Polyhydroxylated Pyrrolidines Research Articles

Total Synthesis of Dragocins A-C through Electrochemical Cyclization.

The first total synthesis of dragocins A-C, remarkable natural products containing an unusual C4' oxidized ribose architecture bridged by a polyhydroxylated pyrrolidine, is presented through a route featuring a number of uncommon maneuvers. Several generations towards the target molecules are presented, including the spectacular failure of a key C-H oxidation on a late-stage intermediate. The final route features rapid, stereocontrolled access to a densely functionalized pyrrolidine and an unprecedented diastereoselective oxidative electrochemical cyclization to forge the hallmark 9-membered ring. Preliminary studies suggest this electrochemical oxidation protocol is generally useful.

Total Synthesis of Dragocins A−C through Electrochemical Cyclization

AbstractThe first total synthesis of dragocins A−C, remarkable natural products containing an unusual C4’ oxidized ribose architecture bridged by a polyhydroxylated pyrrolidine, is presented through a route featuring a number of uncommon maneuvers. Several generations towards the target molecules are presented, including the spectacular failure of a key C−H oxidation on a late‐stage intermediate. The final route features rapid, stereocontrolled access to a densely functionalized pyrrolidine and an unprecedented diastereoselective oxidative electrochemical cyclization to forge the hallmark 9‐membered ring. Preliminary studies suggest this electrochemical oxidation protocol is generally useful.

Mechanistic Insights into Dibasic Iminosugars as pH-Selective Pharmacological Chaperones to Stabilize Human α-Galactosidase.

The use of pharmacological chaperones (PCs) to stabilize specific enzymes and impart a therapeutic benefit is an emerging strategy in drug discovery. However, designing molecules that can bind optimally to their targets at physiological pH remains a major challenge. Our previous study found that dibasic polyhydroxylated pyrrolidine 5 exhibited superior pH-selective inhibitory activity and chaperoning activity for human α-galactosidase A (α-Gal A) compared with its monobasic parent molecule, 4. To further investigate the role of different C-2 moieties on the pH-selectivity and protecting effects of these compounds, we designed and synthesized a library of monobasic and dibasic iminosugars, screened them for α-Gal A-stabilizing activity using thermal shift and heat-induced denaturation assays, and characterized the mechanistic basis for this stabilization using X-ray crystallography and binding assays. We noted that the dibasic iminosugars 5 and 20 protect α-Gal A from denaturation and inactivation at lower concentrations than monobasic or other N-substituted derivatives; a finding attributed to the nitrogen on the C-2 methylene of 5 and 20, which forms the bifurcated salt bridges (BSBs) with two carboxyl residues, E203 and D231. Additionally, the formation of BSBs at pH 7.0 and the electrostatic repulsion between the vicinal ammonium cations of dibasic iminosugars at pH 4.5 are responsible for their pH-selective binding to α-Gal A. Moreover, compounds 5 and 20 demonstrated promising results in improving enzyme replacement therapy and exhibited significant chaperoning effects in Fabry cells. These findings suggest amino-iminosugars 5 and 20 as useful models to demonstrate how an additional exocyclic amino group can improve their pH-selectivity and protecting effects, providing new insights for the design of pH-selective PCs.

Harnessing polyhydroxylated pyrrolidines as a stabilizer of acid alpha-glucosidase (GAA) to enhance the efficacy of enzyme replacement therapy in Pompe disease

To discover small molecules as acid alpha-glucosidase (GAA) stabilizers for potential benefits of the exogenous enzyme treatment toward Pompe disease cells, we started from the initial screening of the unique chemical space, consisting of sixteen stereoisomers of 2-aminomethyl polyhydroxylated pyrrolidines (ADMDPs) to find out two primary stabilizers 17 and 18. Further external or internal structural modifications of 17 and 18 were performed to increase structural diversity, followed by the protein thermal shift study to evaluate the GAA stabilizing ability. Fortunately, pyrrolidine 21, possessing an l-arabino-typed configuration pattern, was identified as a specific potent rh-GAA stabilizer, enabling the suppression of rh-GAA protein denaturation. In a cell-based Pompe model, co-administration of 21 with rh-GAA protein significantly improved enzymatic activity (up to 5-fold) compared to administration of enzyme alone. Potentially, pyrrolidine 21 enables the direct increase of ERT (enzyme replacement therapy) efficacy in cellulo and in vivo.

