[(S)-2-((S)-3-Benzyl-1,4-dioxo-hexahydro-pyrrolo[1,2-a]pyrazin-2-yl)-1-benzyloxymethyl-2-oxo-ethyl]-carbamic acid tert-butyl ester

[(S)-1-((S)-3-Benzyl-1,4-dioxo-hexahydro-pyrrolo[1,2-a]pyrazine-2-carbonyl)-2-methyl-propyl]-carbamic acid tert-butyl ester