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
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
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DOI：
10.1021/ja907252u
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发表时间：
2009-12-09
影响因子：
15
通讯作者：
Kim, Kwan Soo
中科院分区：
文献类型：
--
作者：
Baek, Ju Yuel;Lee, Bo-Young;Kim, Kwan Soo
在O-3、O-4或O-6位具有吸电子的邻三氟甲基苯磺酰基、苄基磺酰基、对硝基苯甲酰基、苯甲酰基或乙酰基的供体与各种受体的甘露糖基化被发现是β-选择性的，除了当供体具有3-O-酰基和6-O-乙酰基时，其提供α-甘露糖苷作为主要产物。3-O-酰基和6-O-乙酰基的α-导向作用归因于它们的远程参与，并且由3-O-三氯乙酰亚氨酰基对异头氧代碳正离子的分子内捕获产生的稳定的双环三氯恶嗪环的分离提供了这种远程参与的证据。三氟甲磺酸根阴离子（甘露糖基氧碳正离子的抗衡阴离子）对于β-选择性是必不可少的，并且通过低温NMR检测到在O-3、O-4或O-6处具有吸电子基团的共价α-甘露糖基三氟甲磺酸根。强吸电子磺酰基在甘露糖基化中表现出更高的β-导向作用，使得α-甘露糖基三氟甲磺酸酯比弱吸电子酰基更稳定。因此，我们提出了β-甘露糖基化的机制和β-导向效应的起源：吸电子基团将稳定α-甘露糖基三氟甲磺酸酯中间体，随后α-三氟甲磺酸酯（或其接触离子对）与受体反应将产生β-甘露糖苷。通过采用在O-4和O-6位具有两个强吸电子苄基磺酰基的供体，实现了空间要求高的受体的β-选择性甘露糖基化。
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 a-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 a-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.