DMAP-Catalyzed Annulation Approach for Modular Assembly of Furan-Fused Chromenes

Copper-Catalyzed Three-Component Tandem Cyclization for One-Pot Synthesis of Indole-Benzofuran Bis-Heterocycles

A one-pot, three-component synthesis of indole-benzofuran bis-heterocycles from terminal alkynes, salicylaldehydes, and indoles has been developed via copper-catalyzed tandem annulation. This catalytic system utilizes readily available starting materials, enabling predictable synthesis of indole-benzofuran bis-heterocycles with broad substrate versatility, excellent regiocontrol, and gram-scale amenability. The reaction proceeds via a sequential pathway involving A3 coupling, 1,4-conjugate addition, and 5-exo-dig cyclization.

Oxidative [4+2] Annulation of Pyrrole-2-carbaldehyde Derivatives with o-Hydroxyphenyl Propargylamines: Syntheses of 5,6,7-Trisubstituted Indolizines

A base promoted oxidative [4+2] annulation of pyrrole-2-carbaldehyde derivatives with o-hydroxyphenyl propargylamines for the synthesis of highly substituted indolizines has been developed. Using DBN as base, a broad range of 5,6,7-trisubstituted indolizines have been prepared in good to excellent yields under mild conditions, and many useful functional groups can be tolerated.

Base-Mediated Cascade Lactonization/1,3-Dipolar Cycloaddition Pathway for the One-Pot Assembly of Coumarin-Functionalized Pyrrolo[2,1-a]isoquinolines

An elegant and highly concise strategy for the construction of coumarin-functionalized pyrrolo[2,1-a]isoquinolines from available propargylamines and isoquinolinium N-ylides has been disclosed. In this reaction, isoquinolinium N-ylides acted as a C2 synthon to form a coumarin ring as well as a 1,3-dipole to construct a pyrrole ring in a single pot. This cascade process involves 1,4-conjugate addition/lactonization/1,3-dipolar cycloaddition to construct four chemical bonds (one C-O bond and three C-C bonds) and two new heterocyclic skeletons. Additionally, most of these compounds showed good fluorescence properties and exhibited high molar extinction coefficient and large Stokes shifts.

Modular Assembly of Pyrrolo[3,4-c]isoquinolines through Rh-Catalyzed Cascade C-H Activation/Annulation of O-Methyl Aryloximes with Maleimides

An efficient and practical strategy for the construction of pyrrolo[3,4-c]isoquinolines via Rh(III)-catalyzed cascade C-H activation and subsequential annulation process from easily available O-methyl aryloximes and maleimides has been disclosed. This facile protocol does not require any inert atmosphere protection with good efficiency in a low loading of catalyst and exhibits good functional group tolerance and broad substrate scope. Notably, the as-prepared products show potential photophysical properties.

Modular access to furo[3,2-c]chromen-4-ones via Yb(OTf)3-catalyzed [3 + 2] annulation of 4-hydroxycoumarins with β-nitroalkenes

A facile and efficient strategy for modular access to furo[3,2-c]chromen-4-ones using 4-hydroxycoumarin and β-nitroalkenes via Lewis acid-catalyzed formal [3 + 2] annulation protocol is described. This reaction proceeds via cascade Michael addition/nucleophilic addition/elimination in the presence of Yb(OTf)3, which involves the formation of two new σ (C-C and C-O) bonds for the construction of a novel furan ring in a single operation. This protocol affords a variety of functional groups, thereby providing a practical and efficient method for the fabrication of a furo[3,2-c]chromen-4-one framework.

Cellulose Functionalization Using N-Heterocyclic-Based Leaving Group Chemistry

There has been continuous interest in developing novel activators that facilitate the functionalization of cellulosic materials. In this paper, we developed a strategy in which trisubstituted triazinium salts act as cellulose preactivators. As leaving groups, these triazinium salts utilize N-heterocycles (pyridine, imidazole, and nicotinic acid). Initially, we optimized the synthetic route for developing these novel cellulose preactivators (triazinium salts), whose structures were confirmed using NMR spectroscopy. The surface zeta potential of cellulose changed from a negative value to a positive one after preactivation due to the cationic nature of these preactivators. To enhance the scope of the study, we functionalized the cellulose-preactivated materials with a series of amine- or hydroxy-containing aliphatic and aromatic hydrocarbons, nucleophilic amino acids (cysteine), colorants (2-aminoanthraquinone and 2-amino-3-methyl-anthraquinone), and biopolymer (zein protein). The treated samples were analyzed using FTIR, time-gated Raman spectroscopy, and reflection spectroscopy, and the success of the functionalization process was validated. To widen the scope of such chemistries, we synthesized four reactive agents containing N-heterocyclic-based leaving groups (pyridine and nicotinic acid) and successfully functionalized cellulose with them in one step. The proposed single- and two-step functionalization approaches will provide opportunities for chemically linking various chemical compounds to cellulose for different applications.

