반응 단계
To a solution of (S)-3-methyl-hex-4-enoic acid ethyl ester* (1.0 g, 6.4 mmol) in 30 mL toluene at −78° C. was added DIBAH (1.0M in THF, 6.4 mL) dropwise over 5 min. The mixture was stirred at −78° C. 45 min at which time 5 drops of methanol were added, resulting in vigorous H2 evolution. Methanol was added until no more gas evolution was observed (ca. 5 mL). At this time the cold bath was removed and ca. 5 mL of sat. aq. Na+ K+ tartrate was added. When the mixture reached room temperature, additional sat. aq. Na+ K+ tartrate and Et2O were added and stirring was continued until the phases were mostly clear (ca. 1 h). The phases were separated, and the organic phase washed with brine, dried (MgSO4), and concentrated to ca. 10 mL total volume owing to volatility concerns. The crude mixture was combined with an additional batch of aldehyde prepared from 10 mmol of the ester by the method described above and the whole used without purification. To a suspension of sodium hydride (60% dispersion in mineral oil) in 25 mL THF was added t-butyl-P,P-dimethylphosphonoacetate (3.0 mL, 15 mmol) dropwise over 1 h such that the evolution of H2 was under control. After the addition was complete, the crude aldehyde in toluene (ca. 20 mL total volume) was added quickly dropwise and the mixture stirred at ambient temperature overnight. The mixture was partitioned between Et2O and sat. aq. NH4Cl, the phases separated, the organic phase washed with brine, dried (MgSO4), and concentrated. Flash chromatography of the residue (0→3→5% EtOAc/hexanes) afforded 1.0 g (29%, two steps) of the unsaturated ester as a pale yellow oil: 1H NMR (CDCl3) δ 6.75 (m, 1H), 5.66 (m, 1H), 5.30 (m, 2H), 2.03-2.29 (m, 3H), 1.58 (d, J=6.1 Hz, 3H), 1.41 (s, 9H), 0.91 (d, J=6.6 Hz, 3H). *(S)-3-methyl-hex-4-enoic acid ethyl ester was prepared from (S)-trans-3-Penten-2-ol [Liang, J.; Hoard, D. W.; Van Khau, V.; Martinelli, M. J.; Moher, E. D.; Moore, R. E.; Tius, M. A. J. Org. Chem., 1999, 64, 1459] via Johnson-Claisen rearrangement with triethylorthoacetate according to the literature protocol [Hill, R. K.; Soman, R.; Sawada, S., J. Org. Chem., 1972, 37, 3737]. (3R,5S)-3-[Benzyl-(1-phenyl-ethyl)-amino]-5-methyl-oct-6-enoic acid tert-butyl ester. To a solution of (S)-(−)-N-benzyl-α-methylbenzylamine (0.60 mL, 2.85 mmol) in 9.0 mL THF at −78° C. was added r-butyllithium (1.6M in hexanes, 1.6 mL) quickly dropwise resulting in a deep pink color. The mixture was stirred at −78° C. for 30 min at which time (5S)-5-Methyl-octa-2,6-dienoic acid tert-butyl ester (0.5 g, 2.38 mmol) in 1.0 mL THF was added slowly dropwise, resulting in a pale tan color which darkened over 3 h. The mixture was stirred 3 h at −78° C., then quenched with sat. aq. NH4Cl. The mixture was allowed to warm to rt and stirred overnight, then partitioned between EtOAc and sat. aq. NH4Cl. The phases were concentrated, and the organic phase dried (MgSO4), and concentrated. Flash chromatography of the residue (3→5% EtOAc/hexanes) provided 0.52 g (52%) of the aminoester as a yellow oil. 1H NMR (CDCl3) δ 7.34 (m, 2H), 7.20 (m, 8H), 5.27 (m, 2H), 3.74 (m, 1H), 3.72 (d, J=15.9 Hz, 1H), 3.41 (d, J=14.9 Hz, 1H), 3.27 (m, 1H), 2.38 (m, 1H), 1.98 (dd, J=3.7, 14.2 Hz, 1H), 1.81 (dd, J=9.3, 14.4 Hz, 1H), 1.54 (d, J=4.9 Hz, 3H), 1.32 (s, 9H), 1.24 (d, J=7.1 Hz, 3H), 0.99 (m, 2H), 0.74 (d, J=6.6 Hz, 3H). (3S,5R)-3-Amino-5-methyl-octanoic acid. To a solution of (3R,5S)-3-[Benzyl-(1-phenyl-ethyl)-amino]-5-methyl-oct-6-enoic acid tert-butyl ester (0.92 g, 2.18 mmol) in 50 mL MeOH was added 20% Pd/C (0.20 g), and the mixture was hydrogenated in a Parr shaker at 48 psi for 23 h. The mixture was filtered and concentrated. To the crude aminoester in 10 mL CH2Cl2 was added 1.0 mL trifluoroacetic acid, and the solution stirred at ambient temperature overnight. The mixture was concentrated, and the residue dissolved in the minimum amount of H2O, and loaded onto DOWEX-50WX8-100 ion exchange resin. The column was eluted with H2O until neutral to litmus, then continued with 5% aq. NH4OH (100 mL). The alkaline fractions were concentrated to provide 0.25 g (66%, two steps) of the amino acid as an off-white solid. 1H NMR (CD3OD) δ 3.41 (m, 1H), 2.36 (dd, J=5.1, 16.6 Hz, 1H), 2.25 (dd, J=8.1, 16.6 Hz, 1H), 1.42 (m, 2H), 1.24 (m, 1H), 1.12 (m, 2H), 1.00 (m, 1H), 0.73 (d, J=6.4 Hz, 3H), 0.68 (t, J=6.8 Hz, 3H). LRMS: m/z 172.1 (M−1).
