Asymmetric syntheses of α-mercapto carboxylic acid derivatives by dynamic resolution of N-methyl pseudoephedrine α-bromo esters

Dynamic resolution of N-methyl pseudoephedrine α-bromo esters in nucleophilic substitution reaction with trityl thiol has been successfully used for the asymmetric preparation of α-mercapto carboxylic acid derivatives up to 97:3 dr. The best results are obtained when α-bromo esters are allowed to equilibrate to the thermodynamic ratios before the addition of sulfur nucleophile. We have shown that the chiral auxiliary can be removed by both reductive cleavage and acidic alcoholysis to provide β-tritylthio alcohol 15 and α-tritylthiolated ester 16, respectively, without detectable racemization.