Cascade Annulation Strategy for Expeditious Assembly of Hydroxybenzo[c]chromen-6-ones and Their Photophysical Property Studies

A 1,8-diazabicyclo[5.4.0]undec-7-ene-promoted cascade double-annulation of ortho-alkynyl quinone methide (in situ generated from modular propargylamine) for constructing of 2-aryl-4-hydroxybenzo[c]chromen-6-ones is developed. This synthetic strategy offers remarkable operational simplicity as it allows the use of benchtop-grade solvents without the need for predrying measures and inert atmosphere protection. Additionally, it demonstrates good functional group compatibility. The photophysical properties of these compounds were also examined, revealing bright fluorescence with high quantum yields.

The L-proline modified Zr-based MOF (Basu-proline) catalyst for the one-pot synthesis of dihydropyrano[3,2-c]chromenes

A novel, reusable, and efficient L-proline-modified Zr-based metal-organic framework (Basu-proline) was designed, synthesized, and characterized by Fourier Transform-Infrared spectroscopy (FT-IR), Energy-Dispersive X-ray spectroscopy (EDX), elemental mapping, Field Emission Scanning Electron Microscopy (FE-SEM), X-ray Diffraction analysis (XRD), Thermo-Gravimetric-Differential Thermal Analysis (TGA-DTA), and N2 adsorption-desorption isotherms. Then, its catalytic performance was assessed in the synthesis of dihydropyrano[3,2-c]chromenes via the one-pot, three-component tandem condensation reaction of 4-hydroxycoumarin, aromatic aldehydes and malononitrile. The Basu-proline catalyst exhibited a better efficiency than some reported protocols regarding higher yields, lower reaction times, and simple separation.

Sulfur-Promoted Oxidative Cyclization of Pentan-1-ones: Direct Access to Tetrasubstituted Furans from Deoxybenzoins and Chalcones

Furan is an important heterocyclic scaffold in natural product, bioorganic, and medicinal chemistry as well as in materials science. The system S8/DABCO/DMSO was found to efficiently mediate the oxidative cyclization of 1,2,3,5-tetraarylpentan-1-ones A, which were obtained in situ as the Michael adducts of chalcones 1 and deoxybenzoins 2, to furan 3. The strategy provided convenient and direct access to tetrasubstituted furans 3 from readily available starting materials with high functional group tolerance.

Temperature-Controlled Chemoselective Synthesis of Multisubstituted 4-Alkynyl/trans 4-Alkenyl Coumarins

A temperature-controlled facile synthesis of multisubstituted 4-alkynyl/trans 4-alkenyl coumarins with a metal salt cascade approach is reported. H₂O serves both as a nucleophile and hydrogen source. The presence of metal salt facilitates the reduction of alkyne. The present protocol bypasses the structural shortcomings of the existing Sonogashira and Heck coupling reactions. In addition, the obtained 2,3-disubstituted coumarins are readily transformed into 2,3-disubstituted dihydrocoumarins, which serve as important building blocks in organic transformations.

Base-Promoted Reaction of 4-Chloro-3-vinyl Coumarins, Phenacylpyridinium Bromides, and Elemental Sulfur: A Designed Approach to Thiopyrano[4,3-c]chromen-5(1H)-ones

A designed synthesis of thiopyrano[4,3-c]chromen-5(1H)-ones was developed based on a base-promoted three-component reaction between 4-chloro-3-vinyl coumarins, phenacylpyridinium bromides, and elemental sulfur. Readily available starting materials, mild conditions, chemoselectivity, operational simplicity, and synthetically useful yields are some highlighted advantages of these transformations.

Radical-Induced Cascade Annulation/Hydrocarbonylation for Construction of 2-Aryl-4H-chromen-4-ones

A robust metal- and solvent-free cascade radical-induced C-N cleavage/intramolecular 6-endo-dig annulation/hydrocarbonylation for the synthesis of the valuable 2-aryl-4H-chromen-4-ones is described. This practical synthesis strategy utilizes propargylamines and air as the oxygen source and green carbonylation reagent, in which propargylamines are activated by the inexpensive and available dimethyl 2,2'-azobis(2-methylpropionate) (AIBME) and (PhSe)2 as the radical initiators. This simple and green protocol features wide substrate adaptability, good functional group tolerance, and amenability to scaling up and derivatizations.

