Current Microwave Chemistry (v.4, #3)

Meet Our Regional Editor by Marcus Vinícius Nora de Souza (187-187).

Background: Improved catalytic effects have been seen using iron-based catalysts after exposure to microwave radiation. It was suspected that these effects would be much more pronounced when thin disks of loose powder are used for the microwave treatment rather than cone shaped sample with a mean thickness of about 10 times that of the disks. <P></P> Methods: Catalyst samples have been subjected to microwave radiation in two bed-shapes: One, very thinly spread powder and two, thicker piles of powder. <P></P> Results: Indeed the larger microwave effects are seen when using the thinly spread samples. <P></P> Conclusion: These observations are important in microwave research in the laboratory and also in any designs of equipment to be used for larger scale microwave treatment of catalytic solids. The effects of bed shape have important implications for using this technique.

Biocatalytic Production of Chiral Benzotriazoles Employing Conventional Heating and Microwave Radiation by Matheus A. Meirelles, Alex H. Jeller, Irlon M. Ferreira, Cristiano Raminelli, Raquel S. Faria, Elisângela de Paula Silveira-Lacerda, André L. M. Porto (191-198).
Background: This paper describes the synthesis of chiral 1,2,3-benzotriazoles by the enzymatic kinetic resolution of &beta;-azidoalcohols using lipase from Candida antarctica B followed by a [3+2] cycloaddition reaction involving benzyne formation from 2-(trimethylsilyl)phenyl triflate and CsF under conventional heating and microwave radiation. (±)-1,2,3-Benzotriazoles were tested in vitro to evaluate their cytotoxic effects on S180 tumor cells using the colorimetric MTT assay. They showed IC50 values comparable to the IC50 value of cisplatin. <P></P> Methods: Synthesis of azidoalcohols and 1,2,3-benzotriazoles were carried out under microwave radiation and conventional heating. Enzymatic kinetic resolution of (±)-azidoalcohols were obtained by Candida antarctica B in orbital shaker. Enzymatic reactions were analyzed using Shimadzu LC-10AD HPLC equipment with UV detector (190-254 nm) and Chiralcel OD-H chiral column (0.46 cm &times; 25 cm; 5 &micro;m). <P></P> Results: Preparation of (±)-azidoalcohols was used the corresponding epoxides and NaN3 in water at 90°C. When the reactions were performed under microwave heating, the compounds were obtained in higher yields (80 and 85%, respectively) and shorter reaction times (10 and 90 min, respectively) versus conventional heating. The (±)-1,2,3-benzotriazoles were produced by the [3+2] cycloaddition reaction between (±)-azidoalcohols and 2-(trimethylsilyl)phenyl triflate in the presence of CsF using acetonitrile as solvent at 70°C under conventional heating or microwave radiation. CAL-B was employed in kinetic resolution of (±)-azidoalcohols yielding enantioenriched products. cytotoxic effects on S180 tumor cells using the colorimetric MTT assay. <P></P> Conclusion: Chiral 1,2,3-benzotriazoles were synthesized from enzymatic kinetic resolution of (±)- azidoalcohols using Candida antarctica B followed by a [3+2] cycloaddition reaction involving the benzyne formation from 2-(trimethylsilyl)phenyl triflate and CsF under conventional heating and microwave radiation. The chiral 1,2,3-benzotriazoles were obtained after 15 min under microwave radiation with 55-56% yields. The (±)-1,2,3-benzotriazoles were tested in vitro to evaluate their cytotoxic effects on S180 tumor cell line using the colorimetric MTT assay and showed IC50 values comparable to the IC50 value of cisplatin.

