Microwave-mediated synthesis of N-containing heterocycles: from batch to continuous flow processes

Microwave-assisted organic synthesis has received increasing attention in recent years as a valuable alternative to the use of conductive heating for accelerating chemical reactions. Current technology has attempted to overcome the limited dimensions of the standing wave cavity by the use of multimode batch or continuous flow reactors that pump the reagents through the microwave chamber. A new prototype continuous flow microwave reactor using either a 10 mL or 80 mL flow cell has been developed for use with a monomodal instrument. This system possessed a number of advantages over commercially available coils or a microreactor. New efficient methods for the microwave-assisted synthesis of pyrazoles, pyrimidines, Bohlmann-Rahtz pyridines and Hantzsch dihydropyridines were developed using ethynyl carbonyl compounds as common precursors and this batch technology was successfully transferred to continuous flow processing using the prototype microwave reactor. The use of an 80 mL tube flow cell allowed the synthesis of 8 g of product in 1 h processing time, showing its potential for the large scale application of this prototype