Synthesis of a nitrogen analogue of salacinol and its alpha-glucosidase inhibitory activity

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.

Novel C-2 Symmetric Molecules as α-Glucosidase and α-Amylase Inhibitors: Design, Synthesis, Kinetic Evaluation, Molecular Docking and Pharmacokinetics

A series of symmetrical salicylaldehyde-bishydrazine azo molecules, 5a–5h, have been synthesized, characterized by ¹H-NMR and ¹³C-NMR, and evaluated for their in vitro α-glucosidase and α-amylase inhibitory activities. All the synthesized compounds efficiently inhibited both enzymes. Compound 5g was the most potent derivative in the series, and powerfully inhibited both α-glucosidase and α-amylase. The IC₅₀ of 5g against α-glucosidase was 0.35917 ± 0.0189 µM (standard acarbose IC₅₀ = 6.109 ± 0.329 µM), and the IC₅₀ value of 5g against α-amylase was 0.4379 ± 0.0423 µM (standard acarbose IC₅₀ = 33.178 ± 2.392 µM). The Lineweaver-Burk plot indicated that compound 5g is a competitive inhibitor of α-glucosidase. The binding interactions of the most active analogues were confirmed through molecular docking studies. Docking studies showed that 5g interacts with the residues Trp690, Asp548, Arg425, and Glu426, which form hydrogen bonds to 5g with distances of 2.05, 2.20, 2.10 and 2.18 Å, respectively. All compounds showed high mutagenic and tumorigenic behaviors, and only 5e showed irritant properties. In addition, all the derivatives showed good antioxidant activities. The pharmacokinetic evaluation also revealed promising results

Optimization of Hyperglycemic Induction in Zebrafish and Evaluation of Its Blood Glucose Level and Metabolite Fingerprint Treated with Psychotria malayana Jack Leaf Extract

A standard protocol to develop type 1 diabetes in zebrafish is still uncertain due to unpredictable factors. In this study, an optimized protocol was developed and used to evaluate the anti-diabetic activity of Psychotria malayana leaf. The aims of this study were to develop a type 1 diabetic adult zebrafish model and to evaluate the anti-diabetic activity of the plant extract on the developed model. The ability of streptozotocin and alloxan at a different dose to elevate the blood glucose levels in zebrafish was evaluated. While the anti-diabetic activity of P. malayana aqueous extract was evaluated through analysis of blood glucose and LC-MS analysis fingerprinting. The results indicated that a single intraperitoneal injection of 300 mg/kg alloxan was the optimal dose to elevate the fasting blood glucose in zebrafish. Furthermore, the plant extract at 1, 2, and 3 g/kg significantly reduced blood glucose levels in the diabetic zebrafish. In addition, LC-MS-based fingerprinting indicated that 3 g/kg plant extract more effective than other doses. Phytosterols, sugar alcohols, sugar acid, free fatty acids, cyclitols, phenolics, and alkaloid were detected in the extract using GC-MS. In conclusion, P. malayana leaf aqueous extract showed anti-diabetic activity on the developed type 1 diabetic zebrafish model.

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.

Amino-functionalized iminocyclitols: synthetic glycomimetics of medicinal interest

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

Biochemical investigation and gene expression analysis of the immunostimulatory functions of an edible Salacia extract in rat small intestine

Roots and bark from plants belonging to genus Salacia of the family Hippocrateaceae (Salacia reticulata, Salacia oblonga, etc.) have been used for traditional Ayurvedic medicine, particularly for the treatment of diabetes. In our study, we evaluated the gene expression profiles in the small intestinal epithelium of rats that were given a Salacia plant extract to gain insight into its effects on the small intestine. In detail, DNA microarray analysis was performed to evaluate the gene expression profiles in the rat ileal epithelium. The intestinal bacterial flora was also studied using T-RFLP (Nagashima method) in these rats. Expressions of many immune-related genes, especially Th1-related genes associated with cell-mediated immunity, were found to increase in the small intestinal epithelium and the intestinal bacterial flora became similar to those in the case with Salacia plant extract administration. Our study thus revealed that Salacia plant extract exerts bioregulatory functions by boosting intestinal immunity.

Facile synthesis of de-O-sulfated salacinols: revision of the structure of neosalacinol, a potent alpha-glucosidase inhibitor

Facile synthesis of de-O-sulfated salacinols (3) was developed by employing the coupling reaction of an epoxide, 1,2-anhydro-3,4-di-O-benzyl-D-erythritol (9) with 2,3,5-tri-O-benzyl-1,4-dideoxy-1,4-epithio-D-arabinitol (10) as the key reaction. The reported structure of a potent alpha-glucosidase inhibitor named neosalacinol (8), isolated recently from Ayurvedic medicine Salacia oblonga, was proved incorrect, and revised to be de-O-sulfated salacinol formate (3c) by comparison of the spectroscopic properties with those of the authentic specimen synthesized. Discrepancies and confusion in the literature concerning the NMR spectroscopic properties of salacinol (1) have also been clarified.

