Stereoselective sulfoxide formation from a thioproline derivative

Abstract Oxidation of the PEP inhibitor, thioprolylpyrrolidine derivative 3 affords a mixture of the two possible trans - and cis -sulfoxide isomers 4 and 5 . 3-Chloroperbenzoic acid oxidation resulted almost exclusive formation of the trans -isomer (α-sulfoxide, 4 , d.e. >99% in CHCl 3 ), whereas when NaIO 4 was used the cis isomer (β-sulfoxide, 5 ) was also obtained, the ratio of the isomers varying with the reaction conditions applied. The structures of the separated isomers 4 and 5 were assigned on the basis of the aromatic solvent-induced shifts (ASIS) in the NMR spectra.