Synthesis and Conformational Analysis of a Sulfonium-Ion Analogue of the Glycosidase Inhibitor Castanospermine

Borane-Trimethylamine Complex: A Versatile Reagent in Organic Synthesis

Borane-trimethylamine complex (Me3N·BH3; BTM) is the most stable of the amine-borane complexes that are commercially available, and it is cost-effective. It is a valuable reagent in organic chemistry with applications in the reduction of carbonyl groups and carbon-nitrogen double bond reduction, with considerable examples in the reduction of oximes, hydrazones and azines. The transfer hydrogenation of aromatic N-heterocycles and the selective N-monomethylation of primary anilines are further examples of recent applications, whereas the reduction of nitrobenzenes to anilines and the reductive deprotection of N-tritylamines are useful tools in the organic synthesis. Moreover, BTM is the main reagent in the regioselective cleavage of cyclic acetals, a reaction of great importance for carbohydrate chemistry. Recent innovative applications of BTM, such as CO2 utilization as feedstock and radical chemistry by photocatalysis, have extended their usefulness in new reactions. The present review is focused on the applications of borane-trimethylamine complex as a reagent in organic synthesis and has not been covered in previous reviews regarding amine-borane complexes.

On the nature of the electronic effect of multiple hydroxyl groups in the 6-membered ring - the effects are additive but steric hindrance plays a role too

Research during the last two decades has shown a remarkable directional component of the substituent effects of hydroxy groups, which has a profound effect on the properties of hydroxylated compounds such as carbohydrates. While the epimerisation of a single hydroxyl function is well studied the consequence of multiple epimerisations is more speculative. In this work the effect of three epimerisations was investigated. To this end epimeric 2-phenyl iminoxylitols that have a phenyl group as a conformational anchor and thus hydroxyl groups in the axial or equatorial position, respectively, were synthesized and their pK_a and conformation were studied. The results show that the large difference in the electronic effect between the axial and equatorial hydroxyls is partially cancelled by counteracting steric hindrance from 1,3-diaxial interactions. Hydrogen bonding does not appear to play any role in the electronic influence of the hydroxyl groups.

Gold(iii) assisted C–H activation of 1,4,7-trithiacyclononane: synthesis and spontaneous resolution of a bicyclic chiral sulfonium salt

A trithiamacrocyclic ligand complex of Au(iii) undergoes a redox-mediated thermal reaction to form a chiral bicyclic sulfonium salt.

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.

Do glycosyl sulfonium ions engage in neighbouring-group participation? A study of oxathiane glycosyl donors and the basis for their stereoselectivity

Neighbouring-group participation has long been used to control the synthesis of 1,2-trans-glycosides. More recently there has been a growing interest in the development of similar strategies for the synthesis of 1,2-cis-glycosides, in particular the use of auxiliary groups that generate sulfonium ion intermediates. However, there has been some debate over the role of sulfonium ion intermediates in these reactions: do sulfonium ions actually engage in neighbouring-group participation, or are they a resting state of the system prior to reaction through an oxacarbenium ion intermediate? Herein, we describe the reactivities and stereoselectivities of a family of bicyclic thioglycosides in which an oxathiane ring is fused to the sugar to form a trans-decalin-like structure. A methyl sulfonium ion derived from one such glycosyl donor is so stable that it can be crystallised from ethanol, yet it reacts with complete stereoselectivity at high temperature. The importance of a ketal group in the oxathiane ring for maintaining this high stereoselectivity is investigated using a combination of experiment and ab initio calculations. The data are discussed in terms of S(N)1 and S(N)2 type mechanisms. Trends in stereoselectivity across a series of compounds are more consistent with selective addition to oxacarbenium ions rather than a shift between S(N)1 and S(N)2 mechanisms.

Fluorine conformational effects in organocatalysis: an emerging strategy for molecular design

Molecular design strategies that profit from the intrinsic stereoelectronic and electrostatic effects of fluorinated organic molecules have mainly been restricted to bio-organic chemistry. Indeed, many fluorine conformational effects remain academic curiosities with no immediate application. However, the renaissance of organocatalysis offers the possibility to exploit many of these well-described phenomena for molecular preorganization. In this minireview, we highlight examples of catalyst refinement by introduction of an aliphatic C-F bond which functions as a chemically inert steering group for conformational control.

