(2S)-2-(2-oxopyrrolidin-1-yl)butanamide (2S)-2-(2-oxo-1-pyrrolidinyl)butanamide (2S)-2-(2-ketopyrrolidin-1-yl)butyramide MLS000759403 SMR000466303 Levetiracetam Etiracetam levo-isomer Keppra XR L-059 SIB-S1 Torleva UCB-L059 LS-137054