Racemic tartaric acid can be prepared in a multistep reaction from maleic acid. In the first step, the maleic acid is epoxidized by hydrogen peroxide using potassium tungstate as a catalyst.
:HOCCHCOH + HO → OCH(COH) 
In the next step, the epoxide is hydrolyzed.
:OCH(COH)  +  HO   →   (HOCH)(COH)