Glycosidase inhibition: assessing mimicry of the transition state

Glycoside hydrolases, the enzymes responsible for hydrolysis of the glycosidic bond in di-, oligo- and polysaccharides, and glycoconjugates, are ubiquitous in Nature and fundamental to existence. The extreme stability of the glycosidic bond has meant these enzymes have evolved into highly proficient catalysts, with an estimated 10(17) fold rate enhancement over the uncatalysed reaction. Such rate enhancements mean that enzymes bind the substrate at the transition state with extraordinary affinity; the dissociation constant for the transition state is predicted to be 10(-22) M. Inhibition of glycoside hydrolases has widespread application in the treatment of viral infections, such as influenza and HIV, lysosomal storage disorders, cancer and diabetes. If inhibitors are designed to mimic the transition state, it should be possible to harness some of the transition state affinity, resulting in highly potent and specific drugs. Here we examine a number of glycosidase inhibitors which have been developed over the past half century, either by Nature or synthetically by man. A number of criteria have been proposed to ascertain which of these inhibitors are true transition state mimics, but these features have only be critically investigated in a very few cases.

Synthesis and evaluation of D-gluco-pyranocyclopropyl amines as potential glucosidase inhibitors

In the recent past sugar-derived cyclopropylamines were proposed as structurally new glycosidase inhibitors. In this Letter we report our efforts in the synthesis of a set of alpha-glucose configured oxabicyclo[4.1.0] heptanes, based on this hypothesis, bearing an amine substituent on the propyl ring and reveal that their inhibitory potential towards a range of mammalian glucosidases is modest.

The effect of electrostatic interactions on conformational equilibria of multiply substituted tetrahydropyran oxocarbenium ions

The three-dimensional structures of dioxocarbenium ions related to glycosyl cations were determined by an analysis of spectroscopic, computational, and reactivity data. Hypothetical low-energy structures of the dioxocarbenium ions were correlated with both experimentally determined (1)H NMR coupling constants and diastereoselectivity results from nucleophilic substitution reactions. This method confirmed the pseudoaxial preference of C-3 alkoxy-substituted systems and revealed the conformational preference of the C-5 alkoxymethyl group. Although the monosubstituted C-5 alkoxymethyl substituent preferred a pseudoequatorial orientation, the C-5-C-6 bond rotation was controlled by an electrostatic effect. The preferred diaxial conformer of the trans-4,5-disubstituted tetrahydropyranyl system underscored the importance of electrostatic effects in dictating conformational equilibria. In the 2-deoxymannose system, although steric effects influenced the orientation of the C-5 alkoxymethyl substituent, the all-axial conformer was favored because of electrostatic stabilization.

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.

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.

New Chain-Extended Analogues of Salacinol and Blintol and Their Glycosidase Inhibitory Activities. Mapping the Active-Site Requirements of Human Maltase Glucoamylase

The synthesis of new chain-extended sulfonium and selenonium salts of 1,4-anhydro-4-thio-(or 4-seleno)-d-arabinitol, analogues of the naturally occurring glycosidase inhibitor salacinol, is described. Nucleophilic attack at the least hindered carbon atom of 4,6-O-benzylidene-2,5-di-O-p-methoxybenzyl-d-mannitol-1,3-cyclic sulfate by 2,3,5-tri-O-p-methoxybenzyl-1,4-anhydro-4-thio-(or 4-seleno)-d-arabinitol gave the sulfonium and selenonium sulfates, respectively. Subsequent deprotection with trifluoroacetic acid yielded the target compounds. In these analogues, an extended polyhydroxylated aliphatic side chain has been incorporated while maintaining the stereochemistry of C-2' and C-3' of salacinol or blintol. These compounds were designed to probe the premise that they would bind with higher affinity to glucosidases than salacinol because the extra hydroxyl groups in the acyclic chain would make favorable polar contacts within the active site. Both target compounds inhibited recombinant human maltase glucoamylase, one of the key intestinal enzymes involved in the breakdown of glucose oligosaccharides in the small intestine, with Ki values in the low micromolar range. Comparison of these values to those of related compounds synthesized in previous studies has provided a better understanding of structure-activity relationships and the optimal stereochemistry at the different stereogenic centers required of an inhibitor of this enzyme. With respect to chain extension, the configurations at C-2' and C-4' are critical for activity, the configuration at C-3', bearing the sulfate moiety, being unimportant. The desired configuration at C-5' is also specified. However, comparison of the activities of the chain-extended analogues with those of salacinol and blintol indicates that there is no particular advantage of the chain-extension relative to salacinol or blintol. These results are similar to those reported earlier for kotalanol, a 7-carbon-extended derivative, versus salacinol against rat intestinal maltase, sucrase, and isomaltase.

