Tertiary allylic methyl(ni-n-butylstannyl) ethers undergo transmetallation and subsequent diastereoselectve [2,3] Wittig rearrangement, wherein chiraliry transfer and olefin selectivity for the [2,3]
sigmatropic event is mediated by a stereogenic center external to the sigrnatropic frnmework.
The [2,3] Wittig rearrangement has become an increasingly important tcol for the stereorational construction of highly-functionalized acyclic and macrocyclic systems as details of the accompanying stereochemical transfer and diastereoselection have emerged12 Recently we reported the diastereoselective rearrangement of a-allyloxy anions derived from tertiary ethers I, which offers a stereocontrolled sigmatropic entry to remotely-functionalized ttisubstituted olefins 2.3 Of considerable interest is the observation that a stereoget+ center outside the sigmatropic framework directs the stereochemical outcome of the electrocychc event for these tertiary substratesp Thus, ethers k, derived from ti diols, rearrange with exclusive diastereoselectivity while the m ethers Ih afford mixtures of H and Z products, a result which is consistent with rearrangement through a conformation involving participation of the vicinal a-alkoxy group in a multidentate coordination3 of the reaction cation. As part of our program to defme the scope of this novel stereocontrol, we have examined the [2,3] Wittig rearrangement of a-lithio ethers &. obtained from uansmetallation of the corresponding methyl triallqlstanyl ethers a.5 We now report that the [2,3] Wittig rearrangement of these systems exhibits a sterecchemical divergence similar to that observed for oxazoline-substituted tertiary ethers.