The present invention relates to methods for synthesizing conformationally constrained peptides and cyclic peptidomimetics obtainable by these methods which are conformationally constrained due an internal cross-link. This cross-link is formed between the side chain of an amino acid residue or analog and a (2S, 4S)4-functionalized proline residue. The invention further relates to the use of (2S, 4S)-4-functionalized proline residues as building units in the synthesis of such peptidomimetics and to the use thereof as antigens, alone or in combination with suitable immunopotentiating delivery systems, for example immunopotentiating reconstituted influenza virosomes to elicit an immune response in a mammal. Moreover, the invention also relates to pharmaceutical compositions containing the same.