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 a sulfonium ion analogue of the glycosidase inhibitor swainsonine

The synthesis of a bicyclic sulfonium ion analogue of a naturally occurring indolizidine alkaloid, swainsonine, in which the bridgehead nitrogen atom is replaced by a sulfonium ion, has been achieved by a multistep synthesis starting from (2S,3S,4R)-2,3-dibenzyloxy-4-formaldehyde-thiolane. The synthetic strategy relies on the intramolecular displacement of a leaving group on a pendant acyclic chain by a cyclic thioether. This bicyclic sulfonium salt provides a candidate with which to further probe the hypothesis that a sulfonium salt carrying a permanent positive charge would be an effective glycosidase inhibitor.

Synthesis of thioswainsonine as a potential glycosidase inhibitor

The synthesis of a bicyclic sulfonium-ion analogue of a naturally occurring glycosidase inhibitor, swainsonine, in which the bridgehead nitrogen atom is replaced by a sulfonium ion, has been achieved by a multi-step synthesis starting from 1,4-anhydro-2,3-di-O-benzyl-4-thio-D-lyxitol. The synthetic strategy relies on the intramolecular displacement of a leaving group on a pendant acyclic chain by a cyclic thioether. This bicyclic sulfonium salt will serve as a candidate to test the hypothesis that a sulfonium salt carrying a permanent positive charge would be an effective glycosidase inhibitor.

Intramolecular 5-endo-Trig Aminomercuration of β-Hydroxy-γ-alkenylamines: Efficient Route to a Pyrrolidine Ring and Its Application for the Synthesis of (+)-Castanospermine and Analogues

The intramolecular aminomercuration reaction of sugar-derived beta-hydroxy-gamma-alkenylamines 8a-c undergoes 5-endo-trig cyclization in high yield. The sugar-substituted pyrrolidines thus obtained were elaborated to the synthesis of polyhydroxylated indolizidine alkaloids, namely, castanospermine 1a, 1-epi-castanospermine 1b, and 8a-epi-castanospermine 1c, having promising glycosidase inhibitory activities.

Stereoelectronic substituent effects

An investigation was carried out on the influence of the stereochemistry of substituents, particularly hydroxyl groups, on their electronic effects in piperidines, carbohydrates (pyranosides), and related compounds. Polar groups, such as OH, OR, and F, were found in the 3 and 4 position to be much more electron-withdrawing when positioned equatorially rather than axially. In contrast, little difference in electronic effects was observed from apolar groups as a result of epimerization. These observations were believed to be caused by differences in charge-dipole interactions and were used to explain why stereoisomeric glycosides hydrolyze with different rates. The conformational changes of hydroxylated piperidines and related compounds as a function of pH were likewise explained from the different substituent effects of axial and equatorial OH groups.

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

A New Class of Glucosidase Inhibitor: Analogues of the Naturally Occurring Glucosidase Inhibitor Salacinol with Different Ring Heteroatom Substituents and Acyclic Chain Extension

Six chain-extended analogues of the naturally occurring glycosidase inhibitor salacinol, with ring-heteroatom variation, were synthesized for structure-activity studies with different glycosidase enzymes. The syntheses involved the reaction of PMB-protected D- and L- seleno-, thio-, and iminoarabinitol with a benzylidene- and isopropylidene-protected 1,3-cyclic sulfate, derived from commercially available D-sorbitol, in 1,1,1,3,3,3-hexafluoro-2-propanol containing potassium carbonate. Deprotection of the products afforded the novel selenonium, sulfonium, and iminium analogues of salacinol containing polyhydroxylated, monosulfated, extended acyclic chains of 6-carbons, differing in stereochemistry at the stereogenic centers and ring-heteroatom constitution. Four of these 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 in the micromolar range, thus providing lead candidates for the treatment of Type 2 diabetes.

