[C:1]1(S(O)(=O)=O)C=CC=CC=1.[C:11]([OH:42])(=[O:41])[CH2:12][CH2:13][C@H:14]([NH:18][C:19]([C:21]1[CH:40]=[CH:39][C:24]([NH:25][CH2:26][C@@H:27]2[NH:38][C:37]3[C:35](=[O:36])[NH:34][C:32]([NH2:33])=[N:31][C:30]=3[NH:29][CH2:28]2)=[CH:23][CH:22]=1)=[O:20])[C:15]([OH:17])=[O:16].[OH-].[Na+].C=O.S(=O)(=O)(O)O>O>[CH2:1]1[N:25]([C:24]2[CH:23]=[CH:22][C:21]([C:19](=[O:20])[NH:18][C@H:14]([C:15]([OH:17])=[O:16])[CH2:13][CH2:12][C:11]([OH:42])=[O:41])=[CH:40][CH:39]=2)[CH2:26][C@@H:27]2[N:38]1[C:37]1[C:35](=[O:36])[NH:34][C:32]([NH2:33])=[N:31][C:30]=1[NH:29][CH2:28]2100 g of pure (6S)-tetrahydrofolic acid benzenesulfonate prepared, for example, by the method described in Swiss Patent Application No. 108, filed January 16, 1991, are suspended in water. The pH is brought to 7.5 by addition of I N sodium hydroxide solution. 15 ml of 37% strength aqueous formaldehyde solution are then added, and the mixture is adjusted to pH 2.5 with 2 N sulfuric acid. The 5,10-methylene-(6R)-tetrahydrofolic acid formed precipitates and is filtered off.
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