Influence of O6 in Mannosylations Using Benzylidene Protected Donors: Stereoelectronic or Conformational Effects?
Influence of O6 in Mannosylations Using Benzylidene Protected Donors: Stereoelectronic or Conformational Effects?
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DOI：
10.1021/jo302455d
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发表时间：
2013-03-15
影响因子：
3.6
通讯作者：
Pedersen, Christian Marcus
中科院分区：
文献类型：
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作者：
Frihed, Tobias Gylling;Walvoort, Marthe T. C.;Pedersen, Christian Marcus
β-甘露糖苷的立体选择性合成及其反应机理已被深入研究，特别是亚苄基保护的甘露糖苷得到了广泛的关注，因为相应的甘露糖基三氟甲磺酸酯通常具有优异的选择性。对于增强的立体选择性的假设是亚苄基将分子锁定在反应性较低的构象中，其中O 6反式到环氧（O 5），这将稳定所形成的α-三氟甲磺酸酯并随后产生β-选择性。在这项工作中，该假设受到挑战，通过使用碳类似物（C7）的亚苄基保护的甘露糖供体，这是在非对映选择性和反应性方面进行了研究，并通过低温NMR在非对映选择性方面，C-7-类似物的行为类似于亚苄基保护的供体，但其低温NMR揭示了几个反应性中间体的形成。其中一个中间体被发现是β-氧锍离子。发现供体的反应性介于“扭转”解除武装和武装供体之间。
The stereoselective synthesis of beta-mannosides and the underlying reaction mechanism have been thoroughly studied, and especially the benzylidene-protected mannosides have gained a lot of attention since the corresponding mannosyl triflates often give excellent selectivity. The hypothesis for the enhanced stereoselectivity has been that the benzylidene locks the molecule in a less reactive conformation with the O6 trans to the ring oxygen (O5), which would stabilize the formed alpha-triflate and subsequent give beta-selectivity. In this work, the hypothesis is challenged by using the carbon analogue (C7) of the benzylidene-protected mannosyl donor, which is investigated in terms of diastereoselectivity and reactivity and by low-temperature NMR In terms of diastereoselectivity, the C-7-analogue behaves similarly to the benzylidene-protected donor, but its low-temperature NMR reveals the formation of several reactive intermediate. One of the intermediates was found to be the beta-oxosulfonium ion. The reactivity of the donor was found to be in between that of the "torsional" disarmed and an armed donor.