Lewis Acid Catalyzed Tandem 1,4-Conjugate Addition/Cyclization of in Situ Generated Alkynyl o-Quinone Methides and Electron-Rich Phenols: Synthesis of Dioxabicyclo[3.3.1]nonane Skeletons

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.

Cooperative Rh/Achiral Phosphoric Acid-Enabled [3+3] Cycloannulation of Carbonyl Ylides with Quinone Monoimines: Synthesis of Benzofused Dioxabicyclo[3.2.1]octane Scaffolds

A cooperative Rh/achiral phosphoric acid-enabled [3+3] cycloaddition of in situ-generated carbonyl ylides with quinone monoimines has been developed. With the ability to build up the molecular complexity rapidly and efficiently, this method furnishes highly functionalized oxa-bridged benzofused dioxabicyclo[3.2.1]octane scaffolds bearing two quaternary centers in good to excellent yields under mild conditions. Moreover, the utility of the current method was demonstrated by gram-scale synthesis and elaboration of the products into various functionalized oxa-bridged heterocycles.

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.

Construction of Spiro[benzo[a]acridine-12,4'-imidazolidine]-2',5'-dione Derivatives via Ring-Opening and Recyclization of Isatins and C-OH Cleavage of 2-Naphthol

An efficient three-component reaction to access spiro[benzo[a]acridine-12,4'-imidazolidine]-2',5'-dione derivatives has been developed through the ring-opening and recyclization process of isatins and dehydroxylation of 2-naphthol, which is different from their conventional reaction modes. Experimental observations suggest that p-toluenesulfonic acid is the key factor that promotes the success of this synthetic strategy. The research provided a novel approach for the construction of spiro compounds from isatins and 2-naphthol in organic synthesis.

Brønsted acid-mediated tandem cyclization of triarylphosphines and in situ generated ortho-alkynyl quinone methides: access to heterocyclic quaternary phosphonium salts

Heterocyclic Quaternary Phosphonium Salts (HQPS) have emerged as promising chemicals for organic synthesis and medicinal chemistry. However, the present synthetic methodology of this type of compound is still limited. Here, we report a deconstructive reorganization strategy based on Brønsted acid-mediated tandem 1,4 addition/intramolecular cyclization of triphenylphosphine derivatives and in situ generated o-AQMs for the first time. This protocol provides a novel approach to heterocyclic quaternary phosphonium salts. The method also features a non-metal catalyst, mild reaction conditions, high efficiency and wide substrate scope. Moreover, a series of obtained heterocyclic phosphonium salts can be converted to isotopically labelled 2-benzofuran compounds directly by simple deuteration reactions.

Methanodibenzo[b,f][1,5]dioxocins as Novel Glutaminase Inhibitor with Anti-Glioblastoma Potential

Glutamine metabolism is an important hallmark of several cancers with demonstrated antitumor activity in glioblastoma cancer cells (GBM). GBM cells regulate glutamine and use it as a major energy source for their proliferation through the glutaminolysis process. Enzymes, such as glutaminase in glutaminolysis, can be targeted by small-molecule inhibitors, thus exhibiting promising anticancer properties. The resistance to glutaminolysis demands the development of new therapeutic molecules to overcome drug resistance. Herein, we have reported a novel library of constrained methanodibenzo[b,f][1,5]dioxocin derivatives as glutaminase (GLS) inhibitors and their anti-GBM potential. The library consisting of seven molecules was obtained through self-condensation of 2'-hydroxyacetophenones, out of which three molecules, namely compounds 3, 5, and 6, were identified with higher binding energy values ranging between -10.2 and -9.8 kcal/mol with GLS (PDB ID; 4O7D). Pharmacological validation of these compounds also showed a higher growth inhibition effect in GBM cells than the standard drug temozolomide (TMZ). The most promising compound, 6, obeyed Lipinski's rule of five and was identified to interact with key residues Arg³⁰⁷, Asp³²⁶, Lys³²⁸, Lys³⁹⁹, and Glu⁴⁰³ of GLS. This compound exhibited the best cytotoxic effect with IC₅₀ values of 63 µM and 83 µM in LN229 and SNB19 cells, respectively. The potential activation of GLS by the best-constrained dibenzo[b,f][1,5]dioxocin in the tested series increased apoptosis via reactive oxygen species production in both GBM cells, and exhibited anti-migratory and anti-proliferative properties over time in both cell lines. Our results highlight the activation mechanism of a dibenzo[b,f][1,5]dioxocin from the structural basis and demonstrate that inhibition of glutaminolysis may facilitate the pharmacological intervention for GBM treatment.

Facile construction of dibenzodioxo[3.3.1]nonanes bearing spirocyclohexadienones via domino [4 + 2] cycloaddition/C(sp3)-H oxidative dehydrogenation coupling reactions

A novel palladium catalyzed homodimerization of ortho-hydroxyphenyl substituted p-QMs has been developed via [4 + 2] cycloaddition/oxidative dehydrogenation coupling domino reactions. An interesting palladium catalyzed intramolecular benzyl C-H oxidation dehydrogenation to form a transannular C(sp³)-O bond was found. This protocol provided an efficient method to construct various dibenzodioxo[3.3.1]nonanes bearing spirocyclohexadienones.

