2-Methylpropene is treated with mCPBA, and the epoxide formed is then treated with $\mathrm{CH}{}_3\mathrm{ONa}$ in methanol, followed by a neutral work-up. The major product is

The first step gives 2,2-dimethyloxirane. Under basic conditions the strong nucleophile methoxide attacks by SN2 at the less hindered CH2 carbon, so oxygen remains on the tertiary carbon. The product is $(\mathrm{CH}{}_3){}_2\mathrm{C}(\mathrm{OH})-\mathrm{CH}{}_2\mathrm{OCH}{}_3$. Acid-catalysed methanolysis would instead give the other regioisomer.