Molecular Basis for Inhibition of GH84 Glycoside Hydrolases by Substituted Azepanes: Conformational Flexibility Enables Probing of Substrate Distortion

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 1C4 conformation in solution according to NMR data. Compared to previously reported branched L-iminosugars, the L-iminosugar C,C-glycosides reported herein were less potent glycosidase inhibitors. However, some of these compounds showed micromolar inhibition of human lysosome β-glucocerebrosidase suggesting that such iminosugars could be useful to access potent CGase inhibitors by adjusting the structure/length of the pseudoanomeric substituents.

Structural variation of the 3-acetamido-4,5,6-trihydroxyazepane iminosugar through epimerization and C-alkylation leads to low micromolar HexAB and NagZ inhibitors

We report the synthesis of seven-membered iminosugars derived from a 3S-acetamido-4R,5R,6S-trihydroxyazepane scaffold and their evaluation as inhibitors of functionally related exo-N-acetylhexosaminidases including human O-GlcNAcase (OGA), human lysosomal β-hexosaminidase (HexAB), and Escherichia coli NagZ. Capitalizing on the flexibility of azepanes and the active site tolerances of hexosaminidases, we explore the effects of epimerization of stereocenters at C-3, C-5 and C-6 and C-alkylation at the C-2 or C-7 positions. Accordingly, epimerization at C-6 (L-ido) and at C-5 (D-galacto) led to selective HexAB inhibitors whereas introduction of a propyl group at C-7 on the C-3 epimer furnished a potent NagZ inhibitor.

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 ubiquitous through 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. Therefore great efforts have been invested in the last three decades to design and synthesize inhibitors of glycosidases leading to a number of drugs currently on the market. Amongst the vast array of structures that have been disclosed, sugars incorporating an amidine moiety have been the focus of many research groups around the world because of their glycosidase transition state-like structure. In this review we report and discuss the structure, the inhibition profile and the use of these molecules including related structural congeners as transition state analogs.

Discovery of a Novel and Brain-Penetrant O-GlcNAcase Inhibitor via Virtual Screening, Structure-Based Analysis, and Rational Lead Optimization

O-GlcNAcase (OGA) has received increasing attention as an attractive therapeutic target for tau-mediated neurodegenerative disorders; however, its role in these pathologies remains unclear. Therefore, potent chemical tools with favorable pharmacokinetic profiles are desirable to characterize this enzyme. Herein, we report the discovery of a potent and novel OGA inhibitor, compound 5i, comprising an aminopyrimidine scaffold, identified by virtual screening based on multiple methodologies combining structure-based and ligand-based approaches, followed by sequential optimization with a focus on ligand lipophilicity efficiency. This compound was observed to increase the level of O-GlcNAcylated protein in cells and display suitable pharmacokinetic properties and brain permeability. Crystallographic analysis revealed that the chemical series bind to OGA via characteristic hydrophobic interactions, which resulted in a high affinity for OGA with moderate lipophilicity. Compound 5i could serve as a useful chemical probe to help establish a proof-of-concept of OGA inhibition as a therapeutic target for the treatment of tauopathies.

Synthesis of Pyrrolidine Monocyclic Analogues of Pochonicine and Its Stereoisomers: Pursuit ofSimplified Structures and Potent β-N-Acetylhexosaminidase Inhibition

Ten pairs of pyrrolidine analogues of pochonicine and its stereoisomers have been synthesized from four enantiomeric pairs of polyhydroxylated cyclic nitrones. Among the ten N-acetylamino pyrrolidine analogues, only compounds with 2,5-dideoxy-2,5-imino-d-mannitol (DMDP) and pochonicine (1) configurations showed potent inhibition of β-N-acetylhexosaminidases (β-HexNAcases); while 1-amino analogues lost almost all their inhibitions towards the tested enzymes. The assay results reveal the importance of the N-acetylamino group and the possible right configurations of pyrrolidine ring required for this type of inhibitors.

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

The synthesis and glycosidase inhibition profile of a series of bicyclic analogs of DNJ and DMJ displaying a similar hydroxyl pattern and a distinct conformation is described.

