Chiral Auxiliary that is Cleaved off after the Reaction

Chiral Auxiliary that is Cleaved off after the Reaction 

Reaction between C2 symmetric diols and a, yS-unsaturated aldehydes yield chiral ethylenic acetals that undergo copper-mediated substitution reactions. With aryl or 

Hydrolysis of the enol ethers obtained from the substitution reaction with the organocopper reagent yielded chiral b-substituted aldehydes with ees of 62 and 73% for the Sn2 and Sn2 products, respectively. 

It was concluded that an oxygen functionality in the C(2)-side chain of the pyr-rolidinyl chiral auxiliary was of great importance for the achievement of high ees. 

On the basis of this conclusion and on NMR studies of complexes of 17b with Lewis acids, a transition state model to explain the observed selectivity was proposed. This involved initial complexation of a cuprate lithium ion to the three different heteroatoms in the substrate, followed by formation of a A-n complexation product from the less hindered si face, the re face being shielded by the pyrrolidine ring (Fig. 8.2). 

SdiemeS,11. Reactions between an ethylenic acetal and organocopper reagents. 

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