| The [1,4]- and [1,2]-Wittig rearrangements of acyclic alpha-silyl and alpha,gamma-disilyl allylic ethers have been studied. Structural and electronic modifications have been introduced to learn the effect that they produce in the [1,4]-/[1,2]-selectivity and diastereoselectivities in some cases. These acyclic substrates in general reacted sluggishly, and therefore most of these reactions show important limitations in term in efficiency and selectivities.;In a similar way, the [1,4]- and [1,2]-Wittig rearrangements of 2-silyl and 4-silyl 5,6-dihydropyrans have been explored, resulting in the discovery of an overall efficient isomerization to cyclopropylsilanes or silyl cyclopentenol structures. The [1,4]-/[1,2]-selectivity can be determined by proper structural and/or electronic modifications at the migrating group or at the allylic portion. The silyl group has been determinant in allowing clean isomerization, presumably due to an electronic contribution, but its steric demand also played a key role in determining the [1,4]-/[1,2]-selectivity and diastereoselectivities of these isomerizations.;The rearrangement of cyclic ethers has been expanded to more complex (bisallylic) substrates, with similar efficiency and selectivities, but larger or shorter rings showed lower reactivity, selectivity and overall efficiency. Comparison with non-silylated analogues provides a better picture of the contribution of silyl groups in these isomerizations. |
