Design and Divergent Synthesis of Aza Nucleosides from a Chiral Imino Sugar

Rational design and synthesis of novel triazole- and tetrazole-fused iminosugars as potential inhibitors of amyotrophic lateral sclerosis (ALS) linked SOD1 aggregation

In this manuscript we report the first example of an iminosugar that inhibits superoxide dismutase fibrillation associated with the amyotrophic lateral sclerosis (ALS). The present work involves synthesis of novel triazole and tetrazole embedded iminosugars, synthesized in 11-13 high yielding steps starting from readily available tri-O-benzyl-D-glucal and proceeding through a concomitant azidation - thermal intramolecular [3 + 2] cycloaddition reaction as the key step. One of these pre-designed iminosugars was found to inhibit fibrillation of SOD1 and also has shown propensity to break pre-formed fibrils. Docking and MD simulation studies suggest that the most probable interaction of this compound is a hydrogen bonding with Arg69, a loop IV residue of SOD1, which has a crucial role in stabilizing the native conformation of SOD1.

An expeditious synthesis of novel DNA nucleobase mimics of (+)-anisomycin

A glycal based expeditious synthesis of novel nucleoside analogues of (+)-anisomycin is reported. Readily available tri-O-benzyl-D-glucal was converted to a partially protected trihydroxypyrrolidine that is used as a common scaffold for the introduction of various nucleobases at the primary hydroxyl centre. Nucleoside analogues possessing all four DNA bases have been synthesized. Selective acetylation at C3 position was carried out with two of these unnatural nucleosides in order to mimic the structure of (+)-anisomycin. Cytotoxicity studies of some of these nucleosides showed that they display weaker activity on HeLa cells than Ara-C.

From Glycals to Nitrogen Heterocycles and Carbocycles via "Cleavage-Intramolecular Recombination Strategy"

Glycals (carbohydrate enol-ethers) have enjoyed profound applications in organic synthesis for more than a century. They not only serve as versatile glycosyl donors or as substrates for Ferrier rearrangement, but also find extensive synthetic applications especially as a "chiral pool" for accomplishing the synthesis of a variety of natural and biologically important compounds. As cyclic enol ethers, they demonstrate high reactivity and are among the most and variously transformable monosaccharide derivatives. The uniqueness of the reactivity of glycals is that they can be synthetically tuned to get a library of derivatives through stereo- and regioselective introduction of a variety of functional groups at C1, C2, C3 as well as C4 carbons of the sugar. We have developed a practical approach for stereoselective mono- and diamination of glycals and over the years utilized these scaffolds for the synthesis of a variety of biologically important nitrogen heterocycles and carbocycles through a "Diversity Oriented Approach". Our synthetic strategy in this direction mainly relied on the cleavage of ring O-C bond of the sugar followed by an "intramolecular recombination" reaction. Utilizing this strategy, we have accomplished the synthesis of several biologically important natural products, their analogues and related unnatural derivatives. Examples of such compounds reported from our group include polyhydroxypyrrolidines, DMDP, anisomycin, steviamine, pochonicine, conduramines, bulgecinine, aminocyclitols, azepanes, 4-hydroxy-D-proline, azanucleosides and their analogues. A personal account highlighting these syntheses is presented here.

Supramolecular Architecture through Self-Organization of Janus-Faced Homoazanucleosides

Design of Janus-faced or double-headed homoazanucleosides with the possibility to undergo self-organization through base pairing has been conceptualized and accomplished. The synthetic strategy demonstrates the unique ability to introduce two similar or complementary nucleobases on opposite arms of a chiral polyhydroxypyrrolidine while also ensuring that their faces are anti to each other to allow only intermolecular interactions between the nucleobases, an essential requisite for self-assembly. Single-crystal X-ray structures were determined for all three types of homoazanucleosides, one possessing two adenine molecules, the other with two thymine moieties, and the third containing both adenine and thymine. The crystal structures of all three display noncovalent interactions, including Watson-Crick base pairing, Hoogsteen H-bonding, and π-π stacking, resulting in unusual supramolecular patterns. The most striking supramolecular motif among them, which emerged from the crystal structure of the homoazanucleoside containing both adenine and thymine, is a left-handed helix formed through Watson-Crick pairing between nucleobases. The present study thus forms a prelude to the design of Janus-faced building blocks to establish helical pillars as well as lateral branches that together define a three-dimensional (3D) lattice. The ready accessibility of these molecules is expected to spur the next generation of discoveries in the design of functional nanomaterials.

