Metal-Free Ring-Expansion Reaction of Six-membered Sulfonylimines with Diazomethanes: An Approach toward Seven-Membered Enesulfonamides

Base mediated aza-[2 + 1] annulation and regioselective aziridine ring-opening cascade: mild synthesis of functionalized β-amino ketones from cyclic N-sulfonyl aldimines and α-carbonyl sulfonium salts

Cyclic N-sulfonyl aldimines are well-known aza-[2C]-synthons for various [2 + n] annulation reactions. Herein we describe a novel base mediated [2 + 1] annulation and a regioselective aziridine ring-opening reaction cascade, which provides an efficient and distinct synthetic strategy from readily available cyclic N-sulfonyl aldimines and α-carbonyl sulfonium salts leading to β-amino ketone derivatives through the corresponding fused tri-substituted aziridines. This one-pot, two-step process involves formation of C-C and C-N bonds and subsequent cleavage of a C-N bond. The features of the developed reaction include the use of mild reaction conditions, broad substrate scope, and excellent yields. The synthetic utility of this approach was demonstrated by gram-scale operation and further product derivatizations.

Copper(I)-Photocatalyzed Diastereoselective Aziridination of N-Sulfonyl Imines with Vinyl Azides: Application to Benzo[f][1,2,3]oxathiazepines Dioxides and Fused Isoxazolines

An in situ generated photoactive copper(I)-complex-catalyzed aziridination reaction of cyclic N-sulfonyl imines with α-aryl-substituted vinyl azides irradiated by blue-LEDs light is reported for the first time. This novel SET process represents a mild, sustainable, and pragmatic method for accessing synthetically resourceful sulfamidate-fused aziridines in acceptable chemical yields with excellent diastereoselectivities. Delightedly, pharmacologically attractive benzo[f][1,2,3]oxathiazepine dioxides and fused isoxazoline frameworks were achieved through our newly developed metal-free based ring-expansion techniques, highlighting the synthetic value of accessed aziridines. Finally, the possible mechanism for [2+1] aza-cyclization was presented based on the conduction of a series of control experiments.

Synthesis of Polycyclic Imidazolidinones via Cascade [3 + 2]-Annulation of β-Oxo-acrylamides with Cyclic N-Sulfonyl Imines

An Et3N-catalyzed cascade [3 + 2]-annulation of β-oxo-acrylamides with cyclic N-sulfonyl ketimines or sulfamate-derived imines is developed under mild reaction conditions, which provides a concise and efficient route to access valuable sultam- or sulfamidate-fused imidazolidinone derivatives in good to excellent yields (80-95% yields) with excellent diastereoselectivities (>20:1 drs). The current protocol features atom economy, a transition-metal-free process, and broad functional group tolerance. Moreover, the asymmetric variant of the [3 + 2]-cycloaddition reaction was achieved in the presence of diphenylethanediamine or quinine-based bifunctional squaramide organocatalysts C-1 and C-11, giving the corresponding chiral polycyclic imidazolidinones in 68-90% yields with 25-94% ees and >20:1 drs in all cases.

Synthesis of Cyclopenta[b]indoles via Sc(III)-Catalyzed Annulation of Vinyl Diazoacetates with Indole-Derived Unsaturated Imines

A Lewis acid Sc(OTf)₃-catalyzed annulation reaction of vinyl diazoacetates with in situ formed α,β-unsaturated imines for the preparation of indole-fused tricyclic rings has been reported. This strategy involves a sequential addition/rebound addition process of vinyl diazoacetates and an in situ dedinitrogenation. This annulation protocol features low-cost catalysts, mild reaction conditions, and facilely prepared substrates.

Synthetic Potential of Regio- and Stereoselective Ring Expansion Reactions of Six-Membered Carbo- and Heterocyclic Ring Systems: A Review

Ring expansion reactions fascinate synthetic chemists owing to their importance in synthesizing biologically active compounds and their efficacy in medicinal chemistry. The present review summarizes a number of synthetic methodologies, including stereoselective and regioselective pathways adopted by scientists, for framing medium- to large-size carbo- and heterocycles involving lactams, lactone, azepine and azulene derivatives via ring expansion of six-membered carbo- and heterocycles that have been reported from 2007–2022. Numerous rearrangement and cycloaddition reactions involving Tiffeneau–Demjanov rearrangement, Aza–Claisen rearrangement, Schmidt rearrangement, Beckmann rearrangement, etc., have been described in this regard.

