Thioamide pseudopeptides

9 Single Position Peptide Bond Modifications 7.9.1 Thioamide Pseudopeptides 

One of the most subtle peptide bond modifications is the insertion of a thioamide /[CSNH] group, in which the carbonyl oxygen has been replaced by a sulfur atom [125, 126], Thioamide formations became more easily accessible with the introduction of the thionating Lawesson s reagent. 

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Table 1 Examples of Thioamide Pseudopeptides and Their Physical Properties19171...

Peptide aldehydes can be synthesized via hydrolysis of thiazolidine precursors with mercury or copper salts (Scheme 10). 56 The Phe-thiazolidine derivative 27 was prepared from reduction of dihydrothiazole ring in 26, which was formed from a (5-hydroxy thioamide cyclization of 25 using the Mitsunobu reaction. N-Terminal Boc and Z groups on thiazolidine pseudopeptides such as Boc-Ala-Phe-thiazolidine and Z-Phe-Tyr-thiazolidine are stable to hydrolysis that affords Boc-Ala-Phe-H and Z-Phe-Tyr-H. 56 ... 

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