Synthesing tetrahydropyran from ethylene oxide
Prepare the tetrahydropyran ether of commercially available 3-bromopropanol using dihydropyran, catalytic PTSA in DCM.

Make the Grignard of the THP-protected bromopropanol, react this with ethylene oxide. Work up and isolate to give the mono-THP-pentane-1,5-diol.

Make the tosylate or mesylate of the mono-THP-pentane-1,5-diol.

Remove the THP group using cat. PTSA in MeOH

Form the alkoxide using LiHMDS in Et2O and cyclisation will occur to give the pyran. Isolate by careful distillation.