To a solution of (8S,10S)-6,8,11-trihydroxy-8-(hydroxyacetyl)-1-methoxy-10-{[(1S,3R,4aS,9S,9aR,10aS)-9-methoxy-1-methyloctahydro-1H-pyrano[4′,3′:4,5][1,3]oxazolo[2,3-c][1,4]oxazin-3-yl]oxy}-7,8,9,10-tetrahydrotetracene-5,12-dione (50 mg, 0.078 mmol) [PNU-159682, compound IIA, prepared as reported in WO 9802446] in 2 ml of dry dimethylformamide kept under argon, (Cyclohex-1-enyloxy)-acetic acid ethyl ester (0.5 mL, [prepared as reported in J. Org. Chem. (1978) 43:1244-1245] and p-toluenesulfonic acid monohydrate (5 mg, 0.026 mmol) were added. The reaction mixture was stirred overnight at room temperature, sodium bicarbonate saturated solution was added (20 mL) and the product extracted with dichloromethane (2×20 mL). The combined organic phases were dried over anhydrous sodium sulfate, filtered, the solvent removed under vacuum and the residue partially purified by flash chromatography (DCM/MeOH 97.5:2.5) to give 30 mg (37%) of the ester intermediate that was used as such in step 2. MS (ESI): 826 [M+H]+. Retention time=7.48 min. method 2
