To a suspension of tert-butyl 2-hydroxyimino-3-oxobutyrate (56.4 g) and potassium carbonate (62.4 g) in ethyl acetate (300 ml) and N,N-dimethylformamide (60 ml) was added benzyl chloroacetate (60 g), and the mixture was stirred at 40° C. for 7 hours. The reaction mixture was poured into ice-water (1000 ml), and the separated organic layer was washed twice with a saturated aqueous solution of sodium chloride and then dried over magnesium sulfate. Removal of the solvent gave tert-butyl 2-benzyloxycarbonylmethoxyimino-3-oxobutyrate (96.3 g) as an oil.
