Abstract
This chapter discusses the seven-membered rings and maximally unsaturated azepines. Cyclization of the β , γ -unsaturated aldehyde by treatment with trifluoroacetic acid, followed by treatment with base (1,4-Diazabicyclo[2.2.2]octane, DABCO or 4-[dimethylamino]pyridine, DMAP) affords 2H-azepine. The compound itself is difficult to isolate but it is remarkably stable in solution, no trace of the thermodynamically favored 3H-azepine being observed after 48h at room temperature. In addition, the bicyclic adduct is formed, probably arising from [2 + 4] cycloaddition of isobutene formed in situ and the intermediate immonium ion. Although diazabicycloundecene (DBU) is well known as a non-nucleophilic base, it is less well recognized as an amidine capable of reactions of such systems. However, under certain circumstances, it is capable of acting as a nucleophile toward a number of strong electrophiles. The successful cycloaddition of dimethyl acetylenedicarboxylate is now added to this list. There has been resurgence in the interest of oxepines due in part to the total synthesis of a number of natural products.
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