To a solution of 11-acetyl-8-carbamoyloxymethyl-4-formyl-14-oxa-1,11-diazatetracyclo[7.4.1.02,7.010,12 ]tetradeca-2,4,6-trien-6,9-diyl diacetate (29 mg) in acetone (2 ml) was added a mixture of chromium trioxide (26 mg), water (76 μl) and sulfuric acid (22 μl) in an ice-water bath. After stirring for 30 minutes in an ice-water bath, isopropyl alcohol (2 ml) was added to the reaction mixture. The resultant mixture was then poured into water (10 ml) and extracted with a mixture of chloroform and methanol (2:1, v/v) (10 ml×5). The extracts were combined and dried over magnesium sulfate, and then evaporated in vacuo. The residue was subjected to preparative thin layer chromatography, which was developed with a mixture of chloroform, methanol and water (65:25:4, v/v) to afford 6,9-diacetoxy-8-carbamoyloxymethyl-14-oxa-1,11-diazatetracyclo[7.4.1.02,7.010,12 ]tetradeca-2,4,6-triene-4-carboxylic acid (5 mg).
