Metal-Free [2 + 2 + 1] Annulation of Alkynes, Nitriles, and Oxygen Atoms: Iodine(III)-Mediated Synthesis of Highly Substituted Oxazoles

Activation of enamine by photoexcited organocatalyst assisted singlet oxygen: synthesis of oxazoles and quinoxalines

Herein, a novel transition-metal-free thiol-based donor-acceptor organophotocatalyst-assisted, singlet-oxygen-mediated tandem oxidative cyclization for the synthesis of substituted oxazoles in moderate-to-good yields is described. The developed method demonstrates applicability for the synthesis of various substituted quinoxalines in good-to-excellent yields. The metal-free methodology shows a practical route for the synthesis of oxazole and quinoxaline derivatives, which are privileged moieties prevalent in various biologically active compounds and natural products. To the best of our knowledge, both the thiol photocatalyst and synthesis of oxazoles by visible-light irradiation are reported for the first time.

Photochemical three-component assembly of tri-substituted oxazoles through a carbenic phosphorus-nitrile hybrid ylide formation/trapping cascade

Construction of complex molecular skeletons with ubiquitous chemical feedstocks in a single transformation is highly appealing in organic synthesis. We report a novel visible-light-induced three-component reaction for the construction of complex 2,4,5-trisubstituted oxazoles, which are valuable in medicinal chemistry, from simple and readily available iodonium-phosphonium hybrid ylides, carboxylic acids, and nitriles. This reaction features a carbenic phosphorus-nitrile hybrid ylide formation/trapping cascade, in which a photo-generated α-phosphonium carbene acts as a sequence trigger. This catalyst- and additive-free transformation exhibits high efficiency and broad substrate scope for synthesizing diverse oxazoles.

Silver-Mediated [2 + 2 + 1] Cyclization of ortho-Propioloylbenzonitriles with Elemental Selenium: Synthesis of 4H-indeno[1,2-c][1,2]selenazol-4-ones

An efficient silver-mediated [2 + 2 + 1] cyclization protocol of ortho-propioloylbenzonitriles with elemental selenium for the synthesis of 4H-indeno[1,2-c][1,2]selenazol-4-ones has been developed. One C-Se bond, one N-Se bond, and one C-C bond were rapidly constructed in one step. The reaction might proceed via the formation of a highly reactive selenoketene intermediate, followed by intramolecular cyclization.

Pd-Catalyzed Asymmetric Oxidative C-H/C-H Cross-Coupling Reaction between Ferrocenes and Azoles

The asymmetric C-H bond functionalization reaction is one of the most efficient and straightforward methods for the synthesis of optically active molecules. Herein, our work discovered a Pd-catalyzed asymmetric oxidative C-H/C-H cross-coupling reaction of ferrocenes with azoles such as oxazoles and thiazoles. Mono-N-protected amino acid as chiral ligands with palladium(II) has been demonstrated as an effective catalytic system in a weakly azine-directed asymmetric C-H bond functionalization reaction. This method offers a powerful strategy for constructing various substituted planar chiral ferrocenes via a dual C-H bond activation pathway in medium yields (up to 70%) with good enantioselectivity (up to 95.3:4.7 er) under mild conditions.

Development of β,β-Dibrominated Secondary Enamides and Their Application to the Synthesis of 5-Br Oxazoles

Herein, we report a base promoted dibromination of enamides using CBr₄ as a bromine source to provide expedient access to β,β-dibrominated secondary enamides. Preliminary synthetic application study shows that these novel products can be readily transformed to 5-Br oxazoles via Cu(I) catalyzed intramolecular cyclization in good yields.

In Situ Generation of N-Triflylimino-λ3-iodanes: Application to Imidation of Phosphines and Catalytic α-Amidation of 1,3-Dicarbonyl Compounds

We describe the imidation of phosphines and α-amidation of 1,3-dicarbonyl compounds using N-triflylimino-λ³-iodane, which is generated in situ from iodosylarene and triflylamide without any other additives. Furthermore, the imino-λ³-iodane catalytically generated from an iodoarene precatalyst with oxone and triflylamide promotes α-amidation of 1,3-dicarbonyl compounds, representing the first method catalyzed by imino-λ³-iodane.

Synthesis of Tetrahydrofuro[2,3-d]oxazoles and Oxazoles by Hypervalent Iodine (III)-Promoted [2 + 2 + 1] Annulation

Reaction of a hypervalent iodine reagent with bistriflimide efficiently promotes three-component regioselective cyclization of tetrahydrofuro[2,3-d]oxazoles and oxazoles from homopropargyl alcohols bearing a phenyl group, with different substituents on the aryl alkyne compounds affecting the selectivity of the resulting product. Utilizing the hydroxyethyl oxazole derivatives obtained in this research could aid in the development of various peroxisome proliferator-activated receptor agonist derivatives.

