Regio-/stereoselective formation of monosulfoxides from thiacalix[4]arenes in all possible conformations

The partial S-oxidation of all four basic conformations of thiacalix4]arene was carried out using NaBO₃·4H₂O as the oxidizing agent. It was found that despite the possible formation of many regio- and stereoisomers, the reactions leading to mono- or disulfoxides were highly regio- and stereoselective depending on the starting conformation. Our results clearly show that the sulfur atoms possessing syn-orientation of the appended phenolic units are remarkably more reactive than the sulfur atoms with anti-oriented aromatic subunits. Moreover, the most easily formed syn-oriented equatorial configuration is highly preferred over the corresponding axial arrangement which was never observed. As demonstrated by the resolution of racemic products, partial S-oxidation represents a very interesting tool to access novel inherently chiral building blocks based on thiacalix4]arenes.