Amino thiol group, protection

At pH 4-5, the reaction is selective for protection of thiol groups in the presence of oc- or e-amino groups. 

Postsynthetic intramolecular alkylation of a cysteine thiol group with a bromo amino acid has also been proposed/431 but this approach is limited by the low coupling efficiency of the bromo amino acid in the peptide synthesis due to a competing intramolecular cyclization reaction. A promising new approach consists of using suitably protected cysteine and homoserine derivatives for the peptide synthesis, whereby the side-chain hydroxy group of homoserine is protected as the TBDMS derivative which is converted on resin into the chloro derivative with freshly prepared triphenylphosphine dichloride. Upon cleavage from the resin, cyclization is performed in solution as shown in Scheme 11144]... 

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. 

N-Tritylamino acids. Direct reaction of trityl chloride with amino acids provides N-trityl derivatives in low yield. A superior route to these derivatives involves reaction of trimethylsilyl esters of N-trimethylsilylamino acids with trityl chloride in refluxing chloroform, which provides the N-protected amino acids in 88-92% yield. Trimethylsilyl-protected hydroxyl or thiol groups are not cleaved under these conditions.1... 

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