Highly Regioselective Migration of the Sulfonyl Group: Easy Access to Functionalized Pyrroles

Divergent Synthesis of Unsymmetrical Bis-heteroaryl Ketones via Base-Promoted Cascade Reactions of 1,2-Alkynedione-Derived N-Propargylic β-Enaminones

Herein we disclose a transition-metal-free, one-pot two-step strategy for the synthesis of unsymmetrical bis-heteroaryl ketones. N-propargylic β-enaminones generated by the Michael addition of propargylamines onto heteroaryl 1,2-alkynediones have been utilized as synthetic equivalents of pyridine or pyrrole scaffolds. The use of alcohol as a solvent resulted in the formation of 2-alkoxylated pyridine scaffold, whereas the use of DMSO promoted the formation of a pyrrole motif.

I2-Cs2CO3 Mediated Intramolecular C2-Amination and Oxidative Rearrangement Cascade of C-3 Phenylthio Indoles: A Route to Synthesize Thiosulfonate-Embedded 2-Iminoindolin-3-ones

An efficient, transition-metal-free protocol employing I2/Cs2CO3 for the synthesis of thiosulfonate containing 2-iminoindolin-3-ones motifs has been developed from C-3 phenylthio indoles. The reaction proceeded through intramolecular cyclization involving C-N bond formation, leading to the formation of indole-fused benzothiazines as a key intermediate. Remarkably, Cs2CO3 played a crucial role in the reaction as an oxygen source, enabling oxidative rearrangement with [1,4]-sulfonyl migration to furnish the final products with the formation of multiple functional groups such as C═O, C═N, and S-SO2.

Regio- and Diastereoselective Radical Dimerization Reactions for the Construction of Benzo[f]isoindole Dimers

This study presents a novel approach for synthesizing benzo[f]isoindole dimers, which involves cascade cyclization and oxidative radical dimerization. Our method allows for the formation of up to five carbon-carbon bonds in a single reaction, exhibiting remarkable diastereoselectivity and regioselectivity. The mechanism and regioselectivity were investigated through a combination of experiments and calculations.

Synthesis of a new fluorophore: wavelength-tunable bisbenzo[f]isoindolylidenes

The creation of new functional molecules is a central task in chemical synthesis. Herein, we report the synthesis of a new type of fluorophore, bisbenzo[f]isoindolylidenes, from easily accessible dipropargyl benzenesulfonamides. Wavelength-tunable fluorophores emitting strong fluorescence of green to red light were obtained in this reaction. Late-stage modifications and incorporation of bioactive molecules into these fluorophores give rise to potential applications in biological studies. Detailed computational and experimental studies were conducted to elucidate the mechanism, and suggest a reaction sequence involving Garratt-Braverman type cyclization, isomerization, fragmentation, dimerization and oxidation.

Base-Promoted [4 + 2] Annulation Reaction of In Situ-Generated Azadienes from N-Propargylamines with Active Methylene Compounds: Access to Highly Functionalized 2-Pyridones

A facile and efficient synthesis of structurally diversified 2-pyridones is demonstrated using the [4 + 2] annulation of in situ generated azadienes from N-propargylamines and active methylene compounds. The reaction is promoted by an inorganic base giving moderate to good yields. The developed methodology is applicable for the direct and formal synthesis of various bioactive molecules. The synthetic utility of the protocol was also illustrated by late-stage functionalization of the products.

Synthesis and Elimination Pathways of 1-Methanesulfonyl-1,2-dihydroquinoline Sulfonamides

A series of new Morita–Baylis–Hillman acetates were prepared and reacted with methanesulfonamide (K2CO3, DMF, 23 °C) to produce tertiary dihydroquinoline sulfonamides in high yields. Subsequent efforts to eliminate the methylsulfonyl group from these derivatives (K2CO3, DMF, 90 °C) as a route to quinolines were met with mixed results. Although dihydroquinoline sulfonamides prepared from ethyl acrylate and acrylonitrile generally underwent elimination to give excellent yields of quinolines, those generated from 3-buten-2-one failed to undergo elimination and instead decomposed. The failure of these ketone substrates to aromatize presumably derives from the enolizable methyl ketone at C-3. Finally, the attempted aromatization of the acrylate-derived 6,7-difluoro-1,2-dihydroquinoline sulfonamide demonstrated that other interesting processes could occur in preference to the desired elimination.

