NMR Studies on Epoxidations of Allenamides. Evidence for Formation of Nitrogen-Substituted Allene Oxide and Spiro-Epoxide via Trapping Experiments

• Published in: Journal of organic chemistry 2002-02, Vol.67 (4), p.1339-1345
• Main Authors: Rameshkumar, C, Xiong, Hui, Tracey, Michael R, Berry, Craig R, Yao, Letitia J, Hsung, Richard P
• Format: Article
• Language: English
• Subjects: Chemistry; Exact sciences and technology; Heterocyclic compounds; Heterocyclic compounds with n hetero atom and also o and/or s, se, te hetero atoms; Organic chemistry; Preparations and properties
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo011048d

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.