Showing papers on "Pyran published in 2014"

Facile and One-Pot Access to Diverse and Densely Functionalized 2-Amino-3-cyano-4H-pyrans and Pyran-Annulated Heterocyclic Scaffolds via an Eco-Friendly Multicomponent Reaction at Room Temperature Using Urea as a Novel Organo-Catalyst

Abstract: A simple, straightforward, and highly efficient multicomponent one-pot synthesis of a pharmaceutically interesting diverse kind of functionalized 2-amino-3-cyano-4H-pyrans and pyran-annulated heterocycles has been developed based on a low-cost and environmentally benign commercially available urea as a novel organo-catalyst. The reaction occurs via tandem Knoevenagel–cyclocondensation of aldehydes, malononitrile, and C–H-activated acidic compounds in aqueous ethanol at room temperature. Following this protocol, it was possible to synthesize 2-amino-3-cyano-pyrano[3,2-c]chromen-5(4H)-ones (4aa–4al), 2-amino-3-cyano-pyrano[4,3-b]pyran-5(4H)-ones (4ba–4be), 2-amino-3-cyano-7,8-dihydro-4H-chromen-5(6H)-one (4ca–4cr), 1H-pyrano[2,3-d]pyrimidine-2,4(3H,5H)-diones (4da–4dd), 2-amino-3-cyano-5,10-dioxo-5,10-dihydro-4H-benzo[g]chromenes (4ea–4ec), 2-amino-3-cyano-4H-pyrans (4fa–4fh), and 1,4-dihydropyrano[2,3-c]pyrazoles (4ga–4gb). The salient features of the present protocol are mild reaction conditions, excellen...

Terminal olefins to chromans, isochromans, and pyrans via allylic C-H oxidation.

Abstract: The synthesis of chroman, isochroman, and pyran motifs has been accomplished via a combination of Pd(II)/bis-sulfoxide C–H activation and Lewis acid co-catalysis. A wide range of alcohols are found to be competent nucleophiles for the transformation under uniform conditions (catalyst, solvent, temperature). Mechanistic studies suggest that the reaction proceeds via initial C–H activation followed by a novel inner-sphere functionalization pathway. Consistent with this, the reaction shows reactivity trends orthogonal to those of traditional Pd(0)-catalyzed allylic substitutions.

Nano α-Al2O3supported ammonium dihydrogen phosphate (NH4H2PO4/Al2O3): preparation, characterization and its application as a novel and heterogeneous catalyst for the one-pot synthesis of tetrahydrobenzo[b]pyran and pyrano[2,3-c]pyrazole derivatives

Abstract: A simple, efficient, and environmentally benign route was developed for the preparation of 2-amino-3-cyano-4-aryl-7,7-dimethyl-5,6,7,8-tetrahydrobenzo[b]pyrans and 6-amino-5-cyano-4-aryl-1,4-dihydropyrano[2,3-c]pyrazoles from the condensation of various aldehydes, malononitrile, and 1,3-dicarbonyl compounds (1,3-cyclohexanedione or 5,5-dimethyl-1,3-cyclohexanedione) or 3-methyl-1-phenyl-2-pyrazoline-5-one, using NH4H2PO4/Al2O3 with good yields. The use of easily available catalyst, shorter reaction times, better yields, the simplicity of the reaction, the heterogeneous system, and the easy work up are the advantages of the present method. Characterization of the catalyst was performed by FT-IR spectroscopy, X-ray diffraction (XRD) techniques, thermal analysis (TG/DTG) and Transmission electron microscopy (TEM).

Nano-titania sulfuric acid-promoted synthesis of tetrahydrobenzo[b]pyran and 1,4-dihydropyrano[2,3-c]pyrazole derivatives under ultrasound irradiation

Abstract: An efficient and convenient procedure has been described for one-pot synthesis of tetrahydrobenzo[b]pyran and 1,4-dihydropyrano[2,3-c]pyrazole derivatives in the presence of nano-titania sulfuric acid (15-nm TSA) as a heterogeneous catalyst. The catalyst is easily prepared from the reaction of nano-titania and chlorosulfonic acid in dichloromethane as solvent. The transmission electron microscopy image of the catalyst showed no detectable changes on the size of the nano-catalyst as a result of chemisorptions of the sulfuric acid. To examine the effect of minimizing the size of TSA particles, the pH analysis of the catalyst with different particle sizes was carried out. The catalyst can be easily recovered and reused for several successive fresh runs without significant loss of activity. Under ultrasound irradiation condition, the reactions proceed smoothly to afford the corresponding products in quantitative yields.