Background: The Terminalia chebula has been reported for the urease inhibition activity while the cardioactive Terminalia arjuna has not been reported for anti-urease activity. The Terminalia arjuna has been reported to lower the nickel toxicity in rice and hence, an attempt has been made by the authors to find the inhibition potential for nickel containing urease enzyme by the bark extract of Terminalia arjuna. <P></P> Objective: The optimization, microwave assisted extraction of bark of Terminalia arjuna and in vitro evaluation of urease inhibitory activity of the extract has been investigated. <P></P> Method: The central composite design was used to optimize the parameters of microwave assisted extraction viz. power, time and solid/solvent ratio. The optimized batch had the values of power 1000W, time 3 minutes and solid/ solvent ratio 1:120. The in vitro urease inhibitory activity of optimized extract was performed spectrophotometrically taking thiourea as standard. <P></P> Results: The percentage yield of optimized extract was found to be 13.6 %. The results of in vitro anti-urease evaluation were obtained spectrophotometrically using thiourea as standard; the microwave assisted extract was found to have better urease inhibition potential (IC50 value) as compared to soxhlet extract and room temperature extract compared with the standard thiourea (IC50 value). <P></P> Conclusion: Hence, the Terminalia arjuna stem bark extract can be used as an effective urease inhibitor and may also be further explored for identification and isolation of the active phytoconstituents responsible for this effect.

Wheat Straw Bio-refining. Part I: Optimization of the Microwave Radiation Process with Sulphuric Acid Pre-treatment by Huseyin Demirhan, Ahmad Fauzi, Vassiliki K Skoulou, Stephanie H. Haywood, Sharif H. Zein (205-218).
Background: This study investigates the microwave-assisted acid extraction of lignin from wheat straw. <P></P> Methods: Two experimental design methods, the Central Composite Design (CCD) and the Response Surface Method (RSM) methods were used to select conditions and optimise the process. The parameters studied were: the stirring rate, H2SO4 concentration, microwave radiation power and residence time. <P></P> Results: The optimum conditions found to achieve the maximum lignin yield (wt%) from wheat straw were: 0.46M H2SO4 under microwave radiation power of 602 W at 65% of stirring rate (3200rpm) and residence time of 39min. These conditions led to a delignification of 78.5% (by weight) of the parent lignin content, which accounted for 11.8wt% on dry biomass basis; this compares to a yield of 3.35wt% for the conventional H2SO4 pre-treatment method under similar conditions. <P></P> Conclusion: This work is a first step to demonstrate the potential of innovative microwave methods to overcome the high cost of acid extraction of lignin from biomass waste.

Cycloaddition Reactions Assisted by Microwave Irradiation: Protic Ionic liquids vs Solvent-free Conditions by Pedro M. E. Mancini, María N. Kneeteman, Mauro Cainelli, Carla M. Ormachea (219-228).
Background: The behavior of N-tosylnitroindoles acting as electrophilic dienophiles in polar Diels-Alder reactions joint to different dienes was analyzed. Reactions were developed under microwave irradiation using toluene or protic ionic liquids as solvents under solvent-free conditions. Good yields in short reaction times were observed. Solvent-free conditions and protic ionic liquids as solvents offered similar results. However, solvent-free conditions favor environmental sustainability. The processes between N-substituted nitroindoles and isoprene were selected for a theoretical study. <P></P> Method: The reactions between the different electrophile-nucleophile pairs were developed under two conditions: (A) reactants and solvents (PILs) were heated in a closed ampoule under conventional thermal conditions, 60°C-48h, except for those developed in toluene that react at 180°-72h; (B) under the effect of MW irradiation in a MW apparatus at 180°C-0.5h. In both the cases, the solvents used were toluene and PILs. All calculations were carried out within the DFT framework using the Gaussian 09 suite of programs. For this purpose, the hybrid functional B3LYP was used together with the 6-31G(d) basis set. <P></P> Results: Experiments were carried out with P-DA process using microwave irradiation instead of conventional heating. Cycloaddition reactions of nitro substituted indoles with different dienes were particularly analyzed using this methodology in the presence of PILs as solvent and under solvent-free conditions. Good yields were obtained in short reaction times. The yields observed in this thermal reaction are lower than those obtained under MW irradiation. This can be explained considering the type of heating produced by MW irradiation. Nitro indoles substituted in the heterocyclic ring and in the benzene moiety were used to compare the behaviors. In both cases, yields and reactivity were similar. A theoretical analysis was performed using Density Functional Theory, which explained the experimental data. <P></P> Conclusion: The N-tosyl-nitroindoles explored in this work reacted as dienophilic electrophiles with dienes of different nucleophilicity. Reactions with different diene/dienophile pairs offered similar products under conventional heating or microwave irradiation. However, under the latter experimental condition, reaction times were significantly short with similar yields. Solvent-free conditions constitute a frequently used tool in microwave systems due to simpler manipulation, low cost, good product separation and for being environmentally friendly. Concerning the DFT study, the theoretically explored system shows that the TSs are unique and asynchronous. Theoretical calculations coincide with the experimental results. Under MW irradiation, in the presence of a PIL, the charge transfer is higher than solvent-free conditions.

