Phosphoric Acid → 2-Iodohexadecanoic acid

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Procedure 3, Step A, for the preparation of methyl 16-=iodohexadecanoate (9), follows Takahashi et al, Appl. Radiat. Isot., 42, 801 (1991), incorporated herein by reference. To a mixture of phosphoric acid, (840 mg, 8.6 mmol) and phosphorus pentoxide (ca 450 mg), potassium iodide (868 mg, 5.2 mmol) and 16-hydroxyhexadecanoic acid (353 mg, 1.3 mmol), available from Aldrich Chemical Co., is added at room temperature and the mixture is heated at 110° C. for 3.5 h. The mixture is cooled to room temperature, and water (25 ml) and ether (120 ml) are added. The ether layer is separated, decolorized by shaking with 10% sodium thiosulfate solution (100 ml), washed with saturated NaCl aqueous solution (200 ml) , and evaporated to give crude 16-=iodohexadecanoic acid (38 mg). To a suspension of the sample in absolute MeOH (30 ml) thionyl chloride (6.5 ml) is added dropwise at 0° C. The mixture is stirred at 0° C. for 15 min and then refluxed for 30 min. After removal of the solvent, the residue is purified by preparative TLC (hexane/CH2Cl2 =7/3) to give 9 (26 mg). The procedure is performed on a 1-mole scale to provide enough material for Step C below.

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