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Stereocontrolled enantioselective total synthesis of the [2+2] quadrigemine alkaloids

  • Author(s): Canham, SM;
  • Hafensteiner, BD;
  • Lebsack, AD;
  • May-Dracka, TL;
  • Nam, S;
  • Stearns, BA;
  • Overman, LE
  • et al.
Abstract

Abstract A unified strategy for enantioselective total synthesis of all stereoisomers of the [2+2] family of quadrigemine alkaloids is reported. In this approach, two enantioselective intramolecular Heck reactions are carried out at the same time on precursors fashioned in four steps from either meso- or (+)-chimonanthine to form the two critical quaternary carbons of the peripheral cyclotryptamine rings of these products. Useful levels of catalyst control are realized in either desymmetrizing a meso precursor or controlling diastereoselectivity in elaborating C2-symmetric intermediates. None of the synthetic quadrigemines are identical with alkaloids isolated previously and referred to as quadrigemines A and E. In addition, we report improvements in our previous total syntheses of (+)- or (-)-quadrigemine C that shortened the synthetic sequence to 10 steps and provided these products in 2.2% overall yield from tryptamine.

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