Microwave promoted methylation of plant polysaccharides

O-Methylation is of outstanding importance in structural polysaccharide chemistry. A novel method for the methylation of polysaccharides using microwave (MW) irradiation is described. Seed gum from Cyamopsis tetragonolobus (Guar) was fully methylated with dimethyl sulphate and sodium hydroxide using 100% microwave power for 4 min in 68% yield. The completely methylated seed gum thus obtained was hydrolyzed by 70% formic acid followed by 0.5N H₂SO₄ under full microwave power for 1.16 and 1.66 min, respectively. The partially methylated monosaccharides were separated and identified.     

Microwave assisted straightforward synthetic method for benzimidazole linked quinoxalinones on soluble polymer support

The simple and efficient method was developed for the synthesis of benzimidazole linked quinoxalinones on soluble polymer support using microwave conditions. The acid catalyzed condensation of 4-fluoro-3-nitrobenzoic acid with polymer immobilized o-phenylenediamine, ipso-fluoro nucleophilic substitution with various primary amines and cyclization with acetyl chloride are the key steps involved in implemented linear synthesis. In key cyclization step, the regioselective N-acylation at secondary amine with chloroacetyl chloride followed by spontaneous intramolecular ring closure through ortho-amine functionality generate the quinoxaline skeleton under microwave irradiation. The removal of polymer support and exposure of quinoxalines for auto-oxidation finally produce the benzimidazole linked quinoxalinone derivatives with high purity and yields.     

One-pot three-component reaction of C60, amino acid and fluorinated benzaldehyde to C60-pyrrolidine derivatives

A series of fluorinated 2,5-disubstituted C₆₀-pyrrolidine derivatives were synthesized via one-pot three-component reaction of C₆₀, amino acid and fluorinated benzaldehyde under reflux in toluene or microwave irradiation. The cis- and trans-isomers could be isolated by chromatography and fully confirmed by ¹H NMR.     

Microwave-assisted reaction between 2-aminobenzoic acids, 2-hydroxybenzaldehydes,and arylboronic acids: a one-pot three-component synthesis of bridgehead bicyclo[4.4.0]boron heterocycles

A one-pot three-component synthesis of 6-aryl-8H-dibenzo[d,h][1,3,7,2]dioxazaborecin-8-ones is described. A mixture of 2-aminobenzoic acid, 2-hydroxybenzaldehyde, and arylboronic acid undergo a 1:1:1 addition reaction in CCl₄ under microwave irradiation to produce bridgehead bicyclo[4.4.0]boron heterocycles in excellent yields.     

Fast preparation of dihydrocyclocitral from citronellal under solventless microwave irradiation

Dihydrocyclocitral, a useful reagent in organic synthesis, has been synthesized in high yield and with high stereoselectivity from citronellal under microwave irradiation in two steps, involving acetic anhydride under base catalysis, then p-toluenesulfonic acid adsorbed on silica gel under solventless conditions (80% yield, reaction time 22 min).     

Straightforward Synthesis of Novel 1-(2'-α-<em>O</em>-D-Glucopyranosyl ethyl) 2-Arylbenzimidazoles

A series of novel 1-(2'-α-O-D-glucopyranosyl ethyl) 2-arylbenzimidazoles has been prepared via one-pot glycosylation of ethyl-1-(2'-hydroxyethyl)-2-arylbenzimidazole-5-carboxylate derivatives. Synthesis of the 2-arylbenzimidazole aglycones from 4-fluoro-3-nitrobenzoic acid was accomplished in four high-yielding steps. The reduction and cyclocondensation steps for the aglycone synthesis proceeded efficiently under microwave irradiation to afford the appropriate benzimidazoles in excellent yields within 2-3 min. Glycosylation of the hydroxyethyl aglycones with the perbenzylated 1-hydroxy- glucopyranose, pretreated with the Appel-Lee reagent, followed by catalytic hydrogenolysis delivered the desired 1-(2'-α-O-D-glucopyranosyl ethyl) 2-aryl-benzimidazoles in a simple and straightforward manner.     

Synthesis of environmentally friendly,magnetic acid-type calix[4]arene catalyst for obtaining Biginelli adducts

An immobilized p-sulfonic acid calix[4]arene was synthesized on the surface of silica-coated Fe₃O₄ nanoparticles. Due to the combination of the magnetic recovery and the acid properties, it acted as a robust, safe and environmentally friendly catalyst for the one-pot synthesis of Biginelli adducts under microwave irradiation and solvent-free conditions. A series of Biginelli adducts were obtained in moderate to excellent yields, and most importantly, the catalyst could be easily recovered by an external magnet and reused five times without significant loss of catalytic activity (ca. 80% yield for all reuses using 0.64 mol% of the catalyst).     

