O=P12OP3(OP(OP(O3)(O1)=O)(=O)O2)=O.P(=O)(O)(O)[OH:16].[CH3:20][O:21][C:22]1[CH:23]=[C:24]([C:30]2[CH:38]=[C:37]([O:39][CH3:40])[C:36]([O:41][CH3:42])=[CH:35][C:31]=2[C:32](O)=O)[CH:25]=[CH:26][C:27]=1[O:28][CH3:29]>O>[CH3:29][O:28][C:27]1[C:26](=[O:16])[C:25]2[C:24](=[CH:23][C:22]=1[O:21][CH3:20])[C:30]1[C:31](=[CH:35][C:36]([O:41][CH3:42])=[C:37]([O:39][CH3:40])[CH:38]=1)[CH:32]=2Polyphosphoric acid which had been newly prepared from diphosphorus pentaoxide (50 g) and phosphoric acid (50 ml) was heated at 100° C. With stirring, 2-(3,4-dimethoxyphenyl)-4,5-dimethoxybenzoic acid (7.5 g) was added thereto as it is (in the crystal form) in several portions. After stirring at 100 to 120° C. for 2 hours, the reaction solution was poured into water (1.5 liter) and extracted with chloroform. The extract was washed with 2% aqueous sodium hydroxide solution, water and a saturated saline solution, then dried over sodium sulfate. The solvent was distilled off, and the residue was purified by silica gel column chromatography (eluent: chloroform) and recrystallized from ethyl acetate-n-hexane to give 5.0 g of 2,3,6,7-tetramethoxyfluorenone as an orange needle-like crystal.
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