General procedure for the synthesis of N-(tert-butoxycarbonyl)-(S)-homoserine tert-butyl ester from N-Boc-L-aspartic acid 1-tert-butyl ester:
1b) Boc-L-Asp-OtBu (3.00 g, 10.4 mmol) was dissolved in 1,2-dimethoxyethane (10 mL) at -15°C and 4-methylmorpholine (1.14 mL, 10.4 mmol) and isobutyl chloroformate (1.35 mL, 10.4 mmol) were added sequentially. The reaction mixture was stirred at -15°C for 10 min before the precipitate was removed by filtration and the precipitate was washed with cold 1,2-dimethoxyethane (20 mL).
A solution of sodium borohydride (0.59 g, 15.6 mmol) in water (5 mL) was slowly added to the filtrate at -15°C. After 5 min, the reaction was terminated by adding water (250 mL). The reaction mixture was extracted with ethyl acetate, the organic layers were combined and dried over anhydrous sodium sulfate. The organic layer was concentrated under reduced pressure to give tert-butyl N-(tert-butoxycarbonyl)-(S)-homoserine.
Yield: 2.70 g, 9.81 mmol, 95% yield.
MS (ESIpos): m/z = 276 [M+H]+.
1H-NMR (400 MHz, chloroform-d): δ[ppm] = 1.46 (s, 9H), 1.48 (s, 9H), 1.51-1.58 (m, 1H), 2.09-2.20 (m, 1H), 3.45 (br.s., 1H), 3.55-3.76 (m, 2H), 4.31-4.40 (m, 1H). 5.35 (d, 1H).
