Tetrahydrofuran → 5-Methylsulfonyltetrazole

To a stirred solution of 5-(methylthio)-1H-tetrazole (300 mg; Aldrich Chemical Company, Inc., Milwaukee, Wis.) in THF (15 mL) and water (15 mL) was added potassium peroxomonosulfate (Oxone®, DuPont Specialty Chemicals, Deepwater, N.J., USA) (3.18 g). After stirring at room temperature for five days, the reaction mixture was filtered and the filtercake was washed with THF. The filtrate was concentrated under reduced pressure. Crude NMR showed this to be a mixture of desired product and starting material, so the mixture was taken up in methanol and treated with potassium peroxomonosulfate (Oxone®, DuPont Specialty Chemicals, Deepwater, N.J., USA). After stirring at room temperature for five days, the reaction mixture was filtered and the filtercake was washed with methanol. The filtrate was concentrated under reduced pressure to afford 0.400 g of impure (I-11a) as a solid which was used in the next step without purification; m/z 146.9 (M−H)−.