Deoxy-nitrosugars. 16th Communication. Synthesis ofN-acetyl-4-deoxyneuraminic acid

Synthesis of 4-O-Alkylated N-Acetylneuraminic Acid Derivatives

The synthesis of 4-O-alkyl analogues of N-acetylneuraminic acid (Neu5Ac) and the scope of the reaction are described. Activated alkyl halides and sulfonates and primary alkyl iodides give products in useful yields. The utility of the methodology is exemplified using a thiophenyl Neu5Ac building block to synthesize a 4-O-alkyl DANA analogue. These results expand the toolbox of Neu5Ac chemistry with value in drug discovery and for the design of novel tools to study the biology of Neu5Ac lectins.

Successful prediction of substrate-binding pocket in SLC17 transporter sialin

Secondary active transporters from the SLC17 protein family are required for excitatory and purinergic synaptic transmission, sialic acid metabolism, and renal function, and several members are associated with inherited neurological or metabolic diseases. However, molecular tools to investigate their function or correct their genetic defects are limited or absent. Using structure-activity, homology modeling, molecular docking, and mutagenesis studies, we have located the substrate-binding site of sialin (SLC17A5), a lysosomal sialic acid exporter also recently implicated in exocytotic release of aspartate. Human sialin is defective in two inherited sialic acid storage diseases and is responsible for metabolic incorporation of the dietary nonhuman sialic acid N-glycolylneuraminic acid. We built cytosol-open and lumen-open three-dimensional models of sialin based on weak, but significant, sequence similarity with the glycerol-3-phosphate and fucose permeases from Escherichia coli, respectively. Molecular docking of 31 synthetic sialic acid analogues to both models was consistent with inhibition studies. Narrowing the sialic acid-binding site in the cytosol-open state by two phenylalanine to tyrosine mutations abrogated recognition of the most active analogue without impairing neuraminic acid transport. Moreover, a pilot virtual high-throughput screening of the cytosol-open model could identify a pseudopeptide competitive inhibitor showing >100-fold higher affinity than the natural substrate. This validated model of human sialin and sialin-guided models of other SLC17 transporters should pave the way for the identification of inhibitors, glycoengineering tools, pharmacological chaperones, and fluorescent false neurotransmitters targeted to these proteins.

An unexpected elimination product leads to 4-alkyl-4-deoxy-4-epi-sialic acid derivatives

A useful, unexpected β,γ-unsaturated-α-keto ester (ethyl (E)-5-acetamido-3,4,5-trideoxy-6,7:8,9-di-O- isopropylidene-D-manno-non-3-en-2-ulosonate 5) was isolated in 91% yield following ozonolysis and chromatographic purification of its enoate ester precursor ethyl 5-acetamido-2,3,4,5-tetradeoxy-6,7:8,9-di-O-isopropylidene-2-methylene- 4-nitro-D-glycero-D-galacto-nononate (6). When the 4R enoate ester (ethyl 5-acetamido-2,3,4,5-tetradeoxy-6,7:8,9-di-O- isopropylidene-2-methylene-4-nitro-D-glycero-D-talo-nononate, 7) was subjected to the same conditions, enone 5 was a minor product (18%) while the major product did not eliminate HNO2 but instead cyclized to form a five-membered ring containing a hemiaminal linkage between C-2 and the amide nitrogen on C-5 (9, 70%). Conjugate addition to enone 5 opens up the potential to generate 4-substituted sialic acid derivatives, a general route to such compounds that has not been previously reported. In a preliminary investigation of such a route, diethylzinc and dimethylzinc were added to enone 5 resulting in generation of 4-alkyl-substituted cyclic hemiaminal structures 11 and 13, which could be deprotected to form 2,7-anhydrosialic acid analogues 14 and 15. These products could then be converted to peracetylated glycals 16 and 17, the 4-methyl-substituted compound 17 being finally deprotected to give a 4-methyl- substituted analogue of the glycal of sialic acid (5-acetamido-2,6-anhydro-3,4,5-trideoxy-4-methyl-D-glycero-D-talo-non-2-enonic acid 18).Key words: conjugate addition, dialkylzinc reagent, sialic acid, ozonolysis, inhibitors.

