Beta-directing effect of electron-withdrawing groups at O-3, O-4, and O-6 positions and alpha-directing effect by remote participation of 3-O-acyl and 6-O-acetyl groups of donors in mannopyranosylations.
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.
Baek
Ju Yuel
JY
Center for Bioactive Molecular Hybrids and the Department of Chemistry, Yonsei University, Seoul 120-749, Korea.
Lee
Bo-Young
BY
Jo
Myung Gi
MG
Kim
Kwan Soo
KS
D019086:Bridged Bicyclo Compounds, Heterocyclic / Q000737:chemistry; D004583:Electrons; D009682:Magnetic Resonance Spectroscopy; D008358:Mannose / Q000138:chemical synthesis / Q000737:chemistry*; D010100:Oxygen / Q000737:chemistry*; D013237:Stereoisomerism; D013696:Temperature
D019086:Bridged Bicyclo Compounds, Heterocyclic; D008358:Mannose; D010100:Oxygen