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.

Characterization of Inhibitors of Postprandial Hyperglycemia from the Leaves of Nerium indicum

Nerium indicum is an India-Pakistan-originated shrub belonging to the oleander family. The ingestion of leaves of N. indicum before a meal is known to effect the lowering of postprandial glucose levels in Type II diabetic patients and this plant is now used as a folk remedy for Type II diabetes in some regions of Pakistan. In the present study, the hot-water extract of N. indicum leaves was found to reduce the postprandial rise in the blood glucose when maltose or sucrose was loaded in rats. It was also found that the extract strongly inhibited alpha-glucosidase, suggesting that the suppression of the postprandial rise in the blood glucose is due to the occurrence of some inhibitors of alpha-glucosidase in the leaves. We, therefore, tried to isolate the active principles from the leaf extract, using alpha-glucosidase-inhibitory activity as the index. Employing Sephadex G-15, silica gel and reversed-phase HPLC, we isolated two active compounds. The UV, mass and NMR spectrometric analyses established that the chemical structures of these compounds are 3-O-caffeoylquinic acid (chlorogenic acid) and its structural isomer, 5-O-caffeoylquinic acid. Both compounds were shown to inhibit alpha-glucosidases in a non-competitive manner. The authentic chlorogenic acid was found to suppress the postprandial rise in the blood glucose in rats and also inhibited the absorption of the glucose moiety from maltose and glucose in the everted gut sac system prepared from rat intestine. These results demonstrate that chlorogenic acid is one of the major anti-hyperglycemic principles present in the leaves of N. indicum. Furthermore, among polyphenol compounds tested, quercetin and catechins were shown to have strong inhibitory activity against alpha-glucosidase.

Leads from Indian medicinal plants with hypoglycemic potentials

Diabetes mellitus is caused due to deficiency in production of insulin by the pancreas, or by the ineffectiveness of the insulin produced. It is a global problem and number of those affected is increasing day by day. The plants provide a potential source of hypoglycemic drugs because many plants and plant derived compounds have been used in the treatment of diabetes. Several medicinal plants have found potential use as hypoglycemic in the Indian system of medicines, including ayurveda. Many Indian plants have been investigated for their beneficial use in different types of diabetes and reports occur in numerous scientific journals. This article aims to provide a comprehensive review on various plant species from Indian biosphere and their constituents, which have been shown to display potent hypoglycemic activity. The use of herbs as hypoglycemic is a major avenue in Indian perspectives particularly for treating diabetes, which require to be explored more effectively as there are so many literatures available on these aspects. This paper describes the chemistry, activity and usage of the constituents isolated from these plants from India for the treatment of diabetes.

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.

Attempted Synthesis of 2-Acetamido and 2-Amino Derivatives of Salacinol. Ring Opening Reactions

The attempted synthesis of the 2-acetamido and 2-amino derivatives of salacinol, a naturally occurring glycosidase inhibitor, is described. Reaction of the protected acetamidothioarabinitol unit with the cyclic sulfate derived from L-erythritol gave the corresponding sulfonium sulfate, which underwent ring opening to give an acyclic amido sulfate. The corresponding reaction of the protected azidothioarabinitol unit with the cyclic sulfate proceeded to give the sulfonium sulfate. However, upon reduction of the azido function to an amine it formed an acyclic ammonium sulfate.

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.

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.

Punica granatum flower extract, a potent alpha-glucosidase inhibitor, improves postprandial hyperglycemia in Zucker diabetic fatty rats

Postprandial hyperglycemia plays an important role in the development of type 2 diabetes and has been proposed as an independent risk factor for cardiovascular diseases. The flowering part of Punica granatum Linn. (Punicaceae) (PGF) has been recommended in Unani literature as a remedy for diabetes. We investigated the effect and action mechanism of a methanolic extract from PGF on hyperglycemia in vivo and in vitro. Oral administration of PGF extract markedly lowered plasma glucose levels in non-fasted Zucker diabetic fatty rats (a genetic model of obesity and type 2 diabetes), whereas it had little effect in the fasted animals, suggesting it affected postprandial hyperglycemia in type 2 diabetes. In support of this conclusion the extract was found to markedly inhibit the increase of plasma glucose levels after sucrose loading, but not after glucose loading in mice, and it had no effect on glucose levels in normal mice. In vitro, PGF extract demonstrated a potent inhibitory effect on alpha-glucosidase activity (IC50: 1.8 microg/ml). The inhibition is dependent on the concentration of enzyme and substrate, as well as on the length of pretreatment with the enzyme. These findings strongly suggest that PGF extract improves postprandial hyperglycemia in type 2 diabetes and obesity, at least in part, by inhibiting intestinal alpha-glucosidase activity.

