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The development of new strategies for synthesis of medium-sized heterocycles has remained a highly attractive but challenging proposition. In the last few years, the application of microwave irradiation has greatly facilitated the construction of such heterocyclic ring systems through a myriad of different synthetic approaches. This feature article illustrates the progress made in the microwave-assisted synthesis of medium-sized heterocycles with an emphasis on examples describing the use of a dedicated microwave synthesizer., (This journal is © The Royal Society of Chemistry 2012)

Microwave-assisted Suzuki-Miyaura and Stille cross-coupling reactions for the synthesis of highly electron-rich and diversely functionalized biaryl intermediates are presented. Microwave-irradiation has been demonstrated to be a very powerful tool for performing difficult transition metal-catalyzed cross-coupling reactions with unfavorably substituted coupling partners. The key biaryl intermediates were used for the microwave-enhanced construction of the 5,6,7,8-tetrahydrobenzo[c,e]azocine skeleton of the Apogalanthamine analogs. The success of the strategy is demonstrated by synthesizing a number of hitherto unknown, diversely functionalized, natural product analogs.

[reaction: see text]. A novel, microwave-enhanced, highly efficient protocol for the synthesis of hitherto unknown (-)-steganacin and (-)-steganone 7-aza analogues containing a 1,2,3-triazole ring has been presented. Microwave irradiation was found to be highly beneficial in promoting the Suzuki reaction and the 1,3-dipolar cycloaddition reaction to generate the highly strained medium-sized ring system of the title molecules.

The "click chemistry" approach has been explored on the 2-(1H)-pyrazinone scaffold for the generation of pharmacologically interesting heterocyclic moieties. Huisgen 1,3-dipolar cycloaddition has been evaluated as the key step for the construction of the 1,2,3-triazole ring at the C-3 position of 2-(1H)-pyrazinones. Two different pathways have been successfully evaluated: (1) via C-C or C-O linkage of the acetylenic part to the C-3 position of the 2-(1H)-pyrazinone scaffold or (2) via azide introduction in the C-3 position. The subsequent application of "click chemistry" resulted in the formation of hitherto unknown skeletons. Microwave irradiation has successfully been applied in different steps of the sequence.

Research has been conducted on 2(1H)-pyrazinones. The study of the inter- and intramolecular hetero-Diels-Alder reactions carried out under controlled microwave irradiation in a series of functionalized 2(1H)-pyrazinones is presented.