Microwave-assisted organic synthesis of 3-substituted-imidazo[1,5-a]pyridines

3-Substituted-imidazo[1,5-a]pyridines were conveniently synthesized in two steps from commercially available picolinic esters under microwave irradiation conditions.     

Synthesis of a β-strand mimetic based on a pyridine scaffold

A synthetic route to a 2,4-disubstituted pyridine as a potential β-strand mimetic has been developed and applied in the synthesis of a tripeptidomimetic of Leu-Gly-Gly. The pyridine scaffold replaces the central glycine, and is substituted with analogues of leucine and glycine in positions 4 and 2, respectively. 2-Fluoro-4-iodopyridine was chosen as the functionalized scaffold and was substituted with protected leucinal in position 4 via a Grignard exchange reaction using iso-propyl magnesium chloride. The glycine moiety was introduced in position 2 via a nucleophilic aromatic substitution reaction (S_NAr) facilitated by microwave irradiation. The synthetic sequence involved 12 steps with an overall yield of 7%.     

Stereospecific cyclodehydration of 1,4-sulfanylalcohols to thiolanes: mechanistic insights

A series of thiolanes were prepared by cyclodehydration of sulfanylalcohols in the presence of catalytic amounts of p-toluenesulfonic acid or by using K10 clay. The sulfur heterocycles were synthesised in good to excellent yields using either a conventional Dean-Stark method or microwave irradiation under solvent-free conditions. The reaction could be performed regio- and stereoselectively and its mechanism was investigated by means of enantio- and diastereomerically enriched substrates. In contrast to previous studies, our results are consistent with an intramolecular S_N2-type mechanism as a general pathway.     

Pretreatment of alfalfa stems by wood decay fungus <Emphasis Type="Italic">Perenniporia meridionalis</Emphasis> improves cellulose degradation and minimizes the use of chemicals

Enzymes of wood decay fungi can be exploited to degrade lignocellulosic wastes for sustainable production of bioethanol. Perenniporia meridionalis was tested for growing at different temperatures on stems of alfalfa. The process aims to produce fermentable sugars and can be divided into the following steps: (1) fungal treatment to degrade lignin, (2) microwave pretreatment in water or in phosphoric acid, and (3) enzymatic hydrolysis of cell wall carbohydrates. Thermogravimetric analysis assessed the biomass content of cellulose and lignin after the fungal treatment. Throughout all steps HPLC analysis of sugars, oligomers and by-products (furfural, hydroxymethylfurfural and acids) was performed. Scanning electron microscopy was used for visual inspection and characterization of the experimental material during the treatments. The P. meridionalis pretreatment enhanced the yield of fermentable sugars obtainable by enzymatic hydrolysis in samples subjected to microwave-assisted pretreatment in water, but not in those in acid medium. This is probably related to the very selective removal of lignin by P. meridionalis, exposing cellulose fibers without depleting them. Furthermore, microwave treatment in water produced less byproducts than in acid medium. By exploiting the P. meridionalis lignin degradation is therefore possible to avoid H₃PO₄ use during the alfalfa stem pre-treatment, reducing economic and environmental impacts.     

Microwave-assisted one-pot synthesis of substituted tetrahydrocarbazole and 8,9,10,11-tetrahydro-7H-pyrido[a]carbazoles

One-pot synthesis of one-substituted tetrahydrocarbazole and 4-substituted 8,9,10,11-tetrahydro-7H-pyrido[a]carbazoles from substituted quinolinylhydrazines and cyclohexanone in acetic acid was performed by microwave irradiation in a controlled temperature with simultaneous cooling system in closed vessel. The optimization procedures of process variables, power, temperature, and irradiation time are reported in detail, and the results from microwave processes are compared with conventional ones.     

Hydrolysis of poly(l-lactic acid) using microwave irradiation

Poly(l-lactic acid) (PLLA) was hydrolyzed using microwave irradiation, and yields of the resultant lactic acid and reaction time were compared with those obtained by conventional heating. In both cases, the reaction temperature was maintained at 170 °C and the weight ratio of PLLA:H₂O was 3:1. Under conventional heating, the lactic acid yield reached saturation after 800 min at 45%, whereas only 120 min was required to reach the same yield level under microwave irradiation. The optical purity under conventional heating decreased with hydrolysis of the PLLA and dropped to 94%ee when the lactic acid yield reached at 45%. Under microwave irradiation, however, the optical purity decreased only after the equilibrium state of hydrolysis was attained. Therefore, to maintain the optical purity at 98%ee, it was necessary to stop microwave irradiation when the lactic acid yield reached 45%.     

