tert-Butyl {(3S)-6-benzyl-1-[(tert-butoxycarbonyl)oxy]-2-oxo-1,2,3,4-tetrahydroquinolin-3-yl}carbamate (81) To a solution of tert-butyl 3-benzyl-N-(diphenylmethylene)-6-nitro-L-phenylalaninate (79) (150 mg, 0.288 mmol) in dichloromethane was added trifluoroacetic acid in equal volume at 0° C., and the reaction was allowed to stir at RT for 18 h. Solvents were removed under reduced pressure, and the residue was taken in water (50 mL), and washed with EtOAc (3×50 mL). The aqueous layer was concentrated under reduced pressure to provide 3-benzyl-6-nitro-L-phenylalanine (80) as a solid (70.2 mg, 59%). LCMS m/z 301.0 (M+1). This crude product (70.2 mg, 0.234 mmol) was dissolved in a 1:1 mixture of THF and MeOH and treated with sodium acetate (325 mg, 2.34 mmol) and tin(II) chloride dihydrate (269 mg, 1.17 mmol) and stirred for 4 h at 0° C. To the reaction mixture was added triethylamine (0.33 mL, 2.4 mmol) and (BOC)2O (132 mg, 0.585 mmol), and the reaction was allowed to warm to RT and stir for 18 h. The reaction mixture was filtered, and the filtrate was washed with water (3×100 mL), washed with saturated aqueous sodium chloride solution, dried over sodium sulfate, filtered, and concentrated in vacuo. Purification via silica gel chromatography (Gradient: 0% to 80% EtOAc in heptane) yielded the product as a solid (40 mg, 36%). 1H NMR (500 MHz, CDCl3) δ 1.47 (br s, 9H), 1.56 (br s, 9H), 2.86-3.00 (m, 1H), 3.33-3.43 (m, 1H), 3.94 (s, 2H), 4.47-4.55 (m, 1H), 5.57 (br s, 1H), 6.9-7.1 (v br s, 1H), 7.04 (s, 1H), 7.11 (br d, J=8 Hz, 1H), 7.18 (d, J=7.6 Hz, 2H), 7.21-7.25 (m, 1H), 7.31 (dd, J=7.8, 7.3 Hz, 2H).
