【作者】 周宜轩；

【导师】 董海；

【作者基本信息】 华中科技大学 ， 材料物理与化学， 2013， 博士

【摘要】 保护及保护基策略从来都是糖化学研究的核心问题之一。保护策略研究的核心是糖羟基的选择性保护问题,其中一种最常用的选择性保护方法是基于有机锡(特别是二丁基氧化锡)的羟基高选择性保护。尽管该方法因操作简便、选择性高已被广泛应用多年,但控制其选择性的机理至今仍不清楚。传统的观点认为其选择性受二丁基氧化锡与羟基生成的缩醛二聚体或多聚体控制。我们的最新研究表明,这种方法的高选择性并非由有机锡缩醛二聚体或多聚体控制,而是由糖环上羟基的立体构型及立体电子效应所决定。本论文将通过深入研究糖环上立体电子效应与选择性之间的关系,进一步研究选择性控制机理。由于新观点排除了有机锡缩醛中间体复杂结构对反应机理研究的干扰,我们可以基于糖环上的立体电子效应,深入研究有机锡方法糖羟基选择性保护的反应机理,从而发展新的糖羟基高选择性保护策略,特别是发展基于绿色无毒有机硅试剂的羟基保护策略,避免剧毒的有机锡试剂的使用。我们主要做了以下几方面的研究。一、基于有机锡选择性糖羟基保护的机理探讨我们研究分析了酰基迁移反应、邻基参与反应和基于有机锡的酰基迁移反应过程,认为这几种反应产物的选择性都受其反应历程中生成的五元环缩醛中间体控制。从而得出结论,有机锡选择性糖羟基保护的高选择性并非由有机锡缩醛二聚体或多聚体控制,而是由糖环上羟基的立体构型及立体电子效应所决定。理论化学计算结果进一步支持了这个结论。在这个理论指导下,我们通过基于有机锡的选择性苄醚化反应研究了卤素阴离子促进该反应的机理。研究表明,能够与锡原子形成稳定Sn-X键的阴离子具有大的促反应活性。由于溴离子适中的促反应活性以及能够在反应过程中再生循环,我们以溴离子为催化助剂进行多苄醚化反应,高效合成了一系列选择性苄醚化的产物。二、基于有机硅选择性糖羟基保护和醋酸根阴离子对羟基的超分子效应研究在上述理论指导下,我们开发了一种基于新型绿色的有机硅试剂的选择性糖羟基保护方法。我们首先用有机硅试剂与相邻两羟基生成硅氧缩醛,接着以醋酸根阴离子作为反应促进剂,有效的对含二醇底物进行选择性乙酰化反应。在考察醋酸根离子对糖选择性乙酰化反应的影响时,发现了醋酸根离子能够和二醇底物形成双氢键,我们利用这种双氢键超分子效应,从而发展了一种以醋酸根离子为催化剂的简便,高效,绿色的高选择性乙酰化方法。更多还原

【Abstract】 Organotin-mediated (especially for dibutyltin oxide) regioselective protection has been extensively used in organic synthesis, especially in carbohydrate synthesis for many years. However, the mechanistic origin of the resulting regioselectivity is still not clear. A general conception had been accepted that the regioselectivity was controlled by complex stannylene structure.In present studies, by comparison of the steric and stereoelectronic effects controlling the geometry of five-membered rings formed from neighboring group participation, from intramolecular acyl group migration, or from orthoester transesterification on pyranoside rings, we proposed a novel theory on the pattern resulting from the reaction with dibutyltin oxide. It is thus suggested that the regioselectivity of organotin-mediated protection is controlled by analogous steric and stereoelectronic effects as in neighboring group participation and acyl group migration, mainly dependent on the stereoelectronic effects of the pyranoside itself, and not related to complex stannylene structures. Based on this theory, the mechanistic origin of the promoted organotin-mediated carbohydrate benzylation by halides was explored by the comparison of the activation ability of halides. The halides, being able to form a stronger bond with tin, showed stronger activation ability. Thus, the catalytic amounts of bromide are preferentially chosen as an activation additive in the reaction with consideration of economy, convenience and moderate activation ability. The results were further applied to mono-and multi-benzylation of additional carbohydrate structures.Based on this theory, we also developed a novel organosilicon-mediated regioselective carbohydrate protection. Dimethyldimethoxysilane or methyl trimethoxysilane was first used to form cyclic1,3,2-dioxasilolane or1,3,2-dioxasilinane intermediates, and subsequent acetate catalyzed monoacylation was efficiently performed by addition of acetic anhydride or acetyl chloride under mild conditions. The reaction exhibited high regioselectivity, resulting in the same protection pattern as in organotin-mediated schemes. We also proposed a principle for supramolecular activation in acylation of hydroxyl group, in which the acylation proceeds following formation of H-bonds between hydroxyl group and anions. By this principle, we develop a regioselective acetylation method for diols and polyols, in which the acetylation is activated by only catalytic amounts of acetate anions.更多还原