Synthesis of Prolylproline

Prolylproline has been synthesized by both classical peptide synthesis method utilizing tert-butoxycarbonyl or trifluoroacetyl protection of the NH group and carbodiimide-promoted peptide bond formation and by opening of the dioxopiperazine ring in octahydrodipyrrolo[1,2-a:1′,2′-d]pyrazine-5,10-dione obtained by thermolysis of proline methyl ester.