A Non-coded β2,2-Amino Acid with Isoxazoline Core Able to Stabilize Peptides Folding through an Unprecedented Hydrogen Bond

Dedicated to Prof. Cesare Gennari on the occasion of his 70th birthday New peptidomimetics containing a beta(2,)(2)-isoxazoline amino acid, i.e. 5-(aminomethyl)-3-phenyl-4,5-dihydroisoxazole-5-carboxylic acid (Isox-beta(2,)(2)AA), were prepared and studied by NMR and theoretical calculation. Although similar amino acid derivatives have already been prepared via 1,3-dipolar cycloaddition reaction, neither experimental details nor characterization were found and they were never used for peptide synthesis. Both enantiomers were inserted in peptide sequences to verify their ability to induce a secondary structure. We found that an unexpected conformation is given by R-Isox-beta(2,)(2)AA, inducing the folding of short peptides thanks to an unprecedented H-bond involving C=N of the isoxazoline side chain of our beta(2,)(2)-AA.