Neu5Ac aldolase isolation

Neu5Ac aldolase has also been used for the synthesis of 3-deoxy-D-mfl7j jo-octulosonic acid (KDO, 17) [16,17,20]. Condensation of o-arabinose (18) with pyruvate gave a mixture of KDO and 4-epi-KDO (19). Wong and coworkers have since reported the isolation of a KDO aldolase which produces KDO with complete stereospecificity at C-4 and also accepts a wide variety of carbohydrate substrates [51]. N-Acetylneuraminate synthase, found in Neisseria meningitidis, has been used to catalyse the condensation of 6-azido-6-deoxy-N-acetylmannosamine with phosphoenolpyruvate to give 9-azido-9-deoxy-Neu5Ac [52]. 

Neu5Ac aldolase has been isolated from many sources including bacteria and mammals. Significant interest toward sialic acid analogs and commercial availability of the aldolase, have led to the intensive studies on this enzyme, which resulted in the knowledge that despite its highly specificity for pyruvate [36] it accepts a number of different acceptor... 

Some observations indicate that the stereochemical outcome of the reaction seems to be under thermodynamic control, and stereochemistry at C-4 can be reversed. For example, in the Neu5Ac aldolase catalyzed synthesis of KDO, a mixture of (5)-C-4 and (7 )-C-4 were isolated when D-arabinose was the substrate (Scheme 5) [47,48]. 

R, = OH, Rj = H, R2 = OH, R4 = CHjOH in Table 3), the enantiomeric compound of the one just reported could be easily prepared. Aldol condensation products were obtained as diastereomeric mixtures from L-sugars, such as L-fucose, L-xylose, L-lyxose, and o-sugars epimeric to o-mannose relative to the 3-position, such as D-allose and o-gulose [46-48]. Table 4 lists the corresponding aldol condensation products isolated as diastereomeric mixtures. Also, 3-deoxy-D-mannose by condensation with pyruvate gave a diastereomeric mixture of 6-deoxy-KDN furanose derivatives [43]. All these results confirm that sialic acid aldolase, similar to other aldolases, exhibits broad specificity toward the electrophilic acceptor on the other hand, only pyruvate was reported acceptable as the donor [10]. But very recently, in contradiction to that, 3-fluoro-Neu5Ac and 3-fluoro-KDN could be prepared by the sialic acid aldolase-catalyzed condensation of 3-fluoropyruvate and Af-acetylmannosamine or o-mannose (Scheme 5) [47]. 

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