Microwave-assisted treatment

A constant interest in the development of new rapid methodologies for the preparation of oxazole hbraries is motivated by their presence in numerous biologically active natural products. Janda and coworkers were hrst to show that oxazoles can be obtained by microwave-assisted treatment of polymer-bound a-acylamino-/f-ketoesters with Burgess reagent [68]. Hydroxybutyl-functionalized /anda/el resin was used for this investigation, with key steps being monitored by on-bead FT-IR. First, a resin-bound acetoacetate was pre-... 

The earliest microwave systems adapted for laboratory work were of the closed-vessel type however, many were superseded by open-vessel systems that were developed to circumvent their shortcomings. In any case, each type of system has its own advantages and disadvantages, so neither can be said to be the better choice for all microwave-assisted treatments of solid samples. The advantages of closed-vessel systems can be summarized as follows ... 

Figure 3.2 Distribution of elements determined by ETAAS after microwave-assisted treatment of various matrices. (From Chakraborty, A.K. et al., Fresenius ]. Anal. Chem., 355, 99, 1996. With permission.)...

As an example of a fully automated system for radionuclide determination, there is a method based on SIA technique developed for Tc determination, including microwave assisted treatment, solid phase extraction (SPE) and online detection (Figure 8.6) [21]. 

Wang Z, Sun X, Lian Z, Wang X, Zhou J, Ma Z (2013) The effects of ultrasonic/microwave assisted treatment on the properties of soy protein isolate/microcrystalline wheat-bran cellulose film. J Food Eng 114(2) 183-191... 

Wang Z, Zhou J, Wang X, Zhang N, Sun X, Ma Z (2014) The effects of ultrasonic/microwave assisted treatment on the water vapor barrier properties of soybean protein isolate-based oleic acid/stearic acid blend edible films. Food Hydrocolloids 35 51-58 Wihodo M, Moraru Cl (2013) Physical and chemical methods used to enhance the structure and mechanical properties of protein films a review. J Food Eng 114(3) 292-302 Woehl MA, Canestraro CD, Mikowski A, Sierakowski MR (2010) Bionanocomposites of thermoplastic starch reinforced with bacterial cellulose nanofibers effect of enzymatic treatment on mechanical properties. Carbohydr Polym 80 866-873 Xu YX, Kim KM, Hanna MA, Nag D (2005) Chitosan-starch composite film preparation and characterization. Ind Crops Prod 21 185-192... 

Scheme 25). The introduction of greater diversity at C-2 and C-4 can be achieved by intercepting the tetrahydropyridinone intermediate in a multi-step sequence involving C-2 ammonolysis, microwave-assisted N-acylation, acid-catalyzed cyclization (to introduce diversity at C-2), treatment with POCI3, and either microwave-assisted nucleophiUc substitution or Suzuki couphng (to introduce diversity at C-4) [89]. 

In this approach [92], the glycosyl /3-azides were prepared according to literature procedure [93] and the acetylenes were synthesized in excellent yields via previously reported Pd-catalyzed microwave-assisted Sonogashira crosscoupling protocol with trimethylsilylacetylene, followed by desilylation upon treatment with TEAR... 

Table 3.10 shows the recovery from PP of Irgafos 168 and its oxidised and hydrolysed by-products by various extraction procedures. As may be observed, One-Step Microwave-Assisted Extraction (OSM) and US lead both to negligible hydrolytic additive degradation. The measured additive decay (by oxidation) is essentially due to the antioxidant activity during the processing (extrusion) step of the polymer and not to the US or microwave heating treatment. 

Similar to the addition of secondary phosphine-borane complexes to alkynes described in Scheme 6.137, the same hydrophosphination agents can also be added to alkenes under broadly similar reaction conditions, leading to alkylarylphosphines (Scheme 6.138) [274], Again, the expected anti-Markovnikov addition products were obtained exclusively. In some cases, the additions also proceeded at room temperature, but required much longer reaction times (2 days). Treatment of the phosphine-borane complexes with a chiral alkene such as (-)-/ -pinene led to chiral cyclohexene derivatives through a radical-initiated ring-opening mechanism. In related work, Ackerman and coworkers described microwave-assisted Lewis acid-mediated inter-molecular hydroamination reactions of norbornene [275]. 

Apart from the preparation of radiotracers, microwave-assisted transformations have also been utilized to carry out simple hydrogen-deuterium exchange reactions. In the case of acetophenone, for example, simple treatment with deuterium oxide as solvent in the presence of molecular sieves at 180 °C for 30 min led to complete... 

