Glycosylium Ions in Superacid Mimic the Transition State of Enzyme Reactions.

The hydrolysis of glycosides is a biochemical transformation that occurs in all living organisms, catalyzed by a broad group of enzymes, including glycoside hydrolases. These enzymes cleave the glycosidic bond via a transition state with substantial oxocarbenium ion character, resulting in short-lived oxocarbenium ion-like species. While such transient species have been inferred through theoretical studies and kinetic isotope effect measurements, their direct spectroscopic characterization remains challenging.

Evaluation of nonnatural L-iminosugar C,C-glycosides, a new class of C-branched iminosugars, as glycosidase inhibitors.

Capitalizing on a previously developed Staudinger/azaWittig/Grignard (SAWG)-ring contraction sequence that furnished protected six-membered L-iminosugar C,C-glycosides bearing an allyl group and various substituents at the pseudoanomeric position, the synthesis and glycosidase inhibition of a small library of six- and seven-membered L-iminosugar C,C-glycosides is reported. Their hydrogenolysis or cyclization by RCM followed by deprotection afforded eleven L-iminosugars including spirocyclic derivatives. All compounds adopt a C conformation in solution according to NMR data.

Stereoselective Access to Iminosugar ,-Glycosides from 6-Azidoketopyranoses.

We report the synthesis of iminosugar ,-glycosides starting from 6-azidoketopyranoses. Their Staudinger-azaWittig-mediated cyclization provided bicyclic ,-acetals, which were stereoselectively opened with AllMgBr to afford β-hydroxyazepanes with a quaternary carbon α to the nitrogen. Their ring contraction via a β-aminoalcohol rearrangement produced the six-membered l-iminosugars with two functional handles at the pseudoanomeric position.

Stereoselective Synthesis of 1--Diethylphosphonomethyl and -difluoromethyl Iminosugars from Sugar Lactams.

A strategy allowing the straightforward synthesis of 1--phosphonomethyl and 1--phosphonodifluoromethyl iminosugars is reported. Conversion of sugar lactams to the corresponding imines with Schwartz's reagent followed by their reaction with LiCHP(O)(OEt) and LiCFP(O)(OEt) stereoselectively afforded the 1,2- and 1,2- glycosyl phosphonates, respectively, in modest to good yields. Application of this methodology to C-2 orthogonally protected sugar lactams paved the way to 2-acetamido- and 2-deoxy-1--phosphonomethyl iminosugars.

Synthesis, conformational analysis and glycosidase inhibition of bicyclic nojirimycin C-glycosides based on an octahydrofuro[3,2-b]pyridine motif.

A set of bicyclic iminosugar C-glycosides, based on an octahydrofuro[3,2-b]pyridine motif, has been synthesized from a C-allyl iminosugar exploiting a debenzylative iodocycloetherification and an iodine nucleophilic displacement as the key steps. The halogen allowed the introduction of a range of aglycon moieties of different sizes bearing several functionalities such as alcohol, amine, amide and triazole. In these carbohydrate mimics the fused THF ring forces the piperidine to adopt a flattened C conformation according to NMR and DFT calculations studies.

Sugar-Derived Amidines and Congeners: Structures, Glycosidase Inhibition and Applications.

Glycosidases, the enzymes responsible for the breakdown of glycoconjugates, including di-, oligo- and polysaccharides, are present across all kingdoms of life. The extreme chemical stability of the glycosidic bond combined with the catalytic rates achieved by glycosidases makes them among the most proficient of all enzymes. Given their multitude of roles in vivo, inhibition of these enzymes is highly attractive with potential in the treatment of a vast array of pathologies ranging from lysosomal storage and diabetes to viral infections.

Synthesis and chelation study of a fluoroionophore and a glycopeptide based on an aza crown iminosugar structure.

Capitalizing on a recently reported iminosugar-based aza-crown (ISAC) accessed by a double Staudinger azaWittig coupling reaction, we have expanded the structural diversity of this new family of sweet cyclam analogs. Replacement of the two secondary amines linking the iminosugar units by two amide bonds obtained a cyclodimerization by with BOP and DIPEA led to a macrocycle that did not demonstrate efficient Zn chelation unlike the parent ISAC. Introduction of two pyrene moieties on the secondary amines of the parent ISAC yielded a new fluoroionophore that selectively binds Hg in methanol.

Insight into the Ferrier Rearrangement by Combining Flash Chemistry and Superacids.

The transformation of glycals into 2,3-unsaturated glycosyl derivatives, reported by Ferrier in 1962, is supposed to involve an α,β unsaturated glycosyl cation, an elusive ionic species that has still to be observed experimentally. Herein, while combination of TfOH and flow conditions failed to observe this ionic species, its extended lifetime in superacid solutions allowed its characterization by NMR-based structural analysis supported by DFT calculations. This allyloxycarbenium ion was further exploited in the Ferrier rearrangement to afford unsaturated nitrogen-containing C-aryl glycosides and C-alkyl glycosides under superacid and flow conditions, respectively.

