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

Abstract

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.

A Photochemical Strategy for the Conversion of Nitroarenes into Rigidified Pyrrolidine Analogues

“…Intrigued by the pioneering result of Lambert we speculate that a second photoexcitation could trigger a disrotatory 4π electrocyclization producing the bicyclic product VII with a cis ring-junction. − It is worth noting that VI is uniquely equipped to undergo this proposed electrocyclic reactivity over other pericyclic reactions. Indeed, thermal disrotatory 6π ring-closure across the whole diene-imine system, a transformation resembling the known heptatriene–norcasdiene equilibrium, was calculated to have a relatively high barrier (Δ G ‡ = 17 kcal mol –1 ). − Furthermore, both photochemical 6π and thermal 4π reactivity can be excluded on the basis that conrotatory orbital movement would provide a structurally not feasible trans ring-junction in the bicyclic products…”

Amine adducts of triallylborane as highly reactive allylborating agents for Cu(i)-catalyzed allylation of chiral sulfinylimines

“…36 However, the most straightforward route of allylboronate activation is, of course, catalysis. Numerous catalytic systems have been elaborated for the allylation of carbonyls and imines, including Lewis acids [37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55] (including Brønsted acids [46][47][48][49][50][51][52][53][54][55] ) and organocatalysts, [56][57][58][59][60][61][62][63][64][65][66] where phenolamines elaborated by Hoveyda were superior (1-6 mol%, time 4-6 h) in terms of both activity (though somewhat lower yields of alkylimines were observed) and selectivity for a wide range of substrates (Scheme 3). [59][60][61][62][63][64]…”

Asymmetric [4 + 2], [6 + 2], and [6 + 4] Cycloadditions of Isomeric Formyl Cycloheptatrienes Catalyzed by a Chiral Diamine Catalyst

“…Substituents can also be used as handles to activate the cycloheptatriene core through catalysis in normal and higher-order cycloadditions. − While heptafulvenes and tropones have been shown to undergo different enantioselective cycloadditions, these molecules are seldom used for reaction control, − which is generally achieved through the other cycloaddend. − To this end, formyl-substituted cycloheptatrienes could serve as promising model substrates, as the formyl group can both activate the system and potentially enable reaction control using, for example, asymmetric aminocatalysis. , Despite this, formyl-substituted cycloheptatrienes have found limited use, which may be in part due to the difficulty in controlling their reactivity. − …”