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
✍ Scribed by Pietro Passacantilli; Clara Centore; Elena Ciliberti; Giovanni Piancatelli; Francesca Leonelli
- Publisher
- John Wiley and Sons
- Year
- 2004
- Tongue
- English
- Weight
- 182 KB
- Volume
- 2004
- Category
- Article
- ISSN
- 1434-193X
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✦ Synopsis
Abstract
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)