Chemoenzymatic Synthesis of Substituted Azepanes by Sequential Biocatalytic Reduction and Organolithium-Mediated Rearrangement

Enantioenriched 2-aryl azepanes and 2-arylbenzazepines were generated biocatalytically by asymmetric reductive amination using imine reductases or by deracemization using monoamine oxidases. The amines were converted to the corresponding N'-aryl ureas, which rearranged on treatment with base with stereospecific transfer of the aryl substituent to the 2-position of the heterocycle via a configurationally stable benzyllithium intermediate. The products are previously inaccessible enantioenriched 2,2-disubstituted azepanes and benzazepines.

From 1,4-Disaccharide to 1,3-Glycosyl Carbasugar: Synthesis of a Bespoke Inhibitor of Family GH99 Endo-α-mannosidase

Understanding the enzyme reaction mechanism can lead to the design of enzyme inhibitors. A Claisen rearrangement was used to allow conversion of an α-1,4-disaccharide into an α-1,3-linked glycosyl carbasugar to target the endo-α-mannosidase from the GH99 glycosidase family, which, unusually, is believed to act through a 1,2-anhydrosugar "epoxide" intermediate. Using NMR and X-ray crystallography, it is shown that glucosyl carbasugar α-aziridines can act as reasonably potent endo-α-mannosidase inhibitors, likely by virtue of their shape mimicry and the interactions of the aziridine nitrogen with the conserved catalytic acid/base of the enzyme active site.

Selective trihydroxylated azepane inhibitors of NagZ, a glycosidase involved in Pseudomonas aeruginosa resistance to β-lactam antibiotics

The synthesis of a series of d-gluco-like configured 4,5,6-trihydroxyazepanes bearing a triazole, a sulfonamide or a fluorinated acetamide moiety at C-3 is described. These synthetic derivatives have been tested for their ability to selectively inhibit the muropeptide recycling glucosaminidase NagZ and to thereby increase sensitivity of Pseudomonas aeruginosa to β-lactams, a pathway with substantial therapeutic potential. While introduction of triazole and sulfamide groups failed to lead to glucosaminidase inhibitors, the NHCOCF₃ analog proved to be a selective inhibitor of NagZ over other glucosaminidases including human O-GlcNAcase and lysosomal hexosaminidases HexA and B.

The price of flexibility - a case study on septanoses as pyranose mimetics

Seven-membered ring mimetics of mannose were studied as ligands for the mannose-specific bacterial lectin FimH, which plays an essential role in the first step of urinary tract infections (UTI). A competitive binding assay and isothermal titration calorimetry (ITC) experiments indicated an approximately ten-fold lower affinity for the seven-membered ring mannose mimetic 2-O-n-heptyl-1,6-anhydro-d-glycero-d-galactitol (7) compared to n-heptyl α-d-mannopyranoside (2), resulting exclusively from a loss of conformational entropy. Investigations by solution NMR, X-ray crystallography, and molecular modeling revealed that 7 establishes a superimposable H-bond network compared to mannoside 2, but at the price of a high entropic penalty due to the loss of its pronounced conformational flexibility. These results underscore the importance of having access to the complete thermodynamic profile of a molecular interaction to "rescue" ligands from entropic penalties with an otherwise perfect fit to the protein binding site.

Efficient Synthesis of Dimeric Oxazoles, Piperidines and Tetrahydroisoquinolines from N-Substituted 2-Oxazolones

A mild and practical method for the construction of heterocycles from N-substituted 2-oxazolones through cascade, BF3 ⋅Et2 O/H2 O-catalyzed reactions involving iminium ion generation and trapping by external or internal olefinic and aryl moieties is described. Mechanistic and computational studies revealed the strong protic acid HBF4 as the initiating catalyst for these cascade reactions. Providing access to novel molecular diversity, these processes may facilitate chemical biology studies, drug discovery efforts and natural products synthesis.

Amino-functionalized iminocyclitols: synthetic glycomimetics of medicinal interest

A review on the syntheses and biological activities of unnatural glycomimetics highlighting the effect of replacement of hydroxyl groups of natural iminosugars by amino functionalities is presented.

γ-Aminoalcohol rearrangement applied to pentahydroxylated azepanes provides pyrrolidines epimeric to homoDMDP

A series of pentahydroxylated pyrrolidines, displaying five contiguous stereogenic centres and epimeric to α-glucosidase inhibitor homoDMDP, have been synthesized. The key step involves a γ-aminoalcohol rearrangement applied to polyhydroxylated azepanes. These five-membered iminosugars demonstrate micromolar inhibition of glycosidases.

