Q4(c)(i). Ethyl levulinate with excess methylmagnesium bromide

Introduction. Ethyl levulinate has a ketone and an ester. Excess Grignard reagent adds once to the ketone and twice to the ester (ester → ketone → tertiary alkoxide). Both ends become tertiary alcohols, which close to a cyclic ether in acid.

Reading of the question: step (2) printed as Et₂O is read as the ether solvent followed by aqueous work-up (H₃O⁺); the unlabelled arrows are acid-catalysed steps.

Scheme: Three equivalents of MeMgBr, then an intramolecular SN1 ring closure.

1. Ketone: MeMgBr adds to C=O (nucleophilic addition) to give a tertiary magnesium alkoxide.
2. Ester: MeMgBr adds to the ester C=O, the tetrahedral intermediate expels EtO⁻ to give a methyl ketone, which at once takes a second MeMgBr. Overall 3 MeMgBr are used.
3. Work-up (H₃O⁺) gives X = 2,5-dimethylhexane-2,5-diol, (CH₃)₂C(OH)CH₂CH₂C(OH)(CH₃)₂.
4. In acid one OH is protonated and leaves as water: Y = the tertiary carbocation (CH₃)₂C+CH₂CH₂C(OH)(CH₃)₂ (via the oxonium ion).
5. The second OH, five atoms away, attacks the cation (5-exo ring closure, intramolecular SN1); loss of H⁺ gives 2,2,5,5-tetramethyltetrahydrofuran.

Conclusion. X = 2,5-dimethylhexane-2,5-diol (from three additions of MeMgBr); Y = the tertiary carbocation formed by loss of water in acid; its ring closure gives 2,2,5,5-tetramethyltetrahydrofuran.

Application. 2,2,5,5-Tetramethyloxolane is a peroxide-resistant ether proposed as a greener solvent in place of toluene; tertiary diols of this kind are also the standard precursors of such hindered ethers.