title: NMR studies on epoxidations of allenamides. Evidence for formation of nitrogen-substituted allene oxide and spiro-epoxide via trapping experiments.
abstract: Two epoxidations of chiral allenamides are described here. While treatment with m-CPBA led to highly stereoselective formation of an alpha-keto aminal that can be useful synthetically, DMDO oxidation led to conclusive evidence for both nitrogen-substituted allene oxide (via mono-epoxidation) and spiro-epoxide (via bis-epoxidation) using intramolecular nucleophilic trapping experiments. NMR studies provide reliable evidence for a 3-oxetanone that can be derived from the spiro-epoxide and also suggest the presence of an allene oxide. Despite a facile second epoxidation as evidenced by the predominant formation of the 3-oxetanone, in the presence of furan, [4 + 3] cycloaddition of the nitrogen-substituted allene oxide or oxyallyl cation with furan occurs faster than the second epoxidation efficiently leading to cycloadducts. This rate difference plays an invaluable role for the success of a stereoselective sequential epoxidation-[4 + 3] cycloaddition reaction via DMDO epoxidations of chiral allenamides.
authors.affiliation: Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, USA.
authors.forename: C
authors.initials: C
authors.lastname: Rameshkumar
authors.forename: Hui
authors.initials: H
authors.lastname: Xiong
authors.forename: Michael R
authors.initials: MR
authors.lastname: Tracey
authors.forename: Craig R
authors.initials: CR
authors.lastname: Berry
authors.forename: Letitia J
authors.initials: LJ
authors.lastname: Yao
authors.forename: Richard P
authors.initials: RP
authors.lastname: Hsung