3-(1,3-Dihydro-4-nitro-1-oxo-2H-isoindol-2-yl)-2,6-piperidinedione; Lenalidomide - Impurity B ; 2,6-Piperidinedione, 3-(1,3-dihydro-4-nitro-1-oxo-2H-isoindol-2-yl)-; 3-(1-Oxo-4-nitro-1,3-dihydro-isoindol-2-yl) piperidine-2,6-dione;