A method for preparing 3-O-alkyl-5,6-O-(1-methylethylidene)-L-ascorbic acid is provided to obtain a reaction intermediate of 5,6-O-(1-methylethylidene)-L-ascorbic acid with high yield and high purity without using expensive DMSO or DMF as a solvent and produce the 3-O-alkyl-5,6-O-(1-methylethylidene)-L-ascorbic acid with industrially satisfactory yield from the reaction intermediate without isolation and purification of the reaction intermediate. A method for preparing 3-O-alkyl-5,6-O-(1-methylethylidene)-L-ascorbic acid comprises the steps of: (a) reacting L-ascorbic acid with acetone using an acidic compound as a catalyst to obtain 5,6-O-(1-methylethylidene)-L-ascorbic acid; and (b) reacting the 5,6-O-(1-methylethylidene)-L-ascorbic acid with an alkylating agent in the presence of a base to obtain the 3-O-alkyl-5,6-O-(1-methylethylidene)-L-ascorbic acid, wherein the step(a) is performed in the presence of 2,2-dimethoxypropane. A method for preparing 5,6-O-(1-methylethylidene)-L-ascorbic acid comprises a step of reacting L-ascorbic acid with acetone using an acidic compound as a catalyst in the presence of 2,2-dimethoxy propane.
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