Racemization

The racemization is a term from the stereochemistry, which describes the conversion of non-racemic mixtures of enantiomers or pure enantiomers of chiral chemical compounds in the racemate ( 1:1 mixture of enantiomers), with an existing enantiomeric excess is reduced to zero. With complete racemization a 1:1 mixture of both enantiomers, a racemate, whose ee = 0 ( the ee is the enantiomeric ) forms. If the ee value ≠ 0, then one speaks of partial (partial ) racemization.

Such methods are used for the recovery of unwanted enantiomers. In a method, the (R ) enantiomer of a racemate obtained by optical resolution, 50 % of the starting material remain as ( S)- enantiomer. By racemization can be the (S) - enantiomer lead again in the racemate or partially racemized and reinsert it as a starting material. If the racemization is carried out simultaneously in situ with the resolution, it is called dynamic resolution.

The mechanism of Racemiserung depends strongly on the nature of the substance. Racemization occurs about as a result of SN1 reactions. Here, a trigonal planar arrangement is produced by the now positively charged carbon atom ( carbenium ion). Therefore, the carbon atom can now be attacked from both sides. As both enantiomers by the reaction arising even when using a single enantiomer as starting material. For example, (R) is 3- bromo-3- methylhexane to water to (R) -3 -methyl-3 -hexanol and (S )-3 -methyl-3 -hexanol in a 50:50 ratio, so it is formed (RS )-3 -methyl-3 -hexanol, a racemate.

An enantiomerically pure secondary alcohol with an stereogenic center at that carbon atom to which the hydroxy group is bound, can be converted then this ketone with sodium borohydride in the racemic secondary alcohol by oxidation to the ketone, and reduction in question, so to be racemized.

Peptide synthesis

In peptide synthesis, the occasionally observed partial racemization of individual amino acid building blocks is an undesirable process because doing two or more peptides of different stereochemistry (diastereomers ) results, which are due to the great chemical similarity often difficult to clean. The diastereomeric peptides thus formed usually have very different physiological - eg pharmacological - effects.