Cyclization of Vinyl Diazo Compounds with Benzofuran-Derived Azadienes Enabled by NaBArF4

An unprecedented cycloaddition of vinyl diazo compounds with benzofuran-derived azadienes catalyzed by rarely independently used NaBAr^F₄ has been established. Benzofuran-fused hydropyridines were constructed with excellent yields and high diastereoselectivity via a Na⁺-catalyzed inverse-electron-demand aza-Diels-Alder reaction. Notably, this transformation also features good compatibility with a one-pot protocol to deliver the spiro[benzofuran-cyclopentene] skeleton, as well as perfect atom economy and simple reaction conditions.

Double Ring Expansion Strategy for Fused 3-Benzazepines: Alternative Synthesis of the Dolby-Weinreb Enamine

A double ring expansion strategy for constructing fused 3-benzazepines is described. The oxidative ring expansion of spiroamine compounds with N-chlorosuccinimide and subsequent ring expansion of the resulting ketiminium ion intermediates with trimethylsilyldiazomethane afforded fused 3-benzazepines in a one-pot operation. Importantly, the Dolby-Weinreb enamine, which is a key synthetic intermediate for harringtonine alkaloids, cephalotaxines, can be accessed from commercial materials in only two steps using our developed method.

Ambident reactivity of 5-aminopyrazoles towards donor-acceptor cyclopropanes

Lewis acid-catalysed reactions of donor-acceptor cyclopropanes with 1,3-disubstituted 5-aminopyrazoles were investigated. Under catalysis with gallium(III) chloride, products of the three-membered ring opening via a nucleophilic attack of the exocyclic amino group were obtained in a chemoselective manner. Oppositely, in the presence of scandium(III) triflate, products of either N-alkylation or C(4)-alkylation, or a mixture of both were formed. The products of the C(4) alkylation were transformed in one step into tetrahydropyrazolo[3,4-b]azepines that are attractive for medicinal chemistry and pharmacology.

Copper-catalyzed reactions of α,β-unsaturated N-tosylhydrazones with diaryliodonium salts to construct N-arylpyrazoles and diaryl sulfones

Herein, an economical copper-catalyzed reaction of α,β-unsaturated N-tosylhydrazones with diaryliodonium salts to construct both N-arylpyrazoles and diaryl sulfones has been developed. Both the p-toluenesulfonyl anion and the 3-arylpyrazole intermediates were formed in situ from N-tosylhydrazones. Subsequently, the former reacted rapidly with diaryliodonium salts to give diaryl sulfones and aryl iodide intermediates, and the latter reacted with aryl iodide to give N-arylpyrazoles under copper-catalyzed conditions. Using unsymmetrical mesityl phenyliodonium salts as substrates, mesityl p-toluenesulfide was obtained as the major product. This reaction took full advantage of the "waste" part of substrates to form an extra diaryl sulfone.

Visible-light-induced cyclization of cyclic N-sulfonyl ketimines to N-sulfonamide fused imidazolidines

A visible-light-induced metal-free cascade cyclization of cyclic N-sulfonyl ketimines with N-arylglycines for the construction of N-sulfonamide-fused imidazolidines was developed. The procedure employed 3 mol% of eosin Y as the photocatalyst at room temperature under visible light irradiation, providing various N-sulfonamide-fused imidazolidines in good yields (32 examples, up to 86% yields).

Pd-Catalyzed Coupling of N-Tosylhydrazones with Benzylic Phosphates: Toward the Synthesis of Di- or Tri-Substituted Alkenes

This study shows that various di- and tri-substituted alkenes with high chemoselectivity were obtained in good to high yields by coupling N-tosylhydrazones (NTHs) with benzylic phosphates as electrophilic partners. The obtained new catalytic system consisted of PdCl₂(CH₃CN)₂/dppp, LiOtBu as a base, and cyclopentyl methyl ether as a green solvent. In addition, we performed a gram-scale transformation using NTH derivatives and benzylic phosphates having a C sp²-Cl bond. The latter was used as a starting point for further postfunctionalization of the key intermediates.

Hydrazones as Substrates in the Synthesis of Isoxazolidines via a KOH-Promoted One-Pot Three-Component Cycloaddition with Nitroso Compounds and Olefins

Hydrazones have been employed as the starting materials in a KOH-mediated one-pot three-component cycloaddition with readily accessible nitroso compounds and olefins to construct various isoxazolidines. Compared with diazo compounds as starting materials, this methodology could afford a wider range of products in good to excellent yields and diastereoselectivities for most substrates, and hydrazones are cheaper, more accessible, and safer substrates. The experimental study shows that the choice of suitable hydrazones is crucial.