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.

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.

Acid-promoted formal [3 + 2] cyclization/N,O-ketalization of in situ generated ortho-alkynyl quinone methides: access to bridged 2,3-cyclopentanoindoline skeletons

An acid-catalyzed formal [3 + 2] cyclization/N,O-ketalization of in situ formed ortho-alkynyl quinone methides (o-AQMs) and tryptophol derivatives was developed.

Total synthesis of myristinins A–F and 3′-hydroxy-5,7-dimethoxy-4-O-2′-cycloflavan by iterative generation of o-quinone methides

An iterative generation of o-quinone methides (o-QMs) and [4+2] cycloaddition followed by inter/intra-molecular Michael addition in a cascade sequence gave expedient access to the total synthesis of myristinins A–F and 3′-hydroxy-5,7-dimethoxy-4-O-2′-cycloflavan and their analogues, respectively.

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.

Lewis Acid Mediated Domino Intramolecular Cyclization: Synthesis of Dihydrobenzo[a]fluorenes

An efficient and facile method for the regioselective synthesis of novel dihydrobenzo[a]fluorenes from readily accessible alkynols is presented. The current strategy triggers the formation of a dual C-C bond intramolecularly via Lewis acid catalysis under mild reaction conditions. Notably, secondary as well as tertiary alcohols bearing an alkyne moiety have been smoothly transformed into the corresponding products. As a result, novel tetracyclic dihydrobenzo[a]fluorenes have been accomplished using this approach.

Copper-Catalyzed Cascade 1,4-Addition/Annulation/Hydrolysis of Propargylamines with 2-Hydroxynaphthalene-1,4-diones: Direct Formation of 12-Phenacyl-11H-benzo[b]xanthenes

A novel and versatile approach to construct 12-phenacyl-11H-benzo[b]xanthene-6,11(12H)-dione derivatives through copper-catalyzed cascade reaction of propargylamines with 2-hydroxynaphthalene-1,4-diones has been developed. The procedure is proposed to go through a sequence of 1,4-conjugate addition, intramolecular nucleophilic addition/dehydration, and hydrolysis of alkyne followed by an enol-ketone tautomerization. The reaction provides a new and highly efficient method for the synthesis of 12-phenacyl-11H-benzo[b]xanthene-6,11(12H)-diones by formation of three new bonds and one heterocycle from readily available starting materials in good to high yields (70-88%) with broad functional group compatibility in a single step.

Synthesis of Sulfonylated Heterocycles via Copper-Catalyzed Heteroaromatization/Sulfonyl Transfer of Propargylic Alcohols

An unprecedented copper-catalyzed heteroaromatization/sulfonyl transfer of propargylic alcohols with isocyanide has been developed. 3-Sulfonyl benzofurans and indoles were produced under Cu(I) catalysis in good to high yields. The developed catalytic methodology provides controlled, modular, and facile access to sulfonyl benzoheterocycle scaffolds.

Direct Access to Bridged Tetrahydroquinolines and Chromanes via an InCl3-Catalyzed Sequential Three-Component Cascade

A sequential three-component cascade process was developed for the synthesis of bridged tetrahydroquinolines and chromanes bearing 2,6-methanobenzo[d][1,3]diazocine and 2,6-methanobenzo[g][1,3]oxazocine scaffolds, respectively, in good yields from readily available materials. The InCl₃-catalyzed reaction progressed via enamine formation, Michael addition, intramolecular cyclization, and intramolecular iminium ion cyclization steps. Notably, this high atom economic approach (-2H₂O) allowed the generation of four new bonds (1 C-C and 3 C-N or 1 C-C, 1 C-O and 2 C-N) and two heterocyclic rings in a single operation.

Selectfluor-Mediated Stereoselective [1 + 1 + 4 + 4] Dimerization of Styrylnaphthols

Stereoselective [1 + 1 + 4 + 4] dimerization of 1-styrylnaphthols has been developed by using Selectfluor as the oxidant for the first time. The reaction was compatible with various functional groups, giving a class of ethanodinaphtho[b,f][1,5]dioxocines with novel 3D skeletons. DFT calculations indicate that this method merges an intriguing stereoselective intermolecular 1 + 1 radical coupling to construct a bridged C-C bond and then an intramolecular [4 + 4] formal cycloaddition of the in situ generated o-quinone methide intermediate.

Mild, Modular, and Convergent Synthesis of Helical Naphtho[2,1-c]chromenes via a Multistep Cyclization/Aromatization Cascade Sequence

Tetracyclic 6H-naphtho[2,1-c]chromenes are expeditiously synthesized through a BF₃·OEt₂-mediated, three-step cascade reaction, creating new central pyran and aromatic rings. The cascade involves the addition of phenol-derived alkynyl substrates to BF₃-activated aldehydes followed by alkyne-Prins cyclization, Friedel-Crafts reaction, and final elimination. Aliphatic and electron-deficient aromatic aldehydes afford the products in 50-74% isolated yields, but benzaldehyde and tolualdehyde resulted in lower yields. X-ray analysis of a p-bromophenyl derivative (5aA) shows the two aromatic moieties are twisted by 28° to create a helical backbone.

