Sulphenyl Protecting Groups

Protection of carbohydrate hydroxyl functions by 0-(2,4-dinitrobenzene-sulphenyl)ation has been studied the group is introduced in better than 70% yield for (154 R = H), and the 5,6-isopropylidene grouping of (154  

4-dinitrobenzenesulphenyl) can be selectively removed by acid hydrolysis the sulphenate is cleaved by Al-HgOAc-MeOH reduction or by hydrogenation over Raney nickel.  

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To obtain block copolymers, we copolymerize successively the NCAs of the two a amino acids. At the end of the polymerization of the first block we protect the NHp terminus by an D-nitrophenyl-sulphenyl group to allow its fractionation and its characterization. After elimination of the protecting group we polymerize the NCA of the second amino acid. 

The reaction of sulphenyl halides with thioethers seems to be a general procedure for the cleavage of a thiol protecting group, provided that a stable cation could be ejected from the thiosulphonium ion intermediate. Thus the thioether linkages between the haem group and the cysteine... 

Dinitrobenzenesulphenyl and o-nitrobenzhydryl esters are carboxyl protective groups which can be removed by photolysis under mild and completely neutral conditions and give the carboxylic acid in high yield (85—98%) [47, 124]. The major by-product in the decomposition of the 2,4-dinitrobenzene-sulphenyl ester (45) is the sulphide (47) formed by electrophilic attack of the cation (46) on the solvent. o-Nitrosobenzo-phenone (48) is formed as the by-product in the decomposition of the nitrobenzhydryl ester. 

Although sulphenyl iodides are generally too reactive to be utilized for protective groups, the conversion of thiols to sulphenyl iodides by the action of iodine or iodine monochloride has been useful in certain situations [164, 165]. The reversible protection of a thiol group in lactic dehydrogenase was also accomplished [166] by treatment of the protein with mercury(ll) chloride. After chemical modification of a histidine residue the chloromercury group was removed by treatment with cysteine. The insoluble nature of these derivatives makes the heavy metal S-protective groups potentially less useful for smaller molecules. 

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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