Asymmetric Deprotonation by BuLi/(−)-Sparteine: Convenient and Highly Enantioselective Syntheses of (<i>S</i>)-2-Aryl-Boc-Pyrrolidines

Highly enantioselective syntheses of (S)-2-aryl-Boc-pyrrolidines (Boc = tert-butoxycarbonyl) can be achieved by treatment of the corresponding (arylmethyl)(3-chloropropyl)-Boc-amines with s-BuLi/(−)-sparteine. The reactions are solvent dependent with the phenyl, p-chlorophenyl, p-fluorophenyl, p-methylphenyl, m-methoxyphenyl, 1-naphthyl, and 2-naphthyl derivatives 1−7 providing 11−17 in yields of 46−75% with enantiomeric excesses of 84−96% in toluene. The 2-thienyl and 3-furyl analogs 8 and 9 afford the (S)-2-heteroaryl-Boc-pyrrolidines 18 and 19 in 51 and 21% yields with 93−96% enantiomeric excesses. The p-methoxyphenyl derivative 10 gives 20 as a racemic product in 42% yield under the same conditions. Reactions of n-BuLi/(−)-sparteine with 1 and 8 give results comparable to those with s-BuLi/(−)-sparteine. Illustrative syntheses of (S)-2-phenyl-(S)-5-methyl-Boc-pyrrolidine (22) and 1,2-(bis-(S)-2-phenylpyrrolindyl)ethane (23) are reported. The mechanism of the reaction is shown to be an asymmetric deprotonation of 1 to give an enantioenriched organolithium intermediate (S)-24 which undergoes cyclization faster than racemization.