[CH2:1]([O:3][C:4](=[O:20])[CH:5]([CH2:18][CH3:19])[C:6](=O)[CH2:7][CH2:8][O:9][CH2:10][C:11]1[CH:16]=[CH:15][CH:14]=[CH:13][CH:12]=1)[CH3:2].C([O-])(=O)C.[NH4+].C([BH3-])#[N:27].[Na+].Cl>C(O)(=O)C.CO>[CH2:1]([O:3][C:4](=[O:20])[CH:5]([CH2:18][CH3:19])[CH:6]([NH2:27])[CH2:7][CH2:8][O:9][CH2:10][C:11]1[CH:16]=[CH:15][CH:14]=[CH:13][CH:12]=1)[CH3:2]
The compound 5-benzyloxy-3-oxo-2-ethyl-pentanoic acid ethyl ester (146.8 g), methanol (1580 ml) and ammonium acetate (406.2 g) are charged in a 6-l five-necked flask equipped with a mechanical stirrer, a condenser, a thermometer, an Argon inlet tube, and a dropping funnel, and the mixture is stirred until a complete solution is obtained. Then sodium cyanoborohydride (16.6 g) is added thereto and, after 5 minutes, glacial acetic acid (330 ml). Stirring is continued for 40 hours at room temperature, then the temperature is lowered to -5° C. and a strong Argon stream is passed through the reaction mixture. 37% HCl (540 ml) is added drop by drop over a period of 20 minutes to lower the pH to about 2.5 and the bubbling Argon stream is continued while the temperature is allowed to rise to room temperature. Ammonium chloride which separates is filtered under vacuum and washed with methanol. The filtrate plus the methanol wash are concentrated to dryness under reduced pressure by heating to 40° -45° C. The residue is taken up with 5% HCl (400 ml) and toluene (900 ml) and the toluene layer is separated and extracted with 5% HCl (three 300 ml portions). The acidic extracts are combined, cooled to about -5° C. and brought to pH 8 by the addition of 50% NaOH (550 ml). The mixture is extracted with methylene chloride (four 300-ml portions) which is then dried over Na2SO4 and concentrated to dryness under reduced pressure yielding 3-amino-5-benzyloxy-2-ethyl-pentanoic acid ethyl ester (99.1 g) which may be used as such in the next step.
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