Efficient Synthesis of the Glucosidase Inhibitor Blintol, the Selenium Analogue of the Naturally Occurring Glycosidase Inhibitor Salacinol

Atom-Economic Synthesis of Unsymmetrical gem-Diarylmethylthio/Seleno Glycosides via Base Mediated C(O)-S/Se Bond Cleavage and Acyl Transfer Approach of Glycosylthio/Selenoacetates

Herein, we invented the Cs₂CO₃-mediated atom economic method that streamlines the scission of the C(O)-S/Se bond involving the in situ generation of an anomeric thiolate/selenolate anion, which reacted with p-QMs to yield novel unsymmetrical gem-diarylmethylthio/seleno glycosides while retaining the anomeric stereochemistry. Notably, the key features of this protocol involve unprecedented long-range acyl transfer (from S/Se to O), thus affording acylation of the final product which is not yet reported by classical methods. This straightforward protocol offers a mild, short reaction time, synthetically simple approach, and compatibility with 8 types of sugar along with phenylthio/benzylseleno esters.

Sweet Selenium: Synthesis and Properties of Selenium-Containing Sugars and Derivatives

In the last decades, organoselenium compounds gained interest due to their important biological features. However, the lack of solubility, which characterizes most of them, makes their actual clinical exploitability a hard to reach goal. Selenosugars, with their intrinsic polarity, do not suffer from this issue and as a result, they can be conceived as a useful alternative. The aim of this review is to provide basic knowledge of the synthetic aspects of selenosugars, selenonium salts, selenoglycosides, and selenonucleotides. Their biological properties will be briefly detailed. Of course, it will not be a comprehensive dissertation but an analysis of what the authors think is the cream of the crop of this interesting research topic.

Glycosyl selenoacetates: versatile building blocks for the preparation of stereoselective selenoglycosides and selenium linked disaccharides

Glycosyl selenoacetate derivatives were prepared by the treatment of glycosyl halide with potassium selenocyanate followed by acetylation of in situ generated glycosyl selenols in one pot. A variety of selenoglycosides and selenium linked disaccharide derivatives were prepared in very good to excellent yields using glycosyl selenoacetates as stable building blocks under mild reaction conditions.

Synthesis, anticancer activity, and SAR analyses of compounds containing the 5:7-fused 4,6,8-triaminoimidazo[4,5-e][1,3]diazepine ring system

Described herein are our limited structure-activity relationship (SAR) studies on a 5:7-fused heterocycle (1), containing the 4,6,8-triaminoimidazo[4,5-e][1,3]diazepine ring system, whose synthesis and potent broad-spectrum anticancer activity we reported a few years ago. Our SAR efforts in this study are mainly focused on judicial attachment of substituents at N-1 and N(6)-positions of the heterocyclic ring. Our results suggest that there is some subtle correlation between the substituents attached at the N-1 position and those attached at the N(6)-position of the heterocycle. It is likely that there is a common hydrophobic binding pocket on the target protein that is occupied by the substituents attached at the N-1 and N(6)-positions of the heterocyclic ligand. This pocket appears to be large enough to hold either a C-18 alkyl chain of N(6) and no attachment at N-1, or a combined C-10 at N(6) and a CH2Ph at N-1. Any alkyl chain shorter or longer than C-10 at N(6) with a CH2Ph attached at N-1, would result in decrease of biological activity.

Expanded potential of seleno-carbohydrates as a molecular tool for X-ray structural determination of a carbohydrate-protein complex with single/multi-wavelength anomalous dispersion phasing

Seleno-lactoses have been successfully synthesized as candidates for mimicking carbohydrate ligands for human galectin-9 N-terminal carbohydrate recognition domain (NCRD). Selenium was introduced into the mono- or di-saccharides using p-methylselenobenzoic anhydride (Tol2Se) as a novel selenating reagent. The TolSe-substituted monosaccharides were converted into selenoglycosyl donors or acceptors, which were reacted with coupling partners to afford seleno-lactoses. The seleno-lactoses were converted to the target compounds. The structure of human galectin-9 NCRD co-crystallized with 6-MeSe-lactose was determined with single/multi-wavelength anomalous dispersion (SAD/MAD) phasing and was similar to that of the co-crystal with natural lactose.