Synthesis, Conformational Analysis, and Complexation Study of an Iminosugar-Aza-Crown, a Sweet Chiral Cyclam Analog.

A new family of chiral symmetric tetraazamacrocycles, coined ISAC for IminoSugar Aza-Crown, incorporating two iminosugars adopting a C conformation is disclosed. Multinuclear NMR experiments on the corresponding Cd complex show that the ISAC is a strong chelator in water and its tetramine cavity adopts a conformation similar to that of the parent Cd-cyclam complex. Similar behavior is observed with Cu in solution, with enhanced stability compared to the Cu-cyclam complex.

Structural and Computational Analysis of 2-Halogeno-Glycosyl Cations in the Presence of a Superacid: An Expansive Platform.

An expansive NMR-based structural analysis of elusive glycosyl cations derived from natural and non-natural monosaccharides in superacids is disclosed. For the first time, it has been possible to explore the consequence of deoxygenation and halogen substitution at the C2 position in a series of 2-halogenoglucosyl, galactosyl, and mannosyl donors in the condensed phase. These cationic intermediates were characterized using low-temperature in situ NMR experiments supported by DFT calculations.

Synthesis and glycosidase inhibition of conformationally locked DNJ and DMJ derivatives exploiting a 2-oxo-C-allyl iminosugar.

A series of analogs of the iminosugars 1-deoxynojirimycin (DNJ) and 1-deoxymannojirimycin (DMJ), in which an extra five or six-membered ring has been fused to the C1-C2 bond have been prepared. The synthetic strategy exploits a key 2-keto-C-allyl iminosugar, easily accessible from gluconolactam, which upon Grignard addition and RCM furnishes a bicyclic scaffold that can be further hydroxylated at the C[double bond, length as m-dash]C bond. This strategy furnished DNJ mimics with the piperidine ring locked in a C conformation with all substituents in axial orientation when fused to a six-membered ring.

Site-Selective Debenzylation of C-Allyl Iminosugars Enables Their Stereocontroled Structure Diversification at the C-2 Position.

A C-2 regioselective debenzylative cycloetherification/reductive elimination sequence applied to perbenzylated C-allyl iminosugars is described. This NIS/TMSOTf-triggered deprotection was successfully applied to five-, six-, and seven-membered C-allyl iminosugars including 1,2 cis and 1,2 trans stereoisomers. It allows rapid introduction of structural diversity at the key C-2 position in a stereoselective manner exploiting the anchimeric assistance of the intracyclic N-benzyl group, giving access to the 2-acetamido and 2-fluoro d-gluco configured C-allyl iminosugars and to the epimeric d-manno derivative.

Defining the S1 Side of Glycosylation Reactions: Stereoselectivity of Glycopyranosyl Cations.

The broad application of well-defined synthetic oligosaccharides in glycobiology and glycobiotechnology is largely hampered by the lack of sufficient amounts of synthetic carbohydrate specimens. Insufficient knowledge of the glycosylation reaction mechanism thwarts the routine assembly of these materials. Glycosyl cations are key reactive intermediates in the glycosylation reaction, but their high reactivity and fleeting nature have precluded the determination of clear structure-reactivity-stereoselectivity principles for these species.

In silico analyses of essential interactions of iminosugars with the Hex A active site and evaluation of their pharmacological chaperone effects for Tay-Sachs disease.

The affinity of a series of iminosugar-based inhibitors exhibiting various ring sizes toward Hex A and their essential interactions with the enzyme active site were investigated. All the Hex A-inhibiting iminosugars tested formed hydrogen bonds with Arg178, Asp322, Tyr421 and Glu462 and had the favorable cation-π interaction with Trp460. Among them, DMDP amide (6) proved to be the most potent competitive inhibitor with a K value of 0.

Multivalency To Inhibit and Discriminate Hexosaminidases.

A set of multivalent polyhydroxylated acetamidoazepanes based on ethylene glycol, glucoside, or cyclodextrin scaffolds was prepared. The compounds were assessed against plant, mammalian, and therapeutically relevant hexosaminidases. Multimerization was shown to improve the inhibitory potency with synergy, and to fine tune the selectivity profile between related hexosaminidases.

Conformational flexibility of the glycosidase NagZ allows it to bind structurally diverse inhibitors to suppress β-lactam antibiotic resistance.