StereoselectiveC-Glycosylation Reactions of Pyranoses: The Conformational Preference and Reactions of the Mannosyl Cation

A systematic study of C-glycosylations of acetals related to mannose and other pyranoses was conducted. The C-5 alkoxyalkyl group provides only a modest influence on stereoselectivity. On the other hand, studies of pentopyranoses bearing alkoxy groups at C-2, C-3, and C-4 showed that the alkoxy groups exerted powerful influences on selectivity. In the case of mannose, the high alpha selectivity observed with C-mannosylation was reversed to high beta selectivity if the C-5 alkoxyalkyl group were removed. An analysis of the conformational preferences of the intermediate oxocarbenium ions, including the mannosyl cation, as well as consideration of steric effects that develop in the transition states for nucleophilic attack provide explanations for these phenomena.

Synthesis of Sulfonium Sulfate Analogues of Disaccharides and Their Conversion to Chain-Extended Homologues of Salacinol: New Glycosidase Inhibitors

Four chain extended homologues of salacinol, a naturally occurring glycosidase inhibitor, were prepared for evaluation as inhibitors of glucosidase enzymes involved in the breakdown of carbohydrates. The syntheses involved the reactions of 1,4-anhydro-2,3,5-tri-O-benzyl-4-thio-D-arabinitol with cyclic sulfate derivatives of different monosaccharides. Debenzylation of the products afforded the novel sulfonium sulfate derivatives of D-glucose, D-galactose, D-arabinose, and D-xylose that are of interest in their own right as glycosidase inhibitors. Reduction to the corresponding alditols then afforded the homologues of salacinol containing polyhydroxylated, acyclic chains of 5- and 6-carbons, differing in stereochemistry at the stereogenic centers. Three of the chain-extended homologues inhibited recombinant human maltase glucoamylase, one of the key intestinal enzymes involved in the breakdown of glucose oligosaccharides in the small intestine, with Ki values in the low micromolar range, of approximately the same magnitude as salacinol, thus providing lead candidates for the treatment of Type 2 diabetes.

Electrostatic interactions in cations and their importance in biology and chemistry

Electrostatic effects exert strongly stabilizing influences on cations, in many cases controlling the conformational preferences of these cations. The lowest energy conformers are ones where the positive charge is brought closest to substituents bearing partial negative charges. These conformational biases, along with stereoelectronic effects, can control the stereoselectivity of reactions involving carbocationic intermediates.

Synthesis and Conformational Analysis of Bicyclic Sulfonium Salts. Structures Related to the Glycosidase Inhibitor Australine

The syntheses of eight sulfonium compounds with structures related to the naturally occurring pyrrolizidine alkaloid, australine, in which the bridgehead nitrogen atom is replaced by a sulfonium ion, are described. The synthetic strategy relies on the intramolecular attack of a cyclic thioether across a terminal double bond in the presence of a suitable electrophile. We postulate that these compounds, having a permanent positive charge on the sulfur atom, will mimic the highly unstable oxacarbenium ion transition state in a glycosidase-catalyzed hydrolysis reaction. The conformational preferences of these compounds, based on analysis of 1H-1H vicinal coupling constants and 1D-NOESY data, are attributed to both steric and electrostatic interactions. These compounds will be used in the study of structure-activity relationships with glycosidase enzymes.

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.

1,3-Dipolar Cycloaddition Reaction of d-Glucose-Derived Nitrone with Allyl Alcohol: Synthesis of 2-Hydroxy-1-deoxycastanospermine Analogues

[reaction: see text] The synthesis and evaluation of glycosidase inhibitory activity of polyhydroxylated indolizidine alkaloids namely 2-hydroxy-1-deoxycastanospermine 3a,b and 2-hydroxy-1-deoxy-8a-epi-castanospermine 3c,d is reported. The key step involves the intermolecular 1,3-dipolar cycloaddition of allyl alcohol to d-glucose-derived nitrone 4, followed by tosylation, that afforded four diastereomeric sugar-substituted isoxazolidines 5a-d with the desired regioselectivity. The one-pot conversion of 5a-d to pyrrolidines 8a-d by hydrogenolysis, removal of 1,2-acetonoide functionality, and hydrogenation afforded corresponding target molecules 3a-d.