Studies towards the synthesis of polyhydroxylated pyrrolidine alkaloids isolated from Broussonetia kazinoki (moraceae)

The syntheses of L-AB1, L-DMDP, and the novel compounds, (−)-phenethyl-L-AB1, (−)-10′-deoxobroussonetine C, (−)-10′-deoxobroussonetine E, (−)-1′-epi-10′-deoxobroussonetine E, and (−)-(6S)-12′-hydroxydodecylmoranoline are reported. These syntheses start from d-xylose employing the Petasis borono-Mannich reaction to stereoselectively introduce the amino group, followed by a chemo- and regioselective O-mesylation to deliver the fully functionalized pyrrolidine moiety after intramolecular SN2-cyclisation. The synthesis of the latter targeted compound involved a ring expansion process of a prolinol moiety to a piperidine derivative under Mitsunobu reaction conditions. An attempted synthesis of desired ent-broussonetine C was unsuccesful due to formation of an unexpected tetrahydrofuran derivative in the final stage of the synthesis. The glycosidase inhibitory activities of four of the new target compounds against a panel of ten glycosidaes is also presented.

Highly stereocontrolled total synthesis of racemic codonopsinol B through isoxazolidine-4,5-diol vinylation.

A new highly diastereoselective synthesis of the polyhydroxylated pyrrolidine alkaloid (±)-codonopsinol B and its N-nor-methyl analogue, starting from achiral materials, is presented. The strategy relies on the trans-stereoselective epoxidation of 2,3-dihydroisoxazole with in situ-generated DMDO, the syn-selective α-chelation-controlled addition of vinyl-MgBr/CeCl3 to the isoxazolidine-4,5-diol intermediate, and the substrate-directed epoxidation of the terminal double bond of the corresponding γ-amino-α,β-diol with aqueous hydrogen peroxide catalyzed by phosphotungstic heteropoly acid. Each of the key reactions proceeded with an excellent diastereoselectivity (dr > 95:5). (±)-Codonopsinol B was prepared in 10 steps with overall 8.4% yield. The antiproliferative effect of (±)-codonopsinol B and its N-nor-methyl analogue was evaluated using several cell line models.

1,4-Dideoxy-1,4-imino-d-arabinitol (DAB) Analogues Possessing a Hydrazide Imide Moiety as Potent and Selective α-Mannosidase Inhibitors.

The synthesis of two polyhydroxylated pyrrolidines as 1,4-dideoxy-1,4-imino-d-arabinitol (DAB) analogues bearing a hydrazide moiety is described. The DAB analogues act as selective and potent inhibitors of α-mannosidase in the submicromolar concentration ranges (Ki values ranging from 0.23 to 1.4 μM).

Synthesis and Glycosidase Inhibition of Broussonetine M and Its Analogues.

