| Abstract
| - The synthesis of tetrahydrofuran Cα-tetrasubstituted amino acids (TAAs) and their effect on theconformation in small peptides are reported. The synthesis starts from the protein amino acid methionine,which is protected at the C and N terminus and converted into the corresponding sulfonium salt byalkylation. Simple base treatment in the presence of an aryl aldehyde leads to the formation oftetrahydrofuran tetrasubstituted Cα-amino acids in a highly diastereoselective (trans/cis ratio up to 97:3)reaction with moderate to good yields (35−78%) depending on the aldehyde used. Palladium-catalyzedcoupling reactions allow a subsequent further functionalization of the TAA. The R,S,S-TAA-Ala dipeptideamide adopts a β-turn type I conformation, whereas its S,R,S isomer does not. The R,S,S-Gly-TAA-Alatripeptide amide shows in the solid state and in solution a conformation of two consecutive β-turn typeIII structures, stabilized by i + 3 → i intramolecular hydrogen bonds.
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