TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: The catalyst is zinc chloride, aluminum chloride, ferric chloride, trimethylaluminum, cerium(III) chloride, tin(IV) chloride, manganese chloride and acetonitrile or triphenylphosphine.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Method: The step (i) comprises adding N,O-dimethylhydroxylamine hydrochloride in organic solvent, at -80 to 35 degrees C, adding catalyst, stirring at room temperature for 10-60 minutes, regulating temperature to -80 to 35 degrees C, adding L-ethyl lactate-organic solvent solution or L-methyl lactate-organic solvent solution, stirring, TLC tracking L-methyl lactate or L-ethyl lactate after reaction is finished, adding diluted hydrochloric acid or saturated ammonium chloride for quenching reaction, adjusting the pH value to 6-7, carrying out organic solvent extraction, drying concentrate to obtain the crude product (S)-2-hydroxy-N-methoxy-N-methyl-propionamide, i.e. compound A, where the mol ratio of N,O-dimethylhydroxylamine hydrochloride and L-methyl lactate and L-ethyl lactate and catalyst is (1-15):(I):(1-15). The step (2) comprises adding crude compound A, benzyl bromide or benzyl chloride, and organic solvent at -80 to 30 degrees C into the reaction kettle, then adding benzyl triethyl ammonium chloride as catalyst, stirring, TLC monitoring until compound A reaction is complete, controlling the temperature to 0-35 degrees C, dropping sodium hydroxide aqueous solution, where the pH is 6-7, reacting for 10-30 hours, carrying out organic solvent extraction, and concentrating to obtain (S)-2-benzyloxy-N-methoxy-N-methyl propionamide, which is compound B, where the mol ratio of crude compound A, benzyl bromide or benzyl chloride, and triethylbenzyl ammonium chloride is 1.0:(0.5-5.0):(0.005-1). The step (iii) comprises adding compound B and organic solvent in the reaction kettle, regulating temperature to -80 to 0 degrees C, adding ethyl group adding Grignard reagent, ethylmagnesium bromide or ethylmagnesium chloride, controlling the temperature at -10 to 10 degrees C, TLC monitoring until compound B reaction is complete, adding ammonium chloride solution until the solution clear, controlling the temperature at 0-30 degrees C, carrying out organic solvent extraction, mixing the organic phase, concentrating to obtain crude (S)-2-benzyloxy-3-pentanone, where the molar ratio of compound B and magnesium chloride or magnesium bromide is 1.0:(0.5-3.0). The mol ratio of N,O-dimethyl hydroxylamine hydrochloride, L-methyl lactate or L-ethyl lactate and catalyst is 5:(1):(1-5). The mol ratio of crude compound A, benzyl bromide or benzyl chloride, triethylbenzyl ammonium chloride is 1.0:(1.8-2.0):(0.05-0.1). The mol ratio of compound B and magnesium chloride or magnesium bromide is 1.0:(1.2-2.0). Preferred Components: The organic solvent is diethyl ether, methyl tert-butyl ether, dichloromethane (preferred), methanol, ethanol, acetone, tetrahydrofuran, methyl tetrahydrofuran or toluene.