Fast and Efficient Synthesis of Maculalactone Derivatives via the Microwave Technique by Daniel Previdi, Mayla Eduarda Rosa, Paulo Marcos Donate (229-237).
Background: The -butyrolactone ring is present in many natural products that exhibit a broad spectrum of biological effects. Tribenzylbutyrolactones, known as maculalactones, are compounds isolated from cyanobacterial species that display diverse biological properties. Given the difficulty in isolating large amounts of maculalactones from natural sources, interest in synthesizing structural analogs of these compounds and extending studies on their biological properties has increased. For this purpose, we have developed a new one-step route based on a multicomponent reaction of the butyrolactone core to synthesize several maculalactone derivatives under microwave radiation as heat source. <P></P> Methods: The maculalactone derivatives were synthesized by a multicomponent reaction; dimethyl 2- benzyl-3-methylenesuccinate, an alkyl or aryl bromide, and an aldehyde were used as starting materials. The crude reaction products were purified by flash column chromatography through silica gel, and the major diastereoisomers were isolated by recrystallization. <P></P> Results: This microwave-assisted synthesis method afforded ten maculalactone derivatives. The microwave radiation technique reduced the reaction time and increased the product yields. These reactions presented good diastereoselectivity, and in all the reactions the major diastereoisomer showed the same relative configuration anti:anti. <P></P> Conclusion: Microwave-assisted multicomponent synthesis proved to be an important strategy to produce several maculalactone derivatives in one step. This method affords good yields within short reaction times.

Microwave-assisted Iodine-catalyzed Rapid Synthesis of 6H-indolo[2,3-b] quinolines: Formal Synthesis of Cryptotackieine by Prakash T. Parvatkar, Perunninakulath S. Parameswaran, Debasish Bandyopadhyay, Sanghamitra Mukherjee, Bimal K. Banik (238-241).
Background: Indoloquinoline alkaloids are of great importance due to their unique structure and various biological activities. Several methods have been developed to synthesize indoloquinolines and among those, one-pot methods are of particular importance due to its simplified reaction procedure. <P></P> Methods: We investigated the one-pot reaction of indole-3-carboxyaldehyde and aniline in presence of catalytic amount of I2 in automated microwave. <P></P> Results: An efficient microwave-mediated tandem synthesis of several linear indoloquinoline alkaloids using 10 mol% iodine as catalyst under mild reaction conditions is described. The reaction involved key steps of imination / nucleophilic addition / annulation / aromatization, all in one pot, in a highly efficient manner under microwave irradiation. <P></P> Conclusion: We have developed an extremely rapid and convenient microwave-assisted method for the synthesis of important class of indoloquinolines using environmentally benign I2 as catalyst in one-pot.

