PURPOSE: To produce a ketal such as 2-ketogulonic acid, etc. useful as a synthetic intermediate of vitamin C, economically, in an industrial scale, by reacting 2-ketogulonic acid (ester) with a ketone in the presence of a ketalization catalyst.

CONSTITUTION: The objective 2,3:4,6-di-O-ketal or 2,3-O-ketal of 2-ketogulonic acid (ester) can be produced by reacting (A) 2-ketogulonic acid or its ester (e.g. 1W18C alkyl ester) with (B) a ketone (preferably 3W7C ketone such as acetone) in the presence of preferably 0.05W10wt% ketalization catalyst (e.g. a halogen containing compound such as hydrogen fluoride) at 20W100°C for 1W8hr. The component B is used usually in large excess as a reaction reagent as well as a solvent. It is especially preferable to carry out the reaction by using the above catalyst together with the same amount of copper, iron or their oxide, etc.