Peroxide-shunt substrate-specificity for the Salmonella typhimurium O2-dependent tRNA modifying monooxygenase (MiaE)

Post-transcriptional modifications of tRNA are made to structurally diversify tRNA. These modifications alter noncovalent interactions within the ribosomal machinery, resulting in phenotypic changes related to cell metabolism, growth, and virulence. MiaE is a carboxylate bridged, nonheme diiron monooxygenase, which catalyzes the O2-dependent hydroxylation of a hypermodified-tRNA nucleoside at position 37 (2-methylthio-N(6)-isopentenyl-adenosine(37)-tRNA) [designated ms(2)i(6)A37]. In this work, recombinant MiaE was cloned from Salmonella typhimurium , purified to homogeneity, and characterized by UV-visible and dual-mode X-band EPR spectroscopy for comparison to other nonheme diiron enzymes. Additionally, three nucleoside substrate-surrogates (i(6)A, Cl(2)i(6)A, and ms(2)i(6)A) and their corresponding hydroxylated products (io(6)A, Cl(2)io(6)A, and ms(2)io(6)A) were synthesized to investigate the chemo- and stereospecificity of this enzyme. In the absence of the native electron transport chain, the peroxide-shunt was utilized to monitor the rate of substrate hydroxylation. Remarkably, regardless of the substrate (i(6)A, Cl(2)i(6)A, and ms(2)i(6)A) used in peroxide-shunt assays, hydroxylation of the terminal isopentenyl-C4-position was observed with >97% E-stereoselectivity. No other nonspecific hydroxylation products were observed in enzymatic assays. Steady-state kinetic experiments also demonstrate that the initial rate of MiaE hydroxylation is highly influenced by the substituent at the C2-position of the nucleoside base (v0/[E] for ms(2)i(6)A > i(6)A > Cl(2)i(6)A). Indeed, the >3-fold rate enhancement exhibited by MiaE for the hydroxylation of the free ms(2)i(6)A nucleoside relative to i(6)A is consistent with previous whole cell assays reporting the ms(2)io(6)A and io(6)A product distribution within native tRNA-substrates. This observation suggests that the nucleoside C2-substituent is a key point of interaction regulating MiaE substrate specificity.

Convenient syntheses of 1,2-trans selenoglycosides using isoselenuronium salts as glycosylselenenyl transfer reagents

Se-glycosyl-isoselenuronium salts such as three and four which can be prepared in one high-yielding step from acetohalogeno sugars proved to be convenient starting materials for the syntheses of a variety of selenoglycosides. Reaction with (ar)alkyl halides proceeds under mild conditions, in short time, at room temperature to afford the corresponding selenoglycosides in good yields. Aryl halides react to appreciable extent only if bearing activating nitro groups on the aromatic ring. Reactions with acylating reagents such as acetic anhydride and benzoyl chlorides furnished anomeric selenoesters some of which were recently proposed as starting compounds for alternative selenoglycoside syntheses. Selenodisaccharides with two different monosaccharide units could also be prepared via reactions of glycosyl-isoselenuronium salts with monosaccharide derivatives bearing primary or secondary triflate groups.

Studies directed toward the stereochemical structure determination of the naturally occurring glucosidase inhibitor, kotalanol: synthesis and inhibitory activities against human maltase glucoamylase of seven-carbon, chain-extended homologues of salacinol

The synthesis of new seven-carbon, chain-extended sulfonium salts of 1,4-anhydro-4-thio- d-arabinitol, analogues of the naturally occurring glycosidase inhibitor salacinol, are described. These compounds were designed on the basis of the structure activity data of chain-extended analogues of salacinol, with the intention of determining the hitherto unknown stereochemical structure of kotalanol, the naturally occurring seven-carbon chain-extended analogue of salacinol. The target zwitterionic compounds were synthesized by means of nucleophilic attack of the PMB-protected 1,4-anhydro-4-thio- d-arabinitols at the least hindered carbon atom of two 1,3-cyclic sulfates differing in stereochemistry at only one stereogenic center. The desired cyclic sulfates were synthesized starting from d-glucose via Wittig olefination and Sharpless asymmetric dihydroxylation. Deprotection of the coupled products by using a two-step sequence afforded two sulfonium sulfates. Optical rotation data for one of our compounds indicated a correspondence with that reported for kotalanol. However, comparison of (1)H and (13)C NMR spectral data of the synthetic compounds with those of kotalanol indicated discrepancies. The collective data from this and published work were used to propose a tentative structure for the naturally occurring compound, kotalanol. Comparison of physical data of previously synthesized analogues with those for the recently isolated six-carbon chain analogue, ponkoranol or reticulanol, also led to elucidation of this structure. Interestingly, both our compounds inhibited recombinant human maltase glucoamylase (MGA), as expected from our previous structure activity studies of lower homologues, with K i values of 0.13 +/- 0.02 and 0.10 +/- 0.02 microM.