PhIO-Mediated Oxidative C═C Bond Cleavage and Reassembly toward Highly Functionalized Oxazolones

An efficient PhIO-mediated oxidative C═C bond cleavage and reassembly of enaminone toward oxazolone with high regioselectivity has been reported. DFT calculations revealed that the reaction proceeded through an oxygen atom transfer, C═C bond cleavage, alkylthio migration, and reassembly cascade. This strategy is highlighted by high atom and step economy with formation of five bonds in one pot and generation of a high-valued oxazolone skeleton under mild conditions.

An umpolung strategy for intermolecular [2 + 2 + 1] cycloaddition of aryl aldehydes and nitriles: a facile access to 2,4,5-trisubstituted oxazoles

We have described the first example of an umpolung strategy for intermolecular [2 + 2 + 1] cycloaddition between two aryl aldehydes and a nitrile under the influence of TMSOTf that proceeds through the formation of N–C, O–C and C–C bonds providing a simple synthetic protocol for obtaining 2,4,5-trisubstituted oxazoles.

An unprecedented intermolecular [2 + 2 + 1] cycloaddition strategy between two aryl aldehydes and a nitrile, wherein one of the aryl aldehydes serves as a carbanion (or equivalent) in the presence of TMSOTf for obtaining oxazole framework is presented.

Ritter-type iodo(iii)amidation of unactivated alkynes for the stereoselective synthesis of multisubstituted enamides

The Ritter reaction, Brønsted- or Lewis acid-mediated amidation of alkene or alcohol with nitrile via a carbocation, represents a classical method for the synthesis of tertiary amides. Although analogous reactions through a vinyl cation or a species alike may offer a route to enamide, an important synthetic building block as well as a common functionality in bioactive compounds, such transformations remain largely elusive. Herein, we report a Ritter-type trans-difunctionalization of alkynes with a trivalent iodine electrophile and nitrile, which affords β-iodanyl enamides in moderate to good yields. Mediated by benziodoxole triflate (BXT), the reaction proves applicable to a variety of internal alkynes as well as to various alkyl- and arylnitriles. The benziodoxole group in the product serves as a versatile handle for further transformations, thus allowing for the preparation of various tri- and tetrasubstituted enamides that are not readily accessible by other means.

Sustainable Access to 5-Amino-Oxazoles and Thiazoles via Calcium-Catalyzed Elimination-Cyclization with Isocyanides

Herein, we report a sustainable, modular, rapid and high-yielding transformation to afford densely functionalized 5-aminooxazoles and thiazoles. The reaction is tolerant to a wide range of functional groups and is typically complete in under 30 min. Furthermore, the described transformation is inherently green in relation to the catalyst and solvent choice as well as producing environmentally benign alcoholic by-products.

Iodonium Cation-Pool Electrolysis for the Three-Component Synthesis of 1,3-Oxazoles

The synthesis of 1,3-oxazoles from symmetrical and unsymmetrical alkynes was realized by an iodonium cation-pool electrolysis of I2 in acetonitrile with a well-defined water content. Mechanistic investigations suggest that the alkyne reacts with the acetonitrile-stabilized I+ ions, followed by a Ritter-type reaction of the solvent to a nitrilium ion, which is then attacked by water. The ring closure to the 1,3-oxazoles released molecular iodine, which was visible by the naked eye. Also, some unsymmetrical internal alkynes were tested and a regioselective formation of a single isomer was determined by two-dimensional NMR experiments.

Retracted Article: Metal-free [2+2+1] cycloaddition polymerization of alkynes, nitriles, and oxygen atoms to functional polyoxazoles

The metal-free [2+2+1] cycloaddition polymerization of alkynes, nitriles, and O-atoms for the regioselective assembly of highly substituted oxazole compounds has been achieved by the use of iodosobenzene (PhIO) with trifluoromethanesulfonic acid (TfOH). The present reaction could be applied to a facile synthesis of polyoxazoles. In this work, the cycloaddition polymerization of 4-cyano-4'-ethynylbiphenyl and PhIO was developed and modified polyoxazole was prepared. All experimental conditions such as polymerization solvent, temperature, catalyst and time were systematically studied. The structure of the obtained polyoxazole was characterized by GPC and NMR, and its thermal properties were studied by TGA. In addition, the good thermal stability of polyoxazoles with unreacted terminal alkynes and cyano groups makes them potentially useful for modifying resins.

