6,7‐Benzotropolone Syntheses Based on Ring‐Closing Metatheses and Four‐Electron Oxidations

Abstract

Four homoallyl ortho‐vinylaryl ketones (10a‐d) – 1,8‐dienes of sorts – were prepared by several approaches. In the presence of 1–2 mol‐% Grubbs‐II catalyst, they ring‐closed to give 6,7‐dihydrobenzocyclohepten‐5‐ones (11a‐d) in 90–96 % yield. With SeO2 the parent compound (11a) delivered benzocyclohepten‐5‐one (13a) and/or selenium‐containing compounds (18–22) but no more than traces of 6,7‐benzotropolone (5a). However, 5a was accessible from compound 11a via the sodium enolate and allowing it to react with a stream of oxygen (43 % yield). The sodium enolates of the substituted 6,7‐dihydrobenzocyclohepten‐5‐ones 11b–d and oxygen underwent analogous 4‐electron oxidations. This furnished the substituted 6,7‐benzotropolones 11b‐d. In contrast, the corresponding lithium enolates were inert towards oxygen. The 6,7‐dihydrobenzocyclohepten‐5‐one 11d was also accessed differently, namely by a Grubbs‐II catalyst‐mediated RCM/C=C migration tandem reaction of the allyl ortho‐allylaryl ketone 73 – another 1,8‐diene of sorts (90 % yield).