(αS,3S,8aS)-Hexahydro-3-methyl-1,4-dioxo-α-(2-phenylethyl)pyrrolo[1,2-a]pyrazine-2(1H)-acetic Acid; (S)-2-((3S,8aS)-3-Methyl-1,4-dioxohexahydropyrrolo[1,2-a]pyrazin-2(1H)-yl)-4-phenylbutanoic Acid; (S)-2-((3S,8aS)-3-Methyl-1,4-dioxohexahydropyrrolo[1,2-a]pyrazin-2(1H)-yl)-4-phenylbutanoic acid; (2S)-2-[(3S,8aS)-3-methyl-1,4-dioxo-6,7,8,8a-tetrahydro-3H-pyrrolo[1,2-a]pyrazin-2-yl]-4-phenylbutanoic acid