2-Alkylidene-4-oxothiazolidine S-oxides: synthesis and stereochemistry

A series of 5-unsubstituted and 5-substituted 2-alkylidene-4-oxothiazolidine-S-oxides were synthesized by the sulfur-oxidation with m-CPBA. The stereochemistry of 5-substituted sulfoxides was determined by means of NMR spectroscopy and DFT theoretical calculations. It was found that the thermodynamically less stable anti-isomer was initially formed in the course of the oxidation, but it underwent epimerization to the mixture enriched in the more stable syn-isomer, during the work-up process. The higher stability of syn-isomers is ascribed to the stronger hyperconjugative σC-H→σ*S-O interaction versus the weaker σC-C→σ*S-O delocalization in their anti-counterparts and to the existence of intramolecular 1,5-CH⋯O hydrogen bonds.