Synthesis of Peracetylated β-D-Glucopyranosyl Thioureas from Substituted 2-Aminobenzo-1 ′ , 3 ′-thiazoles

Some peracetylated glucopyranosyl thioureas containing a heterocyclic ring system, benzo-1,3-thiazole have been prepared by the condensation reaction of tetra-O-acetyl-β-D-glucopyranosyl isothiocyanate and corresponding substituted 2-amino-(6-substituted)benzo-1,3-thiazoles. Investigated heating conditions showed that the solventless microwave-assisted method gave higher yields of these thioureas.


Introduction
Glycosyl isothiocyanates have been widely used as valuable intermediates in the synthesis of glycosyl derivatives 1 .The isothiocyanates and glycosyl isothiocyanates have been the focus of synthetic attention during recent years because of their potential pharmacological properties 2 .Thioureas and their derivatives show strong antibacterial activity and are versatile reagents in organic synthesis 3 .Benzo-1,3-thiazoles are bicyclic ring system with multiple applications.They have diverse chemical reactivity and broad spectrum of biological activity 4 , for examples, substituted 2-aminobenzo-1,3-thiazoles show antitumor 5 and antimalarial activity 6 .Bis-substituted amidino benzo-1,3-thiazoles act as potential anti HIV agents 7 .Some peracetylated glucopyranosyl thioureas containing benzo-1,3-thiazole ring have been obtained in the previous time 8 using conventional heating in solvent (dioxane, toluene or THF).

Experimental
All melting points were recorded on an electrothermal STUART SMP3 (BIBBY STERILIN-UK) apparatus and are uncorrected.The FTIR-spectra was recorded on Magna 760 FT-IR Spectrometer (Nicolet, USA) in form of KBr and using reflex-measure method. 1 H-and 13 C-NMR spectra was recorded on an Avance FT-NMR Spectrometer (Bruker, Germany) at 500.13 MHz and 125.76 MHz, respectively, using DMSO-d 6 as solvent and TMS as an internal standard.Mass spectra were recorded on Micromass AutoSpec Premier Instrument (WATERS, USA) using EI method and on 1100 LC-MSD Trap-SL (Agilent-Technologies, USA) and IONSPECK 910-MS (Varian, USA) using ESI method.

thioureas (in cases of compounds 3a, 3b and 3m) (Method A)
A mixture of corresponding 2-aminobenzothiazoles 2 (2 mmol) and tetra-O-acetyl-β-Dglucopyranosyl isothiocyanate 1 (2 mmol) in dried dioxane (30 mL) and was heated in reflux for 20-25 h.Solvent was removed under reduced pressure to obtain the sticky residue that was triturated with ethanol and recrystallized from a mixture of ethanol and toluene (1:1 in volume) to afford solid compounds 3a, 3b or 3m.

Results and Discussion
We have previously reported on the synthesis of some N-(tetra-O-acetyl-β-Dglucopyranosyl thioureas containing 4,6-diarylpyrimidine components using microwaveassisted method 9 .Hence it is quite interesting to synthesize thioureas having benzothiazole component and glucose moiety.In view of the interest in synthesis of these thioureas, a synthetic method has been involved for use of the microwave-assisted heating instead the conventional one.This method is becoming an increasingly popular method of heating which replaces the classical method because it proves to be a clean, cheap, and convenient method 10 . The required 2-aminobenzo-1',3'-thiazole/6-substituted 2-aminobenzo-1',3'-thiazoles 2 were prepared by previous proceudes [11][12][13] .Tetra-O-acetyl-β-D-glucopyranosyl isothiocyanate 1 was prepared from D-glucose by method described in reference 1,14 .Thioureas 3a-h were synthesized by condensation reaction of isothiocyanate 1 and corresponding amonibenzothiazoles 2a-h (Scheme 1).

Table 1 .
Some investigations of preparation of thioureas 3a and 3b

Table 2 .
Reaction Time for synthesis of thioureas 3a-h