A Metal-Free Tandem C–C/C–O Bond Formation Approach to Diversely Functionalized Tetrasubstituted Furans

A novel heterogeneous biocatalyst based on graphene oxide for synthesis of pyran derivatives

Graphene oxide modified with tryptophan (GO-Trp) has been introduced as a new heterogeneous acid-base biocatalyst for synthesis of some pyran derivatives. GO was prepared according to the Hummer's method and tryptophan as a low-cost green amino acid is covalently bonded to the surface of GO without any organic or toxic reagents in a green way. The new catalyst was characterized by different spectroscopic methods such as Fourier transform infrared, X-ray diffraction (XRD), etc. …. The results of XRD patterns showed an increase in the distance between the GO plates in the presence of the modifying agent which specifies the presence of amino acid between the GO layers. XPS analysis also confirmed successful modification through the presence of C-N bonds in the structure of the catalyst. In addition, improvements in thermal stability and changes in the morphology of the samples were observed using thermogravimetric analysis and Field emission scanning electron microscopy analysis respectively. Evaluation of the catalyst performance in the synthesis of some benzo[b]pyran and pyrano[3,2-c] chromene derivatives showed presentable results. Seven benzo[b]pyran (4a-4g) and five pyrano[3,2-c] chromene (4h-4l) derivatives were synthesized. GO-Trp as a safe, natural and efficient catalyst, could be reused up to 5 runs for synthesis of pyran derivatives without any significant decrease in its potency. High purity of the products and desirable yields are other points that make the present work more attractive.

Controllable Access to Diazo-functionalized 2-Methylene-2,3-dihydrofurans and Diazo-functionalized Furans from Enynones and Diazo Carbonyl Compounds

Using enynones and diazo carbonyl compounds as identical starting materials, methods for chemoselective and regioselective constructs of diazo-functionalized 2-methylene-2,3-dihydrofurans and diazo-functionalized trisubstituted furans have been established in a AgSbF6/DBU/DCE/0 °C system and a AgSbF6/DBU/Et2O·BF3/DCE/0 °C system, respectively. A Lewis acid and organic base cocontrolled reaction for the synthesis of diazo-functionalized trisubstituted furans is infrequent. For diazo-functionalized 2-methylene-2,3-dihydrofuran synthesis, the reaction possesses excellent diastereoselectivity and Z-selectivity. On the basis of Rh2(OAc)4-mediated unique decomposition of diazo-functionalized 2-methylene-2,3-dihydrofurans, an application to diastereoselective construction of a 5-methylene-4,7-dihydro-5H-furo[2,3-c]pyran frame has been achieved for the first time.

Organocatalytic Synthesis of Furan-Embedded Styrene Atropisomers

Herein, we report the first synthesis of furan-embedded styrene atropisomers via the reaction between 1-(aryl-ethynyl)-naphthalen-2-ol and γ-hydroxyenone. The reaction proceeds through in situ furan formation from γ-hydroxyenone. The styrene derivatives were obtained in moderate to good yields with high diastereoselectivities with a catalytic amount of PTSA. Few applications such as triazole formation and cross-coupling reactions have been demonstrated. A preliminary catalytic asymmetric version was also reported.

Preparation of 4-(Nitromethyl)furan Derivatives and Their Application in the Syntheses of Bis(furan-2-yl)oximes

An efficient method for the preparation of tetrasubstituted furans, which contains a nitromethyl group at the 4-position, has been developed. The applications of 4-(nitromethyl)furans on the synthesis of highly functionalized bis(furyl)oxime were explored for the first time.

Unified Approach to Furan Natural Products via Phosphine-Palladium Catalysis

Polyalkyl furans are widespread in nature, often performing important biological roles. Despite a plethora of methods for the synthesis of tetrasubstituted furans, the construction of tetraalkyl furans remains non-trivial. The prevalence of alkyl groups in bioactive furan natural products, combined with the desirable bioactivities of tetraalkyl furans, calls for a general synthetic protocol for polyalkyl furans. This paper describes a Michael-Heck approach, using sequential phosphine-palladium catalysis, for the preparation of various polyalkyl furans from readily available precursors. The versatility of this method is illustrated by the total syntheses of nine distinct polyalkylated furan natural products belonging to different classes, namely the furanoterpenes rosefuran, sesquirosefuran, and mikanifuran; the marine natural products plakorsins A, B, and D and plakorsin D methyl ester; and the furan fatty acids 3D5 and hydromumiamicin.