Cross-metathesis (CM) and Keck asymmetric allylation, which allows access to defined stereochemistry of a remote side chain hydroxyl group, are the key steps in a versatile synthesis of broussonetine M (3) from the d-arabinose-derived cyclic nitrone 14. By a similar strategy, ent-broussonetine M (ent-3) and six other stereoisomers have been synthesized, respectively, starting from l-arabino-nitrone (ent-14), l-lyxo-nitrone (ent-3-epi-14), and l-xylo-nitrone (2-epi-14) in five steps, in 26%–31% overall yield. The natural product broussonetine M (3) and 10’-epi-3 were potent inhibitors of β-glucosidase (IC50 = 6.3 μM and 0.8 μM, respectively) and β-galactosidase (IC50 = 2.3 μM and 0.2 μM, respectively); while their enantiomers, ent-3 and ent-10’-epi-3, were selective and potent inhibitors of rice α-glucosidase (IC50 = 1.2 μM and 1.3 μM, respectively) and rat intestinal maltase (IC50 = 0.29 μM and 18 μM, respectively). Both the configuration of the polyhydroxylated pyrrolidine ring and C-10’ hydroxyl on the alkyl side chain affect the specificity and potency of glycosidase inhibition.

Synthesis and investigation of polyhydroxylated pyrrolidine derivatives as novel chemotypes showing dual activity as glucosidase and aldose reductase inhibitors

Diabetes is a multi-factorial disorder that should be treated with multi-effective compounds. Here we describe the access to polyhydroxylated pyrrolidines, belonging to the d-gluco and d-galacto series, through aminocyclization reactions of two differentially protected d-xylo-hexos-4-ulose derivatives. The prepared compounds proved to inhibit both alpha-glucosidase, responsible for the emergence of hyperglycemic spikes, and aldose reductase, accountable for the development of abnormalities in diabetic tissues. Accordingly, they show the dual inhibitory profile deemed as ideal for diabetes treatment. Significantly, compound 17b reduced the process of cell death and restored the physiological levels of oxidative stress when tested in the photoreceptor-like 661w cell line, thus proving to be effective in an in vitro model of diabetic retinopathy.

Synthesis of bis-Phosphate Iminoaltritol Enantiomers and Structural Characterization with Adenine Phosphoribosyltransferase

Phosphoribosyl transferases (PRTs) are essential in nucleotide synthesis and salvage, amino acid, and vitamin synthesis. Transition state analysis of several PRTs has demonstrated ribocation-like transition states with a partial positive charge residing on the pentose ring. Core chemistry for synthesis of transition state analogues related to the 5-phospho-α-d-ribosyl 1-pyrophosphate (PRPP) reactant of these enzymes could be developed by stereospecific placement of bis-phosphate groups on an iminoaltritol ring. Cationic character is provided by the imino group and the bis-phosphates anchor both the 1- and 5-phosphate binding sites. We provide a facile synthetic path to these molecules. Cyclic-nitrone redox methodology was applied to the stereocontrolled synthesis of three stereoisomers of a selectively monoprotected diol relevant to the synthesis of transition-state analogue inhibitors. These polyhydroxylated pyrrolidine natural product analogues were bis-phosphorylated to generate analogues of the ribocationic form of 5-phosphoribosyl 1-phosphate. A safe, high yielding synthesis of the key intermediate represents a new route to these transition state mimics. An enantiomeric pair of iminoaltritol bis-phosphates (L-DIAB and D-DIAB) was prepared and shown to display inhibition of Plasmodium falciparum orotate phosphoribosyltransferase and Saccharomyces cerevisiae adenine phosphoribosyltransferase (ScAPRT). Crystallographic inhibitor binding analysis of L- and D-DIAB bound to the catalytic sites of ScAPRT demonstrates accommodation of both enantiomers by altered ring geometry and bis-phosphate catalytic site contacts.

Stereoselective synthesis and anticancer activity of broussonetine analogues

The stereoselective synthesis of two broussonetine analogues 14·HCl and ent-14·HCl with differing stereochemistry of the polyhydroxylated pyrrolidine core and a simple C13 alkyl fragment has been achieved. For their construction, the known oxazolidinones 15 and 16 were chosen as appropriate advanced scaffolds. The common hydrophobic side chain was incorporated at an early stage of the synthesis through Grubbs’ cross metathesis chemistry. The required pyrrolidine skeleton was then formed by the cyclization of open chain intermediates 17 and 18. Four synthesized compounds were screened in vitro for antiproliferative/cytotoxic activity against six cancer cell lines by MTT assay. Compounds 14·HCl (HeLa and A-549) and ent-14·HCl (Caco-2 and Jurkat) showed comparable or higher potency than conventional anticancer agent cisplatin on at least two evaluated cancer cells, respectively.