Leveraging the Domino Skeletal Expansion of Thia-/Selenazolidinones via Nitrogen-Atom Transfer in Hexafluoroisopropanol: Room Temperature Access to Six-Membered S/Se,N-Heterocycles

Herein, a highly regioselective domino skeletal-expansion process that transforms 2-aminothiazolidinone into six-membered S,N-heterocycle is developed with the aid of TMS-azide in hexafluoroisopropanol (HFIP) at ambient temperature. Functioning of the C2 tertiary amine as latent reactive group on thiazolidinone moiety was the key to this development, which allowed relay substitution with azide and imparted subsequent ring-expansion under metal/acid free-conditions. The reaction also underscored an intermolecular nitrogen-atom transfer process from TMS-azide leading to final products, where any intermediary azidothiazolidinone was absent. The strategy was extendable to analogous synthesis of Se,N-heterocycles, and furthermore, late-stage drug-modification and follow-up transformations were also performed. Density functional theory calculations and control experiments provided important mechanistic insights and highlighted potential roles of HFIP in the transformation.

Unmasking the reverse reactivity of cyclic N-sulfonyl ketimines: multifaceted applications in organic synthesis

The chemistry related to the exploration of cyclic N-sulfonyl ketimines and their derivatives has attracted significant attention in the last few decades because of their intriguing structures and properties. They serve broadly as reactive synthons in various reactions to create a diverse set of synthetically and biologically attractive molecules. Furthermore, these moieties, which possess multiple heteroatoms (N, O and S), display or can enhance many biological activities. In the case of synthetic reactions, chemists mainly focus on the chemical manipulation of the highly reactive prochiral CN bond of N-sulfonyl ketimines. Besides their traditional role as electrophiles, N-sulfonyl ketimines possess α-Csp³-H protons, and thus behave as potential carbonucleophiles, where they can undergo several C-X (X = C, N and O) bond-forming reactions with different types of electrophiles under various conditions to form a wide range of fascinating asymmetric and non-asymmetric versions of fused heterocycles, carbocycles, spiro-fused skeletons, pyridines, pyrroles, etc. Herein, we highlight the recent examples from our research work and others covering the scope of cyclic N-sulfonyl ketimines as useful carbonucleophiles. In addition, the detailed mechanistic studies of the above-mentioned reactions are also presented.

[2 + 2 + 1] Cycloaddition of N-tosylhydrazones, tert-butyl nitrite and alkenes: a general and practical access to isoxazolines

N-Tosylhydrazones have proven to be versatile synthons over the past several decades. However, to our knowledge, the construction of isoxazolines based on N-tosylhydrazones has not been examined. Herein, we report the first demonstrations of [2 + 2 + 1] cycloaddition reactions that allow the facile synthesis of isoxazolines, employing N-tosylhydrazones, tert-butyl nitrite (TBN) and alkenes as reactants. This process represents a new type of cycloaddition reaction with a distinct mechanism that does not involve the participation of nitrile oxides. This approach is both general and practical and exhibits a wide substrate scope, nearly universal functional group compatibility, tolerance of moisture and air, the potential for functionalization of complex bioactive molecules and is readily scaled up. Both control experiments and theoretical calculations indicate that this transformation proceeds via the in situ generation of a nitronate from the coupling of N-tosylhydrazone and TBN, followed by cycloaddition with an alkene and subsequent elimination of a tert-butyloxy group to give the desired isoxazoline.

Asymmetric Synthesis of 3-Methyleneindolines via Rhodium(I)-Catalyzed Alkynylative Cyclization of N-(o-Alkynylaryl)imines

The first asymmetric synthesis of 3-methyleneindolines from alkynyl imines has been developed via a rhodium-catalyzed tandem process: regioselective alkynylation of the internal alkynes and subsequent intramolecular addition to the imines. The reaction proceeded with unconventional chemoselectivity and provided 3-methyleneindolines with good yields (up to 82% yield) and high enantioselectivities (up to 97% ee). Moreover, this transformation also features mild reaction conditions, perfect atom economy, and a broad substrate scope.

