Thiols protection-deprotection reactions

A general procedure for the iodine reaction in the solid phase is shown in Scheme 9. Both Boc and Fmoc chemistry can be used to assemble the linear S-protected bis-cysteine peptides and for thiol protection the well-established Acm and Trt groups are usually used. Suitable solvents for the thiol-deprotection/oxidation step by iodine to form the disulfide are CH2C12, DMF, or aqueous AcOH. The final deblocking and cleavage from the resin is carried out under standard conditions. Modification at sensitive amino acid residues caused... 

Figure 1.69 SAMSA is an anhydride compound containing a protected thiol. Reaction with protein amine groups yields amide bond linkages. Deprotection of the acetylated thiol produces free sulfhydryl groups for conjugation.

Figure 20.6 Available amine groups on an antibody molecule may be modified with the NHS ester end of SATA to produce amide bond derivatives containing terminal protected sulfhydryls. The acetylated thiols may be deprotected by treatment with hydroxylamine at alkaline pH. Reaction of the thiolated antibody with a maleimide-activated enzyme results in thioether crosslinks.

The general strategy is based on selective deprotection and oxidation of pairwise cysteine residues, as specified by the orthogonal protection scheme selected for this purpose. Since multiple disulfide bonds are formed step-by-step, reaction conditions are required that prevent breaking or scrambling of the disulfide bonds already formed. Therefore, throughout the synthesis, exposure to thiols or alkaline conditions as well as lengthy reaction times that may cause disulfide disproportionation must be avoided. 

Peptides with C-terminal phosphonates, initially reported to have antibacterial properties, have also been found to possess inhibitory properties toward serine proteases)28 The synthesis of peptide phosphonates (Section 15.1.8) usually requires protection of the phos-phonic moiety as a diester, followed by selective deprotection in the final stage. The importance of peptide thiols (Section 15.1.9) is exemplified by captopril, an orally active angiotensin converting enzyme inhibitor used as a treatment for hypertension)29 These peptide thiols are prepared by the reaction of sulfanylalkanoyl amino acids with a-amino esters followed by deprotection of carboxy and sulfanyl groups. Other peptide thiols have been reported to be inhibitors of zinc metalloproteases, collagenases, and aminopeptidases. 

Carboxy terminal amino acid or peptide thiols are prepared from various p-amino alcohols by conversion into a thioacetate (R2NHCHR1CH2SAc) via a tosylate followed by saponification.Several methods have been used to prepare N-terminal peptide thiols, the most common procedure is the coupling of (acetylsulfanyl)- or (benzoylsulfanyl)alkanoic acids or add chlorides with a-amino esters or peptide esters, followed by deprotection of the sulfanyl and carboxy groups. 8 16 Other synthetic methods include deprotection of (trit-ylsulfanyl)alkanoyl peptides, 1718 alkaline treatment of the thiolactones from protected a-sulfanyl acids, 19 and preparation of P-sulfanylamides (HSCH2CHR1NHCOR2, retro-thior-phan derivatives) from N-protected amino acids by reaction of P-amine disulfides with carboxylic acid derivatives, followed by reduction. 20,21 In many cases, the amino acid or peptide thiols are synthesized as the disulfides and reduced to the corresponding thiols by the addition of dithiothreitol prior to use. 

Final purification by use of metal complexes was also applied in the syntheses of the ligands XS4—H4. These ligands exclusively contain thiolate donors and were prepared by Hahn et al. (23) using 2,3-dimercaptobenzoic acid as starting material (Scheme 8). Isopropyl or benzyl protection of the thiol functions, conversion into the acyl chlorides, reaction with a,oo-diamines, and deprotection of the sulfur atoms enabled the connection of two 1,2-benzene-dithiol units via carboxylic acid amide bonds. 

---