C[O:2][C:3](=[O:23])[CH2:4][C:5]1([CH2:21][CH3:22])[C:10]2[NH:11][C:12]3[C:17]([C:9]=2[C:8](=[O:20])[CH2:7][O:6]1)=[CH:16][CH:15]=[CH:14][C:13]=3[CH2:18][CH3:19].C([O-])([O-])=O.[K+].[K+]>CO.O>[CH2:21]([C:5]1([CH2:4][C:3]([OH:23])=[O:2])[C:10]2[NH:11][C:12]3[C:17]([C:9]=2[C:8](=[O:20])[CH2:7][O:6]1)=[CH:16][CH:15]=[CH:14][C:13]=3[CH2:18][CH3:19])[CH3:22]1,8-Diethyl-1,3,4,9-tetrahydro-4-oxopyrano[3,4-b]indole-1-acetic acid methyl ester 17 (1.5 g, 4.8 mmol) was suspended in 30 mL of MeOH and a solution of K2CO3 (3.5 g) in 30 mL of H2O was added. The mixture was heated to reflux, upon which the solution became homogeneous. After refluxing for 2 hours, the solution was cooled to room temperature, and the MeOH removed in vacuo. The aqueous solution was made acidic with 6N HCl, and the resulting cloudy solution was extracted with ether (2×50 mL). The ether layer was washed with 25 mL of saturated NaCl solution, then dried over MgSO4. The MgSO4 was removed and the ether was evaporated. Hexane was gradually added until a slight turbidity was observed. The solution was left at 0° C. overnight and the resultant solid was collected by filtration and dried to provide analytically pure product (1.31 g, 93%), m.p. 200°-202° C.
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