Synthesis of 4-deoxy-4-nitrosialic acid

The synthesis of 4-deoxy-4-nitrosialic acid (3,4,5-trideoxy-4-nitro-D-glycero-beta-D-galacto-non-2-ulopyranosonic acid, 5), was completed in seven steps starting from D-arabinose. Coupling of the 6-carbon fragment, 2-acetamido-1,2-dideoxy-1-nitro-D-mannitol (6) with ethyl alpha-(bromomethyl)acrylate afforded a 2 : 1 mixture of ethyl 5-acetamido-2,3,4,5-tetradeoxy-2-methylene-4-nitro-D-glycero-D-galacto-nononate (9a-S) and ethyl 5-acetamido-2,3,4,5-tetradeoxy-2-methylene-4-nitro-D-glycero-D-talo-nononate (9a-R). This mixture of enones was subjected to ozonolysis, and following reduction of the ozonide, the resultant products cyclised to the pyranosides. The target compound, ethyl 4-deoxy-4-nitrosialate (11a) was isolated by fractional crystallisation. Hydrolysis of the ethyl ester proved problematic; thus, the synthesis was modified by using tert-butyl alpha-(bromomethyl)acrylate. Following ozonolysis of the corresponding tert-butyl enoate esters and diastereomer separation, the tert-butyl ester of 4-nitrosialic acid (11b) could be deprotected under acidic conditions to afford . The target compound is a useful intermediate for synthesis of a variety of C-4 substituted sialic acid derivatives, and it is synthesised by a modular route.

Synthesis of transition-state analogues as potential inhibitors of sialidase from Influenza virus

Sodium 5-acetamido-2,6-anhydro-3,4,5-trideoxy-D-manno-non-2-enonate (2) has been synthesized from N-acetyl-4-deoxy-neuraminic acid methyl ester (4). Sodium 2,6-anhydro-3-deoxy-L-arabino-hept-2-enonate (3), 4-acetamido-2,6-anhydro-3,4-dideoxy-L-arabino-hept-2-enonic acid (18e), and 4-acetamido-2,6-anhydro-3,4-dideoxy-L-ribo-hept-2-enonic acid (18a) have been prepared from L-arabinose. The above compounds were investigated as inhibitors of sialidase from Influenza virus. Only compound 2 showed a significant inhibitory activity (Ki 8 x 10(-2) mM) against the enzyme.

A synthesis of N-acetylneuraminic acid and [6-2H]-N-acetylneuraminic acid from N-acetyl-D-glucosamine

N-Acetylneuraminic acid (Neu5Ac) and [6-2H]-Neu5Ac were prepared from 2-acetamido-2-deoxy-D-glucose (N-acetyl-D-glucosamine). Then Henry reaction of a 1-deoxy-1-nitro derivative of GlcNAc (protected 1-C-nitroanhydro-D-glucitol) with cyclohexylidene-D-glyceraldehyde, followed by successive acetylation and reductive denitration with Bu3SnH, gave an anhydrononitol intermediate (6) diastereo-selectively in high yields. Debenzylidenation of 6 freed its distal primary carbinol group, which was subjected to catalytic oxidation followed by hydrolysis, esterification (diazomethane), and acetylation to give a protected methyl nononate. This ester was transformed into the known methyl N-acetyl-4,7,8,9-tetra-O-acetyl-2,3-dehydroneuraminate (15), which was identical with a sample prepared from Neu5Ac. Neu5Ac was obtained from 15 by bromoetherification (NBS, methanol) followed by reductive debromination with Bu3SnH and hydrolysis. Similarly, the [6-2H]-derivative of 15 was transformed into [6-2H]-Neu5Ac.