Borate as a Phosphate Ester Mimic in Aldolase‐Catalyzed Reactions: Practical Synthesis of L‐Fructose and L‐Iminocyclitols

Abstract

Dihydroxyacetone phosphate (DHAP)dependent aldolases have been widely used for the organic synthesis of unnatural sugars or derivatives. The practicality of using DHAP-dependent aldolases is limited by their strict substrate specificity and the high cost and instability of DHAP. Here we report that the DHAP-dependent aldolase l-rhamnulose 1-phosphate aldolase (RhaD) accepts dihydroxyacetone (DHA) as a donor substrate in the presence of borate buffer, presumably by reversible in situ formation of DHA borate ester. The reaction appears to be irreversible, with the products thermodynamically trapped as borate complexes. We have applied this discovery to develop a practical one-step synthesis of the non-caloric sweetener lfructose. l-Fructose was synthesized from racemic glyceraldehyde and DHA in the presence of RhaD and borate in 92 % yield on a gram scale. We also synthesized a series of l-iminocyclitols, which are potential glycosidase inhibitors, in only two steps.

Chemoenzymatic Synthesis and Inhibitory Activities of Hyacinthacines A1 and A2 Stereoisomers

“…This methodology may become more advantageous with the use of dihydroxyacetone (DHA) and borate as DHAP mimic in RhuA-catalyzed reactions. [26] Modification of FruA and FucA by protein engineering to increase their tolerance towards (R)-and (S)-prolinal and their derivatives is currently in progress. This may lead to new derivatives with optimized inhibitory activity against glycosidases.…”

Enzyme‐Catalyzed Aldol Additions

“…It appears that the DHA-borate ester complex formed in situ might act as a DHAP mimic [99]. Furthermore, under the reaction conditions, the product may be further trapped due to an ensuing formation of vicinal borate diesters, thereby effectively shifting the reaction equilibrium toward the aldol product.…”

Biotransformations in Small‐Molecule Pharmaceutical Development

“…This is beginning to change with the discoveries of fructose-6-phosphate aldolase (FSA) that accepts dihydroxyacetone (DHA) as substrate 163 and that DHAP aldolase can accept DHA when used in borate buffer due to the transient formation of a borate ester that mimics phosphate. 164 As more enzyme kits become commercially available, the screening for a suitable catalyst can now be performed in a matter of hours rather than days or weeks. Furthermore, both the screening and biotransformation can be performed by nonspecialists.…”