β-Directing Effect of Electron-Withdrawing Groups at O-3, O-4, and O-6 Positions and α-Directing Effect by Remote Participation of 3-O-Acyl and 6-O-Acetyl Groups of Donors in Mannopyranosylations
| Autor: | Kwan Soo Kim, Bo-Young Lee, Ju Yuel Baek, Myung Gi Jo |
|---|---|
| Rok vydání: | 2009 |
| Předmět: |
Sulfonyl
chemistry.chemical_classification Magnetic Resonance Spectroscopy Anomer Bicyclic molecule Stereochemistry Chemistry Temperature Oxocarbenium Electrons Stereoisomerism General Chemistry Bridged Bicyclo Compounds Heterocyclic Biochemistry Catalysis Oxygen carbohydrates (lipids) Colloid and Surface Chemistry Mannosylation Intramolecular force Polar effect Mannose Trifluoromethanesulfonate |
| Zdroj: | Journal of the American Chemical Society. 131:17705-17713 |
| ISSN: | 1520-5126 0002-7863 |
| DOI: | 10.1021/ja907252u |
| Popis: | Mannosylations of various acceptors with donors possessing an electron-withdrawing o-trifluoromethylbenzenesulfonyl, benzylsulfonyl, p-nitrobenzoyl, benzoyl, or acetyl group at O-3, O-4, or O-6 positions were found to be beta-selective except when donors had 3-O-acyl and 6-O-acetyl groups, which afforded alpha-mannosides as major products. The alpha-directing effect of 3-O-acyl and 6-O-acetyl groups was attributed to their remote participation, and the isolation of a stable bicyclic trichlorooxazine ring resulting from the intramolecular trapping of the anomeric oxocarbenium ion by 3-O-trichloroacetimidoyl group provided evidence for this remote participation. The triflate anion, counteranion of the mannosyl oxocarbenium ion, was essential for the beta-selectivity, and covalent alpha-mannosyl triflates with an electron-withdrawing group at O-3, O-4, or O-6 were detected by low-temperature NMR. The strongly electron-withdrawing sulfonyl groups, which exhibited a higher beta-directing effect in the mannosylation, made the alpha-mannosyl triflates more stable than the weakly electron-withdrawing acyl groups. We therefore proposed the mechanism for the beta-mannosylation and the origin of the beta-directing effect: the electron-withdrawing groups would stabilize the alpha-mannosyl triflate intermediate, and the subsequent reaction of the alpha-triflate (or its contact ion pair) with the acceptor would afford the beta-mannoside. The beta-selective mannosylation of a sterically demanding acceptor was achieved by employing a donor possessing two strongly electron-withdrawing benzylsulfonyl groups at O-4 and O-6 positions. |
| Databáze: | OpenAIRE |
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