Synthesis of Prenyl‐ and Geranyl‐Substituted Carbazole Alkaloids by DIBAL‐H Promoted Reductive Pyran Ring Opening of Dialkylpyrano[3,2‐a]carbazoles

Abstract: The DIBAL-H promoted reductive pyran ring opening of dialkylpyrano[3,2-a]carbazoles provides a direct access to a broad range of prenyl- and geranyl-substituted carbazoles. Formation of a pyran ring followed by reductive ring opening represents a new method for the introduction of prenyl and geranyl groups. In the course of the present work, we achieved the first total syntheses of the following eight carbazole alkaloids: clauraila-E, 7-hydroxyheptaphylline, 7-methoxyheptaphylline, mukoenine-B (clausenatine-A), mukoenine-A (girinimbilol), mahanimbinol (mahanimbilol), euchrestine-A, and isomurrayafoline-B.

Synthesis, characterization and biological screening of novel 5-imidazopyrazole incorporated fused pyran motifs under microwave irradiation

Abstract: A novel combinatorial library of fused pyran derivatives has been designed and synthesized under microwave irradiation by one-pot three-component cyclocondensation reaction of 5-(1H-imidazol-1-yl)-3-methyl-1-phenyl-1H-pyrazole-4-carbaldehyde with two active methylene compounds and enolizable ketones/phenols in the presence of piperidine as a basic catalyst. All the newly synthesized compounds have been characterized by elemental analysis and various spectroscopic methods. All the final motifs have been screened for their preliminary in vitro antibacterial activity against a panel of pathogenic strains of bacteria and fungi, preliminary in vitro antituberculosis activity against Mycobacterium tuberculosis H37Rv and also for their antimalarial activity against Plasmodium falciparum.

Total Synthesis of 7‐ and 8‐Oxygenated Pyrano[3,2‐a]carbazole and Pyrano[2,3‐a]carbazole Alkaloids via Boronic Acid‐Catalyzed Annulation of the Pyran Ring

Abstract: The boronic acid-catalyzed annulation of citral opens up a short route to oxygenated cyclized monoterpenoid pyranocarbazole alkaloids. Thus, murrayamine-D is available in only three steps and 55% overall yield from the corresponding carbazole precursor.

The photo-Nazarov reaction: scope and application.

Abstract: The reaction conditions and scope of the photo-Nazarov reaction of aryl vinyl ketones were investigated. In contrast to the conventional acid-catalyzed methods, this photolytic electrocyclization proceeds in the neutral or basic conditions. Irradiating substrates bearing various aromatic rings, acid-sensitive groups, cyclohexenyl, cycloheptenyl, and unsaturated pyran with UV-light (254 nm) smoothly yielded hexahydrofluorenones and related structures. This photo-Nazarov reaction could also be applicable to the substrates carrying β-alkyl groups on the enone, which gave corresponding polycyclic rings containing quaternary centers. These photo-electrocyclized products may prove useful for synthesizing a variety of natural products and their derivatives. Further application of this mild photo-Nazarov reaction in the synthesis of taiwaniaquinol B was achieved.

Covalently anchored n-propyl-4-aza-1-azoniabicyclo[2.2.2]octane chloride on SBA-15 as a basic nanocatalyst for the synthesis of pyran heterocyclic compounds

Abstract: In the present study, highly ordered n-propyl-1,4 diazoniabicycle[2.2.2]octane chloride functionalized mesoporous SBA-15, SBA-DABCO, with high surface area and narrow pore-size distribution was synthesized and characterized by TEM, SEM, FT-IR, CHN, TGA, XRD and BET. The organic-modified SBA-15 has hexagonal crystallographic order, pore diameter up to 54.2 A, and the content of DABCO groups up to 1.38 mmol g−1. The catalyst shows an environmentally benign character, which can be easily prepared, stored and recovered without obvious significant loss of activity. The synthesized basic organocatalyst provides a green and useful method for the one-pot synthesis of pyran annulated heterocyclic compounds, and especially spirooxindole pyran compounds in aqueous media. The significant features of the present protocol are simple, environmentally benign, high yields, no chromatographic separation and recyclability of the catalyst.

