Synthesis of Di-, Tri-, and Tetrasubstituted Pyridines from (Phenylthio)carboxylic Acids and 2-[Aryl(tosylimino)methyl]acrylates

Condensation of 4-Tert-butyl-2,6-dimethylbenzenesulfonamide with Glyoxal and Reaction Features: A New Process for Symmetric and Asymmetric Aromatic Sulfones

The synthesis of substituted aza- and oxaazaisowurtzitanes via direct condensation is challenging. The selection of starting ammonia derivatives is very limited. The important step in developing alternative synthetic routes to these compounds is a detailed study on their formation process. Here, we explored an acid-catalyzed condensation between 4-tert-butyl-2,6-dimethylbenzenesulfonamide and glyoxal in aqueous H₂SO₄, aqueous acetonitrile and acetone, and established some new processes hindering the condensation. In particular, an irreversible rearrangement of the condensation intermediate was found to proceed and be accompanied by the 1,2-hydride shift and by the formation of symmetric disulfanes and sulfanes. It has been shown for the first time that aldehydes may act as a reducing agent when disulfanes are generated from aromatic sulfonamides, as is experimentally proved. The condensation between 4-tert-butyl-2,6-dimethylbenzenesulfonamide and formaldehyde resulted in 1,3,5-tris((4-(tert-butyl)-2,6-dimethylphenyl)sulfonyl)-1,3,5-triazinane. It was examined if diimine could be synthesized from 4-tert-butyl-2,6-dimethylbenzenesulfonamide and glyoxal by the most common synthetic procedures for structurally similar imines. It has been discovered for the first time that the Friedel-Crafts reaction takes place between sulfonamide and the aromatic compound. A new synthetic strategy has been suggested herein that can reduce the stages in the synthesis of in-demand organic compounds of symmetric and asymmetric aromatic sulfones via the Brønsted acid-catalyzed Friedel-Crafts reaction, starting from aromatic sulfonamides and arenes activated towards an electrophilic attack.

Bifunctional Urea/Hg(OAc)2-Mediated Synthesis of 4-Aryl-6-oxycarbonyl-2-pyrones and 2-Pyridones from Dithiomalonate and β,γ-Unsaturated α-Keto Esters

Disubstituted 2-pyrones and 2-pyridones were obtained by bifunctional urea-catalyzed Michael addition/lactonization or lactamization followed by a Hg(OAc)₂- or Hg(OAc)₂/DBU-mediated hydrolysis/decarboxylation/dehydrogenation process. This one-pot two-stage protocol enabled the rapid synthesis of 4,6-disubstituted 2-pyrones and 2-pyridones from dithiomalonate and β,γ-unsaturated α-keto esters in practical yields under mild reaction conditions. Additionally, the obtained 2-pyridones were facilely transformed to 2,4,6-trisubstituted pyridines in excellent yields.

Generation and Reactivity of C(1)-Ammonium Enolates by Using Isothiourea Catalysis

C(1)-Ammonium enolates are powerful, catalytically generated synthetic intermediates applied in the enantioselective α-functionalisation of carboxylic acid derivatives. This minireview describes the recent developments in the generation and application of C(1)-ammonium enolates from various precursors (carboxylic acids, anhydrides, acyl imidazoles, aryl esters, α-diazoketones, alkyl halides) using isothiourea Lewis base organocatalysts. Their synthetic utility in intra- and intermolecular enantioselective C-C and C-X bond forming processes on reaction with various electrophiles will be showcased utilising two distinct catalyst turnover approaches.

Access to azonanes via Pd-catalyzed decarboxylative [5 + 4] cycloaddition with exclusive regioselectivity

An efficient approach for the synthesis of azonanes via Pd-catalyzed decarboxylative [5 + 4] cycloaddition has been developed. The reactions feature wide functional group tolerance with exclusive regioselectivity.

Synthesis of 1,4-diazepinone derivatives via a domino aza-Michael/SN2 cyclization of 1-azadienes with α-halogenoacetamides

A novel cyclization of α-halogenoacetamides with 1-azadienes has been developed for the efficient preparation of monocyclic 1,4-diazepinones in one step under transition metal-free conditions. Various α-halogenoacetamides and 1-azadienes are well tolerated and give the desired products in good to excellent yields. This cyclization also demonstrates potential synthetic utility on a gram-scale and further transformation.

Synthetic and mechanistic aspects of sulfonyl migrations

Sulfonyl migrations, frequently described as ‘unusual’ or ‘unexpected’, from the last 20 years, including 1,2-, 1,3-, 1,4-, 1,5-, 1,6- and 1,7-sulfonyl shifts, through either radical or polar processes, either inter- or intramolecularly are reviewed.

