Triethylamine → Methyl L-prolinate

To a solution of L-proline methyl ester hydrochloride (257 mg, 1.43 mmol) in MeOH was added triethylamine (0.2 mL, 1.43 mmol). The solution was concentrated under reduced pressure to give a white solid residue, which was triturated with ether (3×). The collected ether solution was concentrated under reduced pressure to give L-proline methyl ester free amine (220 mg) as a colorless oil. To a solution of above free amine in toluene (5 mL) was added compound 1A (278 mg, 1.3 mmol), followed by 4 Å molecular sieves (1.0 g). The mixture was stirred at room temperature overnight and filtered. The filtrate was diluted with EtOAc, washed with 1 N aqueous HCl, water, brine, dried over MgSO4, concentrated under reduced pressure to give a crude product, which was chromatographed (silica gel) eluting with CH2Cl2, 0.5% MeOH in CH2Cl2 to give the title compound (410 mg, 92% yield) as a yellow foam.