Synthesis of Densely Substituted Sulfonylfurans and Dihydrofurans via Cascade Reactions of α-Functionalized Nitroalkenes with β-Ketosulfones

The reaction of β-ketosulfones with different α-functionalized nitroalkenes affords diversely substituted sulfonylfurans and dihydrofurans. Furthermore, β-ketosulfones react with α-bromonitroalkenes and α-hydrazinonitroalkenes via a cascade Michael addition-cyclization protocol to afford nitrodihydrofurans and hydrazinodihydrofurans, respectively, bearing a key sulfonyl group, in excellent yields with a broad substrate scope. Application of these products has been demonstrated by the synthesis of pyrroles and pyrazoles in good yields. The reaction of β-ketosulfones with nitroallylic acetates yields tetrasubstituted sulfonyl furans through a cascade S_N2'-intramolecular Michael reaction, followed by aromatization. The gram-scale synthesis of a representative example of sulfonylfurans was carried out to demonstrate the synthetic efficiency of the methodology.

Catalytic 1,2-Rearrangements: Organocatalyzed Michael/Semi-Pinacol-like Rearrangement Cascade of 1,3-Diones and Nitroolefins

New types of organocatalytic 1,2-rearrangements, which resemble the Smiles-like or semi-pinacol-like rearrangement, of Michael adducts of 1,3-dicarbonyl-2-alkyl compounds and nitroalkenes have been realized. Unlike the well-known conjugate addition, the reaction affords the 1-phenyl-1-nitroalkanes via unprecedented rearrangement and cascade reactions. Structures of the appropriate products were unambiguously characterized by X-ray crystallography.

2-Activated 1,3-enynes in enantioselective synthesis

The rapid enantioselective synthesis of valuable building blocks and pharmaceutically important compounds from easily accessible precursors is one of the major areas of focus in organic chemistry. In this context, 2-activated 1,3-enyne has emerged as a powerful synthon in recent years for the efficient synthesis of enantioenriched furans, allenes, 4-H-pyrans, and 4-isoxazolines, which are privileged scaffolds in bioactive compounds and natural products. This review will cover the history of the development of 2-activated 1,3-enyne in enantioselective synthesis along with the corresponding mechanisms, which may motivate further development in this area to forge more complex and valuable molecules.

Base-Mediated Tunable Synthesis of 2-Trifluoromethylated Furans and Dihydrofuranols: Extraordinary Stable in Sulfuric Acid

2-Trifluoromethylated furans and dihydrofuranols were tunably synthesized from the cyclization of β-ketonitriles with 3-bromo-1,1,1-trifluoroacetone mediated by bases. In addition, dehydration of dihydrofuranol compounds with concentrated sulfuric acid gave another 2-(trifluoromethyl)furans isomer. The developed methodology exhibits an excellent functional group tolerance for both aromatic and aliphatic β-ketonitriles.

Access to Polysubstituted (Furyl)methylthioethers via a Base-Promoted S-H Insertion Reaction of Conjugated Enynones

A convenient and applicable approach to the construction of diverse functionalized (2-furyl)methylthioether derivatives via base-promoted S-H insertion of conjugated enynones with thiophenols or thiols has been developed. This reaction features readily available starting materials, high atom economy, broad substrate scope, and versatile operation. Moreover, the synthetic utility of this method has been demonstrated by the efficient synthesis of the CNKSPR1 inhibitor precursor and late-stage functionalization of glutathione.

Synthesis of ynones at room temperature catalyzed by copper chloride cryptand complex under solvent free conditions

An air-stable highly efficient reusable CuCl2-cryptand-[2.2.Benzo] catalyst is reported first time for coupling reaction of terminal alkynes with different acyl chlorides in the presence of triethylamine acting both as base and solvent at room temperature to give the corresponding ynones. Easy-going, short reaction time, cost-effective, palladium-, phosphorus- and solvent-free, high yield and recyclability up to 5th times make this method green procedure for ynones' synthesis with wide substrate variety.

Transition-metal-free C–C σ-bond activation of α-aryl ketones and subsequent Zn-catalyzed intramolecular cyclization: synthesis of tetrasubstituted furans

Atom-economical synthesis of tetrasubstituted furans from α-aryl ketones and alkynones through base-promoted cleavage of unstrained C–C single bonds of ketones and subsequent Zn-catalyzed cyclization.