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

An efficient synthesis of blintol, the selenium congener of the naturally occurring glycosidase inhibitor salacinol, and a potent glucosidase inhibitor itself, is described. Unlike our previously reported synthesis, this improved route makes use of p-methoxybenzyl ether protecting groups in the synthesis of one of the two key intermediates, 2,3,5-tri-O-p-methoxybenzyl-1,4-anhydro-4-seleno-D-arabinitol, from L-xylose. The other key intermediate, 2,4-O-benzylidene-L-erythritol-1,3-cyclic sulfate, was successfully prepared from D-glucose instead of the expensive L-glucose. All protecting groups in the resulting adducts were removed with trifluoroacetic acid to yield a mixture of stereoisomers, thereby obviating the problematic deprotection of benzyl ethers by hydrogenolysis. The major stereoisomer, blintol, was then obtained by fractional crystallization.

Absolute Stereostructures of Polypodane-Type Triterpenes, Myrrhanol A and Myrrhanone A, from Guggul-Gum Resin (the Resin of Balsamodendron mukul)

Two new polypodane-type triterpenes, myrrhanol A and myrrhanone A, were isolated from the 50% aqueous methanolic extract of guggul-gum resin [the resin of Balsamodendron (=Commiphora) mukul HOOK]. The structures of the new constituents, including their absolute configurations, were determined on the basis of chemical and physicochemical evidence.

Structural requirements of flavonoids for inhibition of protein glycation and radical scavenging activities

To clarify the structural requirements of flavonoids for formation of advanced glycation end-products (AGEs), various flavonoids were examined. The results suggested the following structural requirements of flavonoids for the inhibition of AGEs formation: (1). as the hydroxyl groups at the 3'-, 4'-, 5-, and 7-positions increased in number, the inhibitory activities became stronger; (2). the activities of flavones were stronger than those of corresponding flavonols, flavanones, and isoflavones; (3). methylation or glucosylation of the 4'-hydroxyl group of flavones, flavonols, and flavanones reduced activity; (4). methylation or glycosylation of the 3-hydroxyl group of flavonols tended to increase activity; (5). glycosylation of the 7-hydroxyl group of flavones and isoflavones reduced activity. In addition, various flavonoids with strong AGEs formation inhibitory activity tended to exhibit strong scavenging activity for 1,1-diphenyl-2-picrylhydrazyl and superoxide anion radicals, with several exceptions.

Structures of new friedelane-type triterpenes and eudesmane-type sesquiterpene and aldose reductase inhibitors from Salacia chinensis

Three new friedelane-type triterpenes named salasones A (1), B (2), and C (3), a new norfriedelane-type triterpene, salaquinone A (4), and a new acylated eudesmane-type sesquiterpene, salasol A (5), were isolated from the 80% aqueous methanolic extract of the stems of Salacia chinensis collected in Thailand. Their stereostructures were elucidated on the basis of chemical and physicochemical evidence. In addition, six constituents, 3beta,22beta-dihydroxyolean-12-en-29-oic acid, tingenone, tingenine B, regeol A, triptocalline A, and mangiferin, were found to show an inhibitory effect on rat lens aldose reductase.

Glycosidase mechanisms

The three-dimensional structure of glycosidases and of their complexes and the study of transition-state mimics reveal structural details that correlate with mechanism. Of particular interest are the transition-state conformations harnessed by individual enzymes and the substrate distortion observed in enzyme-ligand complexes. 3D-structure in synergy with transition-state mimicry opens the way for mechanistic interpretation of enzyme inhibition and for the development of therapeutic agents.

Synthesis of 1,4-anhydro-D-xylitol heteroanalogues of the naturally occurring glycosidase inhibitor salacinol and their evaluation as glycosidase inhibitors

The syntheses of two 1,4-anhydro-D-xylitol heteroanalogues (8 and 9) of the naturally occurring sulfonium ion, salacinol (3), containing a sulfur or nitrogen atom in the ring are described. Salacinol (3) 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 Type 2 diabetes. The synthetic strategy relies on the nucleophilic attack of sulfur or nitrogen analogues of 1,4-anhydro-D-xylitol at the least-hindered carbon of 2,4-O-benzylidene-L-erythritol-1,3-cyclic sulfate. The sulfonium ion 8 inhibited barley-α-amylase (AMY1) and porcine pancreatic-α-amylase (PPA), with Ki values of 109 ± 11 and 55 ± 5 µM, respectively. In contrast, the ammonium ion 9 showed no significant inhibition of either AMY1 or PPA. Compounds 8 and 9 also showed no significant inhibition of glucoamylase.Key Words: glycosidase inhibitors, salacinol analogues, anhydro-D-xylitol heteroanalogues, enzyme inhibition.

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.