Enantioselective, (−)-sparteine-mediated deprotonation of geranyl and neryl N, N-diisopropylcarbamate: configurational stability of the intermediate lithium compounds

Abstract

( E)/( Z)-Isomeric allylic carbamate esters were deprotonated by n-butyllithium/(−)-sparteine in toluene. Trapping experiments with chlorotrimethylsilane afforded the α-substitution products, with ( R)-configuration, revealing that the pro-S proton is removed predominantly to form the corresponding ( S)-lithium·(−)-sparteine derivatives; k S / k R >15:1 and >7:1, respectively. A slow ( S)→( R)-epimerization occurs at −78 °C ( T 1/2>60 min). The allylic double bond is stable to ( Z)–( E) isomerization under these conditions.