Conclusion

Although decarboxylation reaction seems to be a simple one-carbon removing reaction, it is demonstrated that this reaction is a unique and useful reaction in the preparation of optically active carboxylic acids. If the starting material is a racemic carboxylic acid, the optically active compound can be obtained via symmetrization by chemical carboxylation followed by asymmetrization via enzymatic reaction. Accordingly, the whole process can be said as "chemicoenzymatic deracemization" (Fig. 24).

The reactions catalyzed by transketolases are also extremely unique because of the following reason. If one sees only the starting materials and the product, the carbonyl carbon of a ketone is working as a "nucleophile," which cannot happen in ordinary chemical reactions (Fig. 24).

As can be seen from the above examples, the decarboxylation reaction can be said to generate carbanion-equivalent, which is capable of undergoing the enantioselective reactions not only with a proton but also with a carbon electrophile in an aqueous medium. In the future extension of this field, this characteristic point should be utilized for the design of the unique reactions.

In terms of the "carbanion equivalent," the enolase superfamily has a strong relation with decarboxylation reaction. This family is characteristic in its promiscuity. If one is reminded of the phrase "lock and key theory" for the relation between the substrate and the enzyme, the word "promiscuity of the enzyme" may be unbelievable. However, in addition to natural promiscuity, we can change the enzyme to be promiscuous by introducing mutation, especially in the case of the enolase superfamily. This will be one of the challenging problems in future. For that purpose, biotechnology and informatics skill will be essential tool in addition to precise analysis of the reaction mechanism.

Chemical reaction Symmetrization

Chemical reaction Symmetrization

X GOgH y fl COjH

X GOgH y fl COjH

R COj,H

Enzyme

or

Asymmetrization

R C02H homochiral

R-COoH

Enzyme

1 _

* RVC-

0

o

Q-k -

/

Figure 24: Summary of decarboxylation.

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