[CH:1]1([C:6]2[CH:7]=[C:8]3[C:13](=[CH:14][CH:15]=2)[C:12](=[O:16])[NH:11][C:10](=[O:17])[CH2:9]3)[CH2:5][CH2:4][CH2:3][CH2:2]1.[CH3:18][O:19][CH:20](OC)OC>C(O)(=O)C>[CH:1]1([C:6]2[CH:7]=[C:8]3[C:13](=[CH:14][CH:15]=2)[C:12](=[O:16])[NH:11][C:10](=[O:17])[C:9]3=[CH:18][O:19][CH3:20])[CH2:2][CH2:3][CH2:4][CH2:5]1A mixture of 6-cyclopentyl-4H-isoquinoline-1,3-dione (222 mg, 0.97 mmole), 10 mL of acetic acid and trimethylorthoformate (212 mg, 2.0 mmole) is stirred and heated to 90° C. After 2 hours at that temperature the reaction mixture is cooled and the solvents were removed in-vacuo and the residue taken up in 4% methanol in dichloromethane, passed through a short pad of Florisil and eluted with 4% methanol in dichloromethane. The eluate is evaporated and the product is treated with 4:1 hexanes-ethyl acetate and collected by filtration to give a yellow solid, 165 mg, (62%), MS (ES+): m/z 272.2 (M+H).
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