Scope and limitations of the aldol condensation catalyzed by immobilized acylneuraminate pyruvate lyase

Facile synthesis of C5-azido derivatives of thiosialosides and 2,3-dehydro-5-N-acetylneuraminic acid (DANA)

Neuraminic acid (Neu5Ac, also known as sialic acid) is an important monosaccharide found in glycoproteins and glycolipids which plays a vital role in regulation of physiological functions and pathological conditions. The study of sialoglycans has benefitted from the development of glycomimetic probes and inhibitors for proteins and enzymes that interact with and modify neuraminic acid in glycan chains. Methods to access sialoside intermediates with high yield are needed to facilitate the design of new targets. Here, we report the synthesis of C5-azido thiosialosides using a mild method to deprotect the C5-acetamido functional group followed by the use of a diazo-transfer reagent. We examined two diazo-transfer strategies and compared their yields and tolerance of acetate protecting groups. The same methods and comparisons were also performed for the 2,3-dehydro-5-N-acetylneuraminic acid (DANA) scaffold which is commonly used to generate inhibitors of neuraminidase (sialidase) enzymes. We found that C5-azido derivatives of both thiosialosides and DANA could be produced in five or six steps with yields up to 76 % and 83 %, respectively. Diazo-transfer reagents compared in this study were trifluoromethanesulfonyl azide (TfN3) and imidazole-1-sulfonyl azide (ImzSO2N3). We found that both reagents were compatible with this method and showed comparable yields. Finally, we show that C5-azido derivatives can help to avoid O, N-acyl protecting group migration which was observed in C5-NHAc analogs.

Structurally informed site-directed mutagenesis of a stereochemically promiscuous aldolase to afford stereochemically complementary biocatalysts

2-Keto-3-deoxygluconate aldolase from the hyperthermophile Sulfolobus solfataricus is a highly thermostable type I aldolase that can catalyze carbon-carbon bond formation using nonphosphorylated substrates. However, it exhibits poor diastereocontrol in many of its aldol reactions, including the reaction of its natural substrates, pyruvate and D-glyceraldehyde, which afford a 55:45 mixture of D-2-keto-3-deoxygluconate (D-KDGlu) and D-2-keto-3-deoxy-galactonate (D-KDGal). We have employed detailed X-ray crystallographic structural information of this aldolase bound to these diastereoisomeric aldol products to selectively target specific amino acids for mutation for the rapid creation of stereochemically complementary mutants that catalyze either (Re)- or (Si)-facial selective aldol reactions to afford either D-KDGlu or D-KDGal with good levels of diastereocontrol.

Sialidase substrate specificity studies using chemoenzymatically synthesized sialosides containing C5-modified sialic acids

para-Nitrophenol-tagged sialyl galactosides containing sialic acid derivatives in which the C5 hydroxyl group of sialic acids was systematically substituted with a hydrogen, a fluorine, a methoxyl or an azido group were successfully synthesized using an efficient chemoenzymatic approach. These compounds were used as valuable probes in high-throughput screening assays to study the importance of the C5 hydroxyl group of sialic acid in the recognition and the cleavage of sialoside substrates by bacterial sialidases.

Proline organocatalysis as a new tool for the asymmetric synthesis of ulosonic acid precursors

PEP and aldolase mimicry is the key for a direct organocatalytic entry to precursors of ulosonic acids, biomolecules of enormous importance in biology, chemistry and medicine; in the key aldol reaction the dimethylacetal of pyruvic aldehyde is used as phosphoenolpyruvate (PEP) equivalent and the amino acid proline functions as an organocatalyst, imitating the enzyme.

Synthesis of C-3 nitrogen-containing derivatives of N-acetyl-alpha,beta-D-mannosamine as substrates for N-acetylneuraminic acid aldolase

The synthesis of 3-azido-3-deoxy, 3-amino-3-deoxy and 3-N-tert-butyloxycarbonyl-3-deoxy derivatives of 2-acetamido-2-deoxy-alpha,beta-D-mannose (N-acetyl-alpha,beta-D-mannosamine, ManNAc), is presented. The 3-azido-3-deoxy- and 3-N-tert-butyloxycarbonyl compounds were further characterised as their peracetates. A preliminary study has found that these C-3 nitrogen-substituted derivatives of ManNAc not to be substrates for Neu5Ac aldolase.

Formal synthesis of (+)-3-deoxy-D-glycero-D-galacto-2-nonulosonic acid (KDN) via desymmetrization by ring-closing metathesis

[figure: see text] Formal synthesis of the naturally occurring deaminated sialic acid KDN, a potential oncofetal antigen, has been accomplished in 45% overall yield via a novel ketalization/ring-closing metathesis sequence. The rapid introduction of all oxygen functionality in a completely stereocontrolled manner exploited a rigid 6,8-dioxabicyclo[3.2.1]oct-2-ene ring system. This general synthetic strategy should provide access to a number of KDN and sialic acid analogues.