Design, Synthesis and Bioactivity of N-Glycosyl-N′-(5-substituted phenyl-2-furoyl) Hydrazide Derivatives

Condensation products of 5-substituted phenyl-2-furoyl hydrazide with different monosaccharides d-glucose, d-galactose,d-mannose, d-fucose and d-arabinose were prepared. The anomerization and cyclic-acyclic isomers were investigated by 1H NMR spectroscopy. The results showed that, except for the d-glucose derivatives, which were in the presence of β-anomeric forms, all derivatives were in an acyclic Schiff base form. Their antifungal and antitumor activities were studied. The bioassay results indicated that some title compounds showed superior effects over the commercial positive controls.


Introduction
Carbohydrates participate in various vital processes, showing important physiological and biological activities.In a recent review, Becker and coworkers [1] emphasized that the introduction of OPEN ACCESS pharmacophores into sugar templates that possess dense stereochemical information is an excellent strategy for the development of bioactive compounds with rich structural diversity.Free reducing sugars (hemiacetal), which are often available from natural sources, react with α-heteroatom nucleophile hydrazide to give cyclic pyranoside adducts in predominantly β-anomeric form or the open chain hydrazone tautomer [2][3][4][5].In fact, this intrinsic chemoselective condensation reactivity of carbohydrates has been skillfully employed in numerous applications, including biotin labeling [6][7][8], formation of glycoarrays [9][10][11], glycan capture for structural and functional glycomics [12][13][14], and the generation of glycopeptide analogues [15,16].
Hydrazones containing an azometine (Schiff base) -NHN=CH-group possess various bioactivities such as antimicrobial, anticonvulsant, analgesic, antiinflammatory, antiplatelet, antitubercular and antitumoral activities [17].A very famous example is the isonicotinic acid hydrazide (isoniazid) which showed very high inhibitory activity towards Mycobacterium tuberculosis H37Rv in vivo.Researchers synthesized isoniazid-hydrazones derivatives that were reported to have inhibitory activity in mice infected with various strains of M. tuberculosis and also showed less toxicity in these mice than isoniazid [18] because of the blockage of -NH 2 group.These findings further support the growing importance of the synthesis of hydrazide-hydrazones compounds [19].
X was modified with benzoyl g rou p ( 7 ) A: General structure of 5-phenyl-2-furoyl hydrazide derivatives C: X was modified with N-benzamide carbon yl g rou p ( 8 ) D: X was modified with ph en yl meth ylid ene g rou p ( 9 ) Compounds in our p revious work III: X was modified with monosaccharides

Synthesis and Structure Elucidation
Glycosylhydrazides were obtained by condensation of equimolar amounts of the corresponding 5-substituted phenyl-2-furoyl hydrazide with the monosaccharides D-glucose, D-galactose, D-mannose, D-fucose, and D-arabinose in ethanol (Scheme 2).The structures of all the compounds were characterized by 1 H Nuclear Magnetic Resonance (NMR), Infrared Spectroscopy (IR) and High Resolution Mass Spectrometer (HRMS).In the IR spectra, the compounds showed absorption bands around 3200 to 3400 cm −1 originating from the O-H and N-H stretching vibration.The strong bands around 1640 to 1680 cm −1 were carbonyl vibration of the secondary amide.The bands between 1600 and 1620 cm −1 could be assigned to the C= N stretching vibration.Absorption bands around 1610, 1550 and 1475 cm −1 were attributed to the frame vibration of the phenyl and furan ring.Absorption bands around 1430, 1330 and 1260 cm −1 were attributed to the coupled modes of C-C and C-O stretching vibrations of the sugar residues [28,29].The absorption bands around the 1150 and 1085 cm −1 region are characteristic for the pyranose form of hydrazines, and those around the 1075 cm −1 region for the acyclic form of hydrazones [2].
In the 1 H NMR spectra, the ring-chain tautomers of the monosaccharides were obviously determined by the chemical shift of the proton in secondary amide CO-NH and the proton in imine N=CH in DMSO-d 6 solution (Table 1).
If the isomer was a cyclic form, the signal of the amide CO-NH was appeared around 10.20 ppm and there was no imine N= CH signal, meanwhile the other N-NH signal was appeared around 5.87 ppm (Figure 1c).If the isomer was an acyclic form, the signal of the amide CO-NH was down-fileded to 11.50 ppm and the imine N= CH signal was appeared around 7.75 ppm (Figure 1e).The result showed that except the D-Glc, other monosaccharides such as D-Gal, D-Man, D-Fuc and D-Ara hydrazide derivatives were mainly the acyclic tautomers (Figure 1).The chemical shift of the  The ring-chain interconversion of sugar hydrazones in solution usually depended on factors such as the sugar conformation, most probably on the proportion of aldehyde form in solution, the basicity of the hydrazine groups, and the solvent effect of proton-acceptor ability.About the solvent effect on ring-chain interconversion, there were evidence that sugar hydrazones were favorable to be acyclic form in dimethyl sulfoxide-d 6 solution than others, such as pyridine-d 5 , methanol-d 4 and deuterium oxide [2].High pH values also stimulated the ring to chain conversion.The cyclic to acyclic conversion caused by increase in the basicity of hydrazine and the proton-acceptor ability of solvent might be interpreted according to the electronic theory.The strongly basic nitrogen atom of hydrazine group was in high electron-density.The electron could move easily from nitrogen to anomeric carbon of the sugars to form a C=N bong with sp 2 hybridized orbital.The solvent with strong proton-acceptor ability could increase the basicity of the hydrazine group owing to the proton bonding with N-H group.In our study, the same solution and hydrazine were used (Figure 1), suggesting that the ring-chain tautomer was mainly caused due to the different monosaccharides.Some research [2] showed that glucose was prone to be cyclic form especially in dimethylsulfoxide (DMSO) solution.In this study, only glucose derivatives gave the cyclic form, which was consistent with the reported result [2].Although the tautomerism was analyzed by 1 H NMR, these were only the states in DMSO solution and did not reflect in any other solutions or in physiological environment.The equilibrium could move in other solutions, especially under the physiological situation.

