Facile ionic liquid-mediated, three-component sequential reactions for the green, regio- and diastereoselective synthesis of furocoumarins

PIDA-promoted metal-free [3 + 2] heteroannulation of β-ketothioamides with 4-hydroxy coumarins: chemo-/regioselective access to furo[3,2-c]chromen-4-ones at room temperature

Herein, we report a viable protocol to access furo[3,2-c]chromen-4-ones by engaging easily accessible 4-hydroxy coumarins as a three-atom CCO unit and thioamides as a C2 coupling partner, mediated by phenyliodine(III) diacetate (PIDA) at room temperature in a highly efficient and pot-/step-economical manner. This strategy not only avoids potential toxicity and tiresome workup conditions, but also features operational simplicity, a broad substrate scope, good functional group tolerance, high yields, easy scalability and exclusive selectivity. A mechanistic study has shown that this metal-free reaction is triggered by PIDA via activation of the β-carbon of 4-hydroxy coumarin, followed by a nucleophilic addition/intramolecular cyclization/dethiohydration cascade. High-resolution mass spectra (HRMS) study confirms the key intermediates involved during the course of the reaction, elucidating the reaction pathways, and demonstrates the excellent regio- and chemoselectivity of this approach.

Furo[3,2-c]coumarins carrying carbon substituents at C-2 and/or C-3. Isolation, biological activity, synthesis and reaction mechanisms

The isolation, biological activity and synthesis of natural furo[3,2-c]coumarins are presented, covering mainly the developments in the last 35 years. The most relevant approaches toward the synthesis of 2-substituted, 3-substituted and 2,3-disubstituted heterocycles are also discussed, with emphasis on the efficiency of the processes and their mechanisms.

The molecular diversity scope of 4-hydroxycoumarin in the synthesis of heterocyclic compounds via multicomponent reactions

4-Hydroxycoumarins are some of the most versatile heterocyclic scaffolds and are frequently applied in the synthesis of various organic compounds. 4-Hydroxycoumarin-based compounds are important among heterocyclic structures due to their biological and pharmaceutical activities. In this study, we provide an overview on the recent applications of 4-hydroxycoumarin in multicomponent reactions for the synthesis of various heterocyclic compounds during the time period of 2015-2018.

NHC in Imidazolium Acetate Ionic Liquids: Actual or Potential Presence?

Ionic liquids (ILs) are considered in the majority of cases green solvents, due to their virtually null vapor pressure and to the easiness in recycling them. In particular, imidazolium ILs are widely used in many fields of Chemistry, as solvents or precursors of N-heterocyclic carbenes (NHCs). The latter are easily obtained by deprotonation of the C2-H, usually using strong bases or cathodic reduction. Nevertheless, it is known that weaker bases (e.g., triethylamine) are able to promote C2-H/D exchange. From this perspective, the possibility of deprotonating C2-H group of an imidazolium cation by means of a basic counter-ion was seriously considered and led to the synthesis of imidazolium ILs spontaneously containing NHCs. The most famous of this class of ILs are N,N'-disubstituted imidazolium acetates. Due to the particular reactivity of this kind of ILs, they were appointed as “organocatalytic ionic liquids” or “proto-carbenes.” Many papers report the use of these imidazolium acetates in organocatalytic reactions (i. e., catalyzed by NHC) or in stoichiometric NHC reactions (e.g., with elemental sulfur to yield the corresponding imidazole-2-thiones). Nevertheless, the actual presence of NHC in N,N'-disubstituted imidazolium acetate is still controversial. Moreover, theoretical studies seem to rule out the presence of NHC in such a polar environment as an IL. Aim of this Mini Review is to give the reader an up-to-date overview on the actual or potential presence of NHC in such an “organocatalytic ionic liquid,” both from the experimental and theoretical point of view, without the intent to be exhaustive on N,N'-disubstituted imidazolium acetate applications.

Nano γ-Fe2O3-Quinuclidine-Based Catalyst as a Recyclable Organic Base for the Diastereoselective Synthesis of Trans-2,3-Dihydrofuro[3,2-C]Coumarins

A nano γ-Fe2O3-quinuclidine-based catalyst as a new, magnetically recyclable heterogeneous organic base was synthesised and used in the efficient synthesis of a series of 11 trans-3-aroyl-2-(4-halobenzoyl)-2,3-dihydro-4H-furo[3,2-c]chromen-4-ones by the reaction of N-(2-(4-halobenzoyl)-2-oxoethyl)pyridinium bromide with 4-hydroxycoumarin and an aryl glyoxal. The catalyst was easily isolated from the reaction mixture using a magnetic bar and reused at least 10 times without significant decrease in its activity.

A Practical and Diastereoselective Synthesis of Dihydrofurocoumarin from Pyridinium Ylides in Aqueous Medium

A series of 2,3-dihydrofurocoumarins were prepared via a three-component reaction of aldehyde, pyridinium salt and 4-hydroxycoumarin in aqueous ethanol. This method avoids using excessive pyridine and trimethylamine, and has operational simplicity and avoids tedious column chromatography purification. Both aryl aldehydes and aliphatic aldehydes are applicable to this protocol.

