Synthesis of 3-Benzylhexahydropyrrolo[1,2-a]pyrazine-1,4-dione

3-Benzylhexahydropyrrolo[1,2-a]pyrazine-1,4-dione [cyclo(Pro-Phe)] was synthesized by cyclization of prolylphenylalanine and phenylalanylproline methyl esters which were prepared from the corresponding Boc-protected amino acids.