Intramolecular bromonium ion assisted epoxide ring-opening: Capture of the oxonium ion with an added external nucleophile

摘要

9-Oxabicyclo[6.1.0]non-4-ene (1) undergoes intramolecular bromonium ion-assisted epoxide ring-opening using N-bromosuccinimide via a presumed oxonium ion that is subject to stereospecific, nonregioselective capture with added external nucleophiles producing novel bicyclo[4.2.1] and bicyclo[3.3.1] ethers. Carboxylic acids (as catalyzed by tetramethylguanidine), alcohols, water, and halides can all function as effective nucleophiles. Stereospecific direct opening of the bromonium ion with carboxylic acids was found to be a competing process where high dilution disfavors this pathway. Halogen-induced isotopic ~(13)C NMR shifts (Δδ CBr 1.3-1.9 ppb; Δδ CCl 8.6-8.7 ppb) were found to be most useful in unambiguously identifying halogen-bearing carbons, and correlation of these ~(13)C NMR shifts allowed ready assignment of diastereomeric structures. The structure of adducts 6b, 6c, 7b, 7c, 7d, and 8a-d were all elucidated by X-ray crystallography.