Synthesis and Characterization of Some New Nucleoside Analogues from Substituted Benzimidazole via 1 , 3-Dipolar cycloaddition

This paper includes the synthesis of some new nucleoside analogues starting with 2-substituted benzimidazole derivative (7-9), that synthesized by condensation of O-phenylenediamine with p-chloro benzaldehyde and two substituted benzoic acid , which on nucleophilic substitution with propargyl bromide gave a new Nsubstituted compounds (10-12). D-Fructose and D-galactose were chosen as a sugar moiety which were protected, brominated and azotated to give azido sugars (5) and (6), then they were subjected to 1,3-dipolar cycloaddition reaction with N-substuted compounds afforded bloked nucleoside analoges (13-16), which after hydrolysis gave our target the free nucleoside analogues (17-20). All prepared compounds were identified by FT-IR and some of them with 1 H-NMR and 13 C-NMR.


Introduction:
In recent years Nucleoside analogues played pivotal roles in the treatment of viral infections and cancer, nucleosides are active ingredient of one third of the antiviral drugs approved by the US food and drug administration (FDA) [1], thus considered one of the great importance among the compounds with antiviral activity; therefore, to modulate nucleoside or nucleotide activity, the strategy has been modified one of the three major subunit moiety [2].Nucleoside analogues such as Favipiravir, has demonstrated success in treating Ebola virus infections in both cell culture and small animal models [3,4].Benzimidazole is heterocyclic aromatic organic compound.This bicyclic compound consists of the fusion of benzene and imidazole [5].The most prominent benzimidazole compounds in nature is N-ribosyldimethylbenzimidazole, which serves as an axial ligand for cobalt in vitamin B 12 , On the other hand , benzimidazole is an important pharmacophore due to the structural similarty of purine [6].New antiviral drugs and in last year's several biological application like anti-microbial [7], antiviral especially HIV virus [8], antiprotozoal [9], antiinflammatory [10], anthelmintic [11], antihypertensive [12], anti-tumor [13] and anticonvulsant activities [14] and CNSdepressant [15],have been reported for benzimidazole derivatives [16].

Materials and Methods:-Instruments
 Melting points were recorded by Gallen Kamp , England .Melting point apparatus were uncorrected,. Infrared spectra were recorded using Fourier Transform infrared SHIMADZU (8300) (FTIR) infrared spectrometer, Japan,as KBr disc or thin film . 1 H-NMR and 13 C-NMR spectra were recorded on Burker, Ultra shield 300MHz, Jordan, Amman, using tetramethyl silane as internal standard and DMSO-d 6 as a solvent. Biological activity using incubator Memmert. The DMSO-d 6 solvent appeared at 2.5ppm in 1 H-NMR and at 40.45ppm in 13 C-NMR spectrum [12] Chemicals  All chemical starting compounds were obtained from Fluka , Aldrich and BDH

1,2:3,4-Di-O-isopropylideneα-Dgalactopyranose
Anhydrous zinc chloride (9.5 g) was dissolved in 100 ml of acetone with stirring until the zinc chloride was dissolved.Concentrated sulfuric acid (0.32 ml ) was rapidly added drop-wise ,finely powdered, anhydrous Dgalactose (9 g, 0.05 mole ) was quickly added and the mixture was stirred magnetically for 4 hs.A suspension of (16 g) of anhydrous sodium carbonate in (28 ml) of water was added in portions and the mixture was stirred.The suspension was filtered, and the precipitate was washed several times with acetone and filtered.The filtrate and washings are combined, and the solution was evaporated under vacuo.The mixture was extracted 3 times with ether, dried with anhydrous sodium sulfate, filtered, and evaporated to dryness under vacuo to give product (4) as pale yellow syrup.

Hydrolysis of nucleoside analogues
A solution of ( 0.3 g ) of the blocked nucleoside in ( 14 ml ) of ( 0.1 M) methanolic sodium methoxide was refluxed with stirring for 0.5 h., neutralized with acetic acid and evaporated to dryness.The residue was partitioned between water and chloroform and the aqueous phase evaporated to dryness in vacuo, then was recrystallized from ethanol ether .

Results and Discussion
Structurally modified nucleosides represent an important class of medicinal compounds which have been found to behave as therapeutic agents and are currently used in pharmaceuticals as antitumor, antiviral and antibiotic agent.(16) Physical properties of sugars compounds (2-6) were agreed with that in the literature The FT-IR spectrum for compound (2) showed stretching band at 3064cm -1 (C-H arom.), 2929cm -1 (C-H aliph.), a stretching band at 1710 cm -1 (C=O benzoate) , 1573 cm -1 (C=C)arom., and 659.61 cm -1 (C-Br), while a stretching broad band of the hydroxyl group was disappeared(Table 1) .
The FT-IR spectrum of galactose isopropylidene showed a stretching band of (C-O-C) at 1255.57cm -1 appeared for protected group isopropylidene while compound (5) showed in addition of stretching band of (C-O-C) at 1273 cm -1 another strectching band in 648.8 cm -1 for (C-Br )bond while the stretching band for (OH) at 3433 cm -1 was disappeared.Compound (6) spectrum showed a stretching band at 1232 cm -1   for (C-O-C), a stretching band at 2970 cm -1 (C-H aliph.)and 2129 cm -1 for azide.The FT-IR spectrum of benzimidazole compounds (7-9) showed a stretching band at 3400-3429 cm -1 for secondary amine, stretching band at 1502-1587cm - 1 for (C=C)aromatic, 3060-3068 cm -1 for (C-H)aromatic, stretching and at 1610-1631 cm -1 for (C=N).For ferther modification of nucleobase , substituted benzimidazole (7-9) were undergoing nucleophilic substitution with propargyl bromide to give 1-proynyl -2-substituted benzimidazole derivatives (10)(11)(12) .Some of physical properties are listed in Table (4).The FT-IR of compounds (10)(11)(12) showed the disappearance of the secondary amine bands.This was demonstrated substitution of propynyl on nitrogen while acetylenic bond showed stretching bands in the range of 2125-2190 cm -1 .alsoshowed stretching bands in range between 2815 to 2925 cm -1 for C-H aliph.Compound (10) showed a stretching band at 806 for C-Cl.The target nucleoside was prepared through 1,3-dipolar cycloaddition reaction by coupling of nucleobase benzimidazole derivative and sugar moiety using Cu I as a catalyst to give the blocked nucleoside when 1,2,3trizole were configured.Some of physical properties are listed in Table (6).The 1 H-NMR spectrum of 16 showed a singlet at 1.64 ppm for four isopropyledene protons other singlet appeared at 2.5 for methyl toluene, three sugar protons H‫׳‬ 5 ,H‫׳‬ 6 ‫׳׳‪,H‬‬ 6 appeared at 2.7-3.7 ppm.