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

Karl J Bonney, D Christopher Braddock, Andrew J P White, Muhammad Yaqoob

Index: J. Org. Chem. 76(1) , 97-104, (2011)

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Abstract

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.