Synthesis of selenium analogues of the naturally occurring glycosidase inhibitor salacinol and their evaluation as glycosidase inhibitors

Photoredox-Catalyzed Synthesis of C-Benzoselenazolyl/Benzothiazolyl Glycosides from 2-Isocyanoaryl Selenoethers/Thioethers and Glycosyl Bromides

Molecules containing heteroatoms, such as Se and S, play an indispensable role in the discovery and design of pharmaceuticals, whereas Se has been less studied. Here, we described a photoredox strategy to synthesize C-benzoselenazolyl (Bs) glycosides from 2-isocyanoaryl selenoethers and glycosyl bromides. This reaction was carried out under mild conditions with high efficiency. C-Benzothiazolyl (Bt) glycosides could also be synthesized from 2-isocyanoaryl thioethers using this strategy. This method can access novel seleno/thiosugars, which will benefit Se/S-containing drug discovery.

Design and Synthesis of Sulfonium Derivatives: A Novel Class of α-Glucosidase Inhibitors with Potent In Vivo Antihyperglycemic Activities

We report the first attempt of double-spot structural modification on a side-chain moiety of sulfonium-type α-glucosidase inhibitors isolated from genus Salacia. A series of sulfonium salts with benzylidene acetal linkage at the C3' and C5' positions were designed and synthesized. In vitro enzyme inhibition evaluation showed that compounds with a strong electron-withdrawing group attached at the ortho position on the phenyl ring present stronger inhibitory activities. Notably, the most potent inhibitor 21b (1.0 mpk) can exhibit excellent hypoglycemic effects in mice, which can still compete with those of acarbose (20.0 mpk). Molecular docking of 21b demonstrated that besides conventional interacting patterns, the newly introduced benzylidene acetal moiety plays an important role in anchoring the whole molecule in a concave pocket of the enzyme. The successful identification of 21b as a lead compound for new drug discovery may provide a means for structure modification and diversification of the distinguished sulfonium-type α-glucosidase inhibitors.

Synthesis of 4-Selenothreofuranose Derivatives via Pummerer-Type Reactions of trans-3,4-Dioxygenated Tetrahydroselenophenes Mediated by a Selenonium Intermediate

Selenosugars are interesting targets of organic synthesis as they would possess potential biological activities. However, 4-selenotherofuranose derivatives, which have trans configuration for the two dihydroxy substituents at the 2,3-positions and a glycoside bond at the anomeric position, are not available in the current selenosugar library. In this study, racemic 4-selenothreofuranose derivatives were synthesized from trans-3,4-dioxygenated tetrahydroselenophenes in 77-99% yields with the α/β selectivity about 7:3 via oxidation and subsequent seleno-Pummerer rearrangement. The acetoxy group introduced at the anomeric position was then substituted with various nucleophiles, including activated 6-chloropurine, which afforded 4'-selenothreonucleoside derivatives, in the presence of BF₃·OEt₂ or TMSOTf. The stereochemistry of the selenosugar products was established by ¹H NMR spectroscopy as well as X-ray analysis. The similar α/β selectivity observed in the latter glycosylation reaction to that in the former seleno-Pummerer rearrangement suggested the mediation of a common selenonium intermediate (-Se⁺=C<). It was also suggested that an unexpected interaction between the ester protecting group at the 3-position of the selenofuranose ring and the anomeric carbon atom decreases the α/β selectivity.

A review of antidiabetic active thiosugar sulfoniums, salacinol and neokotalanol, from plants of the genus Salacia

During our studies characterizing functional substances from food resources for the prevention and treatment of lifestyle-related diseases, we isolated the active constituents, salacinol (1) and neokotalanol (4), and related thiosugar sulfoniums, from the roots and stems of the genus Salacia plants [Celastraceae (Hippocrateaceae)] such as Salacia reticulata Wight, S. oblonga Wall., and S. chinensis L., and observed their antidiabetic effects. These plant materials have been used traditionally in Ayurvedic medicine as a specific remedy at the early stage of diabetes, and have been extensively consumed in Japan, the United States, and other countries as a food supplement for the prevention of obesity and diabetes. Here, we review our studies on the antidiabetic effects of plants from the genus Salacia, from basic chemical and pharmacological research to their application and development as new functional food ingredients.

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.