Coumarin synthesis via Knoevenagel condensation reaction in 1,1,3,3-N,N,N′,N′-tetramethylguanidinium trifluoroacetate ionic liquid

Coumarin derivatives were synthesized in relatively high yields via Knoevenagel condensation reaction of an ortho-hydroxyaryl aldehyde and an activated β-dicarbonyl C-H acid in the presence of a recyclable ionic liquid 1,1,3,3-N,N,N′,N′-tetramethylguanidinium trifluoroacetate under either classical heating conditions or using microwave irradiation. Application of microwave irradiation decreased the required time by a factor of about 200. The ionic liquid could be recycled several times without loss of efficiency with regards to the reaction times and yields.     

Microwave-assisted rapid and efficient synthesis of C-alkyl imidazoisoquinolinone derivatives

The synthesis of a set of 10-benzyl-2,3-dihydroimidazo[1,2-b]isoquinolin-5(1H)-one and 5-oxo-imidazo[1,2-b]isoquinolin-10-yl)-N-phenylacetamide derivatives was achieved by exposing the corresponding alkylating agent and imidazoisoquinolinone to microwave irradiation and traditional oil bath heating in the presence of K₂CO₃ and DMAP. The microwave technique as well as DMAP as base accelerated the alkylation reaction for 2-6 min giving 79-88% yields.     

Microwave-assisted synthesis of 2-aminothiophene-3-carboxylic acid derivatives, 3H-thieno[2,3-d]pyrimidin-4-one and 4-chlorothieno[2,3-d]pyrimidine

A two-minute microwave irradiation allowed the synthesis of several 2-aminothiophene-3-carboxylic acid derivatives. Their efficient transformation to thieno[2,3-d]pyrimidin-4-one and the corresponding 4-chloro derivative is also reported under microwave irradiation.     

Microwave-assisted synthesis of pyrazolyl bistriazines

Reaction of 6-chloro-N,N′-bispyrazolyl-[1,3,5]triazine-2,4-diamines with 4-aminobenzylamine under microwave irradiation produces bistriazines in excellent yields. The use of a diamine bearing amino groups with different reactivities allowed the reaction to be carried out in two steps and selectively gave monotriazines, bistriazines with identical substituents and differently substituted bistriazines. The structures of these new compounds have been studied by NMR spectroscopy, mass spectrometry and in one case by X-ray crystallogrphy. These new bistriazines have promising applications in supramolecular chemistry based on hydrogen bonds and/or complexation with metals. The presence of a rigid linker can be used for the efficient preparation of extended supramolecular polymers with interesting fluorescence properties by complexation with cyanuric and barbituric acid derivatives.     

A facile one-step synthesis of 5-chloro-imidazo[1,5-a]quinazoline by microwave irradiation

A new and facile method has been developed for the one-step synthesis of 5-chloro-imidazo[1,5-a]quinazoline by cyclization of N-acylanthranilic acid A with 2-amino acetamide B1 or 2-amino-acetonitrile B2 in the presence of POCl₃ under microwave irradiation. 5-chloro-imidazo[1,5-a]quinazolines can be further functionalized by displacement of 5-Cl group.     

Hydrolysis of chitosan under microwave irradiation in ionic liquids promoted by sulfonic acid-functionalized ionic liquids

Hydrolysis of chitosan in ionic liquids was carried out under microwave irradiation (MW) using sulfonic acid-functionalized ionic liquids (SFILs) as catalysts. The effect of microwave power, irradiation time, dosage of SFILs and DMSO was investigated by orthogonal tests. Under the optimal reaction conditions, the yield of total reducing sugars (TRS) reached over 90% within 2 min. The viscosity-average molecular weight of degraded chitosan was determined by viscosity method. The structures of the original and degraded chitosan were characterized by Fourier-transform infrared (FTIR) spectra, X-ray powder diffraction (XRD) analysis and carbon-13 nuclear magnetic resonance spectroscopy (¹³C NMR). The influence of microwave power and irradiation time on the TRS and M_v was further studied. This method can dramatically reduce reaction time.     

Efficient and library-friendly synthesis of 4-<Emphasis Type="Italic">N</Emphasis>-substituted 6-bromopyrido[2,3-<Emphasis Type="Italic">d</Emphasis>]pyrimidines under microwave irradiation

In this paper, a novel synthetic method for 4-N-substituted 6-bromopyrido[2,3-d]pyrimidines was reported. Starting from 2-aminonicotinonitrile, following by bromination of aromatic ring, condensation, cyclization, and Dimroth rearrangement, a series of 21 new pyrido[2,3-d]pyrimidine derivatives was prepared in high yields (up to 98%). In the last two steps, microwave irradiation was used. The structures of synthesized compounds were determined by NMR, IR, and HRMS techniques. The results showed that application of microwave irradiation has a beneficial effect for the preparation of desired products, as it is simple and efficient method for improving the yields and reducing the formation of undesirable by-products.