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Stereochemical Course of Wittig Rearrangements of Dihydropyran Allyl Propargyl Ethers
[2,3]-Wittig
rearrangements of sugar-derived dihydropyran allyl
propargyl ethers located at the 2- or 4-position have been studied
as useful means for extending the carbon chains of the 4- or 2-position
with chirality transfer. The stereochemical course of these reactions
depends on the following factors: (1) deprotonation of <i>pro</i>-<i>R</i> or <i>pro</i>-<i>S</i>-H,
(2) equilibration of the lithiated stereogenic carbanion, (3) conformational
inversion during the rearrangement, and (4) concerted [2,3]- or [1,2]-Wittig
rearrangement. In some cases, a stepwise mechanism that involves the
allyl-C–O bond cleavage is shared as the first step by both
the [2,3]- and [1,2]-Wittig rearrangements. The stereochemical courses
of the rearrangements are compared among the lithiated reactants to
determine the reaction pathways. These mechanisms in the polyoxygenated
dihydropyran ring system were further supported by DFT calculations