Electrophilic cyclization of o-acetoxy- and o-benzyloxyalkynylpyridines: an easy entry into 2,3-disubstituted furopyridines

Pd(II)-Catalyzed Synthesis of Furo[2,3-b]pyridines from β-Ketodinitriles and Alkynes via Cyclization and N-H/C Annulation

A Pd(II)-catalyzed synthesis of furopyridines has been developed from β-ketodinitriles and alkynes via an unusual N-H/C annulation. The participation of both the nitrile groups and the concurrent construction of furan and pyridine rings through the formation of C-C, C═C, C-O, C-N, and C═N bonds are the important features. The synthetic applicability is further demonstrated through a series of postsynthetic alterations.

Synthesis of Substituted Thiophenes through Dehydration and Heterocyclization of Alkynols

A protocol was described for obtaining a variety of substituted thiophenes with functional potential via metal-free dehydration and sulfur cyclization of alkynols with elemental sulfur (S₈) or EtOCS₂K in moderate-to-good yields. The method provides the base-free generation of a trisulfur radical anion (S₃^•-) and its addition to alkynes as an initiator. This research broadens the applications of S₃^•- in the synthesis of sulfur-containing heterocycles.

Synthesis and Structure-Activity Relationships of Imidazopyridine/Pyrimidine- and Furopyridine-Based Anti-infective Agents against Trypanosomiases

Neglected tropical diseases remain among the most critical public health concerns in Africa and South America. The drug treatments for these diseases are limited, which invariably leads to fatal cases. Hence, there is an urgent need for new antitrypanosomal drugs. To address this issue, a large number of diverse heterocyclic compounds were prepared. Straightforward synthetic approaches tolerated pre-functionalized structures, giving rise to a structurally diverse set of analogs. We report on a set of 57 heterocyclic compounds with selective activity potential against kinetoplastid parasites. In general, 29 and 19 compounds of the total set could be defined as active against Trypanosoma cruzi and T. brucei brucei, respectively (antitrypanosomal activities <10 μM). The present work discusses the structure-activity relationships of new fused-ring scaffolds based on imidazopyridine/pyrimidine and furopyridine cores. This library of compounds shows significant potential for anti-trypanosomiases drug discovery.

Access to Substituted Thiophenes through Xanthate-Mediated Vinyl C(sp2)-Br Bond Cleavage and Heterocyclization of Bromoenynes

An environmentally sustainable strategy for the chemoselective heterocyclization of bromoenynes through a transition-metal-free sulfuration/cyclization process is reported. Using inexpensive and safe EtOCS₂K as a thiol surrogate and tetrabutylphosphonium bromide and H₂O as a mixed solvent, the reaction provided a range of substituted thiophenes in moderate to good yields. In addition, 2,3,4,5-tetrasubstituted thiophenes were able to be prepared under mild reaction conditions by electrophilic heterocyclization with NH₄I and EtOCS₂K in good yields.

Gold-Catalyzed Oxidative Cascade Cyclization of 1,3-Diynamides: Polycyclic N-Heterocycle Synthesis via Construction of a Furopyridinyl Core

A facile and practical approach to construct a furopyridinyl motif through a gold-catalyzed cascade cyclization of easily accessible diynamides is described. This strategy offers a straightforward approach to furo[2,3-c]isoquinoline and 6H-furo[3',2':5,6]pyrido[3,4-b]indole derivatives. The reaction could build up four new bonds and two additional heteroaromatic rings via a single operation. The heterocyclic products show promising blue luminous performance with fluorescence quantum yields up to 75%.

Mechanochemical Synthesis of Functionalized Quinolines by Iodine Mediated Oxidative Annulation

An iodine-mediated environmentally benign synthesis of multi-substituted quinoline derivatives is developed using a solvent-free mechanochemical process. Appropriately designed and easily accessible protecting group-free aniline derivatives were used for the oxidative annulation reaction, and a series of quinoline derivatives with variable functionalities were synthesized up to 89 % isolated yield. Importantly, the activator iodine remains in the quinoline molecule and promotes further functionalizations. The present methodology is beneficial with regard to operational simplicity and mild reaction conditions.

One pot Sonogashira-coupling/Wittig olefination procedures

Bromoarylcarbaldehydes, bromoheteroarylcarbaldehydes, and bromoalkenals can be subjected to a one pot Sonogashira coupling–Wittig olefination reaction to give easy access to molecules with extended pi-systems.

Charting the Chemical Reactivity Space of 2,3-Substituted Furo[2,3-b]pyridines Synthesized via the Heterocyclization of Pyridine-N-oxide Derivatives

A concise strategy for the synthesis of 2,3-substituted furo[2,3-b]pyridines is described. Mild, metal-free conditions were successfully applied to produce a range of 2-(alkyl or aryl)-3-ethylcarboxylate-furo[2,3-b]pyridines in yields of 50-91%. Then, the chemical reactivity of this heterocyclic framework was explored to develop straightforward methods for its functionalization. The pyridine moiety reactivity was successfully explored by C-H amination and borylation reactions, although C-H fluorination and radical C-H arylation processes were not as efficient. In addition, while the furopyridine core proved stable under basic conditions, the ring-opening reaction of the furan moiety with hydrazine generated a valuable new pyridine-dihydropyrazolone scaffold.

Iodine-mediated synthesis of heterocycles via electrophilic cyclization of alkynes

Syntheses of heterocyclesvia5-exoor 6-endo-digiodocyclizations from alkynes.

