Chiral Carbanions, 1. Configurational Stability and Reactions of α‐Acyloxy‐Substituted α‐Methylbenzyllithium Compounds

Racemic and optically active 1‐phenylethyl esters of pivalic, (—)‐camphanic and 2,4,6‐triisopropylbenzoic acid were prepared. The esters 7a of pivalic acid were deprotonated with lithium 2,2,6,6‐tetramethylpiperidide (Li‐TMP) to form α‐oxy‐α‐methylbenzyllithium compounds which are partly con‐figurationally stable prior to acylation with starting material. Camphanic ester (–)‐ 7b cannot be deprotonated by Li‐TMP and t BuLi is added to the ester function to afford ketone (–)‐ 12a . 1‐Phenylethyl 2,4,6‐triisopropylbenzoates 16 were transformed within minutes to carbanions 19 by using s BuLi/TMEDA in THF, hexane, toluene and s BuLi without TMEDA in toluene/20% diethyl ether at –78°C. The carbanions are configurationally stable only in toluene/20% diethyl ether and racemise partly in the other solvents. They react with a variety of electrophiles with either retention [MeOD, AcOD, (MeO) 2 CO, CICO 2 Me] or inversion (Me 3 SnCl) of configuration. Carbanions 19 rearrange on warming to –20°C to hydroxy ketone 24 with racemisation.