Iron Nitrate-Mediated Selective Synthesis of 3-Acyl-1,2,4-oxadiazoles from Alkynes and Nitriles: The Dual Roles of Iron Nitrate

A direct strategy for the selective synthesis of 3-acyl-1,2,4-oxadiazoles from alkynes and nitriles has been developed under iron(III) nitrate-mediated conditions. The mechanism includes three sequential procedures: iron(III) nitrate-mediated nitration of alkynes leads to α-nitroketones, dehydration of α-nitroketones provides the nitrile oxides, and 1,3-dipolar cycloaddition of nitrile oxides with nitriles produces 3-acyl-1,2,4-oxadiazoles under iron-mediated conditions. Iron(III) nitrate plays dual roles in the nitration of alkynes and the activation of nitriles, while the formation of pyrimidine/isoxazole byproducts can be efficiently inhibited.

Iodine-Catalyzed Functionalization of Primary Aliphatic Amines to Oxazoles, 1,4-Oxazines, and Oxazinones

Unprecedented I₂-catalyzed α,α-C(sp³)-H, decarboxylative α-C(sp³)-H, lactonized α-C(sp³)-H, and α,β-C(sp³)-H functionalized 5- and 6-annulation as well as α-C(sp³)-H activated 6-lactonization of primary aliphatic amines are devised under aerobic conditions. The metal-free sustainable strategy was employed for the diverse construction of valuable five-and six-membered polycyclic N,O-heteroaromatics such as oxazoles, 1,4-oxazines, and oxazin-2-one with a rapid reaction rate and high yield. The viability of this mild nonmetallic catalysis is successfully verified through syntheses of labile chiral heterocyclic analogues. In contrast to the common practice, this method is not limited to use of prefunctionalized amines, directing groups (DGs) and/or transient DGs, metal catalysts, and traditional oxidants. The possible mechanistic pathway of the annulation reaction is investigated by control experiments and ESI-MS data collected for a reaction mixture of the ongoing reaction. The synthesized new compounds are potent organic nanobuilding blocks to achieve valuable organic nanomaterials of different sizes, shapes, and dimensions, which are under investigation for the discovery of high-tech devices of innovative organic nanoelectronics and photophysical properties.

Synthesis of Multifunctional Aryl(trifloxyalkenyl)iodonium Triflate Salts

A convenient procedure for the synthesis of aryl(trifloxyalkenyl)iodonium triflate salts from commercially available (diacetoxyiodo)benzene, trimethylsilyl trifluoromethanesulfonate, and acetylenes under mild conditions was developed. The obtained multifunctional hypervalent vinyliodonium salts equipped with electrophilic and nucleophilic functions could serve as novel C2 synthons for organic transformations. The structure of the iodonium salts was identified by multidimensional NMR spectroscopy and X-ray crystallography.

Metal-Free Radical [2 + 2 + 1] Cyclization of ortho-Cyanoarylacrylamides with Alkyl Nitriles: Synthesis of Pyrrolo[3,2- c]quinolines

A novel radical [2 + 2 + 1] cyclization of ortho-cyanoarylacrylamides with dual α-C-H bonds in alkyl nitriles has been developed. The reaction provides new facile and straightforward access to cyano-substituted pyrrolo[3,2- c]quinolines, which are important nitrogen-containing polyheterocycles. A possible mechanism for the transformation is proposed.

Gold-catalyzed dual annulation of azide-tethered alkynes with nitriles: expeditious synthesis of oxazolo[4,5-c]quinolines

A gold-catalyzed dual annulation of azide-tethered internal alkynes, which provides convenient access to quinoline derivatives, has been reported.

Ruthenium(ii)-catalyzed C–O/C–S cyclization for the synthesis of 5-membered O-containing and S-containing heterocycles

An efficient and convenient method for the synthesis of oxazole derivatives from enamides has been established via a ruthenium-catalyzed C–O cyclization.

2-Picoline catalyst-triggered [2 + 2 + 2] cycloaddition-type reaction of acetylenedicarboxylates, aldehydes and alkenes

2-Picoline efficiently catalyzes the formation of α,β-enones from acetylenedicarboxylates and aldehydes in the presence of alkenes, thereby leading to pyrans in moderate to good yields with complete regioselectivities. This synthetic method of pyrans is represented as a first example of catalytic and direct [2 + 2 + 2] cycloaddition of three different components under the metal-free conditions.

