Enantioselective Synthesis of an Allenyl Derivative of Pipecolic Acid

Abstract

Addition of a propargylsilane moiety onto chiral cyclic iminium ions occurs with a high level of stereoselectivity intermolecularly as well as intramolecularly. This operation generates allenic α-amino acids precursors. An isomerization leading to a 1,3-dienyl compound precludes the formation of an allenic amino acid in the acyclic series; however, this isomerization did not take place with the cyclic α-amino ester which was thus obtained with a 55 % ee. AM1 calculations explain the difference of reactivity between cyclic and acyclic compounds on a conformational basis.