A solution of L-methionine (2.56 g, 17.2 mmol) in trifluoroacetic acid (15 mL) was added to a solution of benzyl (3R)-1-(2,4-dimethoxybenzyl)-2-oxo-6-phenyl-2,3,4,7-tetrahydro-1H-azepin-3-ylcarbamate (0.84 g, 1.72 mmol) in dichloromethane (20 mL). After 18 h, the mixture was concentrated and water was added. The mixture was extracted with ethyl acetate, washed with water (2×), saturated aqueous sodium bicarbonate (2×), saturated brine, dried over magnesium sulfate, filtered and concentrated. Purification by silica gel chromatography (5% ethyl acetate/hexanes→50% ethyl acetate/hexanes) gave the title compound (0.44 g). MS 337 (M+1).
