Neighboring‐Group Participation in Nitrile‐Forming Beckmann Fragmentation Reactions: Synthesis of Enantiopure ( E )‐2,3‐Di‐ O ‐substituted‐5‐methoxy‐ pent‐4‐enenitriles and Their Conversion into Pyranosylamines

The selective Beckmann fragmentations of multifunctionalised ketoximes have been proven to proceed effectively to give the corresponding nitriles. The chiral ( E )‐1,3,4‐tri‐ O ‐substituted‐6‐methoxy‐hex‐5‐en‐2‐one oxime derivatives, available from glycals and glycosyl glycals, gave enantiopure ( E )‐2,3‐di‐ O ‐substituted‐5‐methoxypent‐4‐enenitriles by treatment with mesyl chloride and triethylamine. The C1−C2 heterolytic fragmentation was completely controlled and directed by the adjacent C1 ether oxygen, which generates a carbonium‐oxonium ion as an active electrofugal group. Unexpectedly, the C3 heteroatom did not assist the cleavage reaction and products derived from C2−C3 fragmentation were never detected. The excellent regio‐ and stereospecificity of the fragmentation reaction, based on the stereochemical outcome, are discussed. A simple synthetic approach to some pyranosylamines is also described. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)