Synthesis of Alkylated Deoxynojirimycin and 1,5-Dideoxy-1,5-iminoxylitol Analogues: Polar Side-Chain Modification, Sulfonium and Selenonium Heteroatom Variants, Conformational Analysis, and Evaluation as Glycosidase Inhibitors

The syntheses of N-alkylated deoxynojirimycin and 1,5-dideoxy-1,5-iminoxylitol derivatives having either a D- or an L-erythritol-3-sulfate functionalized N-substituent are reported. The alkylating agent used was a cyclic sulfate derivative, whereby selective attack of the nitrogen atom at the least hindered primary center afforded the desired ammonium salt. In aqueous solution, these salts were configurationally labile at the ammonium center. Sulfonium and/or selenonium analogues of the ammonium salts were prepared by analogous reactions. The chalcogen salts were obtained as mixtures of diastereomers, separable in some cases, differing only in the stereochemistry at the configurationally stable sulfur or selenium atoms. Proof of configuration and conformation of each compound was obtained by detailed NMR experiments. The compounds are six-membered ring analogues of salacinol, a known sulfonium-salt glucosidase inhibitor. Evaluation of the target compounds for enzyme inhibition of the glucosidase enzyme glucoamylase G2 indicated that these compounds were either inactive or, at best, only weak inhibitors of maltose hydrolysis.

The structure and conformational behavior of sulfonium salt glycosidase inhibitors in solution: a combined quantum mechanical NMR approach

We present the conformational analysis of an inhibitor of alpha-mannosidase, based on a novel sulfonium salt structure (1) that mimics the mannosyl cation intermediate. Because of the number of possible isomeric structures for 1, as well as its complex molecular structure, traditional conformational analysis by NMR was not applicable. Instead, a single experimentally consistent structure was obtained from finite perturbation quantum mechanical calculations of the NMR J-couplings at the B3LYP/6-311G** level. Using a full relaxation matrix analysis, we showed that the quantum-predicted NMR structure was the only isomer that was consistent with the experimental NOE intensities. The results illustrate the potential for finite perturbation calculations to be useful in the analysis of complex charged molecules.

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.

Synthesis and Evaluation of Isourea-Type Glycomimetics Related to the Indolizidine and Trehazolin Glycosidase Inhibitor Families

A practical synthesis of reducing isourea-derived azasugar glycomimetics related to the indolizidine and trehazolin glycosidase inhibitor families with different pK(a) values is disclosed. The polyhydroxylated bicyclic system was built from readily accessible hexofuranose derivatives through a synthetic scheme that involves the preparation of a 5-deoxy-5-carbodiimido adduct by triphenylphosphine-mediated tandem Staudinger--aza-Wittig-type coupling of azide and isothiocyanate precursors, intramolecular cyclization of a transient vic-hydroxycarbodiimide derivative, and nucleophilic addition of the endocyclic nitrogen atom of the generated 2-amino-2-oxazoline intermediate, with a pseudo-C-nucleoside structure, to the masked aldehyde group of the monosaccharide. The last step is pH-dependent so that the final compounds can pivot between the furanose and the 2-oxaindolizidine forms. Nevertheless, the indolizidine tautomer having the R configuration at the aminoacetalic center, fitting the anomeric effect, was the only species detected in solution at neutral or slightly acidic pH when starting from solutions at basic pH. Glycosidase inhibition tests (K(i) values down to 1.9 microM) showed a marked dependence of the selectivity and potency toward alpha- and beta-glucosidases upon the nature of the substituent at the exocyclic isourea nitrogen, shifting from alpha- to beta-selectivity when going from hydrophilic to hydrophobic substituents. Enzyme inhibition is also pH dependent, supporting a dominant role for the uncharged form of the polyhydroxyiminoindolizidine system in the inhibition of beta-glucosidases.

