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

(E)/(Z)-Isomeric allylic carbamate esters were deprotonated by n-butyllithium/(-)-sparteine in toluene. Trapping experiments with chlorotrimethylsilane afforded the alpha-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 degrees C (T-1/2 > 60 min). The allylic double bond is stable to (Z)-(E) isomerization under these conditions. (c) 2005 Elsevier Ltd. All rights reserved.