Synthesis of 3-Deoxy-D-manno2-octulosonic Acid (KDO) and Its Analogs Based on KDO Aldolase-Catalyzed Reactions

3-Deoxy-D-manno-2-octulosonic acid (D-KDO) was synthesized from D-arabinose and pyruvate in 67% yield by using KDO aldolase (EC 4.1.2.23) from Aureobacterium barkerei strain KDO-37-2 (ATCC 49977). Studies on the substrate specificity of the enzyme with more than 20 natural and unnatural sugars indicate that this enzyme widely accepts trioses, tetroses, pentoses, and hexoses as substrates, especially the ones with the R configuration at the 3 position. The substituent on the 2 position had little effect on the aldol reaction. Nine substrates were submitted to the aldol reaction to prepare the products, including d-KDO, 3-deoxy-D-arabino-2-heptulosonic acid (d-DAH), 2-keto-3-deoxy-L-gluconic acid (l-KDG), and 3-deoxy-L-glycero-L-galacto-nonulosonic acid (l-KDN). It appears that the attack of pyruvate took place on the re face of the carbonyl group of acceptor substrates, a facial selection complementary to sialic acid aldolase (si face attack) reactions. The aldolase products can be converted to aldoses via radical-mediated decarboxylation. For example, decarboxylation of pentaacetyl-KDO and hexaacetylneuraminic acid gave penta-O-acetyl-2-deoxy-β-D-manno-heptose and penta-O-acetyl-4-acetamido-2,4-dideoxy-β-D-glycero-D-galacto-octose, respectively.