C(O[CH:5]1[C:22]2[C:17](=[CH:18][CH:19]=[CH:20][CH:21]=2)[CH:16](OC(=O)C)[CH:15]2[CH:6]1[C:7](=[O:36])[C:8]1[C:9]3([S:35][CH2:34][CH2:33][S:32]3)[CH2:10][CH:11]([C:28]([O:30][CH3:31])=[O:29])[CH2:12][C:13]=1[C:14]2=[O:27])(=O)C.[OH-].[Na+]>CO.O>[O:36]=[C:7]1[C:6]2[C:15](=[CH:16][C:17]3[C:22]([CH:5]=2)=[CH:21][CH:20]=[CH:19][CH:18]=3)[C:14](=[O:27])[C:13]2[CH2:12][CH:11]([C:28]([O:30][CH3:31])=[O:29])[CH2:10][C:9]3([S:32][CH2:33][CH2:34][S:35]3)[C:8]1=2A mixture of 26.5 mg (0.05 mmol) of methyl rac-1',2',3',4',5',5a',6',11',11a',12'-decahydro-6',11'-diacetoxy-5',12'-dioxospiro[1,3-dithiolane-2,4'-naphthacene]-2'-carboxylate and 5 mg (0.125 mmol) of sodium hydroxide in 2 ml of methanol was stirred at 20° C. for 1.5 hours. The mixture was then diluted with 10 ml of water and extracted with two 10 ml portions of dichloromethane. The combined dichloromethane extracts were washed with 10 ml of water, dried over magnesium sulphate and evaporated to dryness to give 15 mg (73%) of methyl rac-1',2',3',4',5',12'-hexahydro-5',12'-dioxospiro[1,3-dithiolane-2,4'-naphthacene]-2'-carboxylate in the form of a bright yellow solid of melting point 216° C.
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