Stereodefined quaternary centers

Scheme 10-128 Electrochemical oxidative cyclization provides a route to stereodefined quaternary centers.

Alternatively, a diastereocontroUed formation of quaternary centers could be achieved by the use of an enantiomericaUy pure electrophile with a stereodefined allylmetal species [108]. In this case, a preassociation of the metal with polarized functional groups is supposed to influence the stereochemical outcome. Thus, the stereocontrolled carbocupration of unfunctionalized alkynes followed by homologation and reaction with chiral sulfinyUmines resulted in diastereomerically pure adducts 374 that can be easily deprotected to provide enantiomericaUy enriched homoallylic primary amines possessing an all-carbon quaternary stereocenter (Scheme 10.128). 

This strategy was used to synthesize spirocyclopenteneoxindoles, as well as stereodefined cyclopentenes. Likewise, Hu reported a [3 -f 2] annulation reaction of 2-atylideneindane-l,3-diones with MBH carbonates that proceeded smoothly in the presence of a multifunctional thiourea-phosphine catalysts to produce the corresponding quaternary carbon centered spirocyclic cyclopentenes. A phosphine-triggered tandem [3 + 4] annulation reaction between MBH carbonates and 1,4-diheteroatom dinucleophiles was described, which provided access to saturated seven-membered 1,4-heterocycles. Analogously, MBH carbonates were used as C3 synthons in asymmetric [3 + 2] annulation reactions for the asymmetric synthesis of 3-spirocyclopentene-2-oxindoles. Another kind of annulation reaction for the construction of highly functionalized stereodefined cyclopentene skeletons involved a Rauhut-Currier Domino Reaction. Finally, an efficient asymmetric [3 + 2] cycloaddition reaction between MBH carbonates of isatins and... 

---