Lewis Acids Promoted 3 + 2 Cycloaddition of Oxaziridines and Cyclic Allylic Alcohols through Carbonyl Imine Intermediates

Syntheses of isoxazolidines through the carbonyl imine intermediates are currently limited to monosubstituted olefin substrates. Herein, we reported syntheses of novel bicyclic isoxazolidine-containing compounds through 1,3-dipolar cycloaddition reactions using cyclic allylic alcohols as substrates, which proved challenging in previous reports. Generally, the reaction yields range from good to high, and the reaction substrates tolerate various functional groups, including the cyclopropyl and amine groups. Mechanistic studies suggest that an allylic cation and a carbonyl imine intermediate are involved and responsible for the observed stereochemistry and diastereoselectivity.

5-Annulation of Ketoimines: TFA-Catalyzed Construction of Isoindolinone-3-carboxylates and Development of Photophysical Properties

Herein we have demonstrated the first report on 5-annulation of ketoimines to valuable isoindolinone-3-carboxylates. Instead of commonly used aldimine substrates, relatively less reactive ketoimines are employed for developing a TFA catalyzed organoreductive cyclization to furnish a variety of isoindolinones in excellent yield and reaction rate under mild reaction conditions. This is a metal-free event, which proceeds through a one pot ketoimine formation, hydride transfer from an organic reductant 2-(naphthalen-2-yl)-2,3-dihydrobenzo[ d]thiazole, and followed by five member cyclization sequences through TFA-activation of imine and ester groups. Studies on ESI-MS kinetics, leaving group aptitude, and control experiments led us to propose the mechanistic pathway of the new ketoimine-lactamization reaction. We have shown the synthetic utility of the emerging synthons through easy transformation of isoindolinones to different synthetic analogues. We investigated photophysical properties of the small molecules for their futuristic application as a pharmaceutical and materials, and the heterocycles displayed brilliant fluorescence activity.

Synthesis of Polysubstituted Pyrroles through a Formal [4 + 1] Cycloaddition/E1cb Elimination/Aromatization Sequence of Sulfur Ylides and α,β-Unsaturated Imines

A reaction sequence comprising a formal [4 + 1] cycloaddition, an E1cb elimination, and an aromatization process is described in this work. By doing so, polysubstituted pyrroles were achieved from easily available chemicals, sulfur ylides, and α,β-unsaturated imines. This protocol features mild conditions, high efficiency, and wide substrate scopes.

Fe(II)/Au(I) Relay Catalyzed Propargylisoxazole to Pyridine Isomerization: Access to 6-Halonicotinates

An efficient synthesis of methyl nicotinates/6-halonicotinates by the domino isomerization of 4-propargyl/(3-halopropargyl)-5-methoxyisoxazoles under Fe(II)/Au(I) relay catalysis was developed. It was found that FeNTf₂ is an effective catalyst for first step of the domino isomerization, transformation of isoxazole to 2H-azirine, which is compatible with Ph₃PAuNTf₂, catalyzing the second step.

NaH-Promoted Transformation of Arylthioureas to Aryl-Isothioureas by S-Allylation and S-Benzylation Reactions

A series of 16 substituted aryl-isothioureas were prepared in excellent yields (80–97%) by reacting substituted arylthioureas with allyl bromide or substituted benzyl bromides in the presence of NaH in DMSO at room temperature. The substituent variations on the benzyl reactants had a larger effect on the yields than substituent variations on the allyl reactants. The method provides a facile and convenient preparation of some potentially biologically active compounds.

Tailor-made synthesis of fully alkylated/arylated nicotinates by FeCl3-mediated condensation of enamino esters with enones

A synthetic method for fully alkylated/arylated nicotinates was achieved by FeCl₃-mediated condensation of enamino esters and enones.

Carbene-catalyzed LUMO activation of alkyne esters for access to functional pyridines

A carbene-catalyzed activation of alkyne ester for quick access to functional pyridines is reported. The reaction involves a previously unexplored unsaturated acyl azolium intermediate bearing a carbon–carbon triple bond that shall exhibit rich chemical reactivities.

Isothiourea-catalysed enantioselective pyrrolizine synthesis: synthetic and computational studies

The catalytic enantioselective synthesis of a range of cis-pyrrolizine carboxylate derivatives with outstanding stereocontrol (14 examples, >95 : 5 dr, >98 : 2 er) through an isothiourea-catalyzed intramolecular Michael addition-lactonisation and ring-opening approach from the corresponding enone acid is reported. An optimised and straightforward three-step synthetic route to the enone acid starting materials from readily available pyrrole-2-carboxaldehydes is delineated, with benzotetramisole (5 mol%) proving the optimal catalyst for the enantioselective process. Ring-opening of the pyrrolizine dihydropyranone products with either MeOH or a range of amines leads to the desired products in excellent yield and enantioselectivity. Computation has been used to probe the factors leading to high stereocontrol, with the formation of the observed cis-steroisomer predicted to be kinetically and thermodynamically favoured.