Synthesis of novel homoazanucleosides and their peptidyl analogs

Synthesis of novel homoazanucleosides and their peptidyl analogs as hybrid molecules comprised of amino acids, an iminosugar and natural nucleobases is reported for the first time. A pluripotent amino-substituted chiral polyhydroxypyrrolidine, possessing orthogonally different functional groups on either arm of the pyrrolidine ring, served as an ideal substrate for the synthesis of the proposed peptidyl homoazanucleosides. The acid sensitive primary benzyloxy group, on one arm of the pyrrolidine ring, after selective deprotection, was utilized for the introduction of nucleobases to obtain the homoazanucleosides. The amino group on the other side offered the opportunity to be coupled with amino acids to deliver the desired peptidyl homoazanucleosides. Glycosidase inhibition studies revealed that the acetamido derivatives of homoazanucleosides were found to be sub-millimolar inhibitors of β-N-acetyl-glucosaminidase.

A glycal based approach to the synthesis of (+)-bulgecinine, 3-hydroxy-2,5-dihydroxymethylpyrrolidine and 2-oxapyrrolizidin-3-one

A glycal based synthesis of (+)-bulgecinine, 3-hydroxy-2,5-dihydroxymethylpyrrolidine and 2-oxapyrrolizidin-3-ones proceeding through a common intermediate is reported. The key step in the work presented here is a two-step conversion of 4,6-di-O-benzyl-d-glucal to 2,3-dideoxy-2-tosylamido-d-glucose. This manuscript reports the first carbohydrate based approach to the synthesis of (+)-bulgecinine and the whole sequence has been accomplished with complete stereochemical integrity without the formation of mixture of products in any of these steps.

Novel 1′-homo-N-2′-deoxy-α-nucleosides: synthesis, characterization and biological activity

For the first time, a series of novel 1′-homo-N-2′-deoxy-α-nucleosides containing natural nucleobases as well as 5-fluoro and 5-iodopyrimidine analogs have been synthesized in an efficient manner. Additionally, a high yield protocol for the assembly of a dimeric scaffold containing two sugar moieties linked to the N-1 and N-3 positions of a single pyrimidine base has been accomplished. The structures of the novel homonucleosides were established by a single crystal X-ray structure of 1′-homo-N-2′-deoxy-α-adenosine and NMR studies. The biological activity of these 1′-homo-N-2′-deoxy-α-nucleosides as antiviral (HIV-1 and HBV) and cytotoxic studies was measured in multiple cell systems. The unique structure and easy accessibility of these compounds may allow their use in the design of new nucleoside analogs with potential biological activity and as a scaffold for combinatorial chemistry.

Novel 1′-homo-N-2′-deoxy-α-nucleosides and dimers.

Ring closing metathesis (RCM) approach to the synthesis of conduramine B-2, ent-conduramine F-2, aminocyclopentitol and trihydroxyazepane

The syntheses of conduramine B-2, ent-conduramine F-2, aminocyclopentitol and trihydroxyazepane were accomplished from a common precursor, through a divergent approach using ring closing metathesis (RCM) as the key step. Tri-O-benzyl-d-glucal was converted to 3,4,6-tri-O-benzyl-1,2-dideoxy-2-iodo-1-p-toluenesulfonamido-α-d-mannose. Exposure to NaBH4 in MeOH resulted in a facile 1,2-transposition of the -NHTs group with concomitant glycosylation to give methyl 3,4,6-tri-O-benzyl-2-deoxy-2-p-toluenesulfonamido-β-d-glucoside, which was converted into methyl 6-deoxy-6-iodo-glucoside in three steps. Zinc-mediated Vasella's rearrangement proceeded smoothly to give the pluripotent formyl-olefin, possessing both electrophilic and nucleophilic sites, which was used as a common precursor in our diversity-oriented approach. Vinylation of the carbonyl group followed by RCM and subsequent deprotection resulted in the successful synthesis of conduramine B-2 and ent-conduramine F-2 for the first time. On the other hand, the Wittig reaction of the formyl-olefin affords the diene that undergoes Grubbs' I catalyzed RCM and deprotection/reduction to provide 3-amino-cyclopentan-1,2-diol. Utilizing the nucleophilic site at the nitrogen of the common precursor, base mediated N-allylation was carried out to obtain the corresponding diene that underwent a smooth RCM to afford trihydroxyazepane.

