Retro [1,4]‐Brook Rearrangements of Nonconjugated Secondary α‐Lithio Ethers: Proof of Stereospecificity and of Retention of Configuration at the Carbanion Center

The syntheses of two pairs of diastereomeric γ‐(trialkylsilyloxy) α‐(tributylstannyl) ethers ( syn ‐ and anti ‐ 6b , syn ‐ and anti ‐ 9 ) are described. Treatment of each of these compounds with n BuLi in THF at −78°C furnished α‐lithiated ethers which underwent retro [1,4]‐Brook rearrangements at that temperature within 15–30 min. After aqueous workup, the α‐silyl ethers syn ‐ 14 , anti ‐ 14 , syn ‐ 11 , and anti ‐ 11 , respectively, were isolated isomerically pure in 78–93% yield. Comparison of the stereochemistry of starting materials and products shows that these rearrangements proceed stereospecifically and with retention of configuration at the carbanion center. In a cross–over experiment under similar conditions, a 1:1 mixture of the (tributylstannyl) ethers syn ‐ 9 and syn ‐ 6b gave, via the intermediacy of a 1:1 mixture of the α‐lithiated ethers syn ‐ 13 and syn ‐ 10 , the α‐silyl ethers syn ‐ 11 and syn ‐ 14 exclusively. They represent the outcome of intramolecular rearrangements rather than intermolecular processes.