The hydroxy aldehyde shown is dissolved in an excess of methanol containing a catalytic amount of dry HCl. Which structure is the major product?

5-Hydroxypentanal cyclises spontaneously, the OH adding to the aldehyde to form the six-membered cyclic hemiacetal tetrahydropyran-2-ol. In dry acid and excess methanol the hemiacetal OH is replaced through an oxocarbenium ion by OMe, giving the cyclic mixed acetal 2-methoxytetrahydropyran. The hemiacetal alone is only the intermediate.