Complementary asymmetric alkylation reaction of the lithioenamine derived from 2methoxycarbonylcyclohexanone and (S)-valine tert-butyl ester was examined by employing the various electron oair donatino addTtives in a toluene solvent. in order to clarifv the factors controlling the diastereoface selectivity. " Development of the asymmetric reactions producing either enantiomers is of great importance since the chiral auxiliaries available are usually limited to one enantiomer 1) Some sophisticated methodologies have been devised to produce either enantiomers in the formation of the chiral tertiary carbon center. 2) However, there has been only few reliable methods to produce either enantiomers bearing the chiral quaternary carbon center. 3) We have recently reported a complementary asymmetric synthesis of a,a-dialkyl B-keto esters via the alkylation of the lithioenamines derived from cr-alkyl B-keto esters and (S)-valine tert-butyl ester (Scheme 1).4) The crucial features of the method rely on the dependence of the diastereoface selection on the set of external ligand-toluene solvent systems. When toluene is used as a solvent, the use of one equivalent of HMPA as a ligand 5) leads to top side attack on the one hand, while the use of one to three equivalents of THF, NMe3, or dioxolane leads to bottom side attack on the other hand. The purpose of the present communication is to report a further insight on this phenomena and to provide a mechanistic consideration on this intriguing reaction. Scheme 1 The reaction of the lithioenamine 1 with methyl iodide in toluene was examined. 6, In the absence of HMPA the reaction did not proceed below -50°C. However, in the presence of one equivalent of HMPA, the reaction did proceed at -55°C to give the methylated product (2 (R=Me)) in the enantiomeric purity of over 99% as shown in Table I. Even an addition of a catalytic amount of HMPA (0.1 eq) led to the formation of 2 in 85% ee. Both acceleration of