Pseudo-prolines (ΨPro): direct insertion of ΨPro systems into cysteine containing peptides

The direct conversion of cysteine (Cys) containing peptides into conformationally constrained pseudo-proline (ΨPro) derivatives by intraresidual N,S-acetalisation has been achieved. This post-synthetic modification represents a versatile tool in structure–activity studies of bioactive peptides as exemplified for the immunosuppressive cyclosporine A (CsA) analogue [D-Cys]8CsA.