A three-neck flame-dried flask equipped with a nitrogen inlet and internal thermometer was charged with methyl N-(tert-butoxycarbonyl)-3-[(3S)-2-oxopyrrolidin-3-yl]-l-alaninate (10 g, 35 mmol), THF (200 mL), and chloroiodomethane (10.2 mL, 140 mmol), and the solution was cooled to −77 °C.