Chemoselective synthesis of polyfunctional aminophenyl 2-oxobut-3-enyl-and quinolinylmethyl-C-glycopyranosides from nitrophenyl 2-oxobut-3-enyl C-glycopyranosides under ultrasonic vibration

Chemoselective reduction of nitro group in polyfunc tional nitrophenyl 2-oxobut-3-enyl Cglycopyranosides to the respective aminophenyl 2-ox obut-3-enyl glycopyranosides with SnCl2.2H2O under ultrasonic vibration in good yields was ach ieved successfully. Other potentially reducible groups such as carbonyl, este r, azide, tosyl, alkenic substituents were unaffected during reaction. The 2 ′-nitrophenyl-2-oxobut-3-enyl glycopyranosides as re duction substrates gave 2-quinolinemethyl glycopyranosides via reduction followed by intramolecular cyclocondensation reactions. These β-C-glycopyranosides hold great promise in medicinal chemistry.


Introduction
9][20][21][22] The aminoaryl glycopyranosides possess insulin-like activity and so have chemotherapeutic potential in diabetes, 23 and therefore the new synthetic methods to access these compounds are still in great demand.
Herein, we report the ultrasound-mediated chemoselective reduction of aromatic nitro groups in polyfunctional nitrophenyl 2-oxobut-3-enyl glycopyranosides to the respective anilinyl 2oxobut-3-enyl-or 2-quinolinemethyl β-D-C-glycopyranosides using SnCl 2 .2H 2 O under ambient reaction conditions.The application of the selected compounds of the series for the preparation of many biologically active compounds has also been investigated.Our method does not affect the other functional groups in the sugar moiety and the double bonds and ketone moieties are unchanged except in the case of the 2-nitrophenyl series.Scheme 1. Synthesis of nitrophenyl 2-oxobut-3-enyl-1′-deoxy-C-glycopyranosides.

Results and Discussion
7][38][39] The spectroscopic data of these compounds are similar to those prepared earlier, 36 and most of the newly synthesized compounds follow the same pattern.
To optimize the reaction conditions for the chemoselective reduction, a model substrate (E)-  1.The application of EtOH in the presence of SnCl 2 .2H 2 O (10 eq.) at 30 °C in ultrasonic bath was found to be the most suitable condition to offer the maximum yield (68%) of the desired compound 8b.Increasing the load of catalyst does not affect the yield of the product, however the time for the completion of reaction is slightly reduced (Table 1, entry 4 and 5).Although the conventional stirring at 60 °C and in refluxing condition the yield of the desired product is comparable to that of ultrasonic bath yet the time required in conventional stirring is significantly enhanced (Table 1, entry 7 and 8).It is important to mention here that minor products (detected by TLC) formed during the reaction could not be isolated in pure form to be characterized.
The structural elucidation of compound 8b was carried out on the basis of its spectroscopic data.HRMS of the compound displays m/z.514.1662amu as [M+Na] + peak corresponding to its molecular formulae C 24 H 29 NO 10 .In the 1 H NMR spectrum, the two exchangeable NH 2 protons were observed at δ 3.72 while the two olefinic protons were visible as doublets at  Above all the reactions are conducted in ultrasonic bath expect entries 6-8 which were conducted under conventional stirring.

Experimental Section
General.Commercially available reagent grade chemicals were used as received.All reactions were followed by TLC on Merck Kieselgel 60 F254, with detection by UV light, spraying 20% aq.KMnO 4 solution and/or spraying 4% ethanolic H 2 SO 4 .Column chromatography was performed on Silica Gel (60-120 mesh, E. Merck).IR spectra were recorded as thin films or in KBr solution with a Perkin-Elmer Spectrum RX-1 (4000-450 cm -1 ) spectrophotometer. 1 H and 13 C NMR spectra were recorded on Bruker DRX 400 MHz, 300 MHz, 75 MHz and 100 MHz instruments, respectively, in CDCl 3 and DMSO-d 6 .Chemical shift values are reported in ppm relative to TMS (tetramethylsilane) as the internal reference, unless otherwise stated; s (singlet), d (doublet), t (triplet), dd (double doublet), m (multiplet); J in Hertz.HRMS were performed using a Quattro II (Micromass) instrument.Optical rotations were measured in a 1.0-dm tube with a Rudolf Autopol III polarimeter in CHCl 3 and MeOH."RT" denotes room temperature.