Synthesis and conformation of fluorinated β-peptidic compounds

Experimental and theoretical data indicate that, for α-fluoroamides, the F-C-C(O)-N(H) moiety adopts an antiperiplanar conformation. In addition, a gauche conformation is favoured between the vicinal C-F and C-N(CO) bonds in N-β-fluoroethylamides. This study details the synthesis of a series of fluorinated β-peptides (1-8) designed to use these stereoelectronic effects to control the conformation of β-peptide bonds. X-ray crystal structures of these compounds revealed the expected conformations: with fluorine β to a nitrogen adopting a gauche conformation, and fluorine α to a C=O group adopting an antiperiplanar conformation. Thus, the strategic placement of fluorine can control the conformation of a β-peptide bond, with the possibility of directing the secondary structures of β-peptides.