Protecting Group-Dependent Synthesis of Densely Substituted Dihydropyrroles v/s Pyrroles via 5-Exo-trig Cascade Radical Cyclization to Alkynyl Vinylogous Carbamates

Densely substituted dihydropyrroles could be synthesized with excellent diastereoselectivity via 5-exo-trig cascade radical cyclization to alkynyl vinylogous carbamates. N-Alkyl/acyl protected alkynyl vinylogous carbamates upon radical cyclization using thiophenol gave substituted pyrroles as against dihydropyrroles, which were formed with N-sulfonyl protecting groups. This enabled a rare example wherein both dihydropyrrole and pyrrole rings are assembled in the same reaction. This strategy could be used for the synthesis of an unprecedented adjacent polyheterocyclic system having a furan-thiophene-pyrrole motif. When vinylogous carbamate is embedded in the isoindole moiety, a pyridoisoindole derivative was formed with excellent diastereoselectivity, instead of the expected pyrroloisoindole product.

Assembly of 1,2,3,4-Tetrahydropyrrolo[1,2-a]pyrazines via the Domino Reaction of 2-Imidazolines and Terminal Electron-Deficient Alkynes

The transformation of 2-imidazolines into 1,2,3,4-tetrahydropyrrolo[1,2-a]pyrazines has been realized. A pseudo-three-component reaction of 2-imidazolines with terminal electron-deficient alkynes (2 equiv) first generates imidazolidines, containing an N-vinylpropargylamine fragment. The latter can then undergo a base-catalyzed domino aza-Claisen rearrangement/cyclization reaction sequence, simultaneously constructing pyrrole and pyrazine rings. The process works in a broad substrate scope, delivering pyrrolo[1,2-a]pyrazines in good to excellent yields (45-90%). This two-step approach can be carried out in a one-pot fashion without a noticeable decrease in yield. Remarkably, a three-component protocol for the introduction of two different alkynes has been also developed.

Synthesis of Unsymmetrically Tetrasubstituted Pyrroles and Studies of AIEE in Pyrrolo[1,2-a]pyrimidine Derivatives

Pyrroles are among the most important heterocycles in pharmaceuticals and agrochemicals. Construction of pyrrole scaffolds with different substituents and free NH group however, is challenging. Herein, a metal-free method for...

Divergent Reactivity of δ- and β'-Acetoxy Allenoates with 2-Sulfonamidoindoles via Phosphine Catalysis: Entry to Dihydro-α-carboline, α-Carboline, and Spiro-cyclopentene Motifs

The reactivity of 2-sulfonamidoindoles with acetoxy allenoates under phosphine catalysis depends on the disposition of the acetoxy (OAc) group on the allenoate. In the temperature-controlled [3 + 3] annulations, δ-acetoxy allenoates afforded dihydrocarboline and carboline scaffolds with carbon-nitrogen nucleophilic 2-sulfonamidoindoles, in which allenoate serves as a β-, γ-, and δ-carbon donor. At room temperature (25 °C), dihydro-α-carboline motifs were obtained exclusively through Michael addition, 1,4-proton shift, isomerization, 1,2-proton transfer, phosphine elimination, and aza-Michael addition. The higher temperature (80 °C) cascade protocol using Ph₃P-Cs₂CO₃ combination involves addition-elimination, aza-Claisen rearrangement, tosyl migration, and aromatization as key steps to give α-carbolines containing tosyl functionality at the γ-carbon. In contrast, with β'-acetoxy allenoate, 2-sulfonamidoindole acts only as a carbo-nucleophile in (p-tolyl)₃P-directed [4 + 1] spiro-annulation, leading to five-membered spiro-carbocyclic motifs essentially as single diastereomers (dr >20:1) via chemoselective carbo-annulation.