Peculiarities of supramolecular organization of cyclic ketones with vinylacetylene fragments

1,5-Diaryl(heteroaryl)pentenynones show a tendency to crystallize in acentric space groups due to their inclination to form C-H...O hydrogen-bonded chains instead of dimers and thus exhibit nonlinear optical properties. A series of symmetrical α,α'-bis(3-arylprop-2-yn-1-ylidene)cycloalkanones and unsymmetrical α-(furan-2-ylmethylene)-α'-(3-arylprop-2-yn-1-ylidene)cyclohexanones closely related to pentenynones was synthesized, namely 2,5-bis(3-phenylprop-2-yn-1-ylidene)cyclopentanone, C₂₃H₁₆O, 2,5-bis[3-(4-bromophenyl)prop-2-yn-1-ylidene]cyclopentanone, C₂₃H₁₄Br₂O, 2,6-bis(3-phenylprop-2-yn-1-ylidene)cyclohexanone, C₂₄H₁₈O, 2,6-bis[3-(4-bromophenyl)prop-2-yn-1-ylidene]cyclohexanone, C₂₄H₁₆Br₂O, 4-tert-butyl-2,6-bis(3-phenylprop-2-yn-1-ylidene)cyclohexanone, C₂₈H₂₅O, 4-tert-butyl-2,6-bis[3-(4-methylphenyl)prop-2-yn-1-ylidene]cyclohexanone, C₃₀H₃₀O, 2-(furan-2-ylmethylene)-6-(3-phenylprop-2-yn-1-ylidene)cyclohexanone, C₂₀H₁₆O₂, and 6-(3-butylprop-2-yn-1-ylidene)-2-(furan-2-ylmethylene)cyclohexanone, C₁₀H₂₀O₂, and investigated by means of X-ray diffraction to understand peculiarities of their supramolecular organization. Four of the eight novel compounds crystallize in acentric space groups. Three of these four compounds contain substituents at the para position of the phenyl ring, which affect the charge density on the H(C_Ph) atoms and thus stabilize C_Ph-H...O interactions. The fourth compound realizes the C-H...O hydrogen bonding via H atoms of the furyl ring. The applicability and shortcomings of the Full Interaction Map tool to predict the likelihood of C-H...O and C-H...Br hydrogen-bonded motifs, and the effect of substituents on the phenyl ring on the supramolecular architecture are discussed.

Synthetic Studies toward (±)-Furanocembranoid 1: Construction of the Acyclic Carbon Framework

Herein, we report the synthesis of the entire acyclic carbon framework toward (±)-furanocembranoid 1 via the longest linear sequence of 12 steps from commercially available linalool and diethyl 2-isopropylmalonate. Key to the success of this synthetic approach is a silver-catalyzed enyne-annulation reaction for the formation of 2,4-disubstituted furan motif of unique furanocembranoid 1, isolated from Croton oblongifolius. Construction of macrocycle has also been explored using the ring-closing metathesis reaction.

Tandem Achmatowicz Rearrangement and Acetalization of 1-[5-(Hydroxyalkyl)-furan-2-yl]-cyclobutanols Leading to Dispiroacetals and Subsequent Ring-Expansion to Form 6,7-Dihydrobenzofuran-4(5 H)-ones

Herein, we report a one-pot protocol for the synthesis of dispiroacetals 4 bearing a cyclobutane motif via tandem Achmatowicz rearrangement and acetalization of 1-[5-(hydroxyalkyl)-furan-2-yl]-cyclobutanols 3 with m-CPBA as the oxidant and AgSbF₆ as an additive to promote the cyclization step in an aqueous medium. Dispiroacetals 4 could subsequently undergo Lewis acid-catalyzed ring expansion and skeletal rearrangement to afford 6,7-dihydro-5 H-benzofuran-4-ones 5.

Ultrasonochemical synthesis of 2,3-dihydrofuranediones in aqueous medium

2,3-Dihydrofuranediones were prepared by an efficient one-pot multicomponent Knoevnagel-like condensation between pyrazolecarbaldehydes or indole-3-carbaldehyde, ethyl pyruvate and bromine, followed by an intramolecular cyclization in aqueous medium.

Stereocontrolled Total Synthesis of Nonenolide

Nonenolide (1) was first isolated from the entomopathogenic fungus Cordyceps militaries BCC2816 and exhibited good antimalarial activity against Plasmodium falciparum K1. Structurally, it features a decanolide with a trans-double bond attached to two chiral hydroxy groups, making the total synthesis of the exclusive isomer of 1 more difficult. Herein, we report the successful synthesis of 1 by employing a MacMillan α-hydroxylation to generate three chiral centers in both the key fragments, starting from 1,6-hexanediol and 1,4-butanediol, followed by Steglich esterification of compounds 2 and 3. The exclusive E-isomer was obtained via a ring-closing metathesis of the mono-PMB-protected diene 19. Deprotection provided the required natural product 1.

