Abstract：
We report on the syntheses of NeuAc and NeuGc-containing glycosides via the use of double carbonyl-protected N-acetyl sialyl donors. The 7-O,9-O-carbonyl protection of an N-acyl-5-N,4-O-carbonyl-protected sialyl donor markedly increased the α-selectivity during glycosylation, particularly when glycosylating the C-8 hydroxyl group of sialic acids. The N-acyl carbamates were selectively opened with ethanethiol under basic conditions to provide N-acyl amines. It is noteworthy that N-glycolyl carbamate was more reactive to nucleophiles by comparison with the N-acetyl carbamate due to the electron-withdrawing oxygen in the N-acyl group and however, allowed selective opening of the carbamates without the loss of N-glycolyl groups. To demonstrate the utility of the approach, we began by synthesizing α(2,3) and α(2,6) sialyl galactosides. Glycosylation of the hydroxy groups of galactosides at the C-6 position with the NeuAc and NeuGc donors provided the corresponding sialyl galactoses in good yields with excellent α-selectivity. However, glycosylation of the 2,3-diol galactosyl acceptor selectively provided Siaα(2,2)Gal. Next, we prepared a series of α(2,8) disialosides composed of NeuAc and NeuGc. Glycosylation of NeuGc and NeuAc acceptors at the C-8 hydroxyl group with NeuGc and NeuAc sialyl donors provided the corresponding α(2,8) disialosides, and no significant differences were detected in the reactivities of these acceptors.
摘要：
我们报告了通过使用双羰基保护的N-乙酰基唾液酸供体合成NeuAc和含NeuGc的糖苷。7-O,N-酰基-5-N的9-O-羰基保护，4-O-羰基保护的唾液酸供体显着增加了糖基化过程中的a-选择性，特别是当糖基化唾液酸的C-8羟基时。在碱性条件下用乙硫醇选择性打开N-酰基氨基甲酸酯以提供N-酰基胺。值得注意的是，由于N-酰基中的吸电子氧，与N-乙酰基氨基甲酸酯相比，N-羟酰基氨基甲酸酯对亲核体的反应性更高，允许氨基甲酸酯的选择性打开而不损失N-羟酰基。为了证明该方法的实用性，我们开始合成α(2,3)和α(2,6)唾液酸半乳糖苷。用NeuAc和NeuGc供体在C-6位半乳糖苷的羟基糖基化以良好的产率和优异的a-选择性提供相应的唾液酸半乳糖。然而,2,3-二醇半乳糖基受体的糖基化选择性地提供Siaα(2,2)Gal。接下来,我们制备了一系列由NeuAc和NeuGc组成的α(2,8)二唾液酸苷。用NeuGc和NeuAc唾液酸供体在C-8羟基上的NeuGc和NeuAc受体的糖基化提供了相应的α（2,8）二唾液酸苷，并且在这些受体的反应性中没有检测到显着差异。
