Oxolan-2-one (gamma-butyrolactone) is treated with excess $\mathrm{CH}{}_3\mathrm{MgBr}$ in dry ether, then mild aqueous $\mathrm{NH}{}_4\mathrm{Cl}$ work-up. The product is

The lactone is a cyclic ester. The first $\mathrm{CH}{}_3\mathrm{MgBr}$ opens the ring (alkoxide leaves) to give a methyl ketone bearing a terminal $\mathrm{CH}{}_2\mathrm{O}{}^{-}$. The ketone adds a second mole, giving a tertiary alkoxide. After work-up: $\mathrm{HOCH}{}_2\mathrm{CH}{}_2\mathrm{CH}{}_2-\mathrm{C}(\mathrm{CH}{}_3){}_2\mathrm{OH}$, 2-methylpentane-2,5-diol.