5,5-Dimethylproline dipeptides: an acid-stable class of pseudoproline

Commercially available Fmoc-protected l-amino acids were employed and coupled to l-allylglycine. Cross metathesis with 2-methyl-2-butene using second generation Grubbs’ catalyst gave l-prenylglycine-containing dipeptides. Treatment with trifluoromethanesulfonic acid resulted in cyclisation and subsequent formation of acid-stable 5,5-dimethyl-l-proline dipeptides for direct insertion into linear peptide sequences.