Ketalization of chartreusin (I) with benzaldehyde (II) using p-toluenesulfonic acid in the presence of molecular sieves, yielded the corresponding 3',4'-O-benzylidene derivative as a mixture of endo- and exo-isomers, which were separated by column chromatography. The required exo-isomer (III) was then esterified with 3-ethoxypropionic acid (IV) by means of DCC, producing the title ester.