Sodium halides as the source of electrophilic halogens in green synthesis of 3-halo- and 3,n-dihalobenzo[b]thiophenes

Facile synthesis of diiodoheteroindenes and understanding their Sonogashira cross-coupling selectivity for the construction of unsymmetrical enediynes

Electrophile-promoted cyclizations of functionalized alkynes offer a useful tool for constructing halogen-substituted heterocycles primed for further derivatization. Preinstallation of an iodo-substituent at the alkyne prior to iodo-cyclization opens access to ortho di-iodinated heterocyclic precursors for the preparation of unsymmetrical heterocycle-fused enediynes. This general approach was used to prepare 2,3-diiodobenzothiophene, 2,3-diiodoindole, and 2,3-diiodobenzofuran, a useful family of substrates for systematic studies of the role of heteroatoms on the regioselectivity of cross-coupling reactions. Diiodobenzothiophene showed much higher regioselectivity for Sonogashira cross-coupling at C2 than diiodoindole and diiodobenzofuran. As a result, benzothiophene can be conveniently involved in a one-pot sequential coupling with two different alkynes, yielding unsymmetrical benzothiophene-fused enediynes. On the other hand, the Sonogashira reaction of diiodoindole and diiodobenzofuran formed considerable amounts of di-substituted enediynes in addition to the monoalkyne product by coupling at C2. Interestingly, no C3-monocoupling products were observed for all of the diiodides, suggesting that the incorporation of the 1st alkyne at C2 activates the C3 position for the 2nd coupling. Additional factors affecting regioselectivity were detected, discussed and connected, through computational analysis, to transmetalation being the rate-determining step for the Sonogashira reaction. Several enediynes synthesized showed cytotoxic activity, which is not associated with DNA strand breaks typical of natural enediyne antibiotics.

Visible-light-induced halocyclization of 2-alkynylthioanisoles with simple alkyl halides towards 3-halobenzo[b]thiophenes without an external photocatalyst

A new strategy for the preparation of 3-halobenzo[b]thiophenes via a photo-driven halocyclization/demethylation of 2-alkynylthioanisoles with simple alkyl halides was developed. The reaction can proceed smoothly at room temperature under visible-light irradiation without any external photocatalyst, and the protocol has a range of advantages, including simplicity and mildness of the reaction conditions, good functional-group tolerance, and excellent yields of the products.

Synthesis of Benzo[b]thiophenes via Electrophilic Sulfur Mediated Cyclization of Alkynylthioanisoles

A stable dimethyl(thiodimethyl)sulfonium tetrafluoroborate salt was employed for the electrophilic cyclization reaction of o-alkynyl thioanisoles for the synthesis of 2,3-disubstituted benzo[b]thiophenes. The reaction described herein works well with various substituted alkynes in excellent yields, and a valuable thiomethyl group was introduced with ease. The reaction utilizes moderate reaction conditions and ambient temperature while tolerating various functionalities. To elucidate the mechanism, electrophilic addition reactions using the dimethyl(thiodimethyl)sulfonium tetrafluoroborate salt with diphenylacetylene was demonstrated.

Synthesis and Evaluation of 3-Halobenzo[b]thiophenes as Potential Antibacterial and Antifungal Agents

The global health concern of antimicrobial resistance has harnessed research interest to find new classes of antibiotics to combat disease-causing pathogens. In our studies, 3-halobenzo[b]thiophene derivatives were synthesized and tested for their antimicrobial activities using the broth microdilution susceptibility method. The 3-halo substituted benzo[b]thiophenes were synthesized starting from 2-alkynyl thioanisoles using a convenient electrophilic cyclization methodology that utilizes sodium halides as the source of electrophilic halogens when reacted along with copper(II) sulfate. This environmentally benign methodology is facile, uses ethanol as the solvent, and results in 3-halo substituted benzo[b]thiophene structures in very high yields. The cyclohexanol-substituted 3-chloro and 3-bromobenzo[b]thiophenes resulted in a low MIC of 16 µg/mL against Gram-positive bacteria and yeast. Additionally, in silico absorption, distribution, metabolism, and excretion (ADME) properties of the compounds were determined. The compounds with the lowest MIC values showed excellent drug-like properties with no violations to Lipinski, Veber, and Muegge filters. The time-kill curve was obtained for cyclohexanol-substituted 3-chlorobenzo[b]thiophenes against Staphylococcus aureus, which showed fast bactericidal activity at MIC.

A one-pot successive cyclization-alkylation strategy for the synthesis of 2,3-disubstituted benzo[b]thiophenes

In this study, a new environmentally benign iodine-mediated one-pot iodocyclization/alkylation strategy for the synthesis of benzo[b]thiophene derivatives starting from 2-alkynylthioanisoles was developed. The synthesis of a diverse population of 2,3-disubstituted benzo[b]thiophenes was achieved in high yields by employing moderate reaction conditions using 1,3-dicarbonyl substrates as the nucleophile and various substituted propargyl alcohols as both the cyclization precursor and the alkylating agent. This method resulted in the formation of a series of complex structures obtained in a single step. Additionally, a strategy was devised for the one pot iodocyclization/oxidation of propargyl alcohols into carbonyl functionalized benzo[b]thiophene structures. These green one-pot reaction processes were designed to reduce wastes and byproducts while generating a complex substitution pattern on the benzo[b]thiophene structure. The reported methodologies may be used to synthesize more functionalized benzo[b]thiophene structures that can be used in both biomedical and organic electronic applications.

Recent Progress in the Synthesis of Benzo[b]thiophene

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Due to such a wide range of applicability, the synthesis of benzo[b]thiophene derivatives has attracted intensive research. Numerous mild and efficient approaches for the synthesis of benzo[b]thiophenes have been developed over the years. Different catalysts and substrates have been applied for benzo[b]thiophene synthesis. This review will focus on the studies in the construction of benzo[b]thiophene skeleton, which date back from 2012.

Iodocyclization in Aqueous Media and Supramolecular Reaction Control Using Water-Soluble Hosts

Iodocyclization of 2-alkynylanisoles is an efficient route for synthesizing substituted benzofurans. Reaction efficiency with copper(II) sulfate and sodium iodide in an aqueous slurry under mild conditions is a manifold higher than in organic solvents. Water-soluble hosts of the cyclodextrin family solubilize the compounds in aqueous media and affect the reaction efficiency through conformation control and steric interactions. Computational chemistry and spectral titration provide information on the host-guest complex structure and insight into the mechanistic basis of the observed effects.

Copper-Catalyzed Electrophilic Chlorocyclization Reaction Using Sodium Chloride as the Source of Electrophilic Chlorine

The synthesis of 2,3-disubstituted benzo[b]thiophenes with selective placement of a chlorine moiety at the 3 position while maintaining diversity at the 2 position has only been accomplished by a handful of conditions in the past. The development of a greener, less expensive, and simpler method is paramount for the exploration of innovative compounds for application in medicinal and materials chemistry. Herein, the first reported copper-catalyzed electrophilic chlorocyclization method was developed and employed across diverse substrates to generate highly functionalized 2,3-disubstituted benzo[b]thiophenes and 2,3,5-trisubstituted thiophenes in very high yields. This method was optimized in both ethanol and acetonitrile in a comparative solvent study. The utility of this method was further expanded beyond chlorocyclization by changing the sodium halide to generate bromo- and iodocyclization products in excellent yields.