The Phytochemical Reduction of Ketones

The first suitable representative of this class of substances, 6-methyl-5-heptene-2-one, was investigated by Neuberg and Lewite. This ketone, which is a natural constituent of ethereal oils, can also be obtained by degradation of various aliphatic and cyclic terpenes. By means of fermenting yeast it is converted to 6-methyl-5-heptene-2-ol, 

Unlike the starting material, the product of this reduction contains an asymmetric carbon atom and it was found to be optically active. Since in the case of ketones a Cannizzaro reaction cannot take place, this mechanism for alcohol formation is out of the question. Consideration of the reduction of ketones thus clearly shows that an actual biohydrogenation is involved. 

It is worthy of notice that acetaldehyde, a characteristic product of simultaneous oxidation in the fermentation mixture, was detected in these experiments. Its appearance is undoubtedly connected with the process of reduction. Acetaldehyde, which is formed in the absence of air and cannot therefore be a result of secondary oxidation, occurs in approximately constant ratio to the equivalent amount of the methylheptenol. An obvious relationship thus exists between the hydrogenation and the formation of the acetaldehyde. Presumably a disturbance of the normally correlated processes takes place, in the sense of the theory of fermentation. Whereas acetaldehyde normally is reduced to ethyl alcohol, in this case the intermediary acetaldehyde and the added ketone compete for the hydrogen. It thus follows that at least in the initial stage the amount of acetaldehyde which is not reduced must equal the amount that is displaced as a result of interference by the ketone. 

The question whether similar conditions also play a part during the reduction of aldehydes must be left in abeyance. Such a mechanism cannot be postulated offhand, since an amount of acetaldehyde equivalent 

Conversion of Various Aliphatic, Aromatic and Cylic Ketones 

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