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 nBuLi 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.