The Development of an Aza-C-Glycoside Library Based on a Tandem Staudinger/Aza-Wittig/Ugi Three-Component Reaction

Thioarylation of 6-Amino-2,3,6-trideoxy-d-manno-oct-2-ulosonic Acid (IminoKdo): Access to 3,6-Disubstituted Picolinates and Mechanistic Insights

In this work, we present a metal-free coupling protocol for the regio- and stereoselective C3-thioarylation of 6-amino-2,3,6-trideoxy-d-manno-oct-2-ulosonic acid (iminoKdo). The developed procedure enables the coupling of electron-rich, electron-deficient, and hindered arylthiols, providing a series of C3-modified iminoKdo derivatives in moderate to good yields. Elucidation of active species through controlled experimental studies and time-lapse 31 P NMR analysis provides insights into the reaction mechanism. We demonstrate that, following a tandem Staudinger/aza-Wittig reaction of an azido-containing keto ester, an inseparable equimolar mixture of imine/enamine is formed. The enamine then undergoes a Stork-like nucleophilic attack with the in situ-formed disulfide reagent, resulting in the formation of the coupling products. Additionally, we describe a rarely reported acid-promoted aromatization of the C3-thioarylated iminoKdo skeleton into 3,6-disubstituted picolinates, which are reminiscent of dichotomines.

Stereoselective Synthesis of Nojirimycin α-C-Glycosides from a Bicyclic Acyliminium Intermediate: A Convenient Entry to N,C-Biantennary Glycomimetics

A simple and efficient method for the stereoselective synthesis of nojirimycin α-C-glycoside derivatives has been developed using a bicyclic carbamate-type sp2-iminosugar, whose preparation on a gram scale has been optimized, as the starting material. sp2-iminosugar O-glycosides or anomeric esters serve as excellent precursors of acyliminium cations, which can add nucleophiles, including C-nucleophiles. The stereochemical outcome of the reaction is governed by stereoelectronic effects, affording the target α-anomer with total stereoselectivity. Thus, the judicious combination of C-allylation, carbamate hydrolysis, cross-metathesis, and hydrogenation reactions provides a very convenient entry to iminosugar α-C-glycosides, which have been transformed into N,C-biantennary derivatives by reductive amination or thiourea-forming reactions. The thiourea adducts undergo intramolecular cyclization to bicyclic iminooxazolidine iminosugar α-C-glycosides upon acid treatment, broadening the opportunities for molecular diversity. A preliminary evaluation against a panel of commercial glycosidases validates the approach for finely tuning the inhibitory profile of glycomimetics.

2,3,5-Tri-O-benzyl-d-xylofuranose

The synthesis and crystallization of 2,3,5-tri-O-benzyl-d-xylofuranose permitted us to isolate the alpha anomer with a small contamination of the beta form (ca 10%), whose first crystallographic structure obtained in the P212121 space group was determined at 100 K up to a resolution of sin θmax/λ = 0.71 Å−1 and refined to an R1 value of 0.0171 with a Hirshfeld atom refinement (HAR) approach.

Cyclic Imines in Ugi and Ugi-Type Reactions

Ugi four-component reactions (U-4CRs) are widely recognized as being highly efficient for the synthesis of pseudopeptides. However, the products of these reactions are not so interesting as drug candidates because they are not conformationally restricted enough for a potent interaction with biological targets. One possible way to overcome this problem is to replace amine and oxo components in the U-4CRs with cyclic imines in so-called Joullié-Ugi three-component reactions (JU-3CRs). This approach provides a robust single-step route to peptide moieties connected to N-heterocyclic motifs that are found as core skeletons in many natural products and pharmaceutical compounds. JU-3CRs also provide much better diastereoselectivity than their four-component analogues. We survey here the redesign of many synthetic routes for the efficient preparation of a wide variety of three-, five-, six-, and seven-membered heterocyclic compounds connected to the peptide backbone. Additionally, in the Ugi reactions based on the cyclic imines, α-acidic isocyanides, or azides can be replaced with normal isocyanides or acids, respectively, leading to the synthesis of N-heterocycles attached to oxazoles or tetrazoles, which are of great pharmaceutical significance. This Review includes all research articles related to Ugi reactions based on the cyclic imines to the year 2020 and will be useful to chemists in designing novel synthetic routes for the synthesis of individual and combinatorial libraries of natural products and drug-like compounds.

Stereodivergent access to all four stereoisomers of chiral tetrahydrobenzo[f][1,4]oxazepines, through highly diastereoselective multicomponent Ugi–Joullié reaction

Starting from easily accessible chiral enantiopure 1,2-amino alcohols and salicylaldehydes, a concise route to cyclic imines has been developed. These chiral cyclic imines undergo a highly diastereoselective Ugi–Joullié reaction to give trans tetrahydrobenzo[f][1,4]oxazepines with the introduction of up to 4 diversity inputs. The cis isomer may also be attained, thanks to a thermodynamically controlled base catalysed epimerization. Free secondary amines have been obtained using an unprecedented “removable” carboxylic acid.

