Ten L-5-alkyl-and alkenylthiomethylhydantoin S-oxides (alkyl or alkenyl methyl, ethyl, propyl, isopropyl, allyl, butyl, isobutyl, sec-butyl, pentyl and hexyl) and some related compounds were prepared from L-cystine. L-5-Propylthiomethylhydantoin S-oxide (L-(+)-PHSO) was further split into L-(+)-PHSO and L-(-) PHSO.

The structure-activity relationships of antibacterial L-5-alkylthiomethylhydantoin S-oxides and related compounds prepared were examined. S-Propyl-L-cysteine-S-oxide, its N-carbamoyl derivative and dl-3-alkylthio-2-methylpropionamide 5-oxides which do not have hydantoin ring exhibited far less activities. 5-Alkyl-and alkenylthiomethyl-5-methylhydantoin S-oxides and 5-alkylthioethylhydantoin 5-oxides bearing hydantoin structure did not have either any appreciable activity. From these results, it has strongly been suggested that the structure required for antibacterial activity is summarized as follows; (1) Hydantoin ring and (2) sulfoxide function should be necessary, (3) the hydrogen atom on the 5-C of the hydantoin ring is essential, and (4) the methylene-unit between the sulfur atom and the hydantoin ring is not two (or more), but one.