SnCl<sub>4</sub>- and TiCl<sub>4</sub>-Catalyzed Anomerization of Acylated <i>O</i>- and <i>S</i>-Glycosides: Analysis of Factors That Lead to Higher α:β Anomer Ratios and Reaction Rates
作者:Wayne Pilgrim、Paul V. Murphy
DOI:10.1021/jo101090f
日期:2010.10.15
glucuronic acid or galacturonic acid derivatives were ∼10 to 3000 times faster than those of related glucoside and galactopyranoside counterparts and α:β ratios were generally also higher. Stereoelectronic effects contributed from galacto-configured compounds were up to 2-fold faster than those of corresponding glucosides. The introduction of groups, including protecting groups, which are increasingly electron
讨论了影响SnCl 4和TiCl 4催化的异构化反应速率和立体选择性的因素的定量,以及这如何影响α- O-和α- S-糖脂的合成。SnCl 4催化的18种底物的β- S-和β- O-糖苷的阴离子化反应遵循一级平衡动力学,得到k f + k r值,其中k f是正向反应的速率常数(β→ α)和k r是逆反应的速率常数(α→β)。比较k f + k r值表明,葡萄糖醛酸或半乳糖醛酸衍生物的反应比相关的葡萄糖苷和吡喃半乳糖苷对应物的反应快约10至3000倍,并且α:β比率通常也更高。由半乳糖配置的化合物产生的立体电子效应比相应的糖苷的高达快2倍。越来越多地释放电子的基团(包括保护基)的引入通常导致速率提高。S-糖苷的异构化速度始终快于相应的O-糖苷。与TiCl 4的反应通常比与SnCl 4的反应更快。端基异构体的比例取决于路易斯酸,路易斯酸的当量数,温度和底物。对于TiCl 4促进的反应,观察到O
Photooxidation of thiosaccharides mediated by sensitizers in aerobic and environmentally friendly conditions
作者:Miqueas G. Traverssi、Alicia B. Peñéñory、Oscar Varela、Juan P. Colomer
DOI:10.1039/d0ra09534f
日期:——
A series of β-D-glucopyranosyl derivates have been synthesized and evaluated in photooxidation reactions promoted by visible light and mediated by organic dyes under aerobic conditions. Among the different photocatalysts employed, tetra-O-acetyl riboflavin afforded chemoselectively the respective sulfoxides, without over-oxidation to sulfones, in good to excellent yields and short reaction times. This