| Popis: |
The ability of cyclooctatetraene oxide to undergo two sequential ring contraction events under mild conditions, using Brønsted 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. Grants PID2019-104090RB-100, PID2019-109674GB−I00 and PID2020-118422GB−I00 funded by MCIN/AEI/10.13039/501100011033 and by “ESF Investing in your future” are gratefully acknowledged together with the Basque Government (Grupos IT1558-22) and the Government of Aragón (Grupos Consolidados, E34_20R and a fellowship to M. P.). O. S. thanks MF-URV for a predoctoral grant and J. S. thanks Grant PRE2018-083532 funded by MCIN/AEI/10.13039/501100011033 and by “ESF Investing in your future” for an FPI fellowship. The authors thankfully acknowledge the resources from the supercomputers “Memento” and “Cierzo” and technical expertise and assistance provided by BIFI-ZCAM (Universidad de Zaragoza, Spain). |