Diastereoselective Synthesis of Chromeno[3,2-d]isoxazoles via Brønsted Acid Catalyzed Tandem 1,6-Addition/Double Annulations of o-Hydroxyl Propargylic Alcohols

A Brønsted acid catalyzed tandem process to access densely functionalized chromeno[3,2-d]isoxazoles with good to excellent yields and diastereoselectivities was disclosed. The procedure is proposed to involve a 1,6-conjugate addition/electrophilic addition/double annulations process of alkynyl o-quinone methides (o-AQMs) in situ generated from o-hydroxyl propargylic alcohols with nitrones. Mild conditions, good functional group compatibility, easy scale-up of the reaction, and further product transformation demonstrated its potential application.

A Metal-Free and Redox-Neutral Benzylic C-O Cyclization for Succinct Access of Helical Chromenes

A redox-neutral benzylic C-O cyclization under beneficial transition-metal-free conditions is reported. Key to the success of this process is the utilization of the Tf anion as the leaving group for achieving the redox-neutral transformation. This protocol delivers a series of captivating helical compounds having various functionality in good-to-excellent yields. It is particularly noteworthy that sterically hindered helical compounds are conformationally stable. In addition to simple helical chromenes, the bihelical multiple-ring systems which are potentially useful in material chemistry are also easily attained by employing this method. DFT calculation revealed that quinone intermediate is the key species, among four possible mechanisms, for accomplishing the desired cyclization via an oxa-6π-electrocyclization pathway.

Efficient Synthesis of 3-Sulfonyl-2-sulfonylmethyl-2H-chromenes via Tandem Knoevenagel Condensation/Oxa-Michael Addition Protocol

An organocatalytic [4 + 2] cascade annulation of salicylaldehydes and 1,3-bisarylsulfonylpropenes has been developed. This protocol enables the efficient and straightforward synthesis of a new series of 3-sulfonyl-2-sulfonylmethyl-2H-chromenes that are useful for exploring pharmacologically valued compounds. Further reductive modifications result in 3-desulfonylated chromene or chromane derivatives. This protocol can be expanded to the synthesis of 3-sulfonyl-2-sulfonylmethyl 1,2-dihydroquinoline.

Copper-Catalyzed Cascade Annulation of o-Hydroxyphenyl Propargylamines with Pyrazolin-5-ones to Access Pyrano[2,3-c]pyrazoles

An efficient copper-catalyzed cascade annulation of o-hydroxyphenyl propargylamines and pyrazolin-5-ones is described. This methodology leads to the rapid assembly of a series of valuable pyrano[2,3-c]pyrazoles with good yields across a wide range of substrates in a simple fashion. This novel reaction involves the formation of alkynyl ortho-quinone methides, a 1,4-conjugate addition, and a subsequent 6-endo cyclization process. The mechanistic elucidation is well supported by control experiment and literature precedents.

N-Difluoromethylation of N-pyridyl-substituted anilines with ethyl bromodifluoroacetate

A general protocol for N-difluoromethylation of aniline derivatives is developed. Commercially available ethyl bromodifluoroacetate serves as a difluorocarbene source in the presence of a base. This carbene surrogate is attractive owing to its favorable stability, environmental friendliness and inexpensiveness. This reaction system features notable operational simplicity (bench top-grade solvents can be used without any pre-drying and do not require inert atmosphere protection). A wide range of functional groups in aniline derivatives are well-tolerated, and good-to-excellent product yields are generally obtained.

Titanium-Mediated Domino Cross-Coupling/Cyclodehydration and Aldol-Addition/Cyclocondensation: Concise and Regioselective Synthesis of Polysubstituted and Fused Furans

Titanium enolates, in situ-generated from readily available ketones and titanium tetraisopropoxide, undergo domino cross-coupling/cyclodehydration or domino Aldol-addition/cyclocondensation with α-chloroketones to provide synthetically valuable furan derivatives. The domino process tolerates a variety of cyclic and acyclic ketones and chloroketones, producing polysubstituted furans and bi-, tri-, and tetracyclic fused furans.

Lewis-Acid-Catalyzed Regioselective Construction of Diversely Functionalized Polycyclic Fused Furans

A novel, facile, and efficient Lewis-acid-catalyzed [4 + 1] annulation protocol for the construction of functionalized polycyclic-fused furans is developed. This methodology is free of transition metals and ligands and provides a rapid synthetic route to divergently orientated polycyclic furans in good yields. In addition, this protocol can also be used to synthesize multisubstituted furans.