In a recent study, another method for microwave-assisted heterocycle synthesis leading to a small set of imidazole derivatives has been reported [54], These pharmaceutically important scaffolds were synthesized utilizing polymer-bound 3-N,N-(dimethylamino)isocyanoacrylate. This polymer support was easily prepared by treatment of [4-(bromomethyl)phenoxy]methyl polystyrene with a twofold excess of the appropriate isocyanoacrylate potassium salt in N,N-dimethylformamide (Scheme 7.37). The obtained intermediate was subsequently treated with N,N-di-methylformamide diethyl acetal (DMFDEA) in a mixture of tetrahydrofuran and ethanol to generate the desired polymer-bound substrate. 

A recent study concerned the microwave-assisted parallel synthesis of di- and tri-substituted ureas utilizing dedicated 96-well plates in the CombiCHEM system [60], In a typical procedure, modification of the Marshall resin utilized was achieved by treatment with p-nitrophenyl chloroformate and N-methylmorpholine (NMM) in dichloromethane at low temperatures. The resulting resin was further modified by attaching various amines to obtain a set of polymer-bound carbamates (Scheme 7.48). 

The group of Botta demonstrated the feasibility of their microwave-assisted iodi-nation protocol (see Scheme 6.143 d) toward a polymer-supported substrate [68], An appropriate pyrimidinone attached to conventional Merrifield polystyrene resin was suspended in N,N-dimethylformamide, treated with 2 equivalents of N-iodosuccini-mide (NIS), and subjected to microwave irradiation for 3 min. Treatment of the polymer-bound intermediate with OXONE released the desired 5-iodouracil in almost quantitative yield (Scheme 7.57). 

Microwave-assisted (0.1 or 0.2 Wg-1) convection drying was also applied to osmotically dehydrated blueberries, leading to dried berries that were comparable to freeze-dried ones in much shorter time (Venkatachalapathy and Raghavan, 1998). Frozen blueberries were also dried in a microwave and spouted bed combined dryer (MWSB) after a pretreatment using ethyl oleate and a NaOH dipping solution followed by sucrose osmotic treatment (Feng et al., 1999). Osmotic dehydration prevented the blueberries from... 

In addition to the abovementioned results consisting only the increasing yields (Scheme 6) [38], another example of effective microwave-assisted synthesis of azolopyrimidine carboxamides should be described. Tu et al. [47] reported eco-friendly three-component reaction of 2-aminobenzimidazole, aromatic aldehydes, and some p-dicarbonyl compounds under microwave irradiation (Scheme 8). It was shown that the treatment of the starting materials can be most efficiently carried out at 90°C (200 W MW power) in water medium instead of traditional organic solvents like ethanol, acetic acid, or DMF. 

For example, it was reported in several independent articles that multicomponent treatment of 5-amino-3-methylpyrazles with 1,3-cyclohexandiones and aldehydes under refluxing in EtOH [82, 83], in DMF with methanol [84], or with application of continuous-flow microwave-assisted procedure in DMSO [85] yielded exclusively pyrazoloquinolinones 50 (Scheme 23). On the other hand, the treatment of 3-unsub-stituted 5-aminopyrazoles with cyclic p-diketones or ketosulfones gave mixtures of Hantzsch dihydropyridines 51 and Biginelly dihydropyrimidines 52 in different ratios [86]. 

To decrease the reaction time required for the conventional treatment (from 2 up to 48 h) and to improve the conversion level of the reaction, microwave irradiation was successfully applied for obtaining 4-thiazolidinones substituted with azole ring. Siddiqui et al. [174] reported the efficient microwave-assisted solid-supported approach for one-pot diastereoselective synthesis of some analogs of fungitoxic dibenzyles. 

In order to accelerate sample preparation, new extraction methodologies such as accelerated solvent extraction (ASE) and MAE, based on the use of elevated temperature and pressure to heat the mixture sample-solvent, have been recently developed and applied for PAH extraction from meat [695] and vegetables [696-698]. Garda Falcon et al. [699] used microwave treatment with hexane to accelerate PAH extraction from freeze-dried foods. The fat extracted in this way underwent microwave assisted saponification with ethanolic KOH. Hernandez-Borges et al. [700] combined microwave-assisted hydrolysis and extraction to isolate organic pollutants from mussels, while... 

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