I couldn’t figure out how to turn the five membered ring into 6 membered ring. Pls help
Despite the OP having made no effort to answer this, I was intrigued and wanted to see a solution.
The secondary alcohol is protonated by the strong acid, water leaves to generate a secondary cation. With assistance from the ring oxygen, a methylene shift occurs to generate a more stable cation centred on the oxygen. Solvent removes the methine proton to generate the observed dihydropyran product. This only works because of the presence of the oxygen.