Synthesis of 2H-chromenes: recent advances and perspectives

As one type of chromene, 2H-chromenes represent important oxygen heterocycles, which not only widely exist in natural products, pharmaceutical agents and biologically relevant molecules, but have also been used broadly in materials science and organic synthesis. Two major synthetic strategies have been developed towards such compounds. This review mainly focuses on the recent advances in this field, including benzopyran ring formation involving cyclization reactions and the late-stage functionalization of the parent 2H-chromenes.

Formal (3 + 1 + 1) Carboannulation of Morita-Baylis-Hillman Carbonates with Pyridinium Ylides: Access to Spiro-Cyclopentadiene Oxindoles

An efficient formal (3 + 1 + 1) carboannulation strategy of Morita-Baylis-Hillman (MBH) carbonates with pyridinium ylides was developed for constructing diversely functionalized spiro-cyclopentadiene oxindoles. The reaction initiates with an S_N2' olefination of MBH carbonates with pyridinium ylides. The in situ generated dienes then engage in a challenging (4 + 1) ylide carboannulation, which has been rarely reported before. The reaction features broad substrate scope as well as high chemo- and regioselectivity. (3 + 1 + 1) carboannulation products could be easily transformed into interesting spiro-cyclopenta[c]furan oxindoles.

Assembly of Furazan-Fused Quinolines via an Expeditious Metal-Free [2+2+1] Radical Tandem Cyclization Process

A [2+2+1]-NO-segment-incorporating heteroannulative cascade is described. This versatile method, particularly using modular cyanoarylated ketimine substrates, allows efficient access to structurally diversified quinolines embedded with an oxadiazole core. This metal-free protocol proceeds smoothly at 30 °C, offers easy manipulation of substituents on the quinoline moiety, and tolerates a spectrum of functional groups. Density functional theory calculation revealed that the cyano moiety is crucial to facilitate the early cyclization step in this heteroannulation process and is different from the previously established late cyclization mechanistic interpretation.

Cascade Lactonization/Benzannulation of Propargylamines with Dimethyl 3-Oxoglutarate for Modular Assembly of Hydroxylated/Arene-Functionalized Benzo[c]chromen-6-ones

A DBU-mediated cascade strategy of propargylamines with dimethyl 3-oxoglutarate for constructing a functionalized benzo[c]chromen-6-one core has been achieved. This cascade process presumably involves a sequence of 1,4-conjugate addition, followed by lactonization, alkyne-allene isomerization, enol-keto tautomerization, 6π-electrocyclization, and aromatization. This protocol features mild reaction conditions, simple operation, rich structural diversity, and good functional group tolerance. A photophysical survey reveals that the benzo[c]chromen-6-one products exhibit fluorescence properties and show potential for exploring fluorescent material applications.

A cascade double 1,4-addition/intramolecular annulation strategy for expeditious assembly of unsymmetrical dibenzofurans

Existing synthetic routes for accessing dibenzofuran core have intrinsic regioselectivity, limiting the substitution patterns available in heteropolycyclic arene products. Here we report a double 1,4-conjugate addition/intramolecular annulation cascade reaction between propargylamines and two equivalents of imidazolium methylides that allows efficient access of structurally versatile dibenzofurans. This transition metal-free protocol proceeds smoothly under bench-top air atmosphere and offers easy manipulation of substituents on the dibenzofuran core, and also provides good-to-excellent product yields with good functional group tolerance, particularly the -Br and -Cl groups which are often incompatible with existing metal-catalyzed C-C and/or C-O bond ring-forming processes. It is worth noting that ladder-type π-systems with all-arene quarternary carbon structure can be straightforwardly generated upon simple late-stage functionalization.

Efficient synthesis of tetra- and penta-substituted benzenes via a domino annulation reaction of a pyridinium ylide and chalcone o-enolate

A very simple and highly efficient protocol for synthesizing tetra- and penta-substituted benzene derivatives has been developed.

Multicomponent cascade reactions of HKAs: synthesis of highly functionalized 5H-chromeno[4,3-d]pyrimidines

A novel protocol was developed for the construction of functionalized 5H-chromeno[4,3-d]pyrimidines from 3-formylchromones, heterocyclic ketene aminals, and amidine hydrochlorides via a novel cascade reaction involving ring-opening and 1,3-H shift reactions.