Protection of the Thiol Group

A variant of this procedure is provided by the preparation of S-benzyl-l-cysteine (Expt 5.206). The required thiolate salt is prepared by the reductive cleavage with sodium in liquid ammonia of the disulphide linkage in the amino acid, (S)-cystine, and is alkylated in situ with benzyl chloride. The preparation of this S-benzyl derivative constitutes a method of protection of the thiol grouping in cysteine. 

Y. Wolmann in S. Patai, Protection of the thiol group, Vol. 15, S. 682, Interscience Publ., New York London 1974. 

Their characteristics (after protection of the thiols group by acetylation) are Mn = 5700-12,000 with polydispersity = 1.38-1.65, Taio = 216-248°C, Tg = 45-73°C, and the ability to form transparent films. Thus, polythiourethanes may be used as reactive polymers bearing the thiol groups which are sensitive to radical sources and applied to optical materials. 

The means of protection of various amino acid side chains shall not be dealt with here in any detail, as this is adequately described elsewhere (see General Reading). More important, familiarity with the chemistry of the protection methods for amino and carboxyl functions allows comprehension of the methodology developed for side chain protection. Often, similar or the same protecting groups may be used. As an example, consider the protection of the thiol group of cysteine and the amino functions of ornithine and lysine. 

The reversible protection of the thiol group by conversion to an unsymmetrical disulphide has been accomplished by the action of various sulphenyl derivatives on peptide- or protein-bound cysteine residues. Althou these derivatives are likely to be most useful when only one disu hide bond (or one or more thiol groups) are desired, recent experiments by several laboratories indicates the promise of these S-protective groups which include S-aryl [167], S-ethyl [13], and S-t-butyl mercapto groups [168]. 

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