Conformational analysis of sulfur-containing 6-deoxy-l-hexose derivatives by molecular modeling and NMR spectroscopy. A theoretical study and experimental evidence of intramolecular nonbonded interactions between sulfur and oxygen

6-Deoxy-l-mannose diphenyldithioacetal (1) unexpectedly gave the rearranged products phenyl 3,4-di-O-acetyl-2-S-phenyl-1,2-dithio-6-deoxy-beta-l-glucopyranoside (9) and 3,4-di-O-acetyl-2,5-anhydro-6-deoxy-l-glucose diphenyldithioacetal (10) upon treatment with acetyl chloride, while 6-deoxy-l-mannose ethylenedithioacetal (3) yielded (4aR,6S,7S,8R,8aS)-7,8-diacetyloxy-6-methylhexahydro-4aH-[1,4]dithiino[2,3b]pyran (11), whose structure was further confirmed by X-ray diffraction, and 3,4-di-O-acetyl-2,5-anhydro-l-rhamnose ethylenedithioacetal (12). The geometry of the four rearranged products as well as that of 1-thio-6-deoxy-l-mannopyranosides 5 and 7 and their acetyl derivatives 6 and 8 was studied by density functional theory (B3LYP/6-31G) molecular models, in combination with a Karplus-type analysis of the NMR vicinal coupling constants, revealing that the six-membered ring of pyranosides 5-9 and 11 exists in a slightly distorted chair conformation (6-13% distortion) and that the conformational behavior of the 2,5-anhydro-6-deoxy-l-glucose dithioacetals 10 and 12 is strongly influenced by the presence of stabilizing intramolecular nonbonded sulfur-oxygen 1,4- and 1,5-interactions. Compounds 9-12 were formed by a molecular rearrangement via sulfonium ion intermediates followed by stereoselective intramolecular cyclizations as formulated by the quantum chemical calculations performed in the present study.

Selection of a high-energy bioactive conformation of a sulfonium-ion glycosidase inhibitor by the enzyme glucoamylase G2

Transferred nuclear Overhauser effect and rotating-frame Overhauser enhancement NMR spectroscopies are used to probe the conformation of a bicyclic sulfonium ion, which is an analogue of the naturally occurring glycosidase inhibitor castanospermine, bound to the enzyme glucoamylase G2. Enzyme inhibition assays indicate that the bicyclic sulfonium ion is a slightly better inhibitor (K(i) = 1.32 mM) of glucoamylase G2 than the naturally occurring sulfonium-ion glycosidase inhibitor, salacinol, with a K(i) value of 1.7 mM. The NMR results are interpreted in terms of the selection by the enzyme of a high-energy conformation of the ligand that is already represented in the ensemble of free-ligand conformations.

New synthetic approach to enantiopure bicyclic sulfonium salts: swainsonine analogues

The enantioselective synthesis of bicyclic sulfonium salts 8 or 9, thioanalogues of swainsonine derivatives, is described. The synthetic strategy is based on a stereo- and regiospecific transannular cyclization reaction of nine-membered cyclic sulfides, mediated by Me(3)SiI or carried out under acidic catalysis.

The NH---FC dipole orientation effect for pendant exocyclic CH(2)F

[structure: see text] DFT and MMFF force field calculations for 2 (R = H) predict that two conformers dominate in water (>/=95%) and both sustain a geometry in which the C-F and H-N dipoles align oppositely in a near-planar arrangement. The (1)H NMR spectra (D(2)O and DMSO-d(6)) and X-ray structure for 2 (R = SO(2)NHEt) confirm the predictions in all essentials. A novel single-conformer system is proposed.

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

The syntheses of two selenium analogues (10 and 11) of the naturally occurring sulfonium ion, salacinol (3), are described. Salacinol 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. The synthetic strategy relies on the nucleophilic attack of a 2,3,5-tri-O-benzyl-1,4-anhydro-4-seleno-D-arabinitol at the least hindered carbon of benzyl- or benzylidene-protected D- or L-erythritol-1,3-cyclic sulfate. The use of 1,1,1,3,3,3-hexafluoro-2-propanol as a solvent in the coupling reaction proves to be beneficial. Enzyme inhibition assays indicate that 10 is a better inhibitor (K(i) = 0.72 mM) of glucoamylase than 3, which has a K(i) value of 1.7 mM. In contrast, 11 showed no significant inhibition of glucoamylase. Compounds 10 and 11 showed no significant inhibition of barley-alpha-amylase or porcine pancreatic-alpha-amylase.

