Current Microwave Chemistry - Volume 4, Issue 4, 2017

Background: Quinoxaline is an important nucleus used in the development of novel potentially bioactive structures. Hence, the development of new methodologies that provide access to this scaffold in shorter times and increased yields is of interest. In this work, we review many interesting microwave-assisted methodologies for the synthesis of quinoxalines reported from 2007 to 2015. Methods: The most prominent methods for the synthesis of quinoxalines under microwave irradiation published in articles from 2007-2015 are reviewed. Conclusion: Microwave irradiation proved to be a useful tool, providing many advantages on the synthesis of this class, such as shorter reaction times, higher yields and better selectivity, when compared to classical procedures.

Background: Microwave irradiation in organic synthesis has attracted great interests for scientists to synthesize various biological compounds owing to the advantages of shorter reaction times, higher yields, environmental friendliness and operation simplicity. Methods: Research and online content related to microwave synthesis were reviewed and the key methodologies are brought forward in this review. Guidelines for meaningful synthesis in organic chemistry by microwave irradiation are illustrated. Results: Only those methodologies that involve microwave assisted reaction during synthesis in a combined manner have been discussed. Some recent syntheses that are microwave-accelerated with regard to heterocyclic moieties are summarized. Conclusion: The considerable decrease in reaction time, the smaller overall mass that is heated, the diverse category of compounds that are synthesized, microwave-enabled one-pot synthesis mostly under solvent-free conditions is a more viable technique compared with traditional synthetic methods. In recent years, this technique has attracted great interests for scientists to synthesize various biological compounds owing to the advantages of shorter reaction times, higher yields, environmental friendliness and operation simplicity. The basic principles of green synthesis are taken into account resulting in the design of various new eco-friendly approaches. To address such challenges the application of non-traditional activation methods are utilized, such as microwave irradiation which is a major tool in green synthesis and engineering. Based on the focus of this work, only methods that apply microwave assisted reaction have been discussed in a combined manner.

Background: The potential of microwave-assisted heating for enhancing the alkaline pretreatment of lignocellulosic waste is highlighted in this work. Objective: To maximize separation of fermentable sugars from wheat straw. Methods: Wheat straw hydrolysis assisted by microwave was carried out by setting up a statistical experimental design method and further investigating the main process parameters, namely: temperature (°C), microwave power (W), NaOH concentration (M) and wheat straw pre-treatment time (min) towards maximization of fermentable sugars extraction from wheat straw. The reducing sugars yield (response) of the alkaline pre-treated and microwaved wheat straw in lab scale provided data for building a predictive model which reflected interactions, significance and impact of the process parameters (factors) on the wheat straw hydrolysis yield. Results: SEM and FTIR images of untreated and alkaline pre-treated wheat straw were studied for investigating the morphological changes of wheat straw surface quality and structure resulting from the microwave and/or alkaline pre-treatment. In addition reducing sugars yield of 87%wt. from wheat straw pre-treated at 180°C, 550W microwave power, 0.65M of NaOH for 25min was achieved. This result was significantly higher compared to the one from the straw which was only hydrolysed enzymatically (30 %wt). Conclusion: It was found that shorter pre-treatment times were obtained at higher temperatures, alkali concentration and moderate microwave power levels. The results obtained were further optimized and indicated that the microwave-assisted alkaline pre-treatment of wheat straw technique is an attractive pre-treatment method which reduces the wheat straw pre-treatment time and enhances the hydrolysis yield.

Background: Previous work has shown that iron-based Fischer-Tropsch synthesis (FTS) catalysts can be modified to give a better performance in the synthesis reaction by treatment with microwave radiation. Iron-manganese systems have not previously been studied for this effect but are of interest because of their claimed special FTS performance. Methods: Iron-manganese catalysts were prepared and modified by exposure to microwave radiation. Their catalytic performance in FTS was measured and the surface properties were monitored using the temperature programmed surface reaction (TPSR) of prechemisorbed CO with hydrogen. Results: Only relatively slight changes in the bulk structure of the solids could be seen in microwaved samples, except at higher power levels and longer durations, when some growth in the size of the iron particles was seen. The reduction characteristics of catalysts were also relatively unaffected by microwave radiation. Microwaved catalysts tended to be slightly less active in the FTS reaction but were more stable, with less deactivation being seen. The surface of the catalyst was modified by the microwave radiation, leading to more strongly and more dissociatively adsorbed CO as detected by TPSR. Conclusion: Microwave radiation improves the performance of unsupported iron-manganese catalysts, but the effects seen, although significant, are not as dramatic as those recorded for iron catalysts. A contributory factor may be the absence of potassium which has been implicated in microwave-induced surface modifications of potassium-promoted iron, but the intrinsic microwave absorption properties of manganese-containing samples may also be responsible. The radiation affects surface rather than bulk characteristics.

