Reaction of Grignard reagents with esters
Reaction of a Grignard reagent with an ester is a standard method for producing tertiary alcohols where at least two of the substituents (the "$\small\ce{R_2}$" group attached to the Grignard) are the same.  Judicious choice of the starting ester allows for the preparation of a tertiary alcohol where all 3 substituents are the same. Alternately, you could react 3 equivalents of the Grignard with a cheap dialkyl carbonate (a "double" ester, if you will) to obtain a tertiary alcohol where all 3 groups are the same.



Typically a slight excess of Grignard reagent is used and the tertiary alcohol is produced after acid work-up.  If an excess of Grignard is not used then a mixture (ugh!) of ester, intermediate ketone and alcohol may result.