Preparation method of dextromethorphan

The invention discloses a preparation method of dextromethorphan ((+)-3-methoxy-17-methyl-(9 alpha,13 alpha,14 alpha)-levorphane, I). The method comprises the following steps that a dextromethorphan intermediate (+)-1-(4-methoxy) benzyl-1,2,3,4,5,6,7,8-octahydro-isoquinoline (II) conducts N-benzylation reaction with a benzylation reagent under alkaline conditions to form (+)-1-(4-methoxy) benzyl-N-benzyl-1,2,3,4,5,6,7,8-octahydro-isoquinoline (III); the intermediate (III) is subjected to acid cyclization reaction to form (+)-3-hydroxy-17-benzyl-(9 alpha,13 alpha,14 alpha)-levorphane, (IV); the intermediate (IV) reacts with dimethyl sulfate or methine halide, and is subjected to O-methylation and N-methylation reaction to form (+)-3-methoxy group-17-benzyl-17-methyl-(9 alpha,13 alpha,14 alpha)-levorphane quaternary ammonium salt (V); (V) is subjected to catalytic hydrogenation reaction; benzyl is removed; and dextromethorphan (I) is obtained. Therefore, the preparation method can use a common and cheap methylation reagent to substitute an unusual methylation reagent such as phenyltrimethylammonium hydroxide, and the yield of reaction can be increased.