A solution of Boc2O (619 mg, 2.84 mmol) in acetonitrile (2 mL) was treated successively with a solution of 4-dimethylaminopyridine (33 mg, 0.27 mmol) in acetonitrile (2 mL) and a solution of 348 (365 mg, 2.7 mmol) in acetonitrile (2 mL). The reaction mixture was stirred vigorously for 10 min. After the addition of concentrated sulfuric acid in acetonitrile (40% solution, ρ= 1:03 g cm-3, 0.47 mL), the resulting mixture was stirred for 5 min. Water (0.47 mL) was then added, and the mixture was stirred for a further 5 min. The reaction mixture was then poured into an equal volume of water. The solution was extracted three times with hexane, and the combined hexane layers were dried over magnesium sulfate. Removal of the solvent in vacuo afforded mesityl isocyanate 349 (420 mg, 97%) as colorless crystals; mp 42 °C.
