Reduction, asymmetric phytochemical

Similarly, methyl a-chloroethyl ketone, a, -dichloroacetone and a,a,/3-trichlorobutyraldehyde have been converted phytochemically and in good yield to the corresponding primary or secondary halogenated alcohols. Since all these alcohols show optical rotation, it seems established that the phytochemical reduction generally takes an asymmetric course. (See pp. 80, 81, 88 and 92.)... 

When a racemic substance is hydrogenated or when the reduction leads to the production of centers of asymmetry, the phytochemical reduction will take at first a completely or partially asymmetric course. Examples of such asymmetric reactions are the conversions of pure racemic valeraldehyde, acetaldol, furoin and furil, diacetyl and acetyl-methylcarbinol to optically active alcohols. Occasionally meso forms also arise, as for example in the case off glycols (p. 84). The reasons for the stereochemical specificity of these reactions have not been clarified. This type of phenomenon has frequently been observed in the related intramolecular dismutation of keto aldehydes, especially if enzyme materials of differing origins are used. 

The first experiments made by Neuberg and Nord with the simplest diketone, diacetyl, showed at once that this substance can be hydrogenated phytochemically with comparitive ease. Acetylmethylcarbinol appears as an intermediate (see below), and the end product of reduction is asymmetric and levorotatory. Reduction was effected by the action of fermenting yeast on diacetyl. The 2,3-butanediol that is formed can be isolated by alcohol-ether extraction of the fermentation mixture after concentration in the Faust-Heim apparatus or by steam distillation in an atmosphere of carbon dioxide under ordinary pressure it is then carefully concentrated with the birectifier and obtained in the pure state by final fractionation. 

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