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| - (−)-Sparteine-Mediated Stereoselective IntramolecularCarbolithiation of 4-Substituted 5-Hexynyl Carbamates. Synthesisof Enantiopure 1,3-Difunctionalized Alkylidene Cyclopentanes
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| - The stereoselective carbolithiation of alkynes with external chiral induction has been achieved forthe first time by fusing the concepts of the asymmetric deprotonation (A) with s-BuLi/(−)-sparteine(s-BuLi/1) and the intramolecular carbolithiation (B). Several 4-functionalized 5-hexynyl carbamateswith different terminal substituents have been prepared and efficiently cyclized providingenantiopure highly substituted alkylidene cyclopentanes. The presence of a sterically demandingsubstituent in the propargylic position is the essential feature of these cyclizations in order tosuppress the abstraction of the remaining propargylic proton. Furthermore, in dependence on theterminal substituent, the 6-phenyl-substituted precursors undergo an intramolecular carbolithiationwhereas for the 6-trimethylsilyl-substituted alkynes a subsequent migration of the O-carbamoylgroup onto the vinylic carbanionic center follows.
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