[O:1]=[CH:2][C:3]1[CH:11]=[CH:10][C:8]([OH:9])=[C:5]([O:6][CH3:7])[CH:4]=1.[Na+].[I-:13].OS(O)(=O)=O.II>[OH-].[Na+]>[I:13][C:10]1[C:8]([OH:9])=[C:5]([O:6][CH3:7])[CH:4]=[C:3]([CH:11]=1)[CH:2]=[O:1]Vanillin (28.4 g, 200 mmole (millimole)) was dissolved in 1N NaOH (200 ml) and warmed to 90° C. to avoid precipitation of sodium vanillate. A 2 molar aqueous solution of NaI3.NaI (105 ml, 210 mmole I2) plus 3.55 molar aqueous H2SO4 (5 ml) was added over 3 hours with stirring. The iodine color was discharged, and a pale tan stirrable precipitate formed. The solution was then cooled to room temperature, acidified to pH 2-3 with 20% aqueous H2SO4, and extracted with 10% methanol/90% chloroform. The organic phase was dried (MgSO4) and the solvent stripped to yield 53 g (99%) of 5-iodovanillin, more than 95% pure as analyzed by nuclear magnetic resonance spectroscopy (NMR).
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