NMR Studies on Epoxidations of Allenamides. Evidence for Formation of Nitrogen-Substituted Allene Oxide and Spiro-Epoxide via Trapping Experiments†
journal contributionposted on 31.01.2002, 00:00 by C. Rameshkumar, Hui Xiong, Michael R. Tracey, Craig R. Berry, Letitia J. Yao, Richard P. Hsung
Two epoxidations of chiral allenamides are described here. While treatment with m-CPBA led to highly stereoselective formation of an α-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.