Asymmetric Oxidation of Cyclic Sulfides

For cyclic sulfides, the two lone pairs on the sulfur atom are intrinsically different, because of the ring conformation. Due to this problem, cyclic sulfides are a difficult class of substrates for asymmetric oxidation chemistry. 

In contrast to simple sulfides, cyclic dithioacetals have four sulfur lone pairs in one molecule that can participate in oxidation processes, but the presence of a substituent at the 2-position raises the issue of diastereoselectivity as well as enantioselectivity. Potentially, four stereoisomers of the monoxide 

---

Scheme 19.41 Asymmetric oxidations of cyclic sulfides catalysed by Al(salalen).

---