A novelrearrangement promoted by vitamin B12s of diethyl 1-keto-2-bromomethyl-2-methylsuccinate 10 to diethyl 4-methyl-2-ketoglutarate 11 has been discovered. This can serve as a partial model for the coenzymeB12 dependent enzyme catalyzed interconversion of β-methylaspartate and glutamate. The Schiff base corresponding to 10 does not react with vitamin B12s , but it does rearrange readily under