A solution of aldehyde (methyl 5-oxopentanoate) (1) (3.0 g, 23.1 mmol), in absolute ethanol (100 mL) is added, under stirring, to a solution of dimethyl (3S)-3-[(tert-butoxycarbonyl)amino]-2-oxo-4-phenylbutyl-phosphonate (2) (8.58 g, 23.1 mmol) and K2CO3 (3.2 g, 23.1 mmol), dried at 75° C. for 12 hours, in absolute ethanol (200 mL). The mixture is then stirred at room temperature for 16 hours, the solid residue is filtered off and the solution neutralised with glacial acetic acid. The solvent is removed under reduced pressure and the residue partitioned between ethyl acetate and a saturated solution of NaHCO3. The aqueous phase is extracted with ethyl acetate (2×50 mL) and the pooled organic phases are washed with a saturated NaCl solution and dried over anhydrous Na2SO4. The solvent is removed by distilling under reduced pressure and the product purified by means of flash chromatography on a silica gel column by using a 1:1 mixture of ethyl ether and petroleum ether as eluant. 6.8 g (79%) of a colourless oil are obtained.
