O=P12OP3(OP(OP(O3)(O1)=O)(=O)O2)=O.OP(O)(O)=O.[C:20]([C:24]1[CH:29]=[CH:28][N:27]=[CH:26][CH:25]=1)(=O)[CH2:21][CH3:22].[NH:30]([C:32]1C=[CH:39][C:35]([C:36]([OH:38])=[O:37])=[CH:34][CH:33]=1)N>O>[N:27]1[CH:28]=[CH:29][C:24]([C:20]2[NH:30][C:32]3[C:22]([CH:21]=2)=[CH:39][C:35]([C:36]([OH:38])=[O:37])=[CH:34][CH:33]=3)=[CH:25][CH:26]=120 g of P2O5 were admixed with 12.5 ml of H3PO4 (85%), resulting in a strong increase of the temperature of the reaction mixture. The reaction mixture was then cooled to 60° C., and 8.90 g (65.84 mmol) of 4-propionylpyridine and 4.20 g (27.60 mmol) of 4-hydrazinobenzoic acid were added. The mixture was then stirred at 145° C. for 45 min. The reaction mixture was poured into water, resulting in the precipitation of the yellow product 2-pyridin-4-yl-1H-indole-5-carboxylic acid. This precipitate was filtered off with suction and washed with water until neutral. The 2-pyridin-4-yl-1H-indole-5-carboxylicacid, which was obtained by this method in quantitative yield, was used without further purification for coupling with amino acid derivatives.
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