Synthesis of phosphate derivatives related to the glycosidase inhibitor salacinol

The syntheses of polyhydroxylated imino- and anhydro thio-alditol compounds related to the naturally occurring glycosidase inhibitor, salacinol, containing a phosphate group in the side chain are described. The compounds lack hydroxyl groups on the acyclic side chain and are prototypes of the exact salacinol analogue. The synthetic strategy relies on the Mitsunobu reaction of N- and S-hydroxyalkyl derivatives of 2,3,5-tri-O-benzyl-1,4-dideoxy-1,4-imino-D-arabinitol and 1,4-anhydro-2,3,5-tri-O-benzyl-1-thio-D-arabinitol with dibenzyl phosphate to yield the corresponding protected heteroalditol phosphates. Screening of these compounds against recombinant human maltase glucoamylase (MGA), a critical intestinal glucosidase involved in the processing of oligosaccharides of glucose into glucose itself, shows that they are not effective inhibitors of MGA and demonstrates the importance of the hydroxyl and/or sulfate substituents present on the side chain for effective inhibition. The attempted synthesis of the exact analogue of salacinol by opening of cyclic phosphates is also described.

Synthesis and glycosidase inhibitory activities of chain-modified analogues of the glycosidase inhibitors salacinol and blintol

The synthesis of chain-modified analogues of the naturally-occurring glycosidase inhibitor, salacinol, and its selenium analogue, blintol is described. The modification consists of a frame shift of the sulfate moiety by one carbon atom in the zwitterionic structures as well as an extension of the acyclic chain to five carbons. The target molecules were synthesized by alkylation of 1,4-anhydro-2,3,5-tri-O-p-methoxybenzyl-4-thio (or seleno)-D-arabinitol at the ring heteroatom by 2,3,5-tri-O-p-methoxybenzyl D- or L-xylitol-1,4-cyclic sulfate, followed by deprotection with trifluoroacetic acid. Two of the four compounds inhibit recombinant human maltase glucoamylase, one of the key intestinal enzymes involved in the breakdown of glucose oligosaccharides in the small intestine, with Ki values of 20+/-4 and 53+/-5 microM.

Zwitterionic glycosidase inhibitors: salacinol and related analogues

Natural products with interesting biological properties and structural diversity have often served as valuable lead drug candidates for the treatment of human diseases. Salacinol, a naturally occurring alpha-glucosidase inhibitor, was shown to be one of the active principles of the aqueous extract of a medicinal plant that has been prescribed traditionally as an Ayurvedic treatment for type II diabetes. Salacinol contains an intriguing zwitterionic sulfonium-sulfate structure that comprises a 1,4-anhydro-4-thio-D-arabinitol core and a polyhydroxylated acyclic chain. Due to the unique structural features and its potential to become a lead drug candidate in the treatment of type II diabetes, a great deal of attention has been focused on salacinol and its analogues. Since the isolation of salacinol, several papers describing various synthetic routes to salacinol and its analogues have appeared in the literature. This review is aimed at highlighting the synthetic aspects of salacinol and related compounds as well as their structure-activity relationship studies.

Synthesis of S-alkylated sulfonium-ions and their glucosidase inhibitory activities against recombinant human maltase glucoamylase

The syntheses of nine S-alkylated, cyclic sulfonium-ions with varying alkyl chain lengths, as mimics of N-alkylated imino sugars, and their glucosidase inhibitory activities are described. The target compounds were synthesized by alkylation of 2,3,5-tri-O-benzyl-1,4-anhydro-4-thio-d-arabinitol at the ring sulfur atom using various alkyl halides, followed by deprotection using boron trichloride. Enzyme inhibitory assays against recombinant human maltase glucoamylase (MGA), a critical enzyme in the small intestine involved in the breakdown of glucose oligosaccharides into glucose itself, shows that they are effective inhibitors of MGA with K(i) values ranging from 6 to 75 microM.

