Leucine r-butyl ester

Koga reports that a.P-unsaturated aldimines (89), derived from L-r-leucine r-butyl esters, undergo asymmetric additions with Grignard reagents which is due to formation of a highly ordered magnesium chelate (Scheme 29).75... 

S)-2-Alkyl-2-phenylcycloalkanones have been synthesized in high optical yields using enamines derived from r-leucine r-butyl ester or valine r-butyl ester and 2-phenylcycloalkanones (Scheme 53). ... 

In an unusual variation, Hashimoto and Koga found that Grignard reagents add to the -carbon of im-ines derived from a,3-unsaturated aldehydes (rather than acting as base to form the anions). - High levels of asymmetric induction (91 to 95%) were observed with the r-butyl ester of leucine as the chiral auxiliary (equation 54). 

These data clearly show that the formation of magnesioenamine (18) having the -configuration and attack of alkyl halides under the steric influence of the original chiral center of the tert-leucine tert-butyl ester moiety are responsible for the predominant formation of trans-19 by method B. However, the substituent R introduced is also expected to show a steric effect at the second alkylation stage. Therefore, assuming that the configu-... 

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