Microwave irradiation for accelerating organic reactions -- Part II: Six-, seven-membered, spiro, and fused heterocycles

The use of microwave irradiation (MWI) as a nonconventional source of energy, a consequence of converting electromagnetic energy, had become very attractive for its applications to chemistry and material processing. In organic synthesis, the potential of microwave (MW) as a tool for heating attracted much interest soon after the work of Gedye (86TL279) and Giguere (86TL4945) in 1986. The terminology for the introduction of MW for organic reaction enhancement (MORE) and/or MW-assisted organic synthesis (MAOS) enabled access or parallel synthesis of various classes of compounds in organic chemistry. Although in the beginning some obstacles were faced by chemists using this technology, their desire led to the development of methods for the concurrent use of MW. One of these achievements involves the performance of organic reactions under solvent-free conditions: dry media in open vessels. Moreover, equipment and reactors have been developed and have become commercially available. Nowadays, MW activations are widely used in organic chemistry as shown by the increased number of publications. Available reviews include (91OPP683, 95AJC1665, 95T10403, 97CSR233, 97MI1, 98CSR213, 98CJC525, 98S1213, 99AJC83, 99JHC1565, 99MI1, 99MI2, 99MI3, 99T10851, 00CSR239, 00MI1, 01MI1, 01MI2, 01T4365, 01T9199, 01T9225, 02ACR717, 02MI1, 02MI2, 02MI3, 02T1235, 03MI1, 03MI2, 04H903, 05AHC1). The rapid expansion and popularity of assisting a wide range of organic reactions by exposure to MW have been accompanied by achieving reactions under solvent-free conditions, reducing reaction times, and increasing the yield of products and even selectivity. Moreover, in addition to the economic impact, there are additional advantages such as the use of noncorrosive, inexpensive, and environment-friendly catalysts, thus leading to eco-friendly approaches known as "green chemistry". All these have attracted our attention to review the available literature on the role of MW in the field of heterocyclic chemistry, but owing to the increased number of publications led us first to publish "Microwave Irradiation for Accelerating Organic Reactions. Part I: Three, Four and Five Membered Heterocycles" in a former volume (05AHC1). This new chapter will be the second part including six-, seven-membered, spiro, and fused heterocyclic ring systems. Our survey of the literature on the synthesis and reactions of these heterocycles has been divided according to the number of heteroatoms in the heterocycles. The fused heterocycles are located according to the heterocycle that was built under MWI or as a reaction of these heterocycles acting as precursors. Each type is reviewed by first presenting their methods of preparation of the desired ring followed by its reactions. Heterocycles having either fused benzene or other heterocyclic rings have been located under a separate title when enough literature has been reported. © 2006 Elsevier Inc. All rights reserved.