Microwave-Assisted Synthesis of Acetophenone ( perO-acetylated-β-D-glucopyranosyl )-thiosemicarbazones

−Thirteen new substituted acetophenone (2,3,4,6-tetra-O-acetyl-β-Dglucopyranosyl)-thiosemicarbazones (3) were synthesized by reaction of 2,3,4,6-tetra-O-acetyl-β-D-glucopyranosyl thiosemicarbazide (1) and substituted acetophenones (2). The reaction was performed using microwaveassisted method. The compounds (3) have remarkable antibacterial and antifungal activities against Escherichia coli, Staphylococcus epidermidis and Candida albicans.


Introduction
Thiosemicarbazones are a class of small molecules that have been evaluated over the last 50 years as antivirals and as anticancer therapeutics, as well as for their parasiticidal action against Plasmodium falciparum and Trypanasoma cruzi which are the causative agents of malaria and Chagas's disease, respectively 1 .The chemistry of thiosemicarbazide derivatives of saccharides is interested [2][3][4] .These compounds arouse interest as versatile intermediates for preparing various (e.g., heterocyclic) derivatives as well.Thiosemicarbazones can be used for making electrodes 5 , or complexes formation of metallic ions 6 .Thiosemicarbazones exhibit various biological activities such as antituberculosis 7 , antimicrobial 8 , anti-inflammatory 9 , antiviral, anticonvulsant 10 , antihypertensive 11 , local anesthetic 12 , anticancer 13 , hypoglycemic 14 and cytotoxic activities, among others 15 .
A number of glucosyl thiosemicarbazide derivatives showed significant in vivo antimicroorganisms and in vitro antioxidant activity, which could be used as leads for the development of effective anti-atherosclerotic agents 16 .On the other hand, these molecules can also serve as phosphane-free multidentate ligands for transition-metal catalysis and they are efficient ligands for palladium-catalyzed coupling reactions in air 17 .In the past, some papers have been published for the synthesis of aldehyde/ketone 4-(per-O-acetylated-β-Dglucopyranosyl)-thiosemicarbazones and their corresponding deacetylated analogues 18 .The main synthetic step for the synthesis of these molecules is being the reaction of a glucosyl thiosemicarbazide with a carbonyl compound.
Continuing our studies on the synthesis and the reactivity of peracetated glycopyranosyl isothiocyanates and thiosemicarbazides 19 , we report here a systematic study for the synthesis and spectral characterization of a series of aromatic ketone 4-(β-Dglucopyranosyl)-thiosemicarbazones using microwave-assisted method 20 .

Physical measurements
Melting points were determined by open capillary method on STUART SMP3 instrument (BIBBY STERILIN-UK) and are uncorrected.IR spectra (KBr disc) were recorded on an Impact 410 FT-IR Spectrometer (Nicolet, USA). 1 H and 13 C NMR spectra were recorded on Bruker Avance Spectrometer AV500 (Bruker, Germany) at 500.13 MHz and 125.77MHz, respectively, using DMSO-d 6 as solvent and TMS as an internal standard.The chemical shifts are expressed in δ ppm scale down field from TMS and proton signals are indicated as s = singlet, d = doublet, t = triplet and m = multiplet.HRMS spectra were recorded on mass spectrometer AutoSpec Premier (WATERS, USA) using EI 70 eV.

Results and Discussion
The synthesis of the title compound is outlined in Scheme 1. Condensation of 4-(tetra-O-acetylβ-D-glucopyranosyl)thiosemicarbazide (1) 1c, 6-8 with a number of acetophenones afforded acetophenone 4-(tetra-O-acetyl-β-D-glucopyranosyl)-thiosemicarbazones (3).This syntheses was carried out using minimum amount of solvent (methanol) comparing conventional heating methods (3-5 mL volume vs. 20-30 mL, respectively).Reaction time was from 40 to 50 minutes depending substituent's nature: withdrawing substituents need shorter reaction time than donating ones (Table 1).In the first period of reaction when reaction was starting to irradiate about 10-15 minute, the pasty mixture of reagents in methanol was dissolved and the reaction became homogenous.In the final period of reaction, the solid product appeared and precipitated out.The solid thiosemicarbazones (3) were filtered by suction and recrystallized from ethanol or ethanol/toluene (1:1 in volume) 11,12 .These compounds can be dissolved in ethanol, toluene, chloroform, DMF and had high melting points.Compounds 3b, 3d and 3g have been already reported in references 18 , so other remaining acetophenone 4-(tetra-O-acetyl-β-Dglucopyranosyl)-thiosemicarbazones (3) have been synthesized for the first time.All the products were characterized by IR, 1 H NMR and 13 C NMR, and mass spectra.
The IR spectra of compounds 3 showed characteristic absorptions in the range of 3323-3390 cm −1 and 3298-3332 cm −1 (N−H bond), 1743-1749, 1223-1234 cm −1 and 1038−1098 cm −1 (ester), 1370-1378 cm −1 (C=S), and 1620-1600 cm −1 (C=N bond).In the 1 H NMR spectra of 3, the aromatic proton H-1 is represented as a triplet at the range 5.90-5.95ppm due to the coupling with the N(4")−H and the H-2.The coupling constant J H-1,H-2 (9.0-9.5 Hz) is an evidence which confirms the β configuration in compounds 3 [12][13][14][15][16][17][18] .The NH proton of the thioamide functionality of compounds 3 appeared at 10.71−10.98ppm (singlet) for N(2")H and 8.77−8.58ppm (doublet, J NH,H-1 =9.0−9.5 Hz) for N(4")H.The 13 C-NMR spectra showed the thiocarbonyl carbon atom with chemical shift at δ=179.0−179.7 ppm 18  It has previously been found that thiosemicarbazone derivatives relating to the stereochemistry of the C=N bond, can exist in the E or Z form or as a mixture of E/Z isomers, usually with the E form being the major isomer 23 .E/Z-Isomeric ratios is depended on the substituents' nature and its position on the thiosemicarbazone moiety and the solvent.The correct configuration then can be determined by 1 H NMR spectroscopy, as the N(4")H on NH−N=C group appears at δ=9-12 ppm for the E form, and δ=14-15 ppm for the Z form 23 .Thiosemicarbazones 3 had remarkable anti-microorganism activities (Table 2).In amount of compounds 3 was 20 µg/mL, the biological activities were insignificant for E. coli, S. epidermidis and C. albicans.In higher amount (40 µg/mL and 60 µg/mL) most these compounds had significant biological activities for those microorganisms.

Table 2 .
Exploration of biological activities of thiosemicarbazones 3a-m.