Synthesis of all-L cyclic tetrapeptides using pseudoprolines as removable turn inducers.

Cyclic tetrapeptides have generated great interest because of their broad-ranging biological properties. In order to synthesize these highly strained 12-membered cyclic compounds, a cyclization strategy using pseudoprolines as removable turn inducers has been developed. The pseudoproline derivatives induce a cisoid amide bond in the linear peptide backbone which facilitates cyclization. After cyclization, the turn inducers can be readily removed to afford cyclic tetrapeptides containing serine or threonine residues.