School of Chemistry, University of Leeds, Leeds, LS2 9 JT UK.
Chemistry. 2012 Mar 5;18(10):2987-97. doi: 10.1002/chem.201102861. Epub 2012 Jan 31.
Sulfoxides are frequently used in organic synthesis as chiral auxiliaries and reagents to mediate a wide variety of chemical transformations. For example, diphenyl sulfoxide and triflic anhydride can be used to activate a wide range of glycosyl donors including hemiacetals, glycals and thioglycosides. In this way, an alcohol, enol or sulfide is converted into a good leaving group for subsequent reaction with an acceptor alcohol. However, reaction of diphenyl sulfoxide and triflic anhydride with oxathiane-based thioglycosides, and other oxathianes, leads to a different process in which the thioglycoside is oxidised to a sulfoxide. This unexpected oxidation reaction is very stereoselective and proceeds under anhydrous conditions in which the diphenyl sulfoxide acts both as oxidant and as the source of the oxygen atom. Isotopic labelling experiments support a reaction mechanism that involves the formation of oxodisulfonium (S-O-S) dication intermediates. These intermediates undergo oxygen-exchange reactions with other sulfoxides and also allow interconversion of axial and equatorial sulfoxides in oxathiane rings. The reversibility of the oxygen-exchange reaction suggests that the stereochemical outcome of the oxidation reaction may be under thermodynamic control, which potentially presents a novel strategy for the stereoselective synthesis of sulfoxides.
亚砜通常被用作手性助剂和试剂，用于介导多种化学转化。例如，二苯基亚砜和三氟甲磺酸酐可以激活包括半缩醛、糖醛和硫代糖苷在内的多种糖基供体。这样，醇、烯醇或硫醚被转化为良好的离去基团，随后与受体醇发生反应。然而，二苯基亚砜和三氟甲磺酸酐与基于氧杂硫烷的硫代糖苷以及其他氧杂硫烷的反应会导致不同的过程，其中硫代糖苷被氧化为亚砜。这种出乎意料的氧化反应具有非常强的立体选择性，并且在无水条件下进行，其中二苯基亚砜既充当氧化剂，又充当氧原子的来源。同位素标记实验支持涉及形成氧代二硫鎓（S-O-S）二阳离子中间体的反应机制。这些中间体与其他亚砜发生氧交换反应，并且还允许在氧杂硫烷环中轴向和赤道亚砜的相互转化。氧交换反应的可逆性表明，氧化反应的立体化学结果可能受热力学控制，这为立体选择性合成亚砜提供了一种新策略。
