AbstractAlthough tremendous efforts have been made for the efficient preparation of sialosides, controlling the stereochemical outcome of sialylation reaction still remains one of the most challenging tasks due to the unique chemical structure of sialic acid. We developed a new strategy to statistically analyze the stereoselectivity of sialylation reactions on six types of p‐tolyl thiosialosides in NIS/TfOH system using Relative Reactivity Value (RRV) as the indicator. Analysis of the reaction mechanism showed the formation of the relatively stable glycosyl bromide and glycosyl chloride intermediates from halide‐ and triflate‐containing promotors in the absence of an acceptor. We found that the α/β‐stereoselectivity, yields, and intermediate changes were associated with their donor reactivity. These findings enable to tailor the most suitable building blocks for stereo‐controlled sialylation reactions.
Comparative studies on the O-sialylation with four different α/β-oriented (N-acetyl)-5-N,4-O-carbonyl-protected p-toluenethiosialosides as donors
摘要:
Four types of 5-N,4-O-carbonyl-protected p-toluenethiosialosides were synthesized and their couplings with different acceptors were thoroughly investigated. The results indicate that the sialyl donor structure, the amount of glycosyl acceptor, and the detailed promotion conditions have great influence on the sialylation stereoselectivties and product yields. Under the (p-Tol)(2)SO/Tf2O activation conditions, the glycosylations with simple alcohols provided declined alpha-selectivities and higher yields with increasing the amounts of acceptors from 1.1 equiv to 2.0 equiv. However, the outcome of same sialylation was independent of the relative amounts of sugar alcohol acceptors. With NIS/TfOH as promoter, the alpha-selectivities of the sialylations were significantly improved compared with the cases activated by (p-Tol)(2)SO/Tf2O. In general, the difference in configuration of N-acetylated sialyl donors (D2 and D4) has little effect on the sialylation yield and stereoselectivity. In contrast, the N-deacetylated alpha/beta sialyl donors (D1 and D3) show complex sialylation profiles with different acceptors. (C) 2014 Elsevier Ltd. All rights reserved.
5-<i>N</i>,4-<i>O</i>-Carbonyl-7,8,9-tri-<i>O</i>-chloroacetyl-Protected Sialyl Donor for the Stereoselective Synthesis of α-(2→9)-Tetrasialic Acid
作者:Chang-Ching Lin、Nai-Pin Lin、L. Sk Sahabuddin、Vijaya Raghava Reddy、Li-De Huang、Kuo Chu Hwang、Chun-Cheng Lin
DOI:10.1021/jo100824s
日期:2010.8.6
An efficient stereoselective synthesis of α-(2→9)-tetrasialic acid was achieved using tri-O-chloroacetyl-derivatized sialyl donor and a triol sialyl acceptor. Both the acceptor and the donor were also protected with a cyclic 5-N-4-O-carbonyl protecting group. The donor is highly reactive and enabled α-selective sialylation with various primary, secondary, and tertiary acceptors under in situ activation
使用三-O-氯乙酰基衍生的唾液酸供体和三醇唾液酸受体实现了α-(2→9)-四唾液酸的有效立体选择性合成。受体和供体也都被环状5- N -4- O-羰基保护基保护。供体在原位活化条件(NIS / TfOH,-78°C,乙腈/二氯甲烷)下具有高反应活性,并且能够与各种伯,仲和叔受体进行α-选择性唾液酸化。在温和的反应条件下,容易除去反式稠合的恶唑烷酮环和O-氯乙酰基保护基,以提供完全脱保护的α(2→9)-四唾液酸。