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17.4.1 Product Subclass 1: Oxepins

DOI: 10.1055/sos-SD-017-00971

von Angerer, S.Science of Synthesis, (200417627.

General Introduction

Oxepin (oxepine, IUPAC) attracts attention for several reasons; it is closely related to cycloheptatriene and its aza analogue azepine, it is a potential antiaromatic system with 8π-electrons, it is a valence isomer of benzene oxide, and the formation of isomeric benzene oxide is considered to be the first step of the metabolic oxidation of benzene by the enzyme monooxygenase. Four decades ago, the preparation of oxepin represented a considerable challenge to chemists, however, its synthesis was finally achieved in 1964.[‌1‌] The impetus for this effort centered upon the view that oxepin could be regarded as an analogue of cyclooctatetraene, in the same sense that furan is an analogue of benzene. The possibility of such a relationship came from MO calculations, which suggested that oxepin might possess a certain amount of aromatic character, despite the fact that the molecule does not comply with the (4n+2)-electron requirement for aromaticity. By analogy with the closely related cycloheptatriene/norcaradiene system, it was also postulated that oxepin (1) was a valence tautomer of benzene oxide (2). Other potential valence isomers of oxepin (Scheme 1) include 7-oxanorbornadiene (3), 3-oxatricyclo[3.2.0.02,4]heptene (4) and 3-oxaquadricyclane (5)[‌2‌] (see Section 17.4.1.2).

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References