RETRO [1,4]-BROOK REARRANGEMENTS OF NONCONJUGATED SECONDARY ALPHA-LITHIO ETHERS - PROOF OF STEREOSPECIFICITY AND OF RETENTION OF CONFIGURATION AT THE CARBANION CENTER

The syntheses of two pairs of diastereomeric gamma-(trialkylsilyloxy) alpha-(tributylstannyl) ethers (syn- and anti-6b, syn- and anti-9) are described. Treatment of each of these compounds with nBuLi in THF at -78-degrees-C furnished alpha-lithiated ethers which underwent retro [1,41-Brook rearrangements at that temperature within 15 - 30 min. After aqueous workup, the alpha-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 crossover 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 alpha-lithiated ethers syn-13 and syn-10, the alpha-silyl ethers syn-11 and syn-14 exclusively. They represent the outcome of intramolecular rearrangements rather than intermolecular processes.