An organophotoredox-catalyzed C(sp2)-N cross coupling reaction of cyclic aldimines with cyclic aliphatic amines

An organophotocatalyzed C(sp2)-H/N-H cross-dehydrogenative coupling of cyclic aldimines with aliphatic amines has been developed, which represents the first example of visible-light-induced C-H amination of N-sulfonylated imines. This methodology enables the streamline assembly of amine derivatives via radical mediated C-N bond formation. The current protocol features transition-metal-free, mild conditions, good functional group tolerance and good yields.

Ag-Catalyzed ring-opening of tertiary cycloalkanols for C-H functionalization of cyclic aldimines

We firstly describe a silver-catalyzed direct C-H functionalization of cyclic aldimines with cyclopropanols and cyclobutanols via a radical-mediated C-C bond cleavage strategy. The desired products were generated in decent yields with wide substrate scope under mild reaction conditions. In addition, a gram-scale reaction and synthetic transformation of the product were performed.

iTrEnDi: In Situ Trimethylation Enhancement Using Diazomethane: Improved and Expanded Glycerophospholipid and Sphingolipid Analyses via a Microscale Autonomous Derivatization Platform

Trimethylation enhancement using diazomethane (TrEnDi) is a derivatization technique that significantly enhances the signal intensity of glycerophospholipid species in mass spectrometry (MS) and tandem mass spectrometry (MS/MS) analyses. Here, we describe a novel apparatus that is able to conduct in situ TrEnDi (iTrEnDi) by generating and immediately reacting small amounts of gaseous diazoalkane with analyte molecules. iTrEnDi allows complete and rapid methylation of phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidic acid (PA), and sphingomyelin (SM) in a safe manner by removing any need for direct handling of dangerous diazoalkane solutions. iTrEnDi-modified PC ([PC^Tr]⁺) and PE ([PE^Tr]⁺) showed similar sensitivity enhancements and fragmentation patterns compared to our previously reported methodology. iTrEnDi yielded dimethylated PA ([PA^Tr]), which exhibited dramatically improved chromatographic behavior and a 14-fold increase in liquid chromatography MS (LCMS) sensitivity compared to unmodified PA. In comparison to in-solution-based TrEnDi, iTrEnDi demonstrated a modest decrease in sensitivity, likely due to analyte losses during handling. However, the enhanced safety benefits of iTrEnDi coupled with its ease of use and capacity for automation, as well as its accommodation of more-reactive diazoalkane species, vastly improve the accessibility and utility of this derivatization technique. Finally, as a proof of concept, iTrEnDi was used to produce diazoethane (DZE), a more-reactive diazoalkane than diazomethane. Reaction between DZE and PC yielded ethylated [PC^Tr]⁺, which fragmented via MS/MS to produce a high-intensity characteristic fragment ion, enabling a novel and highly sensitive precursor ion scan.

Base-Mediated Denitrogenative Sulfonylation/Benzannulation of Conjugated N-Sulfonylhydrazones with 3-Formylchromones for the Construction of Polyfunctionalized Biaryl Sulfones

Base-promoted benzannulation of conjugated N-sulfonylhydrazones and 3-formylchromones for the synthesis of diverse biaryl sulfones is described. The approach facilitates new C-C and C-S bond formation via the cascade diazo formation/Michael addition/ring opening/denitrogenative sulfonylation/intramolecular cycloaddition/dehydration and introduces diverse functional groups onto biaryl sulfones. The synthesized compounds are converted to aryl sulfones bearing bioactive benzisoxazole and benzofuran frameworks. Moreover, the synthesized biaryl sulfones possess potent turn-on fluorescence sensing and UV absorbance properties.

Synthesis of Sulfamate-Fused 2-Aminopyrroles via an Isocyanide-Based Three Component [1+2+2] Annulation

An efficient preparation of sulfamate-fused 2-aminopyrroles was achieved through an isocyanide-based three-component [1+2+2] annulation of isocyanides, dialkyl acetylenedicarboxylates, and sulfamate-derived cyclic imines in good to excellent yields (up to 99 %). This reaction proceeds smoothly without any activation or modification of substances under neutral and metal-free conditions. The reaction could also be conveniently performed on a gram scale.

