Switchable Brønsted Acid‐Catalyzed Ring Contraction of Cyclooctatetraene Oxide Towards the Enantioselective Synthesis of Cycloheptatrienyl‐Substituted Homoallylic Alcohols and Oxaborinanes

The ability of cyclooctatetraene oxide to undergo two sequential ring contraction events under mild conditions, using Bronsted acid catalysis, has been studied in detail. We have found that the selectivity can be controlled by the acidity of the catalyst and by the temperature, being able to obtain selectively either the cycloheptatriene carbaldehyde product, arising from a single ring-contraction event, or phenylacetaldehyde that is formed after a second ring contraction process. A complete mechanistic picture of the reaction and a rationale behind the influence of the catalyst is provided based on both experimental and computational data. Finally, this acid-catalyzed ring contraction has been coupled with an in situ enantioselective allylation reaction, delivering enantioenriched cycloheptatrienyl-substituted homoallylic alcohols when it is carried out in the presence of a chiral phosphoric acid catalyst. These homoallylic alcohols have also been converted into enantioenriched oxaborinanes through copper-catalyzed nucleophilic borylation/cyclization protocol.