Aldol reactions mediated by a tetrahedral boronate

2-Deoxy-D-ribose-5-phosphate aldolase (DERA): applications and modifications

2-Deoxy-D-ribose-5-phosphate aldolase (DERA) is a class I aldolase that offers access to several building blocks for organic synthesis. It catalyzes the stereoselective C-C bond formation between acetaldehyde and numerous other aldehydes. However, the practical application of DERA as a biocatalyst is limited by its poor tolerance towards industrially relevant concentrations of aldehydes, in particular acetaldehyde. Therefore, the development of proper experimental conditions, including protein engineering and/or immobilization on appropriate supports, is required. The present review is aimed to provide a brief overview of DERA, its history, and progress made in understanding the functioning of the enzyme. Furthermore, the current understanding regarding aldehyde resistance of DERA and the various optimizations carried out to modify this property are discussed.

Tetrahedral boronates as basic catalysts in the aldol reaction

β-Hydroxyketones are versatile building blocks in organic synthesis, which can be conveniently synthesized from ketones and aldehydes by aldol reactions. Unfortunately, these reactions often suffer from dehydration of the initially formed β-hydroxyketones. Previously, tetrahedral 3,5-difluorophenylborate was shown to be an efficient and selective catalyst for this reaction. The present investigation concerns the catalytic performance of phenyl borates with different substitution patterns in the aldol reaction. It appears that the dehydration reaction can be suppressed by selecting substituents and substituent positions with reduced electron withdrawing effects on the borate function. Optimal suppression of the dehydration of β-hydroxyketones was obtained for compounds corresponding to phenylboronic acids with a pK a > 7. The reactions between benzaldehyde and butanone or 3-pentanone did not show diastereoselectivity, which suggests that the catalysts merely act as bases rather than as templates for the transition state of the aldol reaction. Sterically more demanding ketones were not converted.