Stereochemical Course of [2,3] Anionic and Ylide

Stereochemical Course of [2,3] Anionic and Ylide - J. Am....

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J. Am. Chem. SOC. 1995,117, 11817-11818 11817 Stereochemical Course of [2,3] Anionic and Ylide Rearrangements of Unstabilized a- Aminoorganolithiums Robert E. Gawley,* Qianhui Zhang, and Silvio Campagna Department of Chemistry, University of Miami Coral Gables, Florida 33124-0431 Received April 24, 1995 Reuised Manuscript Receiued October 6, 1995 The [2,3]-sigmatropic (Wittig) rearrangement of allylic ethers has been the subject of considerable interest over the past 50 years, and synthetic applications have increased since Still showed1 that the rearrangement may be initiated by tin-lithium exchange of an a-alkoxy stannane (eq 1).* This observation was important for the synthesis of enantiopure compounds because of the configurational stability of the intermediate a-alkoxyorganolithiums. Recently, three independent studies showed that the reaction is stereospecific and proceeds with inversion of configuration at the lithium-bearing ~arbon.~ [2,3]-Stii/-Wiftig rearrangement BueSn -0-Me BuLi aoLi (,) Me The analogous reaction involving a-aminoorganostannanes (eq 2) is rare, especially among unstabilized carbanions. Usually, the "carbanionic carbon" is stabilized by a double bond, an aromatic ring, or a carbonyl, and when they occur, [2,3]- rearrangements are driven by relief of ring train.^ One example of an aza-Wittig rearrangement of an unstabilized a-aminoor- ganolithium, induced by reductive lithiation of an S,N-acetal, was reported by Broka in 1989.5 In 1993, Coldham reported two examples of aza-[ 1,2]-Wittig rearrangements initiated by tin-lithium exchange, which proceed by a radical mechanism.6 In both cases, the a-aminoorganolithium is acyclic. We are not aware of any previous examples of aza-[2,3]-Wittig rear- rangements of unstabilized a-aminoorganolithiums (eq 2), but we now report that this mechanism can compete with a [1,2]- pathway for 2-lithio-N-allylpyrrolidine. We also find that the aza-[2,3]-Wittig rearrangement proceeds with inversion of configuration at the lithium-bearing carbon, in accord with precedent in the oxygen series3 (2,3]-Aza-Wittig rearrangement? B~~s~A N R e BuLi dN(R)Li (2) When the nitrogen is quaternary, the corresponding organo- (1) Still, W. C.; Mitra, A. Am. Chem. SOC. 1978, 100, 1927-28. (2) Reviews of the [2,3]-Wittig rearrangement: (a) Hoffmann, R. Angew. Chem., Int. Ed. Engl. 1979, 18, 563-640. (b) Hill, R. K. In Asymmetric Synthesis; Morrison, J. D., Ed.; Wiley: New York, 1984; Vol. 3, pp 503- 572. (c) Nakai, T.; Mikami, K. Chem. Rev. 1986,86,885-902. (d) Mikami, K.; Nakai, T. Synthesis 1991, 594-604. (e) Marshall, J. A. In Compre- hensiue Organic Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon: Oxford. 1991: Vol. 3. DD 975-1014. (f, Briickner. R. In ComDrehensive Organic Synthesis; Tros;,'B. M., Fleming,'I., Eds.; Pergamon: Ox'ford, 1991; (3) (a) Vemer, E. J.; Cohen, T. Am. Chem. SOC. 1992,114, 375-377. (b) Hoffmann, R.; Briickner, R. Angew. Chem., Inr. Ed. Engl. 1992, 31, 647-649. (c) Tomooka, K.; Igarashi, T.; Watanabe, M.; Nakai, T. Tetrahedron Lett. 1992, 33, 5795-5798. (4) (a) For examples of aza-[2,3]-Wittig rearrangements of stabilized anions that are driven by the relief of ring strain, see: Durst, T.; Van Den Elzen, R.; LeBelle, M. J.
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Stereochemical Course of [2,3] Anionic and Ylide - J. Am....

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