Peptide-based countermeasures

In a sense this is a perfect example of orthogonal protection schemes in which two different chemical reactions are applied, one for the removal of the transient and the other for the cleavage of the semipermanent blocking groups. Unfortunately this combination can not be readily utilized in the synthesis of peptides with sulfur containing amino acid residues poisoning of the catalyst impedes the process. It is possible to employ countermeasures, such as addition of base or the use of liquid ammonia as solvent, when methionine residues are the source of difficulties, but cysteine residues present a less readily surmountable obstacle. 

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