Synthetic deoxynojirimycin derivatives bearing a thiolated, fluorinated or unsaturated N-alkyl chain: identification of potent α-glucosidase and trehalase inhibitors as well as F508del-CFTR correctors

Synthetic DNJs bearing a thiolated, fluorinated or unsaturated N-substituent exhibit trehalase inhibition or F508del-CFTR correction.

Synthesis of 2-carboxymethyl polyhydroxyazepanes and their evaluation as glycosidase inhibitors

A series of diastereomeric tetrahydroxylated azepanes featuring a carboxymethyl group at the pseudo-anomeric position have been synthesized from a common unsaturated intermediate. Syn- and anti-dihydroxylation reactions were achieved to yield the target compounds after efficient one-step deprotection of carbamate, ester and acetonide groups simultaneously. Screening of these polyhydroxylated azepanes toward a range of commercially available glycosidases was performed and one of the stereoisomers showed potent and selective inhibition toward β-galactosidase (IC50=21 μM).

Selective trihydroxyazepane NagZ inhibitors increase sensitivity of Pseudomonas aeruginosa to β-lactams

AmpC β-lactamase confers resistance to β-lactam antibiotics in many Gram negative bacteria. Inducible expression of AmpC requires an N-acetylglucosaminidase termed NagZ. Here we describe the synthesis and characterization of hydroxyazepane inhibitors of NagZ. We find that these inhibitors enhance the susceptibility of clinically relevant Pseudomonas aeruginosa to β-lactams.

N- and C-alkylation of seven-membered iminosugars generates potent glucocerebrosidase inhibitors and F508del-CFTR correctors

The synthesis and biological evaluation of a library of novel seven-membered iminosugars is reported.

Skeletal rearrangement of seven-membered iminosugars: synthesis of (-)-adenophorine, (-)-1-epi-adenophorine and derivatives and evaluation as glycosidase inhibitors

The mirror image of natural product (+)-adenophorine along with its 1-epi-, 1-homo-analogs and other derivatives have been synthesized and evaluated as glycosidase inhibitors. The synthetic strategy is based on the skeletal rearrangement of tetrahydroxylated C-alkyl azepanes obtained via a Staudinger/azaWittig/alkylation sequence starting from a sugar-derived azidolactol. Several organometallic species have been investigated for the alkylation step including organomagnesium, organolithium, organozinc, organoaluminum and organocerium reagents. While diallylzinc proved to be the most efficient to introduce an allyl substituent, disappointing results were obtained with the other organometallic species leading either to lower yields or no reaction. Enzymatic assays indicate that (-)-adenophorine is a moderate α-l-fucosidase inhibitor.

Carbohydrate force fields

Carbohydrates present a special set of challenges to the generation of force fields. First, the tertiary structures of monosaccharides are complex merely by virtue of their exceptionally high number of chiral centers. In addition, their electronic characteristics lead to molecular geometries and electrostatic landscapes that can be challenging to predict and model. The monosaccharide units can also interconnect in many ways, resulting in a large number of possible oligosaccharides and polysaccharides, both linear and branched. These larger structures contain a number of rotatable bonds, meaning they potentially sample an enormous conformational space. This article briefly reviews the history of carbohydrate force fields, examining and comparing their challenges, forms, philosophies, and development strategies. Then it presents a survey of recent uses of these force fields, noting trends, strengths, deficiencies, and possible directions for future expansion.

Synthesis and properties of septanose carbohydrates

Seven-atom ring sugars, called septanoses, are increasingly the focus of scientific inquiries because of their potential biological activities. This article details the synthesis, conformational analysis, and protein-binding properties of septanose carbohydrates. A distinction is drawn between septanoses that are substituted in the 6-position of the ring and those that are not. When a C-6 substituent is absent, the structure is essentially that of an aldohexose in its septanose, rather than furanose or pyranose, ring form; they may play as-of-yet unexplored roles in glycobiology. Septanoses having a hydroxymethyl group at C-6, on the other hand, are ring-expanded analogues of pyranoses. Syntheses have moved beyond the preparation of seven-membered ring monosaccharides to the development of septanosyl donors. These donors have been used in the synthesis of novel di- and trisaccharides that contain septanoses as well as a variety of glycoconjugates. Low-energy conformations adopted by septanoses have been organized based on ring substitution and stereochemistry. Instances where septanoses have been demonstrated to bind to natural proteins are presented and analyzed. The major conclusion drawn in the chapter is that advances in the synthesis of septanose carbohydrates now enable a detailed investigation of their activity in a number of biological contexts.

