5-Hydroxypentanal, $\mathrm{HO}-\mathrm{CH}{}_2\mathrm{CH}{}_2\mathrm{CH}{}_2\mathrm{CH}{}_2-\mathrm{CHO}$, is dissolved in methanol containing a catalytic amount of dry $\mathrm{HCl}$. The product is

The terminal OH adds intramolecularly to the protonated aldehyde to form the six-membered cyclic hemiacetal (oxan-2-ol). In methanol and acid, its OH is protonated and lost to give the ring oxocarbenium ion, which is trapped by $\mathrm{CH}{}_3\mathrm{OH}$ to give the mixed cyclic acetal 2-methoxyoxane. The lactone would need oxidation.