Short and Diastereoselective Total Synthesis of the Polyhydroxylated Pyrrolidine LAB-1: A Potent α-Glycosidase Inhibitor

We described herein a total synthesis of 1,4-dideoxy-1,4-imino-l-arabinitol [(2S,3S,4S)-2-(hydroxymethyl)pyrrolidine-3,4-diol, LAB-1], a polyhydroxylated pyrrolidine, which has been demonstrated to be a selective and potent α-glycosidase inhibitor. The main features of our approach are its shortness, efficiency, and simplicity. The total synthesis was accomplished in 6 steps with an overall yield of 12%, starting from a chiral optically active Morita–Baylis–Hillman (MBH) adduct prepared (without epimerization), from Garner’s aldehyde. As far as we know, this is the first report describing the total synthesis of this biologically active pyrrolidine by exploring the synthetic versatility of a MBH adduct.

Rapid preparation of (3R,4S,5R) polyhydroxylated pyrrolidine-based libraries to discover a pharmacological chaperone for treatment of Fabry disease

The rapid discovery of a pharmacological chaperone toward human α-Gal A for the treatment of Fabry disease is described. Two polyhydroxylated pyrrolidines with the (3R,4S,5R) configuration pattern underwent rapid substituent diversity by conjugating the primary aminomethyl moiety of each with a variety of carboxylic acids to generate two libraries (2 × 60 members). Our bioevaluation results showed one member with the (2R,3R,4S,5R) configuration pattern and bearing a 5-cyclohexylpentanoyl group as a substituent moiety possessed sufficient chaperoning capability to rescue α-Gal A activity in the lymphocyte of the N215S Fabry patient-derived cell line and other α-Gal A mutants in COS7 cells.

Asymmetric routes toward polyhydroxylated pyrrolidines: Synthesis of 1,4-dideoxy-1,4-imino-d-galactitol and 1,4-dideoxy-1,4-imino-d-glucitol

Herein the total synthesis of the pyrrolidine alkaloids 1,4-dideoxy-1,4-imino-d-galactitol and its diastereoisomer 1,4-dideoxy-1,4-imino-d-glucitol is described, starting from a common optically active precursor. The key step in our approach was the double diastereoselection in the asymmetric dihydroxylation of chiral vinyl azido alcohols, obtained by means of two different regio- and stereoselective nucleophilic openings of the corresponding chiral vinyl epoxide.

Synthesis of polyhydroxylated pyrrolidines from sugar-derived bromonitriles through a cascade addition of allylmagnesium bromide/cyclization/reduction.

The synthesis of polyhydroxylated 2-allylpyrrolidines from sugar-derived bromonitriles in a cascade addition of allylmagnesium bromide/SN2 cyclization/reduction with Zn(BH4)2 is described. The stereochemical course of the reduction step is rationalized. Two of the obtained compounds are transformed into stereoisomers of naturally-occurring iminosugar (+)-lentiginosine. In an alternative approach, 2,2-diallylpyrrolidines are obtained from bromonitriles in a cascade addition of allylmagnesium bromide/SN2 cyclization/addition of another equivalent of allylmagnesium bromide.

Synthesis and Inhibition Study of Bicyclic Iminosugar‐Based Alkaloids, Scaffolds, and Libraries towards Glucosidase

AbstractA small library of bicyclic iminosugar‐based alkaloids and scaffolds possessing a polyhydroxylated pyrrolidine and a varied ring skeleton have been synthesized. Through rapid diversification of the scaffold via an amide coupling with random carboxylic acids, structurally diverse bicyclic iminosugar‐based libraries were prepared with substituent diversity, core diversity, and configurational diversity. This discovery process allowed us to efficiently sieve out potent and specific glycosidase inhibitors, and a bicyclic, conformationally restricted iminosugar was demonstrated to be more potent than the monocyclic ones in this study. The most potent and selective inhibitor discovered was found to have a Ki value of 71 nM against α‐glucosidase.

