Intramolecular carbolithiation reactions of chiral alpha-amino-organolithium species

Abstract

Enantiomerically enriched alpha-amino-organolithium species, in which the lithium atom is attached to a stereogenic carbon centre, have been found to be chemically stable at room temperature in a solvent of very low polarity and undergo intramolecular carbolithiation onto an unactivated alkene. The configurational stability of the chiral organolithium species, bearing a variety of N-alkenyl substituents, was probed by studying the enantiomeric purity of the cyclization products. With N-but-3-enyl-2-lithiopyrrolidine, cyclization to the five-membered ring is more rapid than racemization and a high yield of the pyrrolizidine alkaloid (+)-pseudoheliotridane was obtained with no loss of optical purity. In contrast, with N-pent-4-enyl-2-lithiopyrrolidine, cyclization to the six-membered ring was found to occur with significant loss of optical purity. The cyclization to the six-membered ring was determined to occur with a half-life, t(1/2) approximately 90 min at 23 degrees C. The epimerization of this organolithium species in hexane/Et2O 4:1 was calculated to have a half-life, t(1/2) approximately 30 min at 23 degrees C. Enhanced levels of enantioselectivity for the formation of the indolizidine ring system were obtained using an alkene bearing a terminal phenylthio substituent. With N-[(3-phenylthio)-prop-2-enyl]-2-lithiopyrrolidine, cyclization to the four-membered ring occurs with poor enantioselectivity at low temperature in THF but is highly enantioselective at room temperature in a solvent of very low polarity.