Enantioselective isothiourea-catalysed trans-dihydropyridinone synthesis using saccharin-derived ketimines: scope and limitations

The catalytic enantioselective synthesis of a range of trans-dihydropyridinones from aryl-, heteroaryl- and alkenylacetic acids and saccharin-derived ketimines with good to excellent stereocontrol (15 examples, up to >95 : 5 dr, up to >99 : 1 er) is reported. After extensive optimisation, HyperBTM proved the optimal isothiourea catalyst for this transformation at -78 °C, giving trans-dihydropyridones with generally excellent levels of diastereo- and enantioselectivity.

Organocatalytic Synthesis of Fused Bicyclic 2,3-Dihydro-1,3,4-oxadiazoles through an Intramolecular Cascade Cyclization

Hydrazone-carboxylic acids undergo intramolecular cyclization in the presence of pivaloyl chloride, iPr(2)NEt, and catalytic DABCO to form a range of substituted fused tricyclic 2,3-dihydro-1,3,4-oxadiazoles in high yields.

Enantioselective Synthesis of 3,5,6-Substituted Dihydropyranones and Dihydropyridinones using Isothiourea-Mediated Catalysis

The scope of dihydropyranone and dihydropyridinone products accessible by isothiourea-catalyzed processes has been expanded and explored through the use of 2-N-tosyliminoacrylates and 2-aroylacrylates in a Michael addition-lactonization/lactamization cascade reaction. Notably, to ensure reproducibility it is essential to use homoanhydrides as ammonium enolate precursors with 2-aroyl acrylates, while carboxylic acids can be used with 2-N-tosyliminoacrylates, delivering a range of 3,5,6-substituted dihydropyranones and dihydropyridinones with high enantioselectivity (typically >90 % ee). The derivatization of the heterocyclic core of a 3,5,6-substituted dihydropyranone through hydrogenation is also reported.

Regioselective Pd-Catalyzed Synthesis of 2,3,6-Trisubstituted Pyridines from Isoxazolinones

Substituted pyridines are prevalent heterocycles of fundamental importance. Their efficient regioselective preparation is often still a challenge despite a large number of reported synthetic methodologies. In this letter we report an operationally simple approach that makes use of readily accessible isoxazolinones. The protocol involves a Pd(II)-catalyzed C-regioselective 1,4-addition to vinylketones, followed by a Pd(0)-catalyzed transformation, which is assumed to proceed via vinylnitrene-Pd intermediates. Both hydrogen and air are necessary for the pyridine formation step and could be employed at ratios above the upper explosive limit thus avoiding a safety issue. This new strategy allows an effective, scalable and practical access to various previously unknown 2,3,6-trisubstituted pyridines.

Asymmetric Isothiourea-Catalysed Formal [3+2] Cycloadditions of Ammonium Enolates with Oxaziridines

A highly enantioselective Lewis base-catalysed formal [3+2] cycloaddition of ammonium enolates and oxaziridines to give stereodefined oxazolidin-4-ones in high yield is described. Employing an enantioenriched oxaziridine in this process leads to a matched/mis-matched effect with the isothiourea catalyst and allowed the synthesis of either syn- or anti-stereodefined oxazolidin-4-ones in high d.r., yield and ee. Additionally, the oxazolidin-4-one products have been derivatised to afford functionalised enantioenriched building blocks.

An NHC-catalyzed in situ activation strategy to β-functionalize saturated carboxylic acid: an enantioselective formal [3+2] annulation for spirocyclic oxindolo-γ-butyrolactones

An in situ NHC-catalyzed activation strategy to β-functionalize saturated carboxylic acid was developed. This asymmetric formal [3+2] annulation could deliver spirocyclic oxindolo-γ-butyrolactones from saturated carboxylic acid and isatin in good yields with high to excellent enantioselectivities. The easy availability of the starting materials, direct installation of functional units at unreactive carbon atom and the convergent assembly make this protocol attractive in the field of organic synthesis.

Regio- and diastereoselective construction of 1′,2′-(dihydrospiro[indoline-3,3′-pyrrol]-2′-yl)acrylates through phosphine-catalyzed [4 + 1] annulation of Morita–Baylis–Hillman carbonates with oxindole-derived α,β-unsaturated imines

Phosphine-catalyzed [4 + 1] annulation of Morita–Baylis–Hillman carbonates with oxindole-derived imines has been developed.