藉由醣構築體反應性的不同來合成第一型N-乙醯乳糖胺的四醣分子

Abstract

醣纇和醣綴合物在許多生物過程中扮演重要的角色。含有N-乙醯乳糖胺 (LacNAc) 的聚醣 (glycan) 經常出現在各種醣型 (glycoforms)，例如路易士抗原、ABH血型和凝集素的結合配體。為了進一步了解結構與活性關聯性(structure–activity relationships, SARS)，合成結構上已被定義出含有N-乙醯乳糖胺的寡醣是必須的。然而，酵素合成法雖然具有絕佳的區域 (regio-) 和立體 (stereo-) 選擇性，但化學合成法提供了卓越的靈活性，且也有利於大量的製備。 本實驗室之前已經發展出高效率的合成法，合成含有交錯型保護基 (orthogonal protecting group) 的Gal-β1,3/4-GlcNAc 雙醣分子。為了要更進一步的合成出長鏈的第一型N-乙醯乳糖胺聚醣，且具有高產率，我們利用由Wong教授團隊所提出的一鍋化程式寡醣合成法來合成目標分子。由於醣化反應係基於硫代醣苷 (thioglycosides) 作為醣予體和醣受體，因此醣予體、醣受體和反應產生的產物，其反應性在醣化反應中均扮演著重要的角色，為了避免不必要的副反應發生，瞭解醣予體、醣受體和中間產物的反應性是必須的。首先，我們測量半乳糖醣予體(galactoside donors) 和其相對應雙醣產物的相對反應活性數值 (relative reactivity values, RRVs) ，醣化反應的實驗結果顯示，若想利用醣體反應性的不同一鍋化合成目標分子，則醣予體的相對反應活性必須高於醣受體和其相對應的雙醣體產物，以避免反應複雜化。其次，以重覆性單元為骨架，經由逐步合成法調整反應條件後，利用一鍋化合成法完成第一型N-乙醯乳糖胺四醣體的合成。最後，利用醣化反應中產生的中間體，我們成功的合成出可繼續延長的第一型N-乙醯乳糖胺四醣體。

Carbohydrates and glycoconjugates are crucial in numerous biological processes. N-Acetyllactosamine (LacNAc)-containing glycans are frequently found in various glycoforms, such as Lewis antigens and ABH-blood groups. To further understand the structure–activity relationships (SARs), it is necessary to prepare LacNAc-containing oligosaccharides with structurally defined patterns. Although enzyme synthesis provides the adventages of regio- and stereo- selectivity, chemical synthesis offers excellent flexibility and is suitable for large-scale preparation. We previously developed an efficient method to prepare Gal- β1,3/4-GlcNAc disaccharides with orthogonally protecting groups. To prepare TypeⅠLacNAc -containing saccharides of longer chains in high total yields, we applied one-pot programmable synthesis of oligosaccharides which relies on relative reactivity values (RRVs) of sugar donors to decide how glycans are assembled. Because the reactions utilize S-tolyl glycosides as glycosyl donors and acceptors, it is critical to know the reactivity of donors, acceptors and intermediate products to prevent undesirable reactions. First, We measured the RRVs of galactoside donors and the corresponding disaccharide products. The results indicated that one-pot glycosylation requires the RRVs of glycosyl donors to be higher than those of acceptors and products, to avoid further complications. Furthermore, we accomplished that the stepwise synthesis [1+1+2] to afford typeⅠtetrasaccharides (i.e. Galβ-1,3-GlcNAcβ-1,3-Galβ-1,3-GlcNAc) in an one-pot manner. We also seeked the possibility to use s-tolyl glycosides as glycosyl donors and acceptors, which allows iterative glycosylation to synthesize longer glycan chains.