Synthesis of Polysubstituted Fused Pyrroles by Gold-Catalyzed Cycloisomerization/1,2-Sulfonyl Migration of Yndiamides

Yndiamides (bis-N-substituted alkynes) are valuable precursors to azacycles. Here we report a cycloisomerization/1,2-sulfonyl migration of alkynyl-yndiamides to form tetrahydropyrrolopyrroles, unprecedented heterocyclic scaffolds that are relevant to medicinal chemistry. This functional group tolerant transformation can be achieved using Au(I) catalysis that proceeds at ambient temperature, and a thermally promoted process. The utility of the products is demonstrated by a range of reactions to functionalize the fused pyrrole core.

Direct synthesis of pentasubstituted pyrroles and hexasubstituted pyrrolines from propargyl sulfonylamides and allenamides

Multisubstituted pyrroles are important fragments that appear in many bioactive small molecule scaffolds. Efficient synthesis of multisubstituted pyrroles with different substituents from easily accessible starting materials is challenging. Herein, we describe a metal-free method for the preparation of pentasubstituted pyrroles and hexasubstituted pyrrolines with different substituents and a free amino group by a base-promoted cascade addition-cyclization of propargylamides or allenamides with trimethylsilyl cyanide. This method would complement previous methods and support expansion of the toolbox for the synthesis of valuable, but previously inaccessible, highly substituted pyrroles and pyrrolines. Mechanistic studies to elucidate the reaction pathway have been conducted.

Synthesis of 4-amino-5-allenylisoxazoles via gold(I)-catalysed propargyl aza-Claisen rearrangement

Propargyl aza-Claisen rearrangement of 4-propargylaminoisoxazoles 1 proceeded in the presence of cationic gold(i) catalysts to give 4-amino-5-allenylisoxazoles 2 in good to high yields. The silyl group at the terminal alkyne and a cationic gold(i) catalyst bearing a sterically bulky ligand are essential for the generation of isolable allene intermediates. The N-protection of the generated 4-amino-5-allenylisoxazoles 2 allowed the isolation of 5-allenylisoxazoles 4 that have never been synthesized. N-Propargyl aniline 5 was successfully converted to the corresponding ortho-allenyl aniline 6 under the current reaction conditions.

Methyl (2E)-3-[3-Benzyl-2-(3-methoxy-3-oxoprop-1-yn-1-yl)-2-(1-naphthyl)imidazolidin-1-yl]acrylate

Compounds with propargylamine moiety are useful synthetic precursors of several important classes of nitrogen-containing heterocycles. The title compound, methyl (2E)-3-[3-benzyl-2-(3-methoxy-3-oxoprop-1-yn-1-yl)-2-(1-naphthyl)imidazolidine-1-yl]acrylate, has been prepared by domino-reaction, employing easily available 1-benzyl-2-(1-naphthyl)-4,5-dihydro-1H-imidazole and methyl propiolate in a high 92% yield. The structure of title compound was determined using 1H-NMR, 13C-NMR, UV, FT-IR and HRMS (High-Resolution Mass Spectrometry).

C-H Sulfonylation via 1,3-Rearrangement of Sulfonyl Group in N-Protected 3-Bis-sulfonimidoindole Derivatives Using Fluorine Reagent

A C-H sulfonylation of N-protected 3-bis-sulfonimidoindole derivatives via a 1,3-rearrangement of a sulfonyl group on a bis-sulfonimide moiety using tetra-n-butylammonium fluoride (TBAF) was developed to provide 2-sulfonyl-3-sulfonamidoindole derivatives in high yields as a novel C_sp2-H functionalization of heterocycles. The rearrangement of the sulfonyl group proceeded through an intermolecular addition of the desorbed sulfinyl ion from the bis-sulfonimide moiety in the substrate.

