Protection of thiols

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

Protection of thiols. The r-butyl group has been of limited service for protection of thiols because removal has involved rather drastic treatment with liquid HF. r-Butyl thioethers can be cleaved by treatment with o-nitrobenzene-sulfenyl chloride in acetic acid at 25° to give S-o-nitrophenylsulfeny derivatives. These products are reduced to thiols with NaBH4, HSCH2C02H, or HOCH2CH2SH. 

A very convenient method has been devised [40] for the conversion of thiols to ethyldithio derivatives as a routine procedure for protection of thiol-substituted organic acids. It uses a DMAP-catalysed exchange of the ethylthio group between a thiol and ethyl 2-pyridyl disulfide, prepared from the commercially available 2,2 -dithiobispyridine. Filtration through a macroporous sulfonic acidic resin and evaporation of the solvent yielded the disulfide directly. 

Protection of thiol groups is a substantive issue in peptide chemistry where protection and deprotection of cysteine and the attendant problem of disulfide bond formation is a major challenge. An authoritative review of the subject by Moroder and co-workers can be found in Synthesis of Peptides and Peptidomi-metics (Houben-Weyl), Vol E22a. ... 

A number of other groups can be used for the protection of thiols benzyl thioethers of amino acids and peptides have been cleaved electrochemically at a platinum cathode in liquid ammonia [121], at mercury in MeOH-TMACl [122], and in DMF [123,124], and the trityl group has been used to protect cysteine cysteine was recovered in 90% yield after reduction in DMF [124]. The 4-pyridylmethyl [125,126] and diphenyl-4-pyridylmethyl groups can be cleaved in acid solution [124] at a mercury cathode. 

Protection of thiol groups. Under catalysis by a base (N-methylmorpholine) the reagent reacts with cysteine groups ... 

Protection of thiol groups. Japanese chemists have used the 2-picolyl 1-oxide group for protection of thiols. The protective group is removable by acetic... 

Protection of Thiols as Disulfides. The protection of free thi ols as unsymmetrical ethyl disulfides can be efficlentiy performed with 5-ethyl ethanethiosulfinate In high yield. The advantages of this protocol are (I) It operates with a very inexpensive starting material (ii) it can be performed on a wide variety of substrates such as protected and unprotected thiol containing amino acids... 

In addition to the functional gronps already mentioned, aUyl-based protecting group schemes have been developed for the protection of thiols, especially the side chain thiol of the cysteine residne in peptide synthesis. Becanse aUyl snllides are not snsceptible to Pd-mediated allyl group deprotection and allyl thiocarbonates are nnstable toward nucleophilic attack, the aUoxycarbonylmethyl (Allocam) group was introduced for aUyl-based... 

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