Keywords: Zinc / Carbenoids / Sulfur / Carbanions / Alkenes / Elimination / Rearrangement α-Lithiosulfinyl carbanions react either intermolecularly, after transmetalation into an organocopper derivative in an S N 2-type process, with zinc carbenoids, or intramolecularly by higher-order zincates through a tandem zinc homologation-β-elimination reaction into the corresponding alkenes.
Crossroads: Selective sulfoxide/lithium exchange reactions lead to the unique preparation of diastereo‐ and enantiomerically pure polyalkylated cyclopropanes (see scheme). This reaction allows the independent formation of both diastereoisomers from a common starting material.
[reaction: see text] Secondary alpha-lithiosulfinyl carbanions react either intermolecularly, after transmetalation into an organocopper derivative in an S(N)2-type process with zinc carbenoid, or intramolecularly via higher-order zincate to give, through a tandem zinc homologation-beta-elimination reaction the corresponding alkenes. alpha,alpha-Disubstituted alkenes are only formed from tertiary alpha-lithiosulfinyl carbanions via the 1,2-metalate rearrangement.
In the first part of the lecture, we will discuss the one-pot preparation of chiral homoallylic alcohol and amine derivatives by a copper-catalyzed four-component reaction. In this process, three new carbon-carbon bonds as well as a quaternary and a tertiary chiral center are created with excellent regio- and diastereoselectivities. When the reaction was performed without adding external electrophiles, a β-elimination reaction took place to give polysubstituted allenes in good overall yields. This strategy of zinc-homologation followed by a β-elimination reaction was also synthetically used for the transformation of sp3 sulfoxides into olefins with potential application in asymmetric synthesis. Finally, in the second part of this lecture, the stereoselective preparation of metallated dienes in only two chemical steps from commercially available products will be described. This new strategy is based on a tandem allylic C-H bond activation of a remote ω-double bond followed by an elimination reaction.
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