One-Pot Tandem Approach to Functionalized 3-Hydroxy-2-furanyl-acrylamides

A novel one-pot tandem process involving Knoevenagel condensation, Michael addition, selective amidation, and Paal-Knorr cyclization to diverse functionalized 3-hydroxy-2-furanyl-acrylamides from simple 2-oxoaldehydes and aroylacetonitriles was presented. Attempts were also made to expand the scope of the reaction to different 2-heteroarylfurans. The packing diagram of the molecules viewed down along the α-axis of the unit cell showed a characteristic intramolecular classical O-H···O hydrogen bond between hydroxyl and carbonyl O atoms leading to self-associated (Z)-2-furanyl-acrylamides.

Hydroarylation of unsaturated carbon–carbon bonds in cross-conjugated enynones under the action of superacid CF3SO3H or acidic zeolite HUSY. Reaction mechanism and DFT study on cationic intermediate species

Reactions of enynones with arenes in TfOH-Py or with zeolite HUSY lead to products of hydroarylation of the acetylene bond.

Ultrasound mediated synthesis of dihydropyrano[3,2-d][1,3]dioxin-7-carbonitrile derivatives in H2O/EtOH medium

A one-pot cyclocondensation of 1,3-dioxane-5-one (1) with malononitrile and aromatic aldehydes in aqueous sodium hydroxide under ultrasonic irradiation furnishes a series of pyrano[3,2-d][1,3]dioxin derivatives 3. Reactions are completed after a few minutes and the precipitated products are purified by simple crystallization from ethanol. The reaction with ethyl cyanoacetate instead of malononitrile gives the respective analogous products in high yields.

Synthesis and Characterization of Urofuranoic Acids: In Vivo Metabolism of 2-(2-Carboxyethyl)-4-methyl-5-propylfuran-3-carboxylic Acid (CMPF) and Effects on in Vitro Insulin Secretion

CMPF (2-(2-carboxyethyl)-4-methyl-5-propylfuran-3-carboxylic acid) is a metabolite that circulates at high concentrations in type 2 and gestational diabetes patients. Further, human clinical studies suggest it might have a causal role in these diseases. CMPF inhibits insulin secretion in mouse and human islets in vitro and in vivo in rodents. However, the metabolic fate of CMPF and the relationship of structure to effects on insulin secretion have not been significantly studied. The syntheses of CMPF and analogues are described. These include isotopically labeled molecules. Study of these materials in vivo has led to the first observation of a metabolite of CMPF. In addition, a wide range of CMPF analogues have been prepared and characterized in insulin secretion assays using both mouse and human islets. Several molecules that influence insulin secretion in vitro were identified. The molecules described should serve as interesting probes to further study the biology of CMPF.

Synthesis and biological evaluation of (-)-kunstleramide and its derivatives

Stereoselective total synthesis of (-)-kunstleramide, a cytotoxic dienamide from the bark of Beilschmiedia kunstleri gamble, has been accomplished by using Keck's asymmetric allylation and Trost isomerization as key reactions. Application of the developed strategy for the synthesis of a series of amide analogues (8-22) was also reported. Furthermore, the synthesized compounds were evaluated for their in vitro anti-proliferative activities against human epithelial lung carcinoma (A549), human epithelial cervical cancer (HeLa), human breast adenocarcinoma (MCF7) and human neuroblastoma (IMR32) cell lines using the SRB assay. All the compounds show moderate anti-proliferative activity against all cell lines. Some of the piperazine derivatives (17-22) strongly inhibit the growth of breast cancer cells with IC50 values of 8-20 μM.

An atom- and pot-economical consecutive multi-step reaction approach to polycyclic aromatic hydrocarbons (PAHs)

A novel [4+2]-benzannulation approach to substituted polycyclic aromatic hydrocarbons has been developed under atom-economical, mild and metal-free reaction conditions.

Regio- and stereoselective syntheses of allylic thioethers under metal free conditions

A metal-free, regio and stereoselective synthesis of allylic thioethers using densely functionalized allyl iodides and aryl/alkyl disulfides as coupling partners, provided the resulting allyl aryl thioethers in 62–92% isolated yields is described.