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| - Inter- and Intramolecular Stereoselective Protonation of Enols1,2
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| - Kinetic control of the stereoselectivity of protonation of enolates and other delocalized speciescommonly affords the less stable diastereomer as a consequence of the considerable exothermicityand resulting sp2 transition-state hybridization of the α-carbon. Protonation then is from the lesshindered face of the enolate. The present study is aimed at reversing this phenomenon byintramolecular delivery of the proton. The approach employed required the synthesis of two enolateprecursors, one with a 2-pyridyl group strategically close to the α-carbon and the other with aphenyl group in the same location. The synthesis required 15 steps and involved new methodology.Intramolecular proton transfer, reversing the usual stereoselectivity, was successful. The selectivityproved to depend on several factors including the exo versus endo configuration of the diastereomerreacting, the proton donor employed, and the concentration of the proton donor. A kinetic analysispermitted the determination of the relative reaction orders of the protonation on the two faces ofthe enolate.
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