Question

The answer to this question is A but cannot understand how it occurs... Can anyone show me the mechanism of the reaction?

Answer 1

Well, you know that Grignards typically react with carbonyls....and `A` results after workup.

Explanation:

And the typical reaction is.............

`R_3C^(-)""^(+)MgX+R_1C(=O)R_2rarrR_3C-C(O^(-))R_1R_2`

And Grignards are the premier means of `C-C` bond formation. Now if `R_1` OR `R_2` on the substrate, i.e. the electrophile with which the nucleophilic Grignard reacts, are a leaving group, for instance a halide......i.e. `R_1C(=O)X`, in fact we could add TWO equivs of the carbanion to the carbonyl.

Aqueous workup of `R_3C-C(O^(-))R_1R_2` would give a tertiary alcohol, `R_3C-C(OH)R_1R_2`.

And thus addition of the Grignard to the cyclic ester, which I think is a lactone (someone will correct me if I am wrong), will result in the addition of 2 equiv of Grignard reagent to the ipso carbon.