Enantioselective one-carbon expansion of aromatic rings by simultaneous formation and chromoselective irradiation of a transient coloured enolate

Enantioenriched seven-membered carbocycles are motifs in many molecules of structural and biological interest. We report a simple, practical, transition metal-free and mechanistically unusual method for the enantioselective synthesis of substituted cycloheptatrienes. By forming a coloured enolate with an appropriate absorption band and selectively irradiating in situ, we to initiate a tandem, asymmetric anionic and photochemical ring expansion of readily accessible N-benzylbenzamides. The cascade of reactions leading to the products entails enantioselective benzylic deprotonation with a chiral lithium amide, dearomatizing cyclization of the resulting configurationally defined organolithium to give an extended amide enolate, and photochemically induced formal [1,7]-sigmatropic rearrangement and 6 pi-electrocyclic ring-opening - the latter all evidently being stereospecific - to deliver enantioenriched cycloheptatrienes with embedded benzylic stereocentres.

Automated summary

Enantioenriched seven-membered carbocycles are important motifs in structurally and biologically interesting molecules. This study presents a simple, practical, transition metal-free, and mechanistically unusual method for the enantioselective synthesis of substituted cycloheptatrienes. By generating a colored enolate with an appropriate absorption band and selectively irradiating the reaction in situ, a cascade of reactions is initiated to achieve asymmetric anionic and photochemical ring expansion of readily accessible N-benzylbenzamides.