Protecting groups reduction-labile

Alternatively, both peptide chains could be protected at one cysteine residue as a 5-Acm derivative and at the second cysteine residue by an acid-labile [Trt, Mob, Xan, or Bzl(4-Me)], base-labile (Fm), or reduction-labile (5-tBu) group. Both peptide chains may then be separately converted into the free thiol/Acm-protected form for selective activation of one chain as S-SPy or. S -Npys derivatives by reaction with di(2-pyridyl)disulfide or di[5-nitro(2-pyridyl)]disulfide, or as a sulfenohydrazide derivative by reaction with azodicarbocylic acid derivatives for formation of the first interchain disulfide bridge. 

A scion of the small reductive-elimination orthogonal set is the phenacyl group which, like the 2,2,2-trichloroethyl ester (section 6.5.1), is useful for deprotect-ing the C-terminus of a peptide under conditions that retain such stalwart protecting groups as Fmoc, Boc, and Cbz (see section 8.3). Phenacyl esters are more labile towards nucleophilic attack than standard esters but they are stable towards acidic conditions typically employed for deprotection in peptide synthesis such as HF or trifluoroacetic acid. 

As an alternative to ammonium iodide/TFA, the use of titanium(IV) chloride/sodium iodide in TFA has been proposed for clevage of the acid-labile protecting groups and reduction of Met(O) residues.However, this reagent, besides bearing all the disadvantages of the use of heavy metals in peptides, leads to time-dependent reduction of disulfide bridges as well as to side products at the level of Trp residues. 

Under the same conditions, glycals are also formed from l-thioglycoside-2-xanthates. Phenyl thioglycopyranosides easily undergo reductive lithiation with lithium naphtalenide in tetrahy-drofuran at low temperatures and subsequent elimination of C-2 substituents [85,86]. Such reaction conditions are compatible with labile acid- and base-protecting groups and yields... 

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