Stereoselective synthesis of tetrahydrofuranyl 1,2,3-triazolyl C - nucleoside analogues by ‘click’ chemistry and investigation of their biological activity

The construction of a novel series of enantiopure tetrahydrofuranyl 1,2,3-triazolyl C -nucleoside analogues utilizing click reaction of sugar derived tetrahydrofuranyl alkynes with various ‘in house’ synthesized sugar derived tetrahydrofuranyl azides and an adamantyl azide is described. The biological significance of the synthesized C -nucleosides was highlighted by evaluating them in vitro for anti-bacterial and anti-fungal activity, wherein a number of compounds were found to show excellent anti-bacterial activity and moderate anti-fungal activity.

9][40][41][42][43] The development of its very efficient copper catalyzed click reaction has made this reaction the most widely used method for the synthesis of 1,2,3-triazole. 39,40Consequently in our present study we chose the copper catalyzed click reaction for the synthesis of the targeted 1,2,3-triazolyl C-nucleoside analogues of prototype-I and prototype-II (Figure 2).
The structure of triazolyl C-nucleoside analogue 14 was established by its NMR and mass spectroscopic data.Its 1 H NMR spectrum showed a singlet at δ 7.44 corresponding to the triazolyl proton while the 13 C NMR spectrum showed peaks at δ 121.5 and 147.8 corresponding to CH and qC characteristic to the triazole core unit.Finally the ESI-MS spectrum displayed the [M+H] + peak at m/z 538 and its HRMS spectrum showed the [M] + peak at 537.2490, which confirmed its structure.Once the synthesis of triazolyl C-nucleoside 14 has been successfully achieved we extended the synthetic strategy to access more compounds of this series.Different stereoisomers of THF alkynes and azides were prepared and coupled with each other to obtain a series of isomeric 1,2,3-triazolyl C-nucleoside analogues.Consequently when the stereoisomeric alkynes 13, 17 and 21 were treated with THF azides 9-11 under Huisgen 1,3-dipolar cycloaddition reaction condition, the corresponding tetrahydrofuranyl 1,2,3-triazolyl C-nucleosides analogues 15, 18, 19, 22, 23 and 24 were obtained in 58% -75% yields (Scheme 1).
Interestingly in the case of alkyne 17 derived triazoles 18, 19 and 20 the peak characteristic of CH and qC of the triazole was inexplicably recorded as a very weak signal in 13 C NMR spectrum, but their 1 H NMR, ESI mass and HRMS spectroscopic studies clearly showed that the desired triazoles were formed.The structures of the triazoles 18, 19 and 20 were finally confirmed by recording their HSQC spectra which showed a correlation between carbon signals at 120.4 ppm in case of compounds 18, 19, and 122.4 ppm in compound 20 with their respective triazole CH protons.All the above described tetrahydrofuranyl 1,2,3-triazoly C-nucleoside analogues were evaluated for their in vitro anti-bacterial and anti-fungal activities (See Table 1 and 2 in SI).It was found that the adamantane containing 1,2,3-triazolyl C-nucleoside analogue 25 was active at a MIC of 1.56 μg/ml against S. aureus which is two fold better activity than standard drug gentamycin and three fold better activity than ampicillin while in case of K. pneumoniae it exhibited a MIC of 0.78 μg/ml which is equivalent to the activity of gentamycin.Another adamantane containing analogue 27 was found active at a MIC of 0.78 μg/ml against S. aureus which is three fold better activity than gentamycin, four fold better activity than ampicillin and equal to ciprofloxacin.In case of K. pneumoniae it again exhibited a MIC of 0.78 μg/ml which is equivalent to the activity of gentamycin.However, none of the other 1,2,3-triazolyl C-nucleoside analogues were found to show any anti-bacterial activity.

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The 1,2,3-triazoly C-nucleoside analogues were also screened against a panel of six fungal strains.Only molecules 16, 18, 19 and 20 were found active against T. mentagrophytes at MIC of 25 μg/ml, 12.5 μg/ml, 25 μg/ml and 25 μg/ml respectively.None of the other molecules showed any activity against the fungal strains.

Conclusions
In summary we have disclosed the stereoselective construction of a novel class of 1,2,3-triazolyl C-nucleoside analogues (14-16, 18-20, 22-25 and 27), with multiple points for skeletal diversification, from 'THF alkynes and THF azides, using 'click' chemistry as the key step.The alkynes and azides used in this study were easily accessed from commercially available sugars.
In order to demonstrate the potential biological utility of the synthesized 1,2,3-triazolyl Cnucleoside analogues, they were evaluated for in vitro anti-bacterial and anti-fungal activity.It was found that the adamantane containing 1,2,3-triazolyl C-nucleoside analogues 25 and 27 exhibited excellent activity against S. aureus and K. pneumoniae in a MIC range of 0.78 μg/ml to 1.56 μg/ml which was comparable or better than the activity of many standard drugs while compounds 16, 18, 19 and 20 exhibited moderate anti-fungal activity in a MIC range of 12.5 μg/ml to 25 μg/ml.A detailed study on further elaboration of these 1,2,3-triazolyl C-nucleoside analogues and investigation of their prospective bioactivities is currently underway and would be disclosed in due course.

Supporting Information
Copies of 1 H, 13 C and 2D NMR spectral data of all the synthesized 1,2,3-triazolyl C-nucleoside analogues, general procedure for in vitro anti-bacterial and anti-fungal activity evaluation and tabulated results of in vitro anti-bacterial and anti-fungal activity evaluation of the 1,2,3-triazolyl C-nucleoside analogues.
General procedure for the synthesis of tetrahydrofuranyl 1,2,3-triazolyl C-nucleoside analogues.To a vigorously stirred solution of azide (0.5 mmol) in tert-butyl alcohol (5 mL) was added acetylene (0.6 mmol ) and the reaction was initiated by the addition of a solution of CuSO4.5H2O(0.1 mmol) and sodium ascorbate (0.2 mmol) in distilled water.The colored suspension formed was stirred at the room temperature till the formation of triazole (6 -8h).After completion of the reaction, ice-cold water was added and the aqueous layer was extracted with CHCl3 (3 x15 mL).The combined organic extracts were dried evaporated and purified by column chromatography to afford the pure

Figure 4 .
Figure 4. Azides used in the present study.