(S)-3-(4-Thiazolyl)-2-tert-butoxycarbonylaminopropanoic acid (6)
5b (0.30 g, 1.4 mmol) and 6 N HCl (25 mL) were refluxed for 20 h and then cooled to rt. The reaction
mixture was concentrated in vacuo. The residue was washed with toluene to remove the benzoic acid. The
crude product (0.24 g) was dissolved in THF (25 mL) and adjusted to pH 9 with 3N NaOH solution.
Di-tert-butyl dicarbonate (0.23 g, 1.05 mmol) in THF (5 mL) was added to the mixture and stirred for 40
h at rt. The solvent was removed in vacuo and the residue was adjusted to pH 2 with 3 N HCl, and then
extracted with EtOAc (50 mL), washed with brine and dried using anhydrous MgSO4. The solvent was
concentrated in vacuo and the solid residue was recrystallized from acetone to give 6 (0.18 g, 57%) as a
solid. The enantiomeric purity was 99% ee. [(S)- tr 7.8 min., (R)- tr 9.6 min]. mp 122-123°C, lit.,2 mp
24
24
122-123°C, [α]D +11.0º (c 1.0, MeOH). [α]D +125.9º (c 1.0, CHCl3) lit.,2 [α]D +124.9 (c 1.04,
CHCl3), 1H NMR (CDCl3): δ 1.46 (s, 9H), 3.31-3.39 (m, 1H), 3.49-3.55 (m , 1H), 4.5 (m, 1H), 5.61-5.65
(m, 1H), 7.18 (s, 1H), 8.89-8.91 (m, 1H); IR νmax (CHCl3): 3430, 1710, 1505 cm-1; MS: m/z 273 (9%,
M++1), 259 (4), 257 (4), 217 (90), 199 (100), 173 (28), 153 (6), 127 (50), 111 (3), 99 (23), 71 (18).
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