Gaining insight into the inhibition of glycoside hydrolase family 20 exo-β-N-acetylhexosaminidases using a structural approach

One useful methodology that has been used to give insight into how chemically synthesized inhibitors bind to enzymes and the reasons underlying their potency is crystallographic studies of inhibitor-enzyme complexes. Presented here is the X-ray structural analysis of a representative family 20 exo-β-N-acetylhexosaminidase in complex with various known classes of inhibitor of these types of enzymes, which highlights how different inhibitor classes can inhibit the same enzyme. This study will aid in the future development of inhibitors of not only exo-β-N-acetylhexosaminidases but also other types of glycoside hydrolases.

Binding of 3,4,5,6-tetrahydroxyazepanes to the acid-β-glucosidase active site: implications for pharmacological chaperone design for Gaucher disease

Pharmacologic chaperoning is a therapeutic strategy being developed to improve the cellular folding and trafficking defects associated with Gaucher disease, a lysosomal storage disorder caused by point mutations in the gene encoding acid-β-glucosidase (GCase). In this approach, small molecules bind to and stabilize mutant folded or nearly folded GCase in the endoplasmic reticulum (ER), increasing the concentration of folded, functional GCase trafficked to the lysosome where the mutant enzyme can hydrolyze the accumulated substrate. To date, the pharmacologic chaperone (PC) candidates that have been investigated largely have been active site-directed inhibitors of GCase, usually containing five- or six-membered rings, such as modified azasugars. Here we show that a seven-membered, nitrogen-containing heterocycle (3,4,5,6-tetrahydroxyazepane) scaffold is also promising for generating PCs for GCase. Crystal structures reveal that the core azepane stabilizes GCase in a variation of its proposed active conformation, whereas binding of an analogue with an N-linked hydroxyethyl tail stabilizes GCase in a conformation in which the active site is covered, also utilizing a loop conformation not seen previously. Although both compounds preferentially stabilize GCase to thermal denaturation at pH 7.4, reflective of the pH in the ER, only the core azepane, which is a mid-micromolar competitive inhibitor, elicits a modest increase in enzyme activity for the neuronopathic G202R and the non-neuronopathic N370S mutant GCase in an intact cell assay. Our results emphasize the importance of the conformational variability of the GCase active site in the design of competitive inhibitors as PCs for Gaucher disease.

Imino sugars and glycosyl hydrolases: historical context, current aspects, emerging trends

Forty years of discoveries and research on imino sugars, which are carbohydrate analogues having a basic nitrogen atom instead of oxygen in the sugar ring and, acting as potent glycosidase inhibitors, have made considerable impact on our contemporary understanding of glycosidases. Imino sugars have helped to elucidate the catalytic machinery of glycosidases and have refined our methods and concepts of utilizing them. A number of new aspects have emerged for employing imino sugars as pharmaceutical compounds, based on their profound effects on metabolic activities in which glycosidases are involved. From the digestion of starch to the fight against viral infections, from research into malignant diseases to potential improvements in hereditary storage disorders, glycosidase action and inhibition are essential issues. This account aims at combining general developments with a focus on some niches where imino sugars have become useful tools for glycochemistry and glycobiology.

Towards a stable noeuromycin analog with a D-manno configuration: synthesis and glycosidase inhibition of D-manno-like tri- and tetrahydroxylated azepanes

Noeuromycin is a highly potent albeit unstable glycosidase inhibitor due to its hemiaminal function. While stable D-gluco-like analogs have been reported, no data are available for D-manno-like structures. A series of tri- and tetrahydroxylated seven-membered iminosugars displaying either a D-manno-or a L-gulo-like configuration, were synthesized from methyl α-D-mannopyranoside using a reductive amination-mediated ring expansion as the key step. Screening towards a range of commercial glycosidases demonstrated their potency as competitive glycosidase inhibitors while cellular assay showed selective albeit weak glycoprotein processing mannosidase inactivation.