[1,​4'-​Bipiperidine]​-​1'-​carboxylic acid, (4S)​-​4,​11-​diethyl-​3,​4,​12,​14-​tetrahydro-​4,​12-​dihydroxy-​3,​14-​dioxo-​1H-​pyrano[3',​4':6,​7]​indolizino[1,​2-​b]​quinolin-​9-​yl ester