Asymmetric transformation of (RS)-cysteine via formation of (RS)-4-thiazolidinecarboxylic acids.

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Either (R)- or (S)-cysteine ((R)- or (S)-Cys) was efficiently obtained from (RS)-Cys by the asymmetric transformation via formation of (RS)-4-thiazolidinecarboxylic acid ((RS)-THC) or (RS)-2,2-dimethyl-4-thiazolidinecarboxylic acid ((RS)-DMT) and by using (2R,3R)- or (2S,3S)-tartaric acid ((R)- or (S)-TA), as a resolving agent, in acetic acid. The asymmetric transformation was carried out by combination of crystallization of less soluble salt of (S)-THC or -DMT with (R)-TA (or salt of (R)-THC or -DMT with (S)-TA) and epimerization of soluble diastereomeric salt. The (R)- and (S)-THCs from the less soluble salts gave approximately optically pure (R)- and (S)-Cys’s, respectively, in 64% yield. The asymmetric transformation via formation of (RS)-DMT was more successfully achieved by adding 0.1 molar equivalent of salicylaldehyde; that is, hydrolysis of the obtained less soluble salt gave optically pure (R)- and (S)-Cys’s, respectively, in 80% yield based on the (RS)-Cys used as the starting material.