Thermal Reactions in Heterocyclic Syntheses

Many reactions in heterocyclic multistep syntheses involve thermal condensations. 

MW irradiation of a mixture of 36 and anthranilic acid adsorbed on graphite powder led to the product 37 in high yield (95%) and in a short time (1.5 h) [50]. When this GS/MW process was extended to the reaction between 36 and 5-methylanthrani-lic acid it gave the corresponding cyclized product in 75 % yield. 

Among polyheterocyclic systems, coumarins are synthesized by many routes, including the Pechmann reaction, which involves condensation of phenols with /fke-toesters [51]. This reaction, which has been the most widely applied method, has re- 

Addition of a solid acid catalyst such as Montmorillonite K10 increased the yield significantly under the action of either thermal heating (64%) or MW irradiation (66%) [52 c]. Under the latter conditions the reaction time was reduced. Comparable results were obtained for the synthesis of aminocoumarins 39-42 (Tab. 7.3) [53]. 

Entry Product MW irradiationa Reaction time (min) Yield (%) Conventional heatingh Reaction time (min) Yield (%) 

The three studies described above confirmed that the ratio of the quantity of ma- 

The most important drawback of these procedures is the quantity of product available at the end of the reaction but the high reproducibility of the process is important and enables, in adapted reactors, interesting extension of the scale (multigram). 

Benzothiazoles are important heterocycles and continue to be interesting synthetic targets because several classes of annulated thiazoles and thiazolyl (hetero)arenes have a diverse array of biological activity. 

Here again, use of a small amount of graphite was sufficient to induce strong heating and enable easy work-up. 

Addition of a solid acid catalyst such as Montmorillonite KIO increased the yield significantly under the action of either thermal heating or MW irradiation (e.g. 

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