Fungicidal Activity
The fungicidal activity and fungicidal spectra were evaluated by preliminary bioassay against 19 kinds of fungi (Table 2).
The result showed that the title compounds had broad fungicidal spectra, and most of them possessed good activity against the tested fungi at 50 μg•mL −1 .Compound III-5 (Arabinose derivative) showed good activity against Phytophthora infestans and moderate activity against Phomopsis aspamgi and Cladosporium fulvum.Compound III-4 (Fucose derivative) showed moderate activity against P. aspamgi.Compound III-3 (Mannose derivative) showed good activity against P. infestans and Alternaria alternata.All the compounds had no activity against the Sclerotinia sclerotiorum except compound III-2 (Galactose derivative), which also showed excellent activity against the P. infestans, and moderate activity against A. alternate and P. aspamgi.For the glucose derivatives, fluorinated compounds such as III-6 (2,4-di-F), III-7 (4-F), and III-12 (3-F) showed better activity than the others.Among them, compound III-6 exhibited excellent activity against P. infestans and compound III-7 showed excellent activity against P. aspamgi.Both compounds III-6 and III-12 showed a significant inhibition effect on Valsa mali and C. fulvum.Compound III-12 also showed good activity against A. alternate and P. aspamgi.Compounds III-1 and III-9 showed considerable activity against Phytophthora melonis.Compound III-11 showed some inhibitory effect on the P. melonis.All the compounds showed poor activities against Fusarium graminearum, Pyricularia oryzae, Monilinia ariae, Gloeosporium musarum, Fusarium oxysporum f. sp.Niveum, Botrytis cinerea Pers., Colletotrichum orbiculare, Alternaria dauci and Colletotrichum phomoides.

Antitumor Activity
Antitumor activity of title compounds was checked.The antitumor activity in Table 4 showed that some title compounds had great activity against human promyelocytic leukemic cells (HL-60).In which, the activity of compounds III-3, III-4 and III-5 (IC 50 = 6.9, 1.2, and 19.4 μM) was better than that of the positive control doxorubicin (IC 50 = 28.4μM).Some compounds exhibited good activity against human gastric carcinoma cells (BGC-823), such as III-4, III-5, and III-6 (IC 50 = 6.9, 1.2, and 19.4 μM), but which were lower than that of the doxorubicin (IC 50 = 8.5 μM).Against human nasopharyngeal carcinoma cells (KB), only compound III-3 showed better activity (IC 50 = 9.0 μM) than the positive control doxorubicin (IC 50 = 28.4μM).All the compounds showed poor activity against human hepatocellular carcinoma cells (Bel-7402).The preliminary structure-activity relationship analysis indicated that compounds containing Man (III-3), Fuc (III-4), and Ara (III-5) showed better activity against human promyelocytic leukemic cells (HL-60).Although the compounds containing glucose showed poorer activity against HL-60 than doxorubicin, some of them also possessed considerable activity, and the substituted group had obvious effects on the activity.The compounds with fluoride and nitro group on the ortho (III-6 and III-9) or para-position (III-7 and III-8) showed better activity than the others.It could be deduced that the skeleton of 5-phenyl-2-furoyl hydrazide (hydrazone) displayed important antitumor activity, which was in agreement with our former results [20][21][22][23][24]26,27].

