Mono‐ and bicyclic nitrones have been prepared by this route which allows, as well, the synthesis of nitrones with functional groups in terminal position of the side chain. [ABSTRACT FROM AUTHOR]

A dearomative [3,3′]‐sigmatropic rearrangement that converts N‐alkenylbenzisoxazolines into spirocyclic pyrroline cyclohexadienones has been developed by using the dipolar cycloaddition of an N‐alkenylnitrone and an aryne to access these unusual transient rearrangement precursors. This cascade reaction affords spirocyclic pyrrolines that are inaccessible through dipolar cycloadditions of exocyclic cyclohexenones and provides a fundamentally new approach to novel spirocyclic pyrroline and pyrrolidine motifs that are common scaffolds in biologically‐active molecules. Diastereoselective functionalization processes have also been explored to demonstrate the divergent synthetic utility of the unsaturated spirocyclic products. [ABSTRACT FROM AUTHOR]

A regioselective and convenient methodology was developed to synthesize heterocyclic derivatives, bearing imidazole, piperidines, and azepines rings. The N‐arylnitrones derived from 3‐formylchromones were selected to react with heterocyclic ketene aminal to furnish the structurally attractive and pharmacologically important fused ring heterocycles. The N‐arylnitrone moiety of 3‐formylchromone was used to activate the formyl group for regioselective fused ring heterocycles synthesis, whereas, the effect of substituents at aryl functionality of nitrones were studied to improve the yield of target fused ring heterocyclic products. The synthesized compounds (10‐12) were evaluated for their in vitro cytotoxic and antifungal influences. In cytotoxic (brine shrimp lethality) assay, compound 11e was found to be active with LD50 = 4.1 × 10−6 μg/mL. [ABSTRACT FROM AUTHOR]

A protocol involving cooperative silver‐ and base‐catalyzed diastereoselective cycloaddition of nitrones with isocyanides is described, providing access to a wide range of 2‐imidazolinone derivatives with various functional groups as single diastereomers. Varying the base and temperature of the reaction allowed selective access to both diastereomers of the product. A plausible reaction mechanism is proposed and supported by DFT calculations. [ABSTRACT FROM AUTHOR]

A simple and efficient method for C‐2 deuterium labeling of 3,4‐di‐tert‐butoxypyrroline N‐oxide, a useful chiral building block in azaheterocycles syntheses, is presented. Selective and quantitative deuterium incorporation (> 99 %) was achieved by base‐catalyzed H/D exchange in D2O under mild reaction conditions. A mechanistic pathway based on kinetic and computational data was proposed. The labeled nitrone was used in the synthesis of C‐8a deuterated (1R,2R,8aR)‐lentiginosine. [ABSTRACT FROM AUTHOR]

Summary of main observation and conclusion: A gold(I)‐catalyzed asymmetric intermolecular tandem [3+3]‐cyclization reaction of 2‐(1‐alkynyl)‐2‐ alken‐1‐ones with nitrones has been developed by using Ming‐Phos as a chiral ligand. This method enables access to the stereodivergent synthesis of highly substituted furo[3,4‐d][1,2]oxazines in excellent efficiency and stereoselectivity (up to 99% yield, 99% ee, >20 : 1 dr). [ABSTRACT FROM AUTHOR]

1,2,5‐Oxadiazinanes have a unique heterocyclic skeleton and are expected to be applicable to the synthesis of drugs. However, there have been few reported methodologies to synthesize these structures. We report a facile synthesis of 1,2,5‐oxadiazinanes that is accomplished by the unprecedented photochemical cycloaddition reaction of nitrones with diaminomethanes. The photoreaction is a formal [3+3] cycloaddition, where a nitrone and a diaminomethane act as the 1,3‐dipole and the 1,3‐synthon, respectively. The reaction is significantly accelerated with benzophenone as a photosensitizer. A high degree of diastereoselective addition (d.r. >99 : 1) is observed in the reaction of (Z)‐N‐benzyl‐1‐phenylmethanimine oxide with di(pyrrolidin‐1‐yl)methane. [ABSTRACT FROM AUTHOR]

Summary of main observation and conclusion: A gold(I)‐catalyzed asymmetric intermolecular tandem [3+3]‐cyclization reaction of 2‐(1‐alkynyl)‐2‐ alken‐1‐ones with nitrones has been developed by using Ming‐Phos as a chiral ligand. This method enables access to the stereodivergent synthesis of highly substituted furo[3,4‐d][1,2]oxazines in excellent efficiency and stereoselectivity (up to 99% yield, 99% ee, >20 : 1 dr). [ABSTRACT FROM AUTHOR]