Ni-Pd Catalyzed Cyclization of Sulfanyl 1,6-Diynes: Synthesis of 1'-Homonucleoside Analogues

The Ni-Pd catalyzed addition-cyclization of sulfanyl 1,6-diynes 2-9 with nucleobases is described. The reactions of N-tethered 1,6-diynes with N³-benzoylthymine, N⁴,N⁴-bis(Boc)cytosine, N³-benzoyluracil and N⁶,N⁶-bis(Boc)adenine exclusively afforded the pyrrolylmethyl and furylmethyl nucleotides in good yields. Deprotection of nucleobases was completed by treatment with acids or bases. Furthermore, the reactions of pyrroles and furans with nucleophiles such as alkoxides and amines underwent detosylation and conversion to the alkoxymethyl- and arylaminomethyl-pyrroles and furans in good yields.

Evaluation of antidiabetic effect of total calystegines extracted from Hyoscyamus albus

BACKGROUND

Hyoscyamus albus L. (Solanaceae) an old medicinal plant is a rich source of tropane and nortropane alkaloids which confers to this plant a number of very interesting and beneficial therapeutic effects.

PURPOSE

Calystegines that are polyhydroxylated alkaloids and imino-sugars poccess significant glycosidases inhibitory activities and are therefore good candidats for the treatment of diabetes mellitus.

STUDY DESIGN

Calystegines extracted from Hyoscyamys albus seeds were tested for teir acute oral toxicity and investigated for their in-vivo antidiabetic effect on Streptozotocine induced diabetes in mice.

METHODES

Calystegines were extracted from the seeds plant using an Ion exchange column; the remaining extract was then administrated orally to mice at several single doses for acute toxicity assay. A dose of 130mg/kg streptozotocine was injected to mice to induce diabetes mellitus, and diabetic mice were treated orally during 20days with 10mg/kg and 20mg/kg calystegines and 20mg/kg glibenclamide as the reference drug.

RESULTS

Acute oral toxicity showed that calystegines are not toxic up to a dose of 2000mg/kg with absence of any signs of intoxication and damages in Liver and kidney tissues. The nortropane alkaloids markedly reduced blood glucose levels and lipid parameters of diabetic mice to normal concentrations after 20days of treatment at 10mg/kg and 20mg/kg (p<0.05). Histopathological study of diabetic mice pancreas indicated that calystegines of Hyoscyamus albus have minimized streptozotocine damages on β-cells of islets of langerhans, stimulated β-cells regeneration and improved with this insulin secretion.

CONCLUSION

The findings of this study suggest that calystegines are potent antidiabetic agents with antihyperglicemic and hypolipidemic effects, and a protective fonction on pancreas in streptozotocin induced diabetes in mice.

Synthesis, evaluation of anti-HIV-1 and anti-HCV activity of novel 2',3'-dideoxy-2',2'-difluoro-4'-azanucleosides

A series of 2',3'-dideoxy-2',2'-difluoro-4'-azanucleosides of both pyrimidine and purine nucleobases were synthesized in an efficient manner starting from commercially available L-pyroglutamic acid via glycosylation of difluorinated pyrrolidine derivative 15. Several 4'-azanucleosides were prepared as a separable mixture of α- and β-anomers. The 6-chloropurine analogue was obtained as a mixture of N(7) and N(9) regioisomers and their structures were identified based on NOESY and HMBC spectral data. Among the 4'-azanucleosides tested as HIV-1 inhibitors in primary human lymphocytes, four compounds showed modest activity and the 5-fluorouracil analogue (18d) was found to be the most active compound (EC(50)=36.9μM) in this series. None of the compounds synthesized in this study demonstrated anti-HCV activity.