Starting from easily accessible enantiopure 1,2-aminoalcohols and salicylaldehydes, a concise and diastereodivergent route to tetrahydrobenzo[f][1,4]oxazepines has been developed.

Glycomimetic-based pharmacological chaperones for lysosomal storage disorders: lessons from Gaucher, GM1-gangliosidosis and Fabry diseases

Lysosomal storage disorders (LSDs) are often caused by mutations that destabilize native folding and impair the trafficking of enzymes, leading to premature endoplasmic reticulum (ER)-associated degradation, deficiencies of specific hydrolytic functions and aberrant storage of metabolites in the lysosomes. Enzyme replacement therapy (ERT) and substrate reduction therapy (SRT) are available for a few of these conditions, but most remain orphan. A main difficulty is that virtually all LSDs involve neurological decline and neither proteins nor the current SRT drugs can cross the blood-brain barrier. Twenty years ago a new therapeutic paradigm better suited for neuropathic LSDs was launched, namely pharmacological chaperone (PC) therapy. PCs are small molecules capable of binding to the mutant protein at the ER, inducing proper folding, restoring trafficking and increasing enzyme activity and substrate processing in the lysosome. In many LSDs the mutated protein is a glycosidase and the accumulated substrate is an oligo- or polysaccharide or a glycoconjugate, e.g. a glycosphingolipid. Although it might appear counterintuitive, substrate analogues (glycomimetics) behaving as competitive glycosidase inhibitors are good candidates to perform PC tasks. The advancements in the knowledge of the molecular basis of LSDs, including enzyme structures, binding modes, trafficking pathways and substrate processing mechanisms, have been put forward to optimize PC selectivity and efficacy. Moreover, the chemical versatility of glycomimetics and the variety of structures at hand allow simultaneous optimization of chaperone and pharmacokinetic properties. In this Feature Article we review the advancements made in this field in the last few years and the future outlook through the lessons taught by three archetypical LSDs: Gaucher disease, GM1-gangliosidosis and Fabry disease.

Influence of the configurational pattern of sp(2)-iminosugar pseudo N-, S-, O- and C-glycosides on their glycoside inhibitory and antitumor properties

The synthesis of a complete series of cyclic carbamate-type sp(2)-iminosugar N-, S-, O- and C-octyl pseudoglycosides related to nojirimycin, mannojirimycin and galactonojirimycin, all having the α-pseudoanomeric configuration, is reported. The gem-diamine-type N-pseudoglycosides can be accessed directly from the corresponding reducing sp(2)-imisosugar precursors by reaction with octylamine in methanol, whereas per-O-acetyl or 1-fluoro derivatives were used as pseudoglycosyl donors for the preparation of S-pseudoglycosides or O- and C-pseudoglycosides, respectively. Evaluation of their inhibitory properties against a panel of glycosidases evidenced selectivity profiles that strongly depend on the configurational pattern and the nature of the glycosidic linkage. On the contrary, the antiproliferative activity determined against a panel of tumor cell lines was largely independent of the relative orientation of the hydroxyl groups in the sp(2)-iminosugar moiety. Indeed, sp(2)-iminosugar representatives exhibiting significant growth inhibition potencies were identified in all three configurationally different types of compounds studied, namely α-d-gluco, α-d-manno and α-d-galacto glycoside analogs. Interestingly, none of the compounds affected viability and mortality of normal cells at the used concentrations. Altogether, the results strongly suggest that the anticancer activity of amphiphilic sp(2)-iminosugar glycosides might be unrelated, or not solely related, to their glycosidase inhibitory activity.

Complementary isonitrile-based multicomponent reactions for the synthesis of diversified cytotoxic hemiasterlin analogues

A small family of structural analogues of the antimitotic tripeptides, hemiasterlins, have been designed and synthesized as potential inhibitors of tubulin polymerization. The effectiveness of a multicomponent approach was fully demonstrated by applying complementary versions of the isocyanide-based Ugi reaction. Compounds strictly related to the lead natural products, as well as more extensively modified analogues, have been synthesized in a concise and convergent manner. In some cases, biological evaluation provided evidence for strong cytotoxic activity (six human tumor cell lines) and for potent inhibition of tubulin polymerization.

A sequential synthetic strategy towards unexplored dibenzo[b,f][1,4]thiazepine carboxamides: copper catalysed C–S cyclisation followed by Ugi type 3CC cascade

A two-step diversity oriented synthetic protocol has been developed to access a novel class of dihydrodibenzo[b,f][1,4]thiazepine-11-carboxamides via copper-mediated cyclisation followed by Ugi- Joullié reaction sequence.