Choline Hydroxide as an Efficient Catalyst for the Diastereoselective Synthesis of Trans-2,3-dihydrofuro[3,2-c]Coumarins in an Aqueous Medium

A one-pot, three-component condensation of 2-[2-(4-chlorophenyl)-2-oxoethyl]isoquinolinium bromide with 4-hydroxycoumarin and an aromatic aldehyde in the presence of catalytic amounts of choline hydroxide in refluxing water gave trans-2-(4-chlorobenzoyl)-3-(aryl)-2,3-dihydro-4H-furo[3,2-c]chromen-4-ones in excellent yields and with short reaction times.

A simple, one pot synthesis of furo[3,2-c]chromenes and evaluation of antimicrobial activity

Synthesis of a number of 2-cyano-4-oxo-3-phenyl-3,4-dihydro-2H-furo[3,2-c]chromene-2-carboxylate compounds (5) have been accomplished by a simple, multicomponent one pot reaction and evaluated for in vitro antimicrobial activity against different Gram-positive and Gram-negative bacterial strains. The outcome of the screening study showed that compound 5c exhibited promising activity against Micrococcus luteus MTCC 2470 and Klebsiella planticola MTCC 530. Whereas, compound 5g exhibited excellent activity against Bacillus subtilis MTCC 121, Micrococcus luteus MTCC 2470, Klebsiella planticola MTCC 530, Escherichia coli MTCC 739 and displayed a moderate activity against Staphylococcus aureus MTCC 96 and Candida albicans MTCC 3017 when compared with Ciprofloxacin (standard control).

One-pot multicomponent synthesis of furo[3,2-c]coumarins promoted by amino-functionalized Fe3O4@SiO2 nanoparticles

Amino-functionalized Fe3O4@SiO2 nanoparticles (Fe3O4@SiO2–NH2 NPs) were prepared by treating Fe3O4@SiO2 NPs with (3-aminopropyl)-triethoxysilane. They have been characterized by powder X-ray diffraction, vibrating sample magnetometer, scanning electronic microscopy, energy dispersive X-ray, thermogravimetric analysis and FT-IR spectroscopy. A neat, atom-economical, environmentally benign one-pot multicomponent synthetic route for the synthesis of furo[3,2-c]coumarins in good yields has been devised using the Fe3O4@SiO2–NH2 NPs. The present methodology can be used for the design of libraries and diversity-oriented synthesis, and has potential for biological applications and drug discovery.

3-Methylene-2,4-chromandione in situ trapping: introducing molecular diversity on 4-hydroxycoumarin

3-Methylene-2,4-chromandione trapped in a solid-state stable Mannich adduct, is released in solution.

Construction of dispirocyclohexyl-3,3′-bisoxindole and dispirocyclopentyl-3,3′-bisoxindole via domino cycloaddition reactions of N-benzylbenzimidazolium salts with 2-(2-oxoindolin-3-ylidene)acetates

Two kinds of spirocyclic oxindoles were selectively synthesized from cycloaddition of N-benzyl-N-phenacylbenzimidazolium salts with two molecules of 2-(2-oxoindolin-3-ylidene)acetates.

Pseudo-four component synthesis of mono- and di-benzylated-1,2,3-triazoles derived from aniline

The pseudo-four component click synthesis of dibenzylated 1,2,3-triazoles derived from aniline is reported. The cycloaddition of sodium azide to N-(prop-2-ynyl)-benzenamine (I) in the presence of equimolar amounts of p-substituted benzyl derivatives, yields a mixture of mono- and dibenzylated 1,2,3-triazoles. When two equivalents of the benzyl derivative are added to the multicomponent reaction, the selective preparation of the dibenzylated compounds is achieved. The reactivity of the aniline N-H bond in monobenzylated 1,2,3-triazoles was tested by treatment with one equivalent of a p-substituted benzyl chloride at 40 °C, rendering the dibenzylated derivatives quantitatively.

Nucleophilicity parameters of pyridinium ylides and their use in mechanistic analyses

Kinetics of the reactions of pyridinium, isoquinolinium, and quinolinium ylides with diarylcarbenium ions, quinone methides, and arylidene malonates (reference electrophiles) have been studied in dimethylsulfoxide solution by UV-vis spectroscopy. The second-order rate constants (log k2) were found to correlate linearly with the electrophilicities E of the reference electrophiles, as required by the linear free-energy relationship log k(20°C) = sN(N + E) (J. Am. Chem. Soc. 2001, 123, 9500), which allowed us to derive the nucleophile-specific parameters N and sN for these ylides. Pyridinium substitution is found to have a similar effect on the reactivity of carbanionic reaction centers as alkoxycarbonyl substitution. Agreement between the rate constants measured for the 1,3-dipolar cycloadditions of pyridinium, isoquinolinium, and quinolinium ylides with acceptor substituted dipolarophiles (arylidenemalononitrile and substituted chalcone) and those calculated from E, N, and sN shows that the above correlation can also be employed for predicting absolute rate constants of stepwise or highly unsymmetrical concerted cycloadditions. Deviations between calculated and experimental rate constants by a factor of 10(6) were demonstrated to indicate a change of reaction mechanism.