Ethyl 4-hydroxybutanoate, $\mathrm{HO}-\mathrm{CH}{}_2\mathrm{CH}{}_2\mathrm{CH}{}_2-\mathrm{CO}-\mathrm{OC}{}_2\mathrm{H}{}_5$, is converted completely into 4-methylpentane-1,4-diol using (i) $\mathrm{CH}{}_3\mathrm{MgBr}$ in dry ether; (ii) $\mathrm{H}{}_3\mathrm{O}{}^{+}$. The minimum number of moles of $\mathrm{CH}{}_3\mathrm{MgBr}$ required per mole of ester is

One equivalent is consumed by the acidic O-H group, giving $\mathrm{CH}{}_4$ and an alkoxide. The ester then needs two equivalents: the first gives the ketone after loss of ethoxide, and the second adds to the ketone to give the tertiary alkoxide. Total = 1 + 2 = 3 equivalents; the work-up step gives the 1,4-diol.