Regioselective Synthesis of 3-Trifluoromethylpyrazole by Coupling of Aldehydes, Sulfonyl Hydrazides, and 2-Bromo-3,3,3-trifluoropropene

A general and practical strategy for 3-trifluoromethylpyrazole synthesis is reported that occurs by the three-component coupling of environmentally friendly and large-tonnage industrial feedstock 2-bromo-3,3,3-trifluoropropene (BTP), aldehydes, and sulfonyl hydrazides. This highly regioselective three-component reaction is metal-free, catalyst-free, and operationally simple and features mild conditions, a broad substrate scope, high yields, and valuable functional group tolerance. Remarkably, the reactions could be performed on a 100 mmol scale and smoothly afforded the key intermediates for the synthesis of celecoxib, mavacoxib, SC-560, and AS-136A. Preliminary mechanism studies indicated that a 1,3-hydrogen atom transfer process was involved in this transformation.

Copper-catalyzed carbene insertion and ester migration for the synthesis of polysubstituted pyrroles

A mild synthetic protocol to access 2-ester pyrroles through Cu(OTf)₂-catalysed carbene insertion/ester migration/cyclization of enaminones and α-diazo compounds is reported.

A base-promoted cascade reaction of α,β-unsaturated N-tosylhydrazones with o-hydroxybenzyl alcohols: highly regioselective synthesis of N-sec-alkylpyrazoles

An efficient method for the synthesis of N-sec-alkylpyrazoles via a cascade reaction of α,β-unsaturated N-tosylhydrazones with ortho-hydroxybenzyl alcohols has been developed.

Lewis Acid Triggered Vinylcyclopropane-Cyclopentene Rearrangement

We report a mild Lewis acid induced isomerization of donor-acceptor cyclopropanes, containing an alkenyl moiety and diverse electron-withdrawing group(s) at the adjacent positions, into substituted cyclopentenes. We have found that 1,1,2-trisubstituted cyclopent-3-enes were exclusively obtained in yield of 51-99% when cyclopropanes with a 2-substituted alkenyl group as a donor underwent isomerization. For cyclopropanes bearing a trisubstituted alkenyl group either the corresponding cyclopent-3-enes or isomeric cyclopent-2-enes having two acceptor groups at the C(1) atom were formed, with the reaction selectivity being determined by the applied Lewis acid. We have shown that the reactivity of the donor-acceptor cyclopropane increases with the increase of the electron-donating character of (hetero)aromatic group attached to the alkenyl moiety. The synthetic utility of the developed methodology was also demonstrated through the synthesis of polysubstituted cyclopentane and piperidine derivatives.

Trivalent Y3+ ionic sensor development based on (E)-Methyl-N'-nitrobenzylidene-benzenesulfonohydrazide (MNBBSH) derivatives modified with nafion matrix

(E)-Methyl-N'-nitrobenzylidene-benzenesulfonohydrazide (MNBBSH) compounds were synthesized using a condensation procedure from the derivatives of nitrobenzaldehyde and 4-Methyl-benzenesulfonylhydrazine, which crystallized in ethanol and methanol as well as characterized by FTIR, UV-Vis, 1H-NMR, and 13C-NMR. MNBBSH structure was confirmed using a single crystal X-ray diffraction technique and used for the detection of selective yttrium ion (Y3+) by I-V system. A thin layer of MNBBSH was deposited onto a glassy carbon electrode (GCE) with 5% nafion for the sensitive and selective Y3+ sensor. The modified MNBBSH/GCE sensor is exhibited the better electrochemical performances such as sensitivity, limit of detection (LOD), linear dynamic range (LDR), limit of quantification (LOQ), short response time, and long term storage ability towards the selective metal ion (Y3+). The calibration curve of 2-MNBBSH/GCE sensor was plotted at +1.1 V over a broad range of Y3+ concentration. Sensitivity, LOD, LDR and LOQ of the fabricated sensor towards Y3+ were calculated from the calibration curve and found as 1.90 pAμM-1 cm-2, 10.0 pM, 1.0 nM~1.0 mM and 338.33 mM respectively. The 2-MNBBSH/Nafion/GCE sensor was applied to the selective determination of Y3+ in spiked samples such as industrial effluent and real water samples from different sources, and found acceptable and reasonable results.

Gold-catalyzed ring-expansion through acyl migration to afford furan-fused polycyclic compounds

A gold-catalyzed ring-expansion reaction of alkynones to access furan-fused polycyclic compounds is reported.

Transition-metal-free cascade reaction of α-halo-N-tosylhydrazones, indoles and arylboronic acids

A new transition-metal free three-component coupling of α-halo-N-tosylhydrazones, indoles and arylboronic acids, leading to the synthesis of substituted indole derivatives, has been developed.