different carbanionic nucleophiles can be reacted with -amido sulfones , affording a wide variety of amino derivatives [ 2 - 7 ] .
in addition , considerable research has been conducted on the reaction of nitrogen nucleophiles and ,-unsaturated carbonyls [ 8 - 11 ] .
heteronucleophiles as well as carbanionic reagents can react with -amido sulfones , thus giving the opportunity to prepare a large array of amino derivatives .
furthermore , a series of amino derivatives containing 1 , 3 , 4-oxadiazole ring were synthesized by the reaction of 1 , 3 , 4-oxadiazole-2-thiol with suitably substituted amines and formaldehyde in ethanol . based on these reports , we executed an inorganic base - mediated reaction of 5-subsititued phenyl-1 , 3 , 4-oxadiazole-2-thiols as the source of nitrogen nucleophiles with n - benzoyl--amido sulfones to produce n , n - aminals .
interestingly , the resulting compounds bearing a 1 , 3 , 4-oxadiazole ring are often associated with significant fungicidal and insecticidal activities [ 13 - 15 ] .
nevertheless , as of today , there has been no report on antiviral activities of n - substituted benzamides bearing 1,3,4-oxadiazol unit .
the synthetic route to the title n - substituted benzamides 5 is shown in scheme 1 . in order to prepare the key electrophilic component n - benzoyl--amido sulfones 4 for the final reaction , a three components reaction involving aromatic aldehyde , benzamide and sodium sulfonate was used by modifying the procedure reported by chemla .
the reaction was fast , free of any significant side products formation and the pure product was isolated in moderate yield through recrystallization .
the sulfones were characterized by h nmr , c nmr and ir spectral data . finally , reaction of nitrogen nucleophiles 3 with n - benzoyl--amido sulfones in the presence of basic catalyst koh in ch2cl2 at room temperature afforded the desired n - substituted benzamides 5 bearing 1,3,4-oxadiazol group [ additional file 1 ] .
the effect of different solvents and bases was studied at room temperature with a fixed reaction time of 24 h ( table 1 ) . under these conditions , dichloromethane ( ch2cl2 ) provided the product in higher yield compared to toluene , tetrahydrofuran ( thf ) or acetonitrile . amongst the various bases screened for the experiment , koh gave the best result and higher conversion was achieved when 1.2 equiv of koh was used instead of 1.0 equiv . under optimized conditions ,
the isolated yield of the n - substituted benzamides 5a reached as high as 70% when the reaction mixture was stirred for 24 h in ch2cl2 using 1.2 equiv of koh .
synthetic sequence to n - substituted benzamide analogues 5 containing 1,3,4-oxadiazole ring . effect of different solvents and bases for the synthesis of 5a unless otherwise noted , reactions were carried out with 0.5 mmol ( 1.0 equiv ) of 4a , 0.6 mmol ( 1.2 equiv ) of 3a in 5.0 ml solvent and 1 ml h2o using 0.6 mmol ( 1.2 equiv ) of basic catalyst at room temperature for 24 h. reaction was carried out with 0.5 mmol ( 1.0 equiv ) of koh at room temperature for 24 h. isolated yield after chromatographic purification .
the results of in vivo antiviral activity studies of the n - substituted benzamides 5a-5u against cmv are given in table 2 .
most of the compounds showed promising results in terms of curative bioactivities at 500 g / ml . the comparison of the antiviral activity of the products with commercial reference leads to the following conclusions : ( a ) the antiviral activity is affected by the type of the substituents present in the compound .
the compounds containing 2-fluorophenyl or 4-fluorophenyl group showed better anti - cmv activity compared with those derived from other groups .
in particular , n - substituted benzamides 5o ( r= o - f , r= p - cl ) and 5c ( r= p - f , r= m - cl ) displayed moderate curative rates ( 48.7% and 42.2% , respectively ) against cmv at the concentration of 500 g / ml . these values were comparable to the curative rate ( 53.4% ) shown by the commercial reference ningnanmycin , and superior to other compounds bearing different substituents .
compared with the compounds 5o and 5c bearing suitably substituted aryl substituents , the compounds bearing an unsubstituted phenyl ring , such as 5a and 5n , showed lower inhibitory activities .
( c ) the structural modification caused by changing the substituents ( rand r ) in the phenyl ring have a wide impact on anti - viral activity of the prepared compounds .
bioactivity of various compounds having the same substituents at different positions of the phenyl ring is various .
thus , amongst the compounds 5b , 5c , 5o and 5p containing the same substituents at different positions of the phenyl ring , the compound 5o carrying 4-chloro and 2-fluoro groups in their respective phenyl rings exhibited better bioactivity than others . although these compounds , in general , exhibited slightly lower activity in comparison with the commercial reference ningnanmycin at the concentration of 500 g / ml , some suitably substituted n - substituted benzamides bearing 1,3,4-oxadiazol moiety showed favourable antiviral activity in the preliminary studies .
subtle structural variation might lead to enhancement of activity and should be the direction of future research .
curative effect of the title compounds 5 against cmv in vivo   all antiviral tests were carried out at the concentration of 500 g / ml .
the results of in vivo antiviral activity studies of the n - substituted benzamides 5a-5u against cmv are given in table 2 .
most of the compounds showed promising results in terms of curative bioactivities at 