Graph Theoretical Analysis, Insilico Modeling, Design and Synthesis of Biologically Active Pyrimidines and Quinazolines as Antimicrobial and Antitubercular Agent by Theivendren Panneerselvam, Arumugam Sivakumar, Subramaniyan Arumugam, Murugan Indhumathy, Kunjiappan Selvaraj, Shrinivas D. Joshi, Sheshagiri R. Dixit (242-255).
Backround: Twenty four 4-(4-substituted phenyl)-6-(furan-2-yl)-3,4-dihydro pyrimidin- 2(1H)-one,4-(4-substituted phenyl)-6-(furan-2-yl)-3,4-dihydro pyrimidin-2(1H)-thione and 4-substituted phenyl-3,4,5,6,7,8-hexa hydro quinazolin-2(1H)-one, 4-substituted phenyl-3,4,5,6,7,8-hexa hydro quinazoline-2(1H)-thione derivatives were constitute attractive targets for the development of active antimicrobial as well as antimycobacterial agent. <P></P> Method: Graph Theoretical Analysis was introduced into identified drug target and based on analysis report of human protein NFk&beta;1 chosen as a core drug target. The biologically active compounds were identified by insilico modeling and characterized by spectral and elemental analysis. <P></P> Results: Among the compounds tested, both electron withdrawing and releasing compounds exhibited significant antibacterial and antifungal activities while unsubstituted compounds also showed notable antifungal activity with reference to standard drug Clotrimazole. <P></P> Conclusion: The biologically significant compounds carried a p-fluoro atom in the phenyl ring and the observed results concluded the effectiveness of the lead compounds.

Ligand Free Microwave Assisted Copper-Catalyzed Convenient Synthesis of Substituted Tertiary Amines from Nitroarenes by Kshipra S. Karnik, Aniket P. Sarkate, Sushilkumar S. Bahekar, Pravin S. Wakte (256-261).
Background: A ligand free, microwave assisted, copper catalyzed synthetic procedure for substituted tertiary amines has been developed by using nitroarenes and secondary amines. <P></P> Methods: In the synthetic method, the targeted substituted tertiary amines were synthesized with the help of the microwave by reacting nitroarenes and substituted secondary amines in the presence of copper, potassium carbonate and methanol. <P></P> Results: All substituted tertiary amines were obtained in good yields (88-94%) with the help of a microwave technique (07-12 min) in less time. The protocol adopted was ligand free. <P></P> Conclusion: Microwave assisted technique will be a useful alternative protocol for the preparation of a diverse range of substituted tertiary amines.

Background: Rapid depletion in petroleum resources coupled with environmental pollution from the burning of coal and oil has led to the search for alternative energy sources. Lignocellulostarch biomasses (LCSBs) comprising peels of root crops and vegetables causes environmental hazards due to their non-judicious disposal. They are highly recalcitrant to breakdown necessitating effective pretreatment to exploit their use as bioethanol feedstock. Although microwave (MW)- assisted chemical pretreatment has emerged as a cost-effective process for lignocellulosic biomass, there is no information on its efficacy in deconstructing LCSBs that also contain appreciable quantities of starch. <P></P> Methods: Response surface methodology was adopted to study the impact of three parameters such as MW irradiation power, dilute sulphuric acid (DSA) concentration and irradiation time on the polysaccharide (cellulose, hemicellulose and starch) and lignin changes as well as the reducing sugar (RS) recovery after pretreatment. The optimized system was saccharified using triple enzyme cocktail to assess the RS recovery and saccharification kinetics. <P></P> Results: Microwave power had the greatest influence in deconstructing the LCSBs and maximum quantities of hemicellulose (88%), followed by cellulose (65%), starch (42%) and lignin (32-42%) were removed under RSM optimized conditions (600 W; 0.1M DSA; 7 min irradiation time). Pretreatment Efficiency ranged from 52-62% indicating high RS release from the LCSBs. The residues after optimized pretreatment when saccharified using triple enzyme cocktail containing cellulase, xylanase and Stargen (starch hydrolyzing enzyme complex) for 120 h resulted in very high RS recovery (57-71 g L-1). <P></P> Conclusion: The Overall Conversion Efficiency of carbohydrates to RS was very high (85-96%), indicating that the MW-assisted DSA pretreatment followed by triple enzyme-based saccharification system is an effective strategy for enhancing fermentable sugar yield from lignocellulo-starch biomass.