Lithium borohydride → Benzyl 3,5-dihydroxypentylcarbamate

Methyl 5-(benzyloxycarbonylamino)-3-oxopentanoate (4.5 g, 15.3 mmoles) is dissolved in anhydrous tetrahydrofuran (25 mL) and this solution is added slowly dropwise to an ice-cooled solution of lithium borohydride in anhydrous tetrahydrofuran (2 M, 25 mL, 50 mmoles). When the addition is complete, the cooling bath is removed and the reaction is stirred at ambient temperature for 16 hours. The clear, colorless solution is again cooled in an ice bath, and aqueous hydrochloric acid (1 M, 50 mL) is added slowly dropwise to decompose the excess reducing agent (gas is evolved). When a homogeneous solution is obtained, the mixture is concentrated to remove tetrahydrofuran. Ethyl acetate (100 mL) is added; the mixture is shaken well and poured into a separatory funnel. The layers are allowed to separate and the bottom aqueous layer is drawn off. The aqueous layer is then washed twice more with ethyl acetate (100 mL each), and the combined ethyl acetate solutions are washed with saturated aqueous sodium chloride (50 mL). The ethyl acetate solution is then dried over anhydrous sodium sulfate, filtered and evaporated to a pale brown syrup. The crude product is purified on silica gel (200 mL bed volume) eluting with ethyl acetate (500 mL) then ethyl acetate:methanol (99:1 [v/v], 500 mL then 95:5 [v/v], 1000 mL). Fractions containing pure product by thin layer chromatography are pooled and evaporated to a pale yellow oil. The oil is dried well in vacuo. The yield is 3.6 g (92%). The oil solidifies on standing at 4° C.