Recent Progress in the Construction of Natural De-O-Sulfonated Sulfonium Sugars with Antidiabetic Activities

A group of sulfonium salts equipped with a polyhydroxylated side-chain structure have been isolated and identified as potent α-glycosidase inhibitors. Consequently, they have become an attractive target in diverse research disciplines, including organic synthesis, drug discovery, and chemical biology. To this end, the development of practical and effective synthetic strategies, especially for more bioactive de-O-sulfonated sulfonium salts, is a significant research area in organic synthesis. An ideal synthetic methodology should provide easily accessible intermediates with high chemical stability for the key coupling reaction to diastereoselectively construct the sulfonium cation center. This minireview summarizes recently developed strategies applied in the construction of natural de-O-sulfonated sulfonium sugars: 1) acid-catalyzed de-O-sulfonation of sulfonium sulfate inner salts, 2) a coupling reaction between side-chain fragments containing leaving groups and a thiosugar, 3) a coupling reaction between side-chain fragments containing epoxide structures and a thiosugar, and 4) a two-step sequential S_N 2 nucleophilic substitution between side-chain fragments containing thiol groups and a diiodide derivative.

Practical Route to Neokotalanol and Its Natural Analogues: Sulfonium Sugars with Antidiabetic Activities

An efficient and divergent approach toward the synthesis of all four de-O-sulfonated sulfonium type α-glucosidase inhibitors, originally isolated from plants of genus Salacia, is reported for the first time. The key strategy features a coupling reaction between thiol derivatives and a diiodide counterpart. The newly designed thiol coupling partner presents high chemical stability, while the diiodide partner could be easily obtained with increased overall yields compared with conventional routes. The intermolecular nucleophilic substitution reaction followed by a diastereoselective intramolecular cyclization provided the target five-member sulfonium salt structure, which was connected in an α-orientation to a polyhydroxylated side-chain moiety.

Aspects of extraction and biological evaluation of naturally occurring sugar-mimicking sulfonium-ion and their synthetic analogues as potent α-glucosidase inhibitors from Salacia: a review

A review of the selective inhibitory activities of sulfonium compounds ofSalaciaagainst intestinal α-glucosidases, structural features important for effective inhibition and the toggling approach for controlling starch digestion and glucose release.

Mechanism-based candidate inhibitors of uridine diphosphate galactopyranose mutase (UGM)

Uridine diphosphate-galactopyranose mutase (UGM), an enzyme found in many eukaryotic and prokaryotic human pathogens, catalyzes the interconversion of UDP-galactopyranose (UDP-Galp) and UDP-galactofuranose (UDP-Galf), the latter being used as the biosynthetic precursor of the galactofuranose polymer portion of the mycobacterium cell wall. We report here the synthesis of a sulfonium and selenonium ion with an appended polyhydroxylated side chain. These compounds were designed as transition state mimics of the UGM-catalyzed reaction, where the head groups carrying a permanent positive charge were designed to mimic both the shape and positive charge of the proposed galactopyranosyl cation-like transition state. An HPLC-based UGM inhibition assay indicated that the compounds inhibited about 25% of UGM activity at 500 µM concentration.

First synthesis of fully modified 4'-selenoRNA and 2'-OMe-4'-selenoRNA based on the mechanistic considerations of an unexpected strand break

This study investigated oligonucleotide (ON) synthesis containing 4'-selenoribonucleoside(s) under standard phosphoramidite conditions. Careful operation using a manual ON synthetic system revealed that an unexpected strand break occurred to afford a C2-symmetric homodimer as a byproduct. In addition, this side reaction occurred during I2 oxidation. On the basis of these findings, the first synthesis of fully modified 4'-selenoRNA and 2'-OMe-4'-selenoRNA was achieved using tert-butyl hydroperoxide (TBHP) as the alternative oxidant.

Sulfonium ions as inhibitors of the mycobacterial galactofuranosyltransferase GlfT2

The mycobacterial cell wall possesses a core galactan moiety composed of approximately 30 galactofuranosyl residues attached via alternating β-(1→5) and β-(1→6) linkages.

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.

One-Pot Synthesis of 5-Arylamino-1,3,4-Selenadiazol-2(3H)-Ones from Arylisoselenocyanates

A one-pot reaction of arylisoselenocyanates, hydrazine and bis(trichloromethyl) carbonate, in the presence of a base provides an efficient route for the synthesis of 5-arylamino-1,3,4-selenadiazol-2(3H)-ones in good to excellent yields. A plausible mechanism is proposed for the reaction.