Synthesis of 3-[(4-Nitrophenyl)thio]-Substituted 4-Methylene-1-pyrrolines from N-Propargylic β-Enaminones

A facile and efficient method for the synthesis of 3-[(4-nitrophenyl)thio]-substituted 4-methylene-1-pyrrolines is described. When treated with 4-nitrobenzenesulfenyl chloride in refluxing acetonitrile, N-propargylic β-enaminones produced α-sulfenylated N-propargylic β-enaminones, which, in the presence of sodium hydride or cesium carbonate, underwent nucleophilic cyclization to afford 4-methylene-3-[(4-nitrophenyl)thio]-1-pyrrolines in good to high yields. It was shown for the first time that on N-propargylic β-enaminone systems, α-sulfenylation dominates over the formation of thiirenium ion. This one-pot two-step process was found to be general for a variety of N-propargylic β-enaminones and demonstrated good tolerance to a diversity of aromatic and heteroaromatic groups with electron-withdrawing and electron-donating substituents. This process is also applicable to the cyclization of internal alkyne-tethered N-propargylic β-enaminones. The enrichment of 1-pyrroline core with an aryl sulfide moiety might exhibit potential for the synthesis of molecules of pharmacological interest.

Ruthenium(ii)-catalyzed intermolecular annulation of alkenyl sulfonamides with alkynes: access to bicyclic sultams

A ruthenium-catalyzed allylic C(sp3)-H activation strategy has been employed to develop an intermolecular coupling of alkenyl sulfonamides with alkynes. This protocol features the diastereoselective construction of [3.3.0] and [4.3.0] bicyclic sultams in one step.

Palladium-catalyzed one-pot cycloaddition reactions of thioureas with 3H-indol-3-ones generated in situ from 2-alkynyl arylazides: rapid and efficient access to imidazoloindolines

An efficient palladium-catalyzed one-pot synthesis of imidazoloindolines from 2-alkynyl arylazides under mild reaction conditions has been described.

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.

Base-controlled divergent synthesis of vinyl sulfones from (benzylsulfonyl)benzenes and paraformaldehyde

A tuneable metal-free protocol for the selective preparation of α-substituted vinyl sulfone and (E)-vinyl sulfone derivatives has been described. The base played an important role in the selectivity control of transformation.

Umpolung Reactivity of Ynamides: An Unconventional [1,3]-Sulfonyl and [1,5]-Sulfinyl Migration Cascade

A regioselective sulfonyl/sulfinyl migration cycloisomerization cascade of alkyne-tethered ynamides is developed in the presence of XPhosgold catalyst. This reaction is the first example of a general [1,3]-sulfonyl migration from the nitrogen center to the β-carbon atom of ynamides, followed by umpolung 5-endo-dig cyclization of the ynamide α-carbon atom to the gold-activated alkyne, and final deaurative [1,5]-sulfinylation. This process allows the synthesis of peripherally decorated unconventional 4-sulfinylated pyrroles with broad scope from N-propargyl-tethered ynamides. In contrast, N-homopropargyl-tethered ynamides undergo intramolecular tetradehydro Diels-Alder reaction to provide 2,3-dihydro-benzo[f]indole derivatives. Control experiments and density-functional theory studies were used to study the reaction pathways.

A new strategy for the synthesis of pyridines from N-propargylic β-enaminothiones

A new robust method for the synthesis of 2,4,5-trisubstituted pyridines with high efficiency and broad functional group tolerance is described.

An unprecedented N- to C-sulfonyl migration in the reaction of azomethine amine and allenoates: access to arylsulfonylmethyl substituted pyrazolo[1,5-c]quinazoline and mechanistic studies

A serendipitous discovery of [1,3]-sulfonyl migration has been made in the two-component reaction of azomethine imine and allenoates.

Facile synthesis of substituted diaryl sulfones via a [3 + 3] benzannulation strategy

A metal-free and efficient synthesis of highly substituted diaryl sulfones via a base-mediated [3 + 3] benzannulation strategy.

Ring-opening and cyclization of aziridines with aryl azides: metal-free synthesis of 6-(triflyloxy)quinolines

A metal-free synthesis of 6-(triflyloxy)quinolines has been developed via the ring-opening and cyclization of 2-aryl-1-tosylaziridines with 2-azidobenzaldehydes in the presence of TfOH.

Radical rearrangement of N-sulfonyl-N-aryl propynamides: proceeding with homolytic N–SO2 bond cleavage and 6-endo-dig cyclization toward 3-sulfonyl-2(1H)-quinolinones

A di-tert-butyl peroxide-promoted radical rearrangement of N-sulfonyl-N-aryl propynamides has been developed, giving 2-sulfonyl-3-aryl-2(1H)-quinolinones in moderate to good yields.