Dipivaloyltartaric Acid

The first reported experiments concerned the protonadon of enamines with (2) ,3) )-DPTA leading, after hydrolysis of the iminium salts formed in situ, to opdcally acdve carbonyl compounds. Starting from the (Z)- and ( )-morpholino enamines of 2-phenylpropanal, it was possible to establish, in spite of 

The protonation of lithium enolates of Schiff bases of racemic a-amino esters leads, after the workup, to a-amino acids of (. configuration with ee as high as 70% (eq 2)7  

A first set of experiments, the study of the protonation of enolates obtained from benzaldehyde Schiff bases and Lithium Diisopropylamide, showed that the asymmetric induction was not significantly affected by the size of the R moiety of the amino acid (R = Me, Et, i-Pr, n-Bu, f-Bu, Ph ee = 44-56%). The two main factors improving the enantioselection were the Ar substituent of the Schiff base and the lithium amide used for the deprotonation. The following results (Table 1) indicate clearly that the enantioselectivity increases with the electron-donating power of substituents para to the Schiff base (eq 3), leading to 70% ee with the Schiff base of p-methoxybenzaldehyde derived from phenylglycine.  

The favorable effect of electron-donating substituents X, interpreted as the structure of the enolate becomes more rigid due to the increase of the coordination between the lithium and the nitrogen atoms, was confirmed on a series of a-amino acids.  

The enantioselectivity was dramatically affected by the structure of the lithium amide used for the deprotonation, indicating that the secondary amine liberated during the metalation step participates in the protonation step. - The utilization of chiral lithium amides allowed higher enantioselections than with classical LDA (eq 4) (Table 2).  

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University of Rouen, Mont-Saint-Aignan, France (2/f,3-R)-Dipivaloyltartaric Acid... 

Racemic 2-hydroxy-1,2-diphenylethanone (benzoin) is deprotonated by potassium hydride to give the enediolate, which can be rapidly reprotonated to the enediol by (27 ,3R)-dipivaloyltartaric acid. (S)-Benzoin (2) of 80% ee is formed very slowly from the enediol, whereas the latter is rapidly oxidized to diphenylethanedione (1) in the presence of air148. 

Of special importance is a mixed lithium-magnesium complex, since the individual metal enolates show much lower enantiofacial discrimination. (2/l,3/ )-Dipivaloyltartaric acid is not a useful proton source, however, tert-butyl alcohol, 3,4-dimethyl-5-phenyl-2-imidazolidinone and 2-(Ar-isopropyl-Ar-methylamino)-l-phenylpropanoI (9-H) yield highly enantiomerically enriched (R)- or (S )-oc-damascone (11), especially in the presence of lithium (+)- or (-)-2-(Ar-iso-propyl-A -methylamino)-1 -phenylpropanolate (9-Li) as an auxiliary. 

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