Abstract
The two model peptides Boc-S-Ala-Aib-S-Ala-OMe ( 1a ) and Ac-S-Ala-Aib-S-Ala-OMe ( 1b ) and their monothiated analogues Boc-S(R)-AlaΨ(CSNH)-Aib-S-Ala-OMe ( 3a ), Boc-S-Ala-AibΨ(CSNH)-S-Ala-OMe ( 4a ), AcΨ(CSNH)-S-Ala-Aib-S-Ala-OMe ( 2b ), Ac-S-Ala-AibΨ(CSNH)-S-Ala-OMe ( 4b ),and the dithiated Boc-S-AlaΨ(CSNH)-AibΨ(CSNH)-S-Ala-OMe ( 5a ) are synthesized. Peptide 3a was obtained from the coupling of HCl·H-S-Ala-OMe to 5-2-(1-( tert-butoxycarbonyl-amino) ethyl-4,4-dimethyl-1,3-thiazol-5(4 H)-one ( 11 ). The thioamide analogues 4a (together with 5a ) and 2b were obtained by regioselective thiation of the respective model peptides 1a and 1b using 2,4-bis(4-methylphenyl)-1,3,2,4-dithiadiphosphetane 2,4-disulfide, Lawesson's Reagent (LR). Deprotection of the Boc group of 4a followed by acetylation of the product, afforded 4b . The magnetic nonequivalence of the gem-methyl groups of Aib is discussed.
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