ECHET96 Search CD [Molecules: 16] [Related articles/posters: 022 009 003 046 016 ]

1,3-Anionic Cycloreversions of N-lithioimidazolidines: a new route to 2-azaallyl anions

William H. Pearson,* Michael A. Walters and William G. Harter

Department of Chemistry, University of Michigan, Ann Arbor, Michigan, USA 48109-1055

We have been studying the generation and cycloaddition of 2-azaallyl anions [1], a convenient method for the synthesis of pyrrolidines [2-5]. We have recently been able to apply this chemistry to the synthesis of various alkaloids [6-8]. We found that the most general method for 2-azaallyl anion generation is tin-lithium exchange. However, our early work focused on the deprotonation of imines, the method originally reported by Kauffmann [9]. An early example of our work is shown in Scheme 1, which illustrates the first example of an intramolecular 2-azaallyl anion cycloaddition [2]. Deprotonation of 1 with lithium diisopropylamine (LDA) generated the 2-azaallyl anion 2, which underwent an anionic cycloaddition to produce the N-lithiopyrrolidine 3. Quenching the reaction with water afforded the bicyclic pyrrolidine 4 in good yield.

Ultimately, we found that the deprotonation route to 2-azaallyl anions was limited to non-enolizable imines, thus precipitating our more recent excursion into tin-lithium exchange methodology. However, our studies on deprotonation led us to discover a new route to 2-azaallyl anions, namely the 1,3-anionic cycloreversion of N-lithioimidazolidines (eqn. 1).

This paper describes this discovery and the further use of this method. Perhaps most interesting is that the imidazolidine and deprotonation methods differ in the stereochemical outcome of the cycloadditions.

  1. Introduction (This page)
  2. Observations of imidazolidine intermediates in the deprotonation route to 2-azaallyl anions
  3. Deliberate generation of imidazolidines from 2-azaallyl anions, and their use as 2-azaallyl anion precursors
  4. Synthesis of imidazolidines from diamines, and their use in 2-azaallyl anion generation
  5. An unusual stereochemical complementarity
  6. How general is the imidazolidine fragmentation route to 2-azaallyl anions? and Conclusion
  7. Experimental
  8. References