Diethyl carbonate, $(\mathrm{C}{}_2\mathrm{H}{}_5\mathrm{O}){}_2\mathrm{C}=\mathrm{O}$, is treated with excess $\mathrm{C}{}_2\mathrm{H}{}_5\mathrm{MgBr}$ followed by $\mathrm{H}{}_3\mathrm{O}{}^{+}$. The product is

Two ethoxide groups can leave in succession. The first addition gives ethyl propanoate; the second gives pentan-3-one, and the third equivalent of $\mathrm{C}{}_2\mathrm{H}{}_5{}^{-}$ adds to the ketone to give $(\mathrm{C}{}_2\mathrm{H}{}_5){}_3\mathrm{C}-\mathrm{OMgBr}$. Hydrolysis gives $(\mathrm{C}{}_2\mathrm{H}{}_5){}_3\mathrm{C}-\mathrm{OH}$, 3-ethylpentan-3-ol.