Gold(I)-Catalyzed Stereospecific [4+3]-Cycloaddition Reaction of 1-(Alk-1-ynyl)cyclopropyl Ketones with Nitrones: A Modular Entry to Enantioenriched 5,7-Fused Bicyclic Furo[3,4-d][1,2]oxazepines

摘要

In our previous study, a gold(I)-catalyzed stereoselective formal [4+3]-cycloaddition reaction of 1-(alk-1-ynyl)cyclopropyl ketones with nitrones was found to afford 5,7-fused bicyclic furo[3,4-d][1,2]oxazepines in good to excellent yields with high diastereoselectivity catalyzed by (triphenylphosphine)gold(I) triflate. In order to gain insight into the mechanism of this [4+3] cycloaddition process, an enantioselective version used a MeO-DTBM-BIPHEP derived gold(I) complex as the catalyst, which resulted in the kinetic resolution of the 1-(alk-1-ynyl)cyclopropyl ketones, affording the recovered ketones in high yield with excellent enantioselectivity. An interesting chirality transfer was observed in the gold(I)-catalyzed [4+3] cycloaddition of optically active 1-(alk-1-ynyl)cyclopropyl ketones with nitrones, indicating the reaction undergoes a stereospecific S(N)2-type ring-opening of the cyclopropane.