Synthesis of novel polyhydroxylated pyrrolidine-triazole/-isoxazole hybrid molecules.

A straightforward synthesis of novel, 2-heterocyclyl polyhydroxylated pyrrolidines is described. Stereocontrolled additions of nucleophiles to cyclic nitrones generated the corresponding 2,3-trans adducts, allowing the synthesis of the corresponding pyrrolidines via key intermediates bearing an alkyne and a nitrile oxide. Three hybrid systems, including a pyrrolidine with two isoxazoles and one triazole, are efficiently prepared via 1,3-dipolar cycloaddition. Biological testing of the product alkaloids showed that subtle structural variations have drastic effects on their inhibitory activities against glucosidases.

Sequential Biocatalytic Aldol Reactions in Multistep Asymmetric Synthesis: Pipecolic Acid, Piperidine and Pyrrolidine (Homo)Iminocyclitol Derivatives from Achiral Building Blocks

AbstractA multistep chemoenzymatic synthesis for stereodiverse polyhydroxypipecolic acid analogues, homoiminocyclitols and polyhydroxylated piperidine and pyrrolidine derivatives combining glycine‐dependent aldolases and both D‐fructose‐6‐phosphate aldolase (FSA) or dihydroxyacetone phosphate (DHAP)‐dependent aldolases is presented. The methodology allowed the preparation of known and innovative imine‐derived molecules with a great structural diversity from simple achiral substrates. The strategy consisted of two key aldol addition steps: a first aldol addition of glycine to dimethoxyacetaldehyde catalyzed by L‐ and D‐glycine aldolases and a second aldol addition of DHAP, dihydroxyacetone, hydroxyacetone or glycolaldehyde using FSA or DHAP‐dependent aldolases as catalysts to a conveniently transformed aldol adduct from the first aldol addition. Catalytic reductive amination on the aldol adducts rendered the polyhydroxypipecolic acid analogues, (homo)iminocyclitols and polyhydroxylated pyrrolidine iminocyclitols. The reported strategy is thus designed to create up to five new stereogenic centers in three steps, four of them being controlled in two enzymatic reactions. Moreover, it allowed the installation of diverse functionalities in the molecules. This was possible by taking the full advantage of using aldolases in a multistep approach by virtue of their stereocomplementarity, stereoselectivity and broad substrate tolerance.magnified image

Differential Rapid Screening of Phytochemicals by Leaf Spray Mass Spectrometry

Ambient ionization can be achieved by generating an electrospray directly from plant tissue (“leaf spray”). The resulting mass spectra are characteristic of ionizable phytochemicals in the plant material. By subtracting the leaf spray spectra recorded from the petals of two hibiscus species H. moscheutos and H. syriacus one gains rapid access to the metabolites that differ most in the two petals. One such compound was identified as the sambubioside of quercitin (or delphinidin) while others are known flavones. Major interest centered on a C19H29NO5 compound that occurs only in the large H. moscheutos bloom. Attempts were made to characterize this compound by mass spectrometry alone as a test of such an approach. This showed that the compound is an alkaloid, assigned to the polyhydroxylated pyrrolidine class, and bound via a C3 hydrocarbon unit to a monoterpene.

Sugar-derived cyclic imines: one-pot synthesis and direct functionalization

A simple method for the synthesis of sugar-derived imines by a Schwartz's reagent reduction of easily available sugar lactams has been described. A direct addition of nucleophiles to the generated in situ cyclic imines and subsequent deprotection of hydroxyl function allows to convert sugar lactams in polyhydroxylated pyrrolidines and piperidines.