Seven-membered ring heterocycles reviews

This review covers the years 2001-2002. Seven-membered heterocycles continued to be a focus of considerable activity over this period, with fused systems a particular highlight. Ring closing metathesis methodology has been adapted to further syntheses of seven-membered rings. 

Seven-membered rings are featured in three of the chapters of the present volume, namely, the benzazepines (S. Kasparek), 1,5-benzodiazepines (D. M. G. Lloyd and H. P. Cleghorn), and 2,3-dihydro-l,4-diazepines (D. M. G. Lloyd, H. P. Cleghorn, and D. R. Marshall). Recent advances in oxazole chemistry are described by R. Lakhan and B. Ternai, and H.-J. Timpe surveys the heteroaromatic A-imines. The final chapter is a review of aromaticity (M. J. Cook, A. R. Katritzky, and P. Linda), and it concentrates on the heterocyclic aspects of this controversial subject. 

Eight-membered rings can be obtained by [4+4]-cycloadditions of 1,3-dienes [1] via diradicals or other intermediates. Synthesis of such compounds has been achieved by thermal, [2] photochemical, [3] and by metal-catalyzed [4] processes these reactions have been the subject of extensive mechanistic [5] and theoretical [5c] studies. Their strategic applications in natural product synthesis have been reviewed. [5d] The thermal version has generated little interest, except in orthoquino-dimethane dimerizations and in cycloreversions the Cope rearrangement of 1,2-divinyl-cyclobutanes [3] is more commonly used. [4+4]-Cycloadditions are also used with 1,3-dipoles or mesoionic heterocycles for the synthesis of six- and seven-membered rings. Sometimes also [6+4]-cycloadditions are... 

The formation of four- (146), five- (147)-(150), six- (151)-(153), and seven-membered (154) heterocycles, in which not only sulfur but also phosphorus together with the p-methoxyphenyl substituent (An) of LR are incorporated into the ring, has been comprehensively reviewed. ... 

After an initial inorganic example and two aliphatic compounds, drugs based on single carbocyclic rings are discussed, followed by di-, tri- and tetracyclic systems. Five-, six- and seven-membered heterocycles complete the review, with simple before more complex molecules within each section. 

The non-planar polyene nature of azepines renders them susceptible to a variety of intra-and inter-molecular pericyclic processes. The azepine-benzeneimine valence isomerization has been discussed in Section 5.16.2.4, and the ring contractions of azepines to benzenoid compounds in the presence of electrophiles is covered in Section 5.16.3.3. In this section the thermal and photochemical ring contractions of azepines to bicyclic systems, their dimerizations and their isomerizations via sigmatropic hydrogen shifts are discussed. Noteworthy is a recent comprehensive review which compares and contrasts the many and varied valence isomerizations, dimerizations and cycloadditions of heteroepins (conjugated seven-membered heterocycles) containing one, two and three heteroatoms (81H(15)1569). 

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