β-Turn modulation by the incorporation of c6Ser into Xaa-Pro dipeptide

The enantiomerically pure (1R,2S)- and (1S,2R)-1-amino-2-hydroxycyclohexane-1-carboxylic acids (c6Ser) 1 and 2 were incorporated into dipeptide Xaa-Pro by a resolution method, involving the aminolysis of a racemic 2-tert-butyl-5(4H)-oxazolone intermediate with the hydrochloride derivative of l-Pro-NHMe. Once the model dipeptides Piv-(1R,2S)-c6Ser-l-Pro-NHMe (6) and Piv-(1S,2R)-c6Ser-l-Pro-NHMe (7) were formed and separated, their X-ray analysis allowed the characterisation of a non-folded structure for peptide 6 and an unexpected type I β-turn with a trans-Pro at the i+2 position for peptide 7.