Microwave irradiation Pechmann coumarin synthesis

For the synthesis of coumarins, the Pechmann reaction [145] is one of the most popular synthetic routes. As the reaction is conventionally carried out at high temperature, two microwave-assisted versions have been recently described. Besson and co-workers described the cyclocondensation of different m-amino phenols 226 with /1-ketoesters 227 on graphite/montmorillonite KIO support (Scheme 83). The use of graphite was crucial in the development of the reaction conditions. In fact, microwave irradiation of the reagents using different conditions gave poor results in terms of yields and purity. The optimized conditions, using a monomode microwave system, employed... 

Some 7-aminocoumarins-4-carboxylates [60 a] were synthesized by the Pechmann reaction, by microwave irradiation of the reactants on solid supports (graphite-K10) (Scheme 8.41). Synthesis of unsubstituted coumarins (C-4 position) has been also reported [60 b]. 

The long-established Pechmann synthesis of coumarins has been revisited. The reaction of phenols with ethyl acetoacetate (EAA) occurs in better yield at lower temperatures and milder conditions in ionic liquids which act as both solvent and Lewis acid catalyst <01TL9285> and in solvent free reactions catalysed by 4-TsOH <01CL110>. Microwave irradiation of aminophenols and EAA on a solid support of graphite and montmorillonite KIO rapidly gives good yields of 4-substituted 7-aminocoumarins <01TL2791>. 

Rajitha, B., Naveen Kumar, V., Someshwar, R, Venu Madhav, J., Narsimha Reddy, P., Thirupathi Reddy, Y. 2006. Dipyridine copper chloride catalyzed coumarin synthesis via Pechmann condensation under conventional heating and microwave irradiation. ARX/VOC 2006(12) 23-27. 

Thirupathi Reddy, Y., Vqayakumar, N. S., Peter, A. C., Pavan, K. D., Narsimha Reddy, R, Rajitha, B. 2008. Ceric ammonium nitrate (CAN) An efficient catalyst for the coumarin synthesis via Pechmann condensation using conventional heating and microwave irradiation. Synth. Commun. 38 2082-2088. 

---