General Information
All the melting points were determined with a Cole-Parmer melting point apparatus while the thermometer was uncorrected.IR spectra were recorded on a NEXUS-470 FTIR (Nicolet) spectrometer with KBr pellets. 1 H NMR spectra were recorded with Bruker DPX300 (Bruker, Billerica, MA, USA) and JEOL JNM-ECA500 instrument (JEOL Ltd., Tokyo, Japan), while tetramethylsilane was used as the internal standards.HRMS was performed with Bruker APEX IV instrument (Bruker, Billerica, MA, USA).Analytical thin-layer chromatography (TLC) was carried out on precoated plates (silica gel 60 F254).The developing solvents were chloroform and methanol (v/v: 6/1), and spots were visualized with ultraviolet (UV) light.

General Synthetic Procedure for the Title Compounds III
A mixture of monosaccaride I (0.05 mol) and 5-substituted phenyl-2-furoyl hydrazide II (0.05 mol) reacted in ethanol under reflux for 6-8 h.The reaction was catalyzed by drops of acetic acid.After cooling, the solvent was removed under reduced pressure, and the solid was recrystallized from ethanol to obtain the target compounds III.
All the synthesized compounds were solid.Their structures were confirmed by 1 H NMR, IR, and ESI-HRMS.

Fungicidal Activity
The fungicidal activity and antifungal spectra of title compounds were evaluated using the mycelium growth rate test at 50 μg•mL −1 .The fungi were provided and maintained by Department of Applied Chemistry, College of Science, China Agricultural University, Beijing, China.The commercial fungicides mancozeb, carbendazim, thiram, and hymexazol were used as positive controls.EXCEL2007 was applied to analyze bioassay data.Analysis of difference significance was performed using SPSS v.18.0 (SPSS Inc., Chicago, IL, USA).
The fungicidal activity of title compounds was assessed using the radial growth test on potato dextrose agar (PDA).The compounds were dissolved in acetone and mixed with sterile molten PDA to obtain different concentrations of 50, 25, 12.5, 6.25, 3.13, 1.56 and 0.78 μg•mL −1 .Three replicates were used per treatment.After an incubation period of 96 h at 23 °C under a regular 12:12 h light:dark regimen, mycelial growth diameters were measured and the inhibition percentages relative to the control with 1% acetone were calculated.The rate was calculated according to the following formula: where I 1 is the inhibition rate, 1 D is the average area of mycelia in the blank test, and 0 D is the average area of mycelia in the presence of compounds.The EC 50 and EC 80 values were calculated using log-probit analysis, and the results were given in Table 1.

Antitumor Activity
All the title compounds were dissolved in DMSO and screened for preliminary anticancer activity against four different cell lines: a human promyelocytic leukemic cell line (HL-60), a human hepatocellular carcinoma cell line (Bel-7402), a human gastric carcinoma cell line (BGC-823), and a human nasopharyngeal carcinoma cell line (KB) at concentration gradient of 0.1, 1.0, 10, 50 and 100 μM.The commercial drug doxorubicin was used as positive control in the bioassay.Three replicates were performed.

Conclusions
In summary, a novel series of N-glycosyl-N'-(5-substituted phenyl-2-furoyl) hydrazide derivatives were synthesized in good yields.The anomerization and ring-chain isomers were confirmed by 1 H NMR spectroscopy.Their antifungal and antitumor tests indicated that some title compounds showed superiority over the commercial positive controls during the present studies.Comparing title compounds III with lead compounds B (diacylhydrazides), C (semicarbazides), and D (acylhydrazones) (Scheme 1), it can be seen that the modification with sugars in hydrazide molecules is important for improving the antifungal and antitumor activity.These compounds could be lead compounds for further development.

Scheme 1 .
Scheme 1. Designed strategy for title compounds.
glucose 3.89 ppm and the coupling constant of 8.25 Hz indicated that the glucose was in the presence of β isomer.

Table 3 .
Fungicidal activity of title compounds.

Table 4 .
Antitumor activity of the title compounds.