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
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 fragmentationreaction, based on the stereochemical