A Grignard reagent with a protected alcohol

4-Bromobutan-1-ol cannot be converted into a Grignard reagent: the OH would destroy it as soon as it formed. It is first protected as the tetrahydropyranyl ether with dihydropyran and a catalytic amount of p-toluenesulfonic acid.

The protected bromide forms the Grignard reagent with magnesium in THF, which adds to acetone. The final aqueous acid treatment protonates the alkoxide and, at the same time, hydrolyses the THP acetal: 5-methylhexane-1,5-diol is obtained in a single work-up.