Metal-free iodine(iii)-promoted synthesis of 2,5-diaryloxazoles

A nonmetal-catalyzed oxidative cyclization to achieve 2,5-disubstituted oxazoles from inexpensive and readily available substituted chalcone, (diacetoxyiodo)benzene (PIDA) and ammonium acetate (NH4OAc) at room temperature is described. The reaction forms a variety of 2,5-diaryloxazoles in good to excellent yields with broad substrate scope under mild conditions without the requirement of ligands and additional bases.

Copper-Catalyzed Oxygenation Approach to Oxazoles from Amines, Alkynes, and Molecular Oxygen

A novel and efficient oxygenation approach to trisubstituted oxazoles via a copper-catalyzed aerobic oxidative dehydrogenative annulation of amines, alkynes, and O₂ has been developed. This transformation combines dioxygen activation and oxidative C-H bond functionalization and provides a practical protocol for the preparation of oxazole derivatives, which are privileged units found in various bioactive compounds or other natural products. ¹⁸O-labeling experiments were conducted to reveal that oxygenation was involved in this chemistry.

Multicomponent reactions: a sustainable tool to 1,2- and 1,3-azoles

The present review outlines the recent advancements and pioneering efforts on the synthesis of 1,2/1,3-azoles employing a multicomponent strategy.

Synthesis of 2,5-disubstituted oxazoles via cobalt(iii)-catalyzed cross-coupling of N-pivaloyloxyamides and alkynes

An efficient synthesis of 2,5-disubstituted oxazoles via cobalt(iii)-catalyzed cross-coupling of N-pivaloyloxyamides and alkynes is described herein.

Benziodoxole Triflate as a Versatile Reagent for Iodo(III)cyclization of Alkynes

The utility of benziodoxole triflate, derived from α,α-bis(trifluoromethyl)-2-iodobenzyl alcohol, as a versatile reagent for iodo(III)cyclization via electrophilic activation of alkyne, is reported herein. The reagent promotes cyclization of alkynes tethered to a variety of nucleophilic moieties, affording benziodoxole-appended (hetero)arenes such as benzofurans, benzothiophenes, isocoumarins, indoles, and polyaromatics under mild conditions. This unprecedented class of (hetero)aryl-I^III compounds proved easy to purify, stable, and amenable to various synthetic transformations.

Development of a method for the synthesis of 2,4,5-trisubstituted oxazoles composed of carboxylic acid, amino acid, and boronic acid

A novel method for the synthesis of trisubstituted oxazoles via a one-pot oxazole synthesis/Suzuki–Miyaura coupling sequence has been developed. One-pot formation of 5-(triazinyloxy)oxazoles using carboxylic acids, amino acids and a dehydrative condensing reagent, DMT-MM, followed by Ni-catalyzed Suzuki–Miyaura coupling with boronic acids provided the corresponding 2,4,5-trisubstituted oxazoles in good yields.

A Heterogeneous Gold(I)-Catalyzed [2 + 2 + 1] Annulation of Terminal Alkynes, Nitriles, and Oxygen Atoms Leading to 2,5-Disubstituted Oxazoles

The first heterogeneous gold(I)-catalyzed [2 + 2 + 1] annulation of terminal alkynes, nitriles, and oxygen atoms has been achieved by using an MCM-41-immobilized phosphine-gold(I) complex as catalyst and 8-methylquinoline N-oxide as oxidant under mild conditions, yielding a variety of 2,5-disubstituted oxazoles in good to excellent yields with broad substrate scope. The new heterogeneous gold(I) catalyst can easily be recovered by simple filtration of the reaction solution and recycled for at least eight times without significant loss of activity.

Dehydrogenative Aromatic Ring Fusion for Carbazole Synthesis via C-C/C-N Bond Formation and Alkyl Migration

An intermolecular dehydrogenative annulation (IDA) for carbazole synthesis via sequential C-C/C-N bond formation with a selective alkyl group migration is reported. Using the hypervalent iodine(III) reagent PhI(OAc)₂ (PIDA), in a one-pot operation, up to five C(sp2)-H bonds, one N(sp3)-H bond functionalization, and one alkyl (Me, Et) group migration could all be achieved from non-prefunctionalized 1,3,5-trialkylbenzenes and anilides under ambient laboratory conditions. Mechanistically, it is shown that PIDA reacts with anilides to generate a nitrenium ion or an equivalent carbenium ion which influences the second aromatic ring to be activated for C-C/C-N bond formation. Strategically, regioselective fusion of arenes to anilides is described.