Background: Modified pyrimidines is a very important pharmacophore core of naturally occurring and synthetic bioactive compounds, interacting with the synthesis and function of nucleic acids, interfering with biosynthetic pathways and competing for the same binding sites of naturally occurring pyrimidines. Method: C5-arylalkynyl glucopyranonucleosides 3, 5 and -3´-C-trifluoromethyl-β-D-allopyranonucleosides of uracil 13, were synthesized using the one-step Sonogashira reaction under microwave irradiation and subsequent deprotection. Hydrogenation of acetylated 2a,e,i,j,k,n with NiCl2 and NaBH4, gave C5-phenylethenyl 6n and C5-arylethyl 8a,e,i,j,k uracil pyranonucleosides, which upon deacetylation, afforded the unprotected pyranonucleosides 7n and 9a,e,i,j,k, respectively. Results: The newly synthesized compounds were evaluated for their potential cytostatic activities in cell culture. Conclusion: The results of the cytostatic examinations indicated that compounds 2a,e,i,j and 12c,d are suitable candidates for further biological studies on lead drug and structural optimization.

Background: The origin of the microwave acceleration organic reactions is still not completely clear. According to the selective heating mechanism of microwave irradiation, the heat accumulated in the polar reactants may generate localized superheating, resulting in acceleration of the certain reactions. Methods: In this study, the Hammett linear relationship of a “one-pot” saponification with a series of mixed butyl substituted benzoates was selected as a molecular level probe to explore the microwave selective heating effects in intermolecular reactions. If there is a “hot spots” effect, the Hammett linear relationship will be distorted. Heating curves at 20 W of microwave power for 0.1 mol/L solution of butyl 4-nitrobenzoate, butyl 4-chlorobenzoate, butyl benzoate, butyl 4-methylbenzoate, and 4- methoxybenzoate solutions in solvent CCl4 show that butyl 4-nitrobenzoate can act as ‘molecular radiators' in the reaction mixture. We performed this “one-pot” saponification in three solvent systems with different polarity to identify this effect. The microwave experiments were performed under the constant temperature mode and constant power mode. Results: In polar solvent butanol, the Hammett linear relationship does not change. Large amount of solvent absorbs the microwave energy and shields the differences of the microwave absorbing ability among different butyl substituted benzoates. In the medium polar solvent THF, the strong polar reactant butyl 4-nitrobenzoate can play as ‘molecular radiators’ which means that its instance temperature is higher than the surrounding. In this case, the Hammett linear relationship remains unchanged under microwave irradiation in both constant temperature mode and constant power mode. However, there is a rate acceleration under microwave irradiation (measured as the reaction constant ρ is increased). Finally, we attempted to conduct the reactions in nonpolar solvent CCl4. Unfortunately, the reactions did not happen due to poor solubility of butyl substituted benzoates. Conclusion: In this study, we found that there is no measurable effective temperature gradient of different polar reactants but a whole acceleration in reaction rate. The acceleration is attributed to the thermal effect.

Background: Application of microwave irradiation to heterogeneous catalysis has not been explored intensively due to arching phenomena of metals-supported catalysts in presence of flammable solvents. Hence, solvent free organic microwave synthesis has attracted great interests because of high efficiency, shorter reaction times and eco-friendly nature. Methods: This is the report of an efficient and expeditious green protocol for SNAr reaction of 2- bromo-3, 5-dinitrothiophene with amines and thiophenols using microwave heating. The solvent-free, chromatography-free method is highly sustainable along with the recyclability point of the solid support simple neutral Al2O3. Results: Approximately 100% conversion takes place just only in 2-3 min under microwave irradiation at 40°C. Conclusion: Several N-substituted 3, 5-dinitrothiophene-2-amine and S-substituted 3, 5-dinitro-2-sulfanylthiophene derivatives have been synthesized with good to excellent yields within very short time in environmentally benign manner. The synthesized products show very interesting morphological feature (microporous tube) towards privileged nanotechnology. Another mentionable thing is that the products are fully organic microporous tubes whose synthesis is really challenging.