Glycosyl Formates: Glycosylations with Neighboring-Group Participation

Protected 2-O-benzyolated glycosyl formates were synthesized in one-step from the corresponding orthoester using formic acid as the sole reagent. Glucopyranosyl, mannopyranosyl and galactopyranosyl donors were synthesized and their glycosylation properties studied using model glycosyl acceptors of varied steric bulk and reactivity. Bismuth triflate was the preferred catalyst and KPF₆ was used as an additive. The 1,2-trans-selectivities resulting from neighboring-group participation were excellent and the glycosylations were generally high-yielding.     

Use of Trifluoromethanesulfonic Acid in Fischer Glycosylations

Abstract

The Fischer glycosylation₁ is one of the standard reactions in carbohydrate chemistry, in which a reducing sugar is reacted under acid catalysis with a simple alcohol to give a glycoside. Hydrochloric acid is the classical catalyst, but other proton, Lewis acid, or acid form ion echange resins have also been used. Now, the use of trifluoromethanesulfonic acid (triflic acid) as a catalyst is communicated.     

Phenanthroline‐Catalyzed Stereoretentive Glycosylations

Carbohydrates are essential moieties of many bioactive molecules in nature. However, efforts to elucidate their modes of action are often impeded by limitations in synthetic access to well‐defined oligosaccharides. Most of the current methods rely on the design of specialized coupling partners to control selectivity during the formation of glycosidic bonds. Reported herein is the use of a commercially available phenanthroline to catalyze stereoretentive glycosylation with glycosyl bromides. The method provides efficient access to α‐1,2‐cis glycosides. This protocol has been performed for the large‐scale synthesis of an octasaccharide adjuvant. Density‐functional theory calculations, together with kinetic studies, suggest that the reaction proceeds by a double S_N2 mechanism.     

Baylis‐Hillman Reaction of Arylaldehydes with Phenyl Vinyl Ketone,Phenyl Acrylate,and Phenyl Thioacrylate

In the Baylis‐Hillman reaction of aryl aldehydes with phenyl vinyl ketone, we found that the diadduct 4 was exclusively formed, and that the yield of 4 can reach 80% with increasing amounts of phenyl vinyl ketone. But, for phenyl acrylate and phenyl thioacrylate, only the normal Baylis‐Hillman adduct was obtained. The substituent effects were also examined, and a plausible reaction mechanism was proposed for the formation of 4 .     

Glycosylations with a septanosyl fluoride donor lacking a C2 protecting group

Septanosyl fluorides, prepared from protected pyranoses, were used as donors in glycosylation reactions. The fluorides were synthesized by the addition of vinyl Grignard to the pyranoses followed by ozonolysis and then DAST-mediated fluorination. Activation in the presence of nucleophiles then provided the product glycosides. High α-stereoselectivity was observed for glycosylations using a donor that had a free C2 hydroxyl group; a model where the hydroxyl group participates to guide the stereochemical outcome is proposed.     

Stereoretentive Reactions at the Anomeric Position: Synthesis of Selenoglycosides

Reported is the stereospecific cross‐coupling of anomeric stannanes with symmetrical diselenides, resulting in the synthesis of selenoglycosides with exclusive anomeric control. The reaction proceeds without the need for directing groups and is compatible with free hydroxy groups as demonstrated in the preparation of glycoconjugates derived from mono‐, di‐, and trisaccharides and peptides (35 examples). Given its generality and broad substrate scope, the glycosyl cross‐coupling method presented herein can find use in the synthesis of selenium‐containing glycomimetics and glycoconjugates.     

Kdo Glycosylations and Their Application in Oligosaccharide Synthesis**

Higher carbon saccharide 3-deoxy-d -manno-oct-2-ulosonic acid (Kdo) is a structural unit of bacterial lipopolysaccharides (LPSs) and capsular polysaccharides (CPSs). Kdo is present in the inner core region of LPSs, and this region is structurally conserved. Being non-mammalian in origin, Kdos are effectively recognized by the native and adaptive immune systems. Therefore, the synthesis of new Kdo derivatives and neoglycoconjugates is highly important for the development of vaccines. This review highlights recent accomplishments related to α-glycosylations, β-glycosylations and C-glycosylations of Kdos and their application to the stereoselective synthesis of inner core oligosaccharides.     

含有苯基的青蒿素类似物的合成

为考察引入苯基的青蒿素类似物的抗疟活性, 我们设计了化合物7. 从苯甲醛出发, 经过10步反应, 制得光化学反应底物环烯醚16. 16经光氧化反应得到非期望产物17. 17的化学结构由X射线晶体衍射法测定.     

苯乙烯与N-对位取代苯基马来酰亚胺的乳液共聚

合成了苯乙烯(St)与 N-对位取代基马来酰亚胺的3种二元共聚物乳液.在苯乙烯与N-对位取代苯基马来酰亚胺(N-p-RPhMI)的最大共聚比内,通过种子滴加乳液聚合制得高稳定性、高固含量(40%)、低粘度的共聚物乳液.研究了 N-p-RPhMI 苯环对位上取代基团对共聚物乳液的性能以及共聚物热性能影响.结果表明:N-p-RPhMI 的加入提高了乳液的产率,并且随着取代基团极性的增强,乳液产率提高,乳胶粒的平均粒径增大,乳液的表观粘度降低;共聚物的热分解温度随着取代基团极性的增加而提高,但取代基极性对共聚物的玻璃化转变温度(Tg)的影响明显.同时,St/N-对甲氧基马来酰亚胺(N-p-MOPhMI)体系中助溶剂的加入对共聚物乳液性能影响很大,使共聚物的热起始分解温度升高,但对共聚物的 Tg 基本无影响.     

紫外分光光度法测定苯基硅油的苯基含量

采用水解缩合法合成了苯基硅油,并进行傅立叶红外(FTIR)、氢谱(1H NMR)表征.根据氢谱的积分,计算得产品的苯基含量.基于苯基基团在紫外区的特征吸收,以自制苯基硅油为标准品,建立了一种紫外分光光度法测定苯基硅油中苯基含量的方法.以二氯甲烷为溶剂,264 nm为最大吸收波长,苯基浓度在0.277×10-3～2.13×10-3 mmol· mL-1范围内,符合比尔定律,其R2 =0.99946.实际样品测定的相对标准偏差小于1.81％,平均回收率为99.0％左右.该方法线性相关性好、精密度及准确度高、便捷、成本低,适合科研及工业生产中苯基含量的日常分析.     

Nucleophile‐Directed Stereocontrol Over Glycosylations Using Geminal‐Difluorinated Nucleophiles

The glycosylation reaction is the key transformation in oligosaccharide synthesis, but it is still difficult to control in many cases. Stereocontrol during cis‐glycosidic linkage formation relies almost exclusively on tuning the glycosylating agent or the reaction conditions. Herein, we use nucleophile‐directed stereocontrol to manipulate the stereoselectivity of glycosylation reactions. Placing two fluorine atoms in close proximity to the hydroxy group of an aliphatic amino alcohol lowers the oxygen nucleophilicity and reverses the stereoselectivity of glycosylations to preferentially form the desired cis‐glycosides with a broad set of substrates. This concept was applied to the design of a cis‐selective linker for automated glycan assembly. Fluorination of an amino alcohol linker does not impair glycan immobilization and lectin binding as illustrated by glycan microarray experiments. These fluorinated linkers enable the facile generation of α‐terminating synthetic glycans for the formation of glycoconjugates.