NagZ is an N-acetyl-β-d-glucosaminidase that participates in the peptidoglycan (PG) recycling pathway of Gram-negative bacteria by removing N-acetyl-glucosamine (GlcNAc) from PG fragments that have been excised from the cell wall during growth. The 1,6-anhydromuramoyl-peptide products generated by NagZ activate β-lactam resistance in many Gram-negative bacteria by inducing the expression of AmpC β-lactamase. Blocking NagZ activity can thereby suppress β-lactam antibiotic resistance in these bacteria.

HF-Induced Intramolecular C-Arylation and C-Alkylation/Fluorination of 2-Aminoglycopyranoses.

Internal C-aryl and C-alkyl glycosides derived from 2-aminoglycopyranoses have been synthesized, exploiting a HF-mediated stereoselective intramolecular glycosylation. These conditions are compatible with acetate protecting groups and allow introduction of aromatics with various electronic distributions at the anomeric position. This strategy also provides straightforward entry to original fluorinated sugar-azacycle hybrids via a tandem internal C-glycosylation/fluorination reaction starting from 2-N-allyl/propargyl glycopyranoses.

New Broad-Spectrum Antibacterial Amphiphilic Aminoglycosides Active against Resistant Bacteria: From Neamine Derivatives to Smaller Neosamine Analogues.

Aminoglycosides (AGs) constitute a major family of potent and broad-spectrum antibiotics disturbing protein synthesis through binding to the A site of 16S rRNA. Decades of widespread clinical use of AGs strongly reduced their clinical efficacy through the selection of resistant bacteria. Recently, conjugation of lipophilic groups to AGs generated a novel class of potent antibacterial amphiphilic aminoglycosides (AAGs) with significant improved activities against various sensitive and resistant bacterial strains.

Synthesis of pyrrolidine-based analogues of 2-acetamidosugars as N-acetyl-D-glucosaminidase inhibitors.

A ring-contraction strategy applied to β-azido,γ-hydroxyazepanes yielded after functional group manipulation new tetrahydroxylated pyrrolidines displaying an acetamido moiety, one of these iminosugars demonstrating low micromolar inhibition on N-acetylglucosaminidases.

Synthesis of 1,2-trans-2-acetamido-2-deoxyhomoiminosugars.

The first synthesis of 1,2-trans-homoiminosugars devised as mimics of β-D-GlcNAc and α-D-ManNAc is described. Key steps include a regioselective azidolysis of a cyclic sulfite and a β-amino alcohol skeletal rearrangement applied to a polyhydroxylated azepane. The β-D-GlcNAc derivative has been coupled to serine to deliver an iminosugar C-amino acid.

Synthesis of 1,2-cis-homoiminosugars derived from GlcNAc and GalNAc exploiting a β-amino alcohol skeletal rearrangement.

The synthesis of 1,2-cis-homoiminosugars bearing an NHAc group at the C-2 position is described. The key step to prepare these α-D-GlcNAc and α-D-GalNAc mimics utilizes a β-amino alcohol skeletal rearrangement applied to an azepane precursor. This strategy also allows access to naturally occurring α-HGJ and α-HNJ.

Synthesis and conformational analysis of bicyclic mimics of α- and β-D-glucopyranosides adopting the biologically relevant ²,⁵B conformation.

The synthesis of three conformationally locked d-glucopyranoside analogs displaying the hydroxyl pattern of the parent sugar is described. A two carbon bridge connecting the C-2 and C-5 atoms of the pyranose ring allows a torsion of the sugar ring toward a (2,5)B conformation as confirmed by conformational analysis. This conformation is strongly believed to be adopted by the oxacarbenium ion-like transition state of several inverting glucosidases.

A peptide nucleic acid-aminosugar conjugate targeting transactivation response element of HIV-1 RNA genome shows a high bioavailability in human cells and strongly inhibits tat-mediated transactivation of HIV-1 transcription.

The 6-aminoglucosamine ring of the aminoglycoside antibiotic neomycin B (ring II) was conjugated to a 16-mer peptide nucleic acid (PNA) targeting HIV-1 TAR RNA. For this purpose, we prepared the aminoglucosamine monomer 15 and attached it to the protected PNA prior to its cleavage from the solid support. We found that the resulting PNA-aminoglucosamine conjugate is stable under acidic conditions, efficiently taken up by the human cells and fairly distributed in both cytosol and nucleus without endosomal entrapment because cotreatment with endosome-disrupting agent had no effect on its cellular distribution.

Access to L- and D-iminosugar C-glycosides from a D-gluco-derived 6-azidolactol exploiting a ring isomerization/alkylation strategy.

A flexible synthetic access to six-membered L- and D-iminosugar C-glycosides is reported starting from the easily available 6-azido-6-deoxy-2,3,4-tri-O-benzyl-D-glucopyranose precursor. This methodology involves a highly diastereoselective tandem ring enlargement/alkylation and a stereocontrolled ring contraction. It allows an efficient synthesis of iminosugar C-glycosides displaying structural diversity at both C-1 and C-6.