Diastereoselective lithiation and substitution of (S)-N-benzylprolinol andrac-N-benzylpiperidine-2-methanol via the carbamate esters: Kinetic resolution by means of (-)-sparteine-mediated deprotonation

The (S)-N-benzylprolinol carbamate (S)-8 is deprotonated by sec-butyllithiu m with the removal of the pro-R-H and the intermediate lithium compound 10 is trapped by several electrophiles. Complete ul-diastereotopos-differentat ion is achieved irrespective of the supporting additive [TMEDA, (-)-spartei ne, or ether]. For the homologous piperidine derivative rac-9, the substrat e-directed diastereoselectivity is less pronounced (similar to 6:1). Thus, an efficient kinetic resolution is observed by means of sec-butyllithium/(- )sparteine, leading to the preferential consumption of the (+)-(R)-9 with a bstraction of the pro-S-H, followed by its substitution by electrophiles. E nantioenriched (-)-(S)-9 remains unchanged.