Cyclic synthesis using microwaves

Dharman MM, Choi H-J, Kim D-W et al (2011) Synthesis of cyclic carbonate through microwave irradiation using silica-supported ionic liquids Effect of variation in the silica support. Catal Today 164(1) 544—547... 

Cyclic ureas have many applications as intermediates in the preparation of biologically active molecules. The conventional methods involve cyclization of 1,2-diamines with phosgene or oxidative carbonylation of diamines. Varma and coworkers developed a direct synthesis of cyclic ureas from urea and diamines in the presence of ZnO using microwaves. The major advantage of the method is that the reaction is accelerated by exposure to microwave irradiation the byproducts were, moreover, easily eliminated compared with traditional methods [32]. 

As for the solid support, several polymer-supported amines were tested (Fig. 2). For either the pyrazole and isoxazole synthesis, the best results were given by aniline-functionalized cellulose, which has also the advantage of a relatively low cost. For the 2-aminopyrimidine library, polystyrene-based piperazine and piperidine gave products with a much higher purity compared with other secondary non-cyclic or primary amines, hi both cases, the resins could be reused for up to four times before degradation in their behavior was observed. This reusability could be further enhanced (up to 10 cycles for cellulose-based aniline) when the microwave-assisted protocols were used. 

Cyclic ketene acetals, which have utility as co-polymers with functional groups capable of cross-linking, etc., have been prepared by the elimination of HX from 2-halomethyl-l,3-dioxolanes. Milder conditions are used under phase-transfer conditions, compared with traditional procedures, which require a strong base and high temperatures. Solid liquid elimination reactions frequently use potassium f-butoxide [27], but acceptable yields have been achieved with potassium hydroxide and without loss of any chiral centres. The added dimension of sonication reduces reaction times and improves the yields [28, 29]. Microwave irradiation has also been used in the synthesis of methyleneacetals and dithioacetals [30] and yields are superior to those obtained with sonofication. 

Diels-Alder cyclizations are thermal reactions, and recent significant developments in the use of microwave irradiation to pyridine synthesis have been reported, including a series of pyrazolo[3,4- ]pyridines from nitroalkenes and pyrazolylimines with reaction times of 5-10min (Equation 115) <2000T1569>. This method appears to be general to this bicyclic class for example, replacement of the substituent on the imine allows for a C-2-unsubstituted pyridine. A range of cyclic and acyclic dienes have also been reported. 

The synthesis of functionalized 8-hydroxy-l,2,3,4-tetrahydroquinolines 77 has been reported by Dai and co-workers [119] in moderate to good yields via an aza-Diels-Alder reaction of an 2-aminophenol, a substituted benzaldehyde and a cyclic alkene catalyzed by TFA under controlled microwave irradiation in acetonitrile. In general, when electron deficient aromatic aldehydes were used, the adducts could be isolated in 39-59% yields with a predominance of the trans isomer (Scheme 58). 

The synthesis of a series of pyrimido[l,2-a]benzimidazoles 98 has been reported by Dandia and co-workers [136] through a MCR of 2-aminobenzimidazole, mal-ononitrile or ethylcyanoacetate and a carbonyl compound in water. Cyclic as well as acyclic carbonyl compounds could be used. The reaction highly benefits from microwave (domestic oven) irradiation in terms of rate and the compounds 98 were obtained in good yields (Scheme 75). 

Figure 2. Synthesis of cyclic anhydride from dicarboxylic acid and isopropenyl acetate using KSF and microwaves [44],...

Synthesis of cyclic acetals Cyclic ketals (potential cosmetics ingredients) have been obtained in excellent yields from a cineole ketone under the action of microwave in solvent-free conditions or in toluene. The results reported compared very favorably with those obtained by use of conventional heating (Eq. (39), Table 4.11) [123]. 

Phuruangrat, A., Thongtem, X, and Thongtem, S. Synthesis of nanocrystalline metal molybdates using cyclic microwave radiation. Materials Science-Poland,28(2), 557-563 (2010). 

ABSTRACT Synthesis of nanostructured LiCoOj and TiO via microwave assisted and conventional hydrothermal methods, respectively, is presented. The physical properties of the resulting material are characterised via X-ray powder diffraction and scanning electron microscopy. Cyclic voltammetry and Galvanostatic Intermittent Titration Technique are used to study differences of the electrochemical properties compared to bulk material. 

Thionation of Carboxylate Esters and Lactones. The reaction of esters and lactones with LR represents a well-established method for the synthesis of the corresponding thiocarbony 1 deriva-tives. - In fact, LR is the much superior reagent for the thionation of esters as compared with phosphorus pentasullide. Open-chain thionoesters have been conveniently obtained. Interestingly, natural triglycerides also smoothly react with LR to yield pure tristhionotriglycerides. Even better results are obtained with carboxylate esters if the improved solubilized LR or microwave irradiation are applied. The method has particularly been used for the preparation of cyclic thionoesters. Obviously there are very few structural or other restrictions that inhibit the formation of y-,S-, or macrocyclic thionolactones. Once more, microwave irradiation significantly improves the re-... 

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