Facile synthesis of sulfonium ion derivatives of 1,5-anhydro-5-thio-l-fucitol as potential α-l-fucosidase inhibitors

Five sulfonium ion derivatives with 1,5-anhydro-5-thio-L-fucitol as a core structure were efficiently synthesized as potential alpha-L-fucosidase inhibitors. The key unit, the tri-O-benzyl derivative of L-fucitol, was readily synthesized from methyl alpha-D-mannopyranoside. Alkylation with methyl iodide or 5-methoxycarbonyl-1-pentyl iodide in acetonitrile containing AgBF4 afforded the corresponding alkylated sulfonium tetrafluoroborates. Alternatively, ring opening of three 1,3-cyclic sulfates in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) containing K2CO3 afforded the corresponding zwitterionic sulfonium sulfates.

Synthesis of new analogues of salacinol containing a pendant hydroxymethyl group as potential glycosidase inhibitors

The synthesis of new analogues of the naturally occurring glycosidase inhibitor, salacinol, and its ammonium analogue, ghavamiol is described. These analogues contain an additional hydroxymethyl group at C-1, which was intended to form additional polar contacts within the active site of glycosidase enzymes. The target zwitterionic compounds were synthesized by means of nucleophilic attack at the least hindered carbon atom of 2,4-O-benzylidene-l (or d)-erythritol 1,3-cyclic sulfate by 2,5-anhydro-1,3:4,6-di-O-benzylidene-2,5-dideoxy-5-thio (or 1,5-imino)-l-iditol.

Design and synthesis of selenonium and sulfonium ions related to the naturally occurring glucosidase inhibitor salacinol

Four series of analogues of the naturally occurring glucosidase inhibitor salacinol were synthesized for structure–activity studies with different glycosidase enzymes. The target zwitterionic compounds were synthesized by means of nucleophilic attack at the least-hindered carbon atom of the 1,3-cyclic sulfates derived from D-glucose and D-mannose by the isopropylidene-protected 1,4-anhydro-4-thio- and seleno-D-allitols and the 4-thio- and seleno-L-allitols. Deprotection of the coupled products afforded the novel sulfonium and selenonium ions containing polyhy droxylated acyclic chains of four and six carbons, with different stereochemistry at the stereogenic centers and with 1,4-anhydro-4-seleno or 4-thio-D- or L- alditol heterocyclic rings. The compounds showed no significant activity against recombinant human maltase glucoamylase (MGA), a critical intestinal glucosidase involved in the processing of oligosaccharides of glucose into glucose itself.Key words: glycosidase inhibitors, zwitterionic, selenonium salts, sulfonium salts, cyclic sulfates, L-ascorbic acid, D-gulonic-γ-lactone.

Inhibition of recombinant human maltase glucoamylase by salacinol and derivatives

Inhibitors targeting pancreatic alpha-amylase and intestinal alpha-glucosidases delay glucose production following digestion and are currently used in the treatment of Type II diabetes. Maltase-glucoamylase (MGA), a family 31 glycoside hydrolase, is an alpha-glucosidase anchored in the membrane of small intestinal epithelial cells responsible for the final step of mammalian starch digestion leading to the release of glucose. This paper reports the production and purification of active human recombinant MGA amino terminal catalytic domain (MGAnt) from two different eukaryotic cell culture systems. MGAnt overexpressed in Drosophila cells was of quality and quantity suitable for kinetic and inhibition studies as well as future structural studies. Inhibition of MGAnt was tested with a group of prospective alpha-glucosidase inhibitors modeled after salacinol, a naturally occurring alpha-glucosidase inhibitor, and acarbose, a currently prescribed antidiabetic agent. Four synthetic inhibitors that bind and inhibit MGAnt activity better than acarbose, and at comparable levels to salacinol, were found. The inhibitors are derivatives of salacinol that contain either a selenium atom in place of sulfur in the five-membered ring, or a longer polyhydroxylated, sulfated chain than salacinol. Six-membered ring derivatives of salacinol and compounds modeled after miglitol were much less effective as MGAnt inhibitors. These results provide information on the inhibitory profile of MGAnt that will guide the development of new compounds having antidiabetic activity.