New organometallic ruthenium(II) complexes containing chelidonic acid (4-oxo-4H-pyran-2,6-dicarboxylic acid): synthesis, structure and in vitro biological activity

Abstract: Two new bivalent organometallic ruthenium complexes [Ru(HL)(CH3CN)(CO)(PPh3)2] (3) and [Ru(HL)(CH3CN)(CO)(AsPh3)2] (4), (HL = 4-oxo-4H-pyran-2,6-dicarboxylic acid) were synthesized, structurally characterized and their biological activities (anti-microbial, DNA–protein interactions, antioxidant and cytotoxic activity studies (MTT, LDH release and NO release)) have been investigated and compared with that of appropriate precursor complexes [RuHCl(CO)(PPh3)3] (1), [RuHCl(CO)(AsPh3)3] (2) and the ligand H2L. The crystal structure of the complex 3 was solved by a single crystal X-ray diffraction technique, which revealed that it is a distorted octahedral with HL as a dibasic bidentate donor and the chelator was observed to undergo C–H activation at one of the ortho positions, leading to the formation of a five membered metallacycle. The in vitro antimicrobial activity was carried out using the well diffusion method against different species of pathogenic bacteria and fungi and complex 4 exhibited a better activity in inhibiting the growth of the tested organisms. DNA–protein interactions of the complexes have been examined by photophysical studies, which revealed that the complexes can bind with DNA through non-intercalation and the complexes strongly quench the intrinsic fluorescence of bovine serum albumin, through a static quenching process. The free radical scavenging ability, assessed by a series of in vitro antioxidant assays involving the DPPH radical, hydroxyl radical, nitric oxide radical, superoxide anion radical, hydrogen peroxide and a metal chelating assay showed that the new complexes 3 and 4 possess excellent radical scavenging properties over 1, 2, H2L, and the standard drugs, vitamin C and BHT. The in vitro cytotoxic activities of the compounds have been validated against A549 cells via an MTT assay, LDH release, NO release and the values were compared with that of the standard drug cisplatin. The results indicated that the new complexes, specifically the complex 3, displayed a higher cytotoxic activity in inhibiting the growth of A549 cells, outperforming by five fold, the standard drug cisplatin.

Tuning the reactivity of oxygen/sulfur by acidity of the catalyst in Prins cyclization: oxa- versus thia-selectivity.

Abstract: An unprecedented oxa- versus thia-selectivity has been observed in Prins cyclization of 6-mercaptohex-3-en-1-ol with aldehydes In the presence of a stoichiometric amount of strong Lewis or Bronsted acids, the reaction provides the hexahydro-2H-thieno[3,2-c]pyran skeleton predominantly via oxonium-Prins cyclization In contrast, a catalytic amount of weak Lewis or Bronsted acids provides the hexahydro-2H-thiopyrano[4,3-b]furan preferentially through thionium-Prins cyclization

Gold(III) chloride catalyzed synthesis of chiral substituted 3-formyl furans from carbohydrates: application in the synthesis of 1,5-dicarbonyl derivatives and furo[3,2-c]pyridine.

Abstract: This report describes a gold(III)-catalyzed efficient general route to densely substituted chiral 3-formyl furans under extremely mild conditions from suitably protected 5- (1-alkynyl)-2,3-dihydropyran-4-one using H2O as a nucleophile.The reaction proceeds through the initial formation of an activated alkyne–gold(III) complex intermediate, followed by either a domino nucleophilic attack/anti-endo-dig cyclization, or the formation of a cyclic oxonium ion with subsequent attack by H2O. To confirm the proposed mechanistic pathway, we employed MeOH as a nucleophile instead of H2O to result in a substituted furo[3,2-c]pyran derivative, as anticipated. The similar furo[3,2-c]pyran skeleton with a hybrid carbohydrate–furan derivative has also been achieved through pyridinium dichromate (PDC) oxidation of a substituted chiral 3-formyl furan. The corresponding protected 5-(1-alkynyl)-2,3-dihydropyran-4-one can be synthesized from the monosaccharides (both hexoses and pentose) following oxidation, iodination, and Sonogashira coupling sequences. Furthermore, to demonstrate the potentiality of chiral 3-formyl furan derivatives, a TiBr4-catalyzed reaction of these derivatives has been shown to offer efficient access to 1,5-dicarbonyl compounds, which on treatment with NH4OAc in slightly acidic conditions afforded substituted furo[3,2-c]pyridine.

