1,4-Etheno-1H-2,3-benzodiazepine-2,3-dicarboxylic acid,6-[[(1,3-dihydro-1,3-dioxo-2H-isoindol-2-yl)acetyl]oxy]-4,5-dihydro-4,7,8-trimethoxy-5-oxo-, diethyl ester