Synthesis of zwitterionic selenonium and sulfonium sulfates from D-mannose as potential glycosidase inhibitors

Four chain-extended analogues of the naturally occurring glycosidase inhibitor salacinol were synthesized for structure–activity studies with different glycosidase enzymes. The syntheses involved the reaction of isopropylidene-protected 1,4-thio- and 1,4-seleno-D-talitols and 1,5-thio- and 1,5-seleno-L-gulitols, derived from D-mannose, with a benzylidene- and isopropylidene-protected 1,3-cyclic sulfate, also derived from D-mannose. Deprotection of the products afforded the novel selenonium and sulfonium sulfates composed of heterocyclic five- and six-membered ring core structures with pendant polyhydroxylated, acyclic chains of six carbon atoms.Key words: glycosidase inhibitors, zwitterionic selenonium and sulfonium sulfates, cyclic sulfate

Disaccharide digestion: clinical and molecular aspects

Sugars normally are absorbed in the small intestine. When carbohydrates are malabsorbed, the osmotic load produced by the high amount of low molecular weight sugars and partially digested starches in the small intestine can cause symptoms of intestinal distention, rapid peristalsis, and diarrhea. Colonic bacteria normally metabolize proximally malabsorbed dietary carbohydrate through fermentation to small fatty acids and gases (ie, hydrogen, methane, and carbon dioxide). When present in large amounts, the malabsorbed sugars and starches can be excreted in the stool. Sugar intolerance is the presence of abdominal symptoms related to the proximal or distal malabsorption of dietary carbohydrates. The symptoms consist of meal-related abdominal cramps and distention, increased flatulence, borborygmus, and diarrhea. Infants and young children with carbohydrate malabsorption show more intense symptoms than adults; the passage of undigested carbohydrates through the colon is more rapid and is associated with detectable carbohydrates in copious watery acid stools. Dehydration often follows feeding of the offending sugar. In this review we present the clinical and current molecular aspects of disaccharidase digestion.

Synthesis of New Nitrogen Analogues of Salacinol and Deoxynojirimycin and Their Evaluation as Glycosidase Inhibitors

The synthesis of two enantiomerically pure iminosugars, analogues of 1-L-deoxynojirimycin (l-DNJ) and 1-D-deoxymannojirimycin (DMJ), was achieved using cyclic sulfate substituted isoxazoline derivatives. The piperidine ring was formed via the reduction of an isoxazoline into an amine which underwent a spontaneous intramolecular cyclization by reaction with the cyclic sulfate moiety. The nucleophilic attack of these two trisubstituted piperidines and morpholine on L- and D-erythritol-1,3-cyclic sulfates gave six new nitrogen analogues of salacinol. The inhibitory properties of the synthesized salacinol analogues were evaluated on several commercial glycosidases.

Synthesis and biological evaluation of deoxy salacinols, the role of polar substituents in the side chain on the α-glucosidase inhibitory activity

Three analogs (5, 6, and 7) lacking polar substituents in the side chain of a naturally occurring alpha-glucosidase inhibitor, salacinol (1a), were synthesized by the coupling reaction of a thiosugar, 1,4-dideoxy-1,4-epithio-D-arabinitol (3), with cyclic sulfates (8, 9, and 10), and their alpha-glucosidase inhibitory activities were examined. All these simpler analogs (5, 6, and 7) showed less inhibitory activity compared to 1a, and proved the importance of cooperative role of the polar substituents for the alpha-glucosidase inhibitory activity. A practical synthetic route to 3 starting from D-xylose is also described.

Synthesis of d-lyxitol and d-ribitol analogues of the naturally occurring glycosidase inhibitor salacinol

The synthesis of analogues of the naturally occurring glycosidase inhibitor, salacinol, in which the D-arabinitol ring has been replaced by D-lyxitol or D-ribitol, is described. Salacinol is one of the active principles in the aqueous extracts of Salacia reticulata, which are traditionally used in India and Sri Lanka for the treatment of Type II diabetes. The synthetic strategy relies on the nucleophilic attack of 1,4-anhydro-2,3,5-tri-O-p-methoxybenzyl-4-thio-D-lyxitol or 1,4-anhydro-2,3,5-tri-O-p-methoxybenzyl-4-thio-D-ribitol at the least hindered carbon of the benzylidene-protected L-cyclic sulfate derived from L-erythritol. Screening of these compounds against recombinant human maltase glucoamylase (MGA), a critical intestinal glucosidase involved in the processing of oligosaccharides of glucose into glucose itself, shows that they are not effective inhibitors of MGA and demonstrates the importance of the d-arabinitol configuration in the heterocyclic ring for effective inhibition.