Microwave assisted method

The use of microwave irradiation for this reaction, compared to conventional thermal heating, was investigated. Chloroform used as solvent under the conventional heating did only allow a temperature of 60 °C and a direct comparison between the two methods is therefore somewhat unfair imder these circumstances. Nevertheless, the microwave-assisted method is attractive and proved useful for both primary and secondary amines resulting in highly substituted pyrazolo ring-fused pyridones 40 in 68-86% yields within only 10 min. 

Fig. 10 2-Pyridone carboxylic acids 64 can be selectively decarboxylated to the saturated derivatives 65 via a reagent-free microwave-assisted method. Decarboxylation was also conducted under conventional heating but then copper cyanide was required resulting in mixtures of saturated and unsaturated 2-pyridones...

Scheme 15 Other microwave-assisted methods for the synthesis of imidazoles...

Scheme 22 Improved microwave-assisted method for the synthesis of 1,2,4-triazines...

Li and co-workers introduced a rapid and efficient microwave-assisted method to prepare new disubstituted 1,3,4-thiazoles from 1,4-disubtituted thiosemicarbazides with the objective to obtain biologically active molecules. The intermediate l-aryloxyacetyl-4-(4-methoxybenzoyl)thiosemicarbazide was irradiated in an excess of glacial acetic acid in a domestic microwave oven and led to the formation of 2-(methoxybenzoyl-5-aryloxymethyl)-l,3,4-dithiazoles in good yields [30] (Scheme 20). 

Obviously, some reactions are more suitable than others to be further developed into microwave-assisted methods. The following sections will cover some of the latest efforts made in this area but hopefully this previous section has shown that there are yet many other methods that are transformable into the more high throughput type of chemistry that MAOS can offer. 

Various methods have been used to synthesize metal oxide nanopartides. The methods discussed here indude hydrothermal alteration, reflux, and microwave-assisted methods. 

Microwave irradiation greatly improved the reaction between the lactim ethers 157 and anthranilic acid, as regards yields, reaction time and also the integrity of the stereocenter, as shown by the results obtained for compounds 158 (R1 = (R)-Me R4 = H R2 = Me, 4-MeO-Bn, (CH2)2Ph, 2-naphthylmethyl) and 158 (R =(R)-Me R4 = Me R2 = indol-3-ylmethyl), prepared by both the conventional thermal and the microwave-assisted methods <2004SL803>. 

A microwave-assisted method to prepare novel 8-mercapto-3-methyl-7-alkyl xanthines 68 has been reported. Compared to conventional synthetic routes, the new method has significantly shortened synthetic steps and reaction time <06TL775>. 

In the context of preparing potential inhibitors of dihydrofolate reductase (DHFR), the group of Organ has developed a rapid microwave-assisted method for the preparation of biguanide libraries (Scheme 6.174) [330]. Initial optimization work was centered around the acid-catalyzed addition of amines to dicyandiamide. It was discovered that 150 °C was the optimum temperature for reaction rate and product recovery, as heating beyond this point led to decomposition. While the use of hydrochloric acid as catalyst led to varying yields of product, evaluation of trimethylsilyl chloride in acetonitrile as solvent led to improved results. As compared to the protic... 

The development in microwave chemistry has been remarkable during the last few years from the first reports in which, typically, domestic ovens were used to modern applications with state-of-the-art single-mode cavities. We believe that it is today possible today to develop robust microwave-assisted methods for nearly any reaction that needs an external heat source and we have proved it is possible to perform transition-metal catalyzed reactions very cleanly and selectively. 

Perez, R., Perez, E.R., Suarez, M., Gonzalez, L., Loupy, A. et al, Synthesis of aminotoluenesulfonamide derivatives using conventional heating or microwave-assisted methods, Org. Prep. Proced. Int., 1997, 29, 671. 

Oufnac, D.S. Xu, Z. Sun, T. Sahliov, C. Prinyawiwatkul, W. Godber, J.S. 2007. Extraction of antioxidants from wheat bran using conventional solvent and microwave-assisted methods. Cereal Chem. 84 125-129. 

Zigoneanu, I.G. Williams, L. Xu, Z. Sabliov, C.M. 2008. Determination of antioxidant components in rice bran oil extracted by microwave-assisted method. Biores. Tech. 99 4910-4918. 

A solventless Suzuki coupling reaction has been developed using both thermal and microwave-assisted methods. A potassium fluoride-alumina mixture is utilized along with palladium powder. The KF acts as a base. ... 

Overall, microwave-assisted methods are effective approaches in analysis of proteins/peptides, especially when used with other analytical techniques, such as HPLC and MS methods. 

Recently, 2-alkyl- and 2-aryl-substituted benzimidazole derivatives 15 have been synthesized from 1,2-diaminobenzene dihydrochloride 13 and its corresponding acids 14 in the presence of polyphosphoric acid using microwave-assisted methods (Scheme 3). The reaction time required for the synthesis of benzimidazole derivatives 15 was reduced to minutes by this method compared to conventional synthesis, which required up to four hours of heating to complete the reaction. Furthermore, it was found that the application of microwave irradiation increased yields by 10-50% (Table 1). It has been... 

One-pot synthesis of 6-mercaptopurines from 4,5-diamino-6-chloropurine, an aldehyde and elemental sulfur has been reported. The key advantage of this procedure is that H2S was generated in situ <04TL2321>. An efficient conversion of 6-cyanopurines into 6-alkoxyformimidoylpurines has been developed <040BC1019>. Synthesis of diverse purine libraries has been optimized by a microwave assisted method using minivials <04JCO171>. 

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