Synthesis under Microwave Dielectric Heating

The catalysis of ammonium-based protic and Bronsted ILs for biodiesel synthesis under microwave dielectric heating has been investigated. Ammonium-based ILs. According with... 

Westman, J. and Orrling, K., Cascade synthesis with (triphenylphosphoranylidene)-ethenone as a versatile reagent for fast synthesis of heterocycles and unsaturated amides under microwave dielectric heating, Comb. Chem. High Throughput Screening, 2002, 5, 571-574. 

The methodology was successfully extended to a one-pot total synthesis of complex heterocyclic systems such as pyrazino [2,1-b] quinazolines 79, encountered in nature as alkaloids 80-82 (Scheme 50) [125]. To assemble the pyrazino[2,l-fo]quinazoline core, N-Boc protected amino acid 76 was employed instead of carboxylic acid 72 (Scheme 49) in the synthesis of the corresponding intermediate benzoxazinones 77. The subsequent reaction with an amine moiety of another amino acid ester 78 was accompanied by concomitant cleavage of the N-Boc protecting group and diketopiperazine-like cyclization (for the one-pot deprotection-cyclization reaction of N-Boc dipeptide esters to afford 2,5-piperazinedione under microwave dielectric heating, see [128]) to afford the target heterocycle 79. Hence, the total... 

Legeay JC, Eynde JJV, Bazureau JP (2005) Ionic liquid phase technology supported the three component synthesis of Hantzsch 1,4- dUiydropyridines and Biginelh 3,4-dihydropyrimidin-2(l//)-ones under microwave dielectric heating. Tetrahedron 61 12386-12397... 

An interesting helpful support on the development and interpretation of further possible synthetic procedures will come from the second part of the chapter, whereby wet-chemical synthesis of colloidal metallic single or hybrid nanostructures carried out under microwave dielectric heating will be reported. This is described as another way to selectively deliver thermal energy inside the reaction domain frequently dominated by a series of "secondary effects" and it will be analyzed with the aim to evidence, whether exist, possible mechanistic differences with the convectional heating. 

Sun et al. have also reported microwave-assisted S5mthetic process for bis-benzimidazolyl benzoxazole synthesis. IL-grafted 3-amino-4-hydroxy benzimidazolyl 100 was subjected with thiocarbonyl diimidazole under micro-wave irradiation followed by S-alkylation by various alkyl bromides at ambient temperature to afford IL-bound bis-benzimidazolyl benzoxazole 108 and subsequently basic methanolysis to remove IL support (Scheme 12) [45]. Present strategy is useful to obtain IL-intermediate and final product by simple operation, and short reaction time is observed under microwave dielectric heating. 

H. Hakkou, J.J.V. Eynde, J. Hamelin, J.P. Bazureau, Ionic liquid phase organic synthesis (loOPOS) methodology appUed to the three component preparation of 2-thioxo tetrahydrop)frimidin-4-(ll-f)-ones under microwave dielectric heating. Tetrahedron (2004) 3745-3753. 

In recent years, parallel to the emergence of SPOS, microwave-mediated organic synthesis has come to hght and has developed into a popular field [24-31]. The main advantage of microwave dielectric heating compared to other conventional methods, such as hot plate, oil bath or isomantle, is the tremendous rate enhancement generally observed under microwave irradiation conditions. Various theories have been proposed to explain the source of the rapidity of microwave chemistry [32,33]. However, the gener-... 

D. M. P. Mingos, A. G. Whittaker, Microwave Dielectric Heating Effects in Chemical Synthesis in Chemistry under Extreme or non-Classical Conditions,... 

In this book we have tried to assemble a selection of authors to shine light on the underlying principles of microwave dielectric heating, how this dielectric heating has been used in chemistry to give us microwave-assisted organic synthesis applied on a wide variety of reaction types as well as on how microwave-assisted organic synthesis has impacted the chemistry research within industry. 

A remarkable improvement of the cycloaddition process was observed also in the synthesis of isoxazolidine 37 under microwave irradiation conditions [70] (Scheme 23). Thus, microwave dielectric heating of nitrone and allyl alcohol for merely 1 hour was equivalent in terms of yields to the oil-bath heating for as long as 15 days. The addition of Zn(OTf)2 not only improved the diastereoselectivity of the process, but also resulted in a substantial reduction of the reaction time. Still, microwave irradiation at 120 °C delivered the diastereomerically pure isoxazolidine 37-czs in just 15 minutes,... 

The ILP-bound P-oxo esters 29(a, b) with PF anion are the preferred precursors because after microwave dielectric heating, the excess of j3-oxo esters 28(a, b) and eventually unreacted starting ILP 27a were eliminated easily by washing with AcOEt. With the selected ILP-bound j3-oxo esters 29(a, b) with PF anion, Legeay and coworkers have examined the polyhydroquinoline synthesis under neat conditions (Fig. 12.50). 

A two-step, one-pot process has been developed for the synthesis of aza-indoles under dielectric heating conditions [31]. In the first step, aminopy-ridines and ketones were condensed either at room temperature (in the case of aza-indoles 19-20) or under dielectric heating at 160-220 °C to yield intermediate enamines 18. Subsequent microwave-assisted intramolecular Heck reactions furnished the corresponding 4-, 5-, 6- or 7-azaindoles in moderate to good yields (Scheme 9). 

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