PURPOSE: To obtain the titled compound useful as an intermediate for synthesizing caphalosporin, β-lactam antibiotic, by one process simply and easily, by oxidizng a 2,2-dimethylthiazolidine derivative as a raw material with oxygen in the presence of a photosensitizer.

CONSTITUTION: A 2,2-dimethylthiazolidine derivative shown by the formula I (R1 is H, or lower alkyl; R2 is amino-protecting group) is oxidized with oxygen in the presence of a photosensitizer in a polar solvent such as methanol, ethanol, acetonitrile, etc. to give a compound shown by the formula II. Preferably Rose Bengale, or methylene blue having good solubility in the polar solvent used in the above-mentioned reaction is used as the photosensitizer. Halogen-, tungsten-, sodium-lamp or mercury vapor lamp is used as a light irradiation lamp. An amount of the photosensitizer added is about 0.5W1wt% based on the raw material compound. The reaction is advanced smoothly by adding DMSO to it.