Tetrahydrofuran Cα-Tetrasubstituted Amino Acids: Two Consecutive β Turns in a Crystalline Linear Tripeptide

Abstract

The synthesis of tetrahydrofuran Cα-tetrasubstituted amino acids (TAAs) and their effect on the conformation 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 by alkylation. Simple base treatment in the presence of an aryl aldehyde leads to the formation of tetrahydrofuran 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-catalyzed coupling reactions allow a subsequent further functionalization of the TAA. The R,S,S-TAA-Ala dipeptide amide adopts a β-turn type I conformation, whereas its S,R,S isomer does not. The R,S,S-Gly-TAA-Ala tripeptide amide shows in the solid state and in solution a conformation of two consecutive β-turn type III structures, stabilized by i + 3 → i intramolecular hydrogen bonds.