Electronically modified amine substituted alkynols for regio-selective synthesis of dihydrofuran derivatives

An efficient approach has been developed for the regio-selective synthesis of iodo-dihydrofurans from amino alkynols. These dihydrofurans have been further diversified by C–C, C–N coupling reactions.

Copper-promoted hydration and annulation of 2-fluorophenylacetylene derivatives: from alkynes to benzo[b]furans and benzo[b]thiophenes

An efficient copper-promoted hydration reaction and its application in the synthesis of benzo[b]furan and benzo[b]thiophene derivatives is presented starting from readily available 2-fluorophenylacetylene derivatives. The key annulation step involves the hydration of the C-F bond of 2-fluorophenylacetylene derivatives followed by an intramolecular annulation to afford benzo[b]furan and benzo[b]thiophene derivatives. Moreover, structurally important 2,2'-bisbenzofuran scaffolds are provided in good yields.

A novel one-pot method for the synthesis of substituted furopyridines: iodine-mediated oxidation of enaminones by tandem metal-free cyclization

A novel iodine-mediated oxidative tandem cyclization reaction of simple enaminones has been developed for the synthesis of substituted furopyridines through C–C/C–N/C–O bond formation in a one-pot procedure.

Three steps in one pot: synthesis of linear bilateral extended 2,2':6',2″-terpyridineruthenium(II) complexes

Metal complexes of linear bilateral extended 2,2':6',2″-terpyridine [2,6-bis(2-substituted furo[2,3-c]pyridin-5-yl)pyridine] are topological tridentate analogues of 5,5'-functionalized 2,2'-bipyridine. The present methodology provides access to linear bilateral extended 2,2':6',2″-terpyridineruthenium(II) complexes starting from 5,5″-bis(methoxymethoxy)-4,4″-bis(substituted ethynyl)-2,2':6',2″-terpyridines via one-pot cleavage of the methoxymethyl (MOM) protecting group, cycloisomerization, and metal complexation in the presence of Ru(DMSO)4Cl2. In this reaction, ruthenium(II) likely plays a triple role, acting as a Lewis acid to cleave the MOM ether, a catalyst facilitating 5-endo-dig cyclization, and a coordination metal center, being an integral part of the final product.

Direct assembly of 3,4-difunctionalized benzofurans and polycyclic benzofurans by phenol dearomatization and palladium-catalyzed domino reaction

A method to directly convert 2-alkynylphenols to 3,4-difunctionalized benzofurans and polycyclic benzofurans was developed. This protocol involves a hypervalent-iodine-mediated oxidative dearomatization to break the aromaticity of 2-alkynylphenols, and a palladium-catalyzed domino reaction to install two functional groups at the C3 and the C4 positions and restore the aromaticity of benzofurans.

An expedient approach to pyrrolo[3,2-c]quinolines via regioselective formation of the pyrrole nucleus over indoles

The regioselective synthesis of pyrrolo[3,2-c]quinolines from protected 2-alkynylanilines by Heck and intramolecular Michael addition has been described.

Cyclization of homopropargyl chalcogenides by copper(II) salts: selective synthesis of 2,3-dihydroselenophenes, 3-arylselenophenes, and 3-haloselenophenes/thiophenes

Copper(II) halide mediated cyclization of homopropargyl chalcogenides gave three types of chalcogenophene derivatives. Selective product formation was achieved by controlling solvent, temperature, and atmosphere. By using CuBr2 and 1,2-dichloroethane at room temperature under ambient atmosphere, 4-bromo dihydroselenophene derivatives were obtained, whereas CuBr2 and 1,2-dichloroethane at reflux gave selectively 2-substituted selenophenes. When 1,2-dichloroethane was replaced by dimethylacetamide, 3-halo-selenophenes were obtained exclusively. The versatility of chalcogenophenes was also studied by reaction of 3-haloselenophenes with terminal alkynes under Sonogashira conditions affording the cross-coupled products. In addition, the reaction of 3-haloselenophenes with boronic acids gave the corresponding Suzuki-type products in good yields.

Exploring the potential of diarylacetylenediols as hydrogen bonding catalysts

In the course of a search for new classes of hydrogen bonding catalysts, we have examined diarylacetylenediols as potential catalysts for the Diels-Alder reaction. General and efficient methods have been developed for the preparation of these diols. Their structures were systematically modified, and increased activity was observed for those possessing an electron-withdrawing group on the aryl groups. The electron-deficient diarylacetylenediol catalysts, while more active, undergo spontaneous cyclization to the corresponding benzo[b]furans. A mechanism is postulated to explain this facile transformation. Computational studies performed on 2-ethynylphenol help to explain the effect of the alkyne on the conformation and hydrogen bond donating ability of the adjacent OH group. Finally, the crystal structure of one of the diols is reported, and it displays an intricate network of intermolecular hydrogen bonds.

[5C + 1N] annulations: two novel routes to substituted dihydrofuro[3,2-c]pyridines

Two novel routes based on [5C + 1N] annulations for the synthesis of 2,3-dihydrofuro[3,2-c]pyridines are described. Ammonium acetate (NH(4)OAc) is used as an ammonia source in both routes. The first route utilizes 1-acyl-1-[(dimethylamino)alkenoyl]cyclopropanes as a five-carbon 1,5-bielectrophilic species and combines the [5C + 1N] annulation and regioselective ring-enlargement of cyclopropyl ketone into one pot, whereas the second route utilizes 3-acyl-2-[(dimethylamino)alkenyl]-4,5-dihydrofurans as the five-carbon synthons, which involves a sequential intermolecular aza-addition, intramolecular aza-nucleophilic addition/elimination, and dehydration reaction.