2,​3-​Naphthalenedicarboxy​lic acid, 1-​(3,​4-​dihydroxyphenyl)​-​1,​2-​dihydro-​6,​7-​dihydroxy-​, 3-​[(1R)​-​1-​carboxy-​2-​(3,​4-​dihydroxyphenyl)​ethyl] 2-​[(1S)​-​1-​carboxy-​2-​(3,​4-​dihydroxyphenyl)​ethyl] ester, (1S,​2R)​-