Diastereoselective Oxidations of a Thioether Appended with a Neighboring Carboxylic Acid Group

Diastereoselective oxidations of endo acid 5a, 6-endo-(methylthio)bicyclo[2.2.1] heptane-2-endo-carboxylic acid, and endo ester 5b, methyl 6-endo-(methylthio)bicyclo[2.2.1]heptane-2-endo-carboxylate, with the bromine complex of l,4-diazabicyclo[2.2.2]octane in aqueous acetic acid to the corresponding sulfoxides are reported. The relative stereochemistry of the major diastereomeric product obtained by such oxidation of endo acid 5a was unequivocally established by X-ray crystallographic analysis. Endo acid sulfoxide 7a crystallizes in the monoclinic space group P2₁/ c with a = 8.554 (1) A, b = 8.989 (2) A, c = 12.575 (2) A, β- 98.64 (1)°, and Z = 4. The structure was solved by direct methods. Full-matrix least-squares refinement led to a conventional R factor of 0.038 after several cycles of anisotropic refinement. The predominant sulfoxide formed in the oxidation of endo acid 5a and endo ester 5b with the bromine complex of l,4-diazabicyclo[2.2.2]octane is of the opposite diastereomeric series. This difference in stereochemistry is ascribed to neighboring-group participation in the oxidation of endo acid 5a by the carboxylate moiety. Diastereoselective oxidations of endo acid 5a and endo ester 5b with m-chloroperoxybenzoic acid to the corresponding sulfoxides are reported. The results suggest that the carboxylic acid group does not direct attack by the peracid on the thioether.