To a 3 L multinecked flask equipped with an overhead stirrer, nitrogen inlet, and internal thermometer was charged N-(tert-butoxycarbonyl)-3-[(3S)-2-oxopyrrolidin-3-yl]-l-alanine (190 g, 698 mmol) followed by dichloromethane (1100 mL) and the solution was cooled to 0 °C employing an ice/NaCl bath.