Chiral acids and bases

The asymmetric reactions discussed below have one feature in common, namely that the substrates have two reaction sites and a plane of symmetry, and the chiral reagent reacts preferentially with one of the two enantiotopic groups. 

The first example involves the asymmetric protonationl l of a hydroxydicarboxylate. When the disodium salt of 4-hydroxypimelic acid (44) is protonated with (5)-camphorsulphonic acid (45) under optimal conditions (3.5 mM in ethanol, -78°) the (5)-lactone (46) is formed with an enantiomeric excess of 94%. The authors believe that the reaction proceeds by enantiotopic protonation of the / e-carboxylate, followed by lactonisation. 

A deracemisation is defined as the conversion of a racemic compound into an enantiomerically-enriched form. (Note this is not a resolution, in which the two enantiomers are separated in a deracemisation, one enantiomer is converted into the other.) An example of this is provided by the following asymmetric reaction.t  

Racemic benzoin is converted to its dianion (47), which of course is achiral, being planar. On enantioselective protonation with 

Deprotonation of prochiral ketones with chiral amide bases such as (50), derived from the readily available enantiomerically pure 1-phenylethylamine, provides an efficient asymmetric synthesis of silyl enol ethers (51). 

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