5-Membered rings dihydro-, aromatization

In contrast to the 1,4-dithiocin system, 1,4-dioxocin (1) is well-known and has been characterized as an olefinic compound by its spectra as well as its chemical behavior.5-6 The reason why 1,4-dioxocin in contrast to 1.4-dihydro-1.4-diazocine (see Section 1.4.) and 4//-l,4-oxazocinc (sec Section 1.12.), does not qualify as a 107r-aromatic species, is the less pronounced tendency of oxygen atoms for 7t-electron delocalization. An X-ray analysis of the 6-substituted 1,4-dioxocin 2 confirms the presumed nonplanar conformation of the 1,4-dioxocin structural element.9 The eight-membered ring exhibits a twisted boat-chair confirmation. 

Other fused ring systems in which C-protonation in the five-membered ring leads to pyridinium or quinolinium aromatic rings, respectively, are 2-phenyl-2-pyrindine (Anderson and Harrison, 1964) and 3-acyl-l,2-dihydro-4H-j3-quinindines (Kholodov et al., 1969). 

The five-membered ring systems can also behave as electron-rich dienophiles in [n4 + 2] reactions. Indole and 1,2,4,5-tetrazines yield indolopyridazines (262) via [ 4 + 2] cycloaddition followed by extrusion of nitrogen and aromatization, whilst the dihydro adduct,... 

Dipolar cycloadditions can, of course, only produce five-membered rings. Addition of dipolarophiles can generate tetrahydro, dihydro or aromatic oxidation level heterocycles, as illustrated above. Alkene dipolarophiles, with a group that can be eliminated following cycloaddition, give the same result as equivalent alkyne dipolarophiles, for example enamines as the dipolarophile, interact with azides, as the 1,3-dipole, with subsequent elimination of the amine, affording 1,2,3-triazoles. ... 

The main chemical aspect in which compounds with a nitrogen in a five- or six-membered ring differ from their acyclic counterparts is that they can be dehydrogenated to the corresponding aromatic system. Dihydro-aromatic systems naturally show Ihe greatest tendency to aromatise, indeed one of the important reducing coenzymes, NADPH (a 1,4-dihydropyridine), makes use of this tendency - it is a hydride donor ... 

An elegant and efficient synthesis of 6,11-dihydro-ll-ethyl-57/-dibenz[6,c]azepine derivatives 53 has been described which involves a BFs-catalysed aromatic amino-Claisen rearrangement of 51a-d to 52a-d followed by an intramolecular alkene Friedel-Crafts alkylation (acid catalysed) to access the 7-membered ring in 53 in high yield. With the amino-Claisen rearrangement of 51e, an inseparable mixture of 54e and 54e was obtained, since in this case both ortho positions in 56 are free for the rearrangement <04SL2721>. 

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. 

For six-membered rings, the corresponding 1,5-dicarbonyl precursor has to contain a C-C double bond in order to lead directly to the aromatic system (though it is relatively easy to dehydrogenate the dihydro-heterocycle which is comparably obtained if a saturated 1,5-dicarbonyl compound is employed). 

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