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 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.