1,1′-Butylenebis(3-methyl-3H-imidazol-1-ium) dihydrogensulfate as a halogen-free and reusable binuclear Brönsted ionic liquid catalyzed the synthesis of pyrano[4,3-b]pyran derivatives

Abstract: 1,1′-Butylenebis(3-methyl-3H-imidazol-1-ium) dihydrogensulfate as an efficient, halogen-free and reusable binuclear Bronsted ionic liquid catalyzed the synthesis of pyrano[4,3-b]pyran derivatives at 60 °C under solvent-free conditions. This method has the advantages of high yield, clean reactions, simple methodology and short reaction time. The ionic liquid could be recycled without significant loss of activity. The structure of the compounds was confirmed by IR, 1H NMR, 13C NMR, mass and elemental analysis.

Green synthesis of phosphoryl-2-oxo-2H-pyran via three component reaction of trialkyl phosphites.

Abstract: An effective one-pot synthesis of dialkoxyphosphoryl-2-oxo-2H-pyran derivatives by three-component reaction of alky bromides and dialkyl acetylenedicarboxylates in the presence of trialkyl phosphite is described. The reactions were performed under solvent-free conditions at 50°C and neutral conditions and provided good yields of products.

Microwave-Assisted Copper-Powder-Catalyzed Coupling and Cyclization of β-Bromo-α,β-unsaturated Carboxylic Acids with 1,3-Diketones Leading to 2H-Pyran-2-ones

Abstract: β-Bromo-α,β-unsaturated carboxylic acids were coupled and cyclized with 1,3-diketones by microwave irradiation in the presence of a catalytic amount of copper powder and base to give the corresponding 2H-pyran-2-ones in good to high yields.

Perchloric acid modified-cellulose: a versatile, novel and biodegradable heterogeneous solid acid catalyst for single-pot synthesis of novel bis-pyran annulated heterocyclic scaffolds under solvent-free conditions

Abstract: A one-pot multi-component, green, and highly efficient protocol has been developed for the synthesis of novel bis-pyran annulated heterocyclic scaffolds (4a–r) using cellulose–perchloric acid (CPA) as an eco-friendly, biodegradable and recyclable catalyst in excellent yields under solvent-free conditions. Other remarkable features of this environmentally benign protocol are short reaction time, a wide range of functional group tolerance, use of inexpensive heterogeneous catalyst, and high yield of products via a simple experimental and work-up procedure as compared to the conventional methods. The cellulose–HClO4 (CPA) catalyst is characterized by using FTIR, powder XRD and SEM–EDX analyses. The stability of the catalyst was evaluated by TG/DTA characterization techniques. The catalyst can be recycled several times with a slight loss of its catalytic activity.

Synthesis and Conformational Analysis of Cyclic Homooligomers from Pyranoid ε‐Sugar Amino Acids

Abstract: New pyranoid e-sugar amino acids were designed as building blocks, in which the carboxylic acid and the amine groups were placed in positions C2 and C3 with respect to the tetrahydropyran oxygen atom. By using standard solution-phase coupling procedures, cyclic homooligomers containing pyranoid e-sugar amino acids were synthesized. Conformation analysis was performed by using NMR spectroscopic experiments, FTIR spectroscopic studies, X-ray analysis, and a theoretical conformation search. These studies reveal that the presence of a methoxy group in the position C4 of the pyran ring produces an important structural change in the cyclodipeptides. When the methoxy groups are present, the structure collapses through interresidue hydrogen bonds between the oxygen atoms of the pyran ring and the amide protons. However, when the cyclodipeptide lacks the methoxy groups, a U-shape structure is adopted, in which there is a hydrophilic concave face with four oxygen atoms and two amide protons directed toward the center of the cavity. Additionally, we found important evidence of the key role played by weak electrostatic interactions, such as the five-membered hydrogen-bonded pseudocycles (C5) between the amide protons and the ether oxygen atoms, in the conformation equilibrium of the macrocycles and in the cyclization step of the cyclic tetrapeptides.