S-adenosylmethionine conformations in solution and in protein complexes: conformational influences of the sulfonium group

S-Adenosylmethionine (AdoMet) and other sulfonium ions play central roles in the metabolism of all organisms. The conformational preferences of AdoMet and two other biologically important sulfonium ions, S-methylmethionine and dimethylsulfonioproprionic acid, have been investigated by NMR and computational studies. Molecular mechanics parameters for the sulfonium center have been developed for the AMBER force field to permit analysis of NMR results and to enable comparison of the relative energies of the different conformations of AdoMet that have been found in crystal structures of complexes with proteins. S-Methylmethionine and S-dimethylsulfonioproprionate adopt a variety of conformations in aqueous solution; a conformation with an electrostatic interaction between the sulfonium sulfur and the carboxylate group is not noticeably favored, in contrast to the preferred conformation found by in vacuo calculations. Nuclear Overhauser effect measurements and computational results for AdoMet indicate a predominantly anti conformation about the glycosidic bond with a variety of conformations about the methionyl C(alpha)-C(beta) and C(beta)-C(gamma) bonds. An AdoMet conformation in which the positively charged sulfonium sulfur is near an electronegative oxygen in the ribose ring is common. Comparisons of NMR results for AdoMet with those for the uncharged S-adenosylhomocysteine and 5'-methylthioadenosine, and the anionic ATP, indicate that the solution conformations are not dictated mainly by molecular charge. In 20 reported structures of AdoMet.protein complexes, both anti and syn glycosidic torsional angles are found. The methionyl group typically adopts an extended conformation in complexes with enzymes that transfer the methyl group from the sulfonium center, but is more folded in complexes with proteins that do not catalyze reactions involving the sulfur and which can use the sulfonium sulfur solely as a binding site. The conformational energies of AdoMet in these crystal structures are comparable to those found for AdoMet in solution. The sulfonium sulfur is in van der Waals contact with a protein heteroatom in the structures of four proteins, which reflects an energetically favorable contact. Interactions of the sulfonium with aromatic rings are rarely observed.

Synthesis of nitrogen analogues of salacinol and their evaluation as glycosidase inhibitors

The syntheses of two nitrogen analogues (11 and 12) of the naturally occurring sulfonium ion, salacinol (7) are described. The latter compound 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. The synthetic strategy relies on the nucleophilic attack of a 1,4-dideoxy-1,4-imino-D- or L-arabinitol at the least hindered carbon of 2,4-O-benzylidene D- or L-erythritol-1,3-cyclic sulfate. The nitrogen analogues bear a permanent positive charge and serve as mimics of the sulfonium ion. We reasoned that these ammonium derivatives should function in a manner similar to that of known glycosidase inhibitors of the alkaloid class such as castanospermine (4) and deoxynojirimycin (5). Enzyme inhibition assays indicate that salacinol (7) is a weak (K(i) = 1.7 mM) inhibitor of glucoamylase, whereas compounds 11 and 12 inhibit glucoamylase with K(i) values in the range approximately 10-fold higher. The nitrogen analogues 11 and 12 showed no significant inhibitory effect of either barley alpha-amylase (AMY1) or porcine pancreatic alpha-amylase (PPA) at concentrations of 5 mM. In contrast, salacinol (7) inhibited AMY1 and PPA in the micromolar range, with K(i) values of 15 +/- 1 and 10 +/- 2 microM, respectively.

A new class of glycosidase inhibitor: synthesis of salacinol and its stereoisomers

Salacinol (4) 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. The syntheses of salacinol (4), the enantiomer of salacinol (5), and a diastereomer (7) are described. The synthetic strategy relies on the selective nucleophilic attack of 2,3,5-tri-O-benzyl-1,4-anhydro-4-thio-D- or L-arabinitol at C-1 of 2,4-O-benzylidene D- or L-erythritol-1,3-cyclic sulfate. The work serves to resolve the ambiguity about the exact structure of salacinol and establishes conclusively the structure of the natural product.