Sulfuric Acid → tert-Butyl 5-oxo-L-prolinate

A pressure flask is charged with (S)-(−)-2-pyrrolidone-5-carboxylic acid (25 g, 193.6 mmol) and dichloromethane (100 mL) and the stirred suspension is cooled in a dry ice acetone bath. Liquid 2-methylpropene (150 mL) is added followed by concentrated sulfuric acid (0.5 mL). The pressure flask is sealed and the reaction mixture is stirred at room temperature for 48 h. The pressure flask is cooled in a dry acetone bath and the reaction mixture is diluted with dichloromethane (500 mL) and treated with a saturated solution of sodium bicarbonate until the effervescence is ceased. The organic layer is separated, washed with saturated solution of sodium bicarbonate (300 mL), brine (300 mL), dried over anhydrous Na2SO4 and solvent removed in vacuo to give the title compound as a white crystalline solid. The crude material is used without purification.