Abstract
A number of N-acetyl-S-glycosyl cysteine derivatives have been prepared through the development of a general and simply applicable synthetic pathway, by modifying existing literature methods. The coupling of N-acetyl-L-cysteine and a carbohydrate is desirable as it may improve the efficacy of the labile N-acetyl-L-cysteine as a drug. The S-glycosyl cysteines prepared are as follows: N-acetyl-S-D-glucopyranosyl-L-cysteine, alpha and beta-anomers, N-acetyl-S-beta-D-ribopyranosyl-L-cysteine, N-acetyl-S-alpha-D-mannopyranosyl-L-cysteine and N-acetyl-O-methyl-S-(2,3,4,6-tetra-O-benzoyl-beta-D-galactopyranosyl)-L-cysteine. The coupling reaction was designed to yield both a and beta-anomers in the same step, and this was observed in the synthesis of the glucose derivatives. However, the other carbohydrates chosen appear to couple more selectively. The preparation of N-acetyl-O-methyl-S-alpha-D-glucopyranosyl-L-cysteine was also carried out by a different method, but this proved to be more involved and resulted in lower yields. The stability of N-acetyl-S-beta-D-glucopyranosyl-L-cysteine towards the thioglycosidase enzyme myrosinase was studied. N-acetyl-S-beta-D-glucopyranosyl-L- cysteine was found to be stable to hydrolysis by myrosinase, but some inhibition of a standard sinigrin-myrosinase hydrolysis was observed. The thioglycosidic linkage (-SH) of N-acetyl-S-beta-D-glucopyranosyl-L-cysteine is stable to hydrolysis in acidic media, which is contrary to previous work reported in the literature. However, the hydrolysis of the amide group of the cysteine side chain occurred in acidic solution giving the apparently acid-stable S-beta-D-glucopyranosyl-L-cysteine. A kinetic study of this acetyl cleavage was undertaken and a second order rate constant of 5.96 +/- 0.24 x 10-4 dm3 mol-1 hours-1 was obtained.