Towards the elusive structure of kotalanol, a naturally occurring glucosidase inhibitor

This Highlight describes the detailed approach used to determine the absolute stereochemistry of the stereogenic centers in the acyclic side chain of kotalanol, a naturally occurring glucosidase inhibitor isolated from the plant Salacia reticulata. The plant extract itself is used in Ayurvedic medicine for the treatment of Type 2 diabetes. We highlight the syntheses of proposed candidates based on structure-activity relationships, the total synthesis of kotalanol, and crystallographic studies of kotalanol and its de-O-sulfonated derivative complexed with recombinant human maltase glucoamylase (MGA), a critical intestinal glucosidase involved in the breakdown of glucose oligomers into glucose.

Synthesis of aza- and thia-spiroheterocycles and attempted synthesis of spiro sulfonium compounds related to salacinol

The synthesis of aza- and thia-spiroheterocycles and the attempted synthesis of spiro sulfonium compounds related to salacinol are described. The binding of the nanomolar inhibitor swainsonine to Drosophila Golgi alpha-mannosidase II (dGMII) involves a large contribution of interactions between the six-membered ring of the inhibitor and the hydrophobic pocket within the enzyme active site. Salacinol, a naturally occurring sulfonium ion, is one of the active principles in the aqueous extracts of Salacia reticulata that are traditionally used in Sri Lanka and India for the treatment of diabetes. Spiro aza- and thia-heterocycles and a spiro analogue of salacinol were designed with the expectation that the hydrocarbon portions would make hydrophobic contributions to binding. The former sets of compounds were synthesized successfully but the salacinol analogue proved to be elusive. The stereochemistry of the final compounds was determined by means of 1D-NOESY experiments. The aza- and thia-heterocycles were not effective inhibitors of Golgi alpha-mannosidase II or human maltase glucoamylase.

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 of analogues of salacinol containing a carboxylate inner salt and their inhibitory activities against human maltase glucoamylase

The syntheses of analogues of the naturally occurring glycosidase inhibitor, salacinol, containing a carboxylate inner salt are described. Salacinol is a sulfonium ion with an internal sulfate counterion. The synthetic strategy relies on the nucleophilic attack of 1,4-anhydro-2,3,5-tri-O-benzyl-4-thio-D- or L-arabinitol at the least hindered carbon of 4,5-anhydro-2,3-O-isopropylidene-D-ribonic acid benzyl ester to yield coupled adducts. Deprotection of the coupled products gives the target compounds. The compound derived from D-arabinitol inhibits recombinant human maltase glucoamylase, one of the key intestinal enzymes involved in the breakdown of glucose oligosaccharides in the small intestine, with a Ki value of 10+/-1 microM.

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.

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.

Synthesis of C-5-thioglycopyranosides and their sulfonium derivatives from 1-C-(2′-oxoalkyl)-5-S-acetylglycofuranosides

1-C-(2'-oxoalkyl)-5-S-acetylglycofuranosides of L-arabinose, D-ribose, and D-xylose were converted to 1-C-(2'-oxoalkyl)-5-thioglycopyranosides by base treatment. The transformation was achieved through beta-elimination to an acyclic alpha,beta-conjugated aldehyde (ketone or ester), followed by an intramolecular hetero-Michael addition by the 5-thiol group. The cycloaddition was highly stereoselective in favor of an equatorial 1-C-substitution. The resultant C-5-thioglycopyranosides were further converted to the sulfonium salts by treatment with cyclic sulfate and methyl iodide. Two sulfonium isomers were obtained due to the presence of both S-axial and S-equatorial substitutions. We observed that the chemical shifts of both C-1 and C-5 in the S-axial substituted sulfonium sugars are always shifted up-field (5-10 ppm) in comparison to those in the S-equatorial substitutions (deltaC 49-53 ppm vs 42-45 ppm at C-1 and 37-42 ppm vs 32-35 ppm at C-5), which provides an easy way for determination of the stereochemistry.

Synthesis of a novel class of sulfonium ions as potential inhibitors of UDP-galactopyranose mutase

Two sulfonium salts of 1,4-anhydro-4-thio-D-galactitol, with structures related to the known sulfonium salt glycosidase inhibitor, salacinol, have been synthesized as potential inhibitors of UDP-galactopyranose mutase. The synthetic strategy relies on the alkylation reaction of 1,4-anhydro-2,3,5,6-tetra-O-benzyl-4-thio-D-galactitol at the sulfur atom with 2,4-O-benzylidene-D- or -L-erythritol-1,3-cyclic sulfate. In each case, the reaction proceeded stereoselectively to yield only one stereoisomer at the stereogenic sulfur atom. The effect of the polar solvent, 1,1,1,3,3,3-hexafluoroisopropanol (HFIP), in promoting high-yielding reactions is highlighted. The target compounds are then obtained by hydrogenolysis.