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

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 SeO 2 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).