Propanoic acid, 3-​[2-​[2-​[2-​[2-​[[2-​(2,​6-​dioxo-​3-​piperidinyl)​-​2,​3-​dihydro-​1,​3-​dioxo-​1H-​isoindol-​4-​yl]​amino]​ethoxy]​ethoxy]​ethoxy]​ethoxy]​-