Synthesis of 6,12-Disubstituted Methanodibenzo[b,f][1,5]dioxocins: Pyrrolidine Catalyzed Self-Condensation of 2′-Hydroxyacetophenones

The preparation of unprecedented 6,12-disubstituted methanodibenzo[b,f][1,5]dioxocins from pyrrolidine catalyzed self-condensation of 2′-hydroxyacetophenones is herein described. This method provides easy access to this highly bridged complex core, resulting in construction of two C–O and two C–C bonds, a methylene bridge and two quaternary centers in a single step. The intricate methanodibenzo[b,f][1,5]dioxocin compounds were obtained in up to moderate yields after optimization of the reaction conditions concerning solvent, reaction times and the use of additives. Several halide substituted methanodibenzo[b,f][1,5]dioxocins could be prepared from correspondent 2′-hydroxyacetophenones.

Iron-catalyzed domino Knoevenagel-hetero-Diels-Alder reaction: facile access to oxabicyclo[3.3.1]nonene derivatives

A Fe-catalyzed domino Knoevenagel-hetero-Diels-Alder reaction of alkenyl aldehydes and 1,3-diketones has been developed. It provides straightforward access to a series of oxabicyclo[3.3.1]nonene derivatives in promising yields with excellent diastereoselectivity and functional group tolerance. The resultant bridged dihydropyrans can be smoothly converted into chromene derivatives, thus highlighting the synthetic utility of this method.

A novel multicomponent quadruple/double quadruple domino reaction: highly efficient synthesis of polyheterocyclic architectures

A novel multicomponent quadruple domino reaction (MCQDR) for the assembly of structurally complex molecular architectures via the formation of three rings and three contiguous stereogenic centers has been accomplished with high regio- and diastereoselectivity. Solvents, catalysts and work-up were not required to obtain the target molecules. In addition, this new protocol is also extended for the multicomponent double quadruple domino reaction (MCDQDR) to create novel polyheterocyclic architectures in an orthogonal manner.

FeCl3-promoted tandem 1,4-conjugate addition/6-endo-dig cyclization/oxidation of propargylamines and benzoylacetonitriles/malononitriles: direct access to functionalized 2-aryl-4H-chromenes

An efficient and concise procedure has been developed for the synthesis of functionalized 2-aryl-4H-chromenes based on a tandem reaction of propargylamines and benzoylacetonitriles/malononitriles in the presence of FeCl3 as an environmentally friendly promoter. This reaction involves a highly efficient tandem sequence consisting of 1,4-conjugate addition, 6-endo-dig cyclization, and oxidation. This protocol tolerates a variety of functional groups, thereby providing a practical and efficient method for the fabrication of 2-aryl-4H-chromene skeletons.

Intramolecular Benzannulation of 3-Sulfonyl-2-benzylchromen-4-ones: Synthesis of Sulfonyl Dibenzooxabicyclo[3.3.1]nonanes

In this work, a concise route for the synthesis of sulfonyl dibenzo-oxabicyclo[3.3.1]nonanes by a two-step route is described, including (i) NaBH₄/LiCl-mediated reduction of 3-sulfonyl-2-benzylchromen-4-ones and (ii) sequential BF₃·OEt₂-mediated intramolecular annulation of the resulting 3-sulfonyl-2-benzylchroman-4-ols. A plausible mechanism is proposed and discussed herein. This protocol provides a highly effective stereocontrolled aryl-hydroxyl Friedel-Crafts-type cross-coupling to construct the tetra- or pentacyclic bridged framework. The use of various reaction conditions is investigated for an efficient transformation.

Asymmetric organocatalytic conjugated addition of pyrazolin-5-ones to ortho-quinomethanes: construction of vicinal tertiary and all-carbon quaternary stereocenters

An organocatalytic conjugated addition of pyrazolin-5-ones to ortho-quinomethanes was developed to construct the vicinal tertiary and all-carbon quaternary stereocenters.

A ZnI2-catalyzed regioselective cascade 1,4-conjugate addition/5-exo-dig annulation pathway for one-pot access to heterobiaryl frameworks

Facile access to π-extended heterobiaryl compounds via a non-cross-coupling strategy has been achieved.

Synthesis of 4-styrylcoumarins via FeCl3-promoted cascade reactions of propargylamines with β-keto esters

A versatile and highly regioselective FeCl₃-promoted tandem cyclization reaction of in situ generated alkynyl o-quinone methides (o-AQMs) with β-keto esters has been developed.

Lewis acid-catalyzed tandem cyclization of in situ generated o-quinone methides and arylsulfonyl hydrazides for a one-pot entry to 3-sulfonylbenzofurans

Lewis acid-mediated one-pot tandem cyclization of o-QMs with arylsulfonyl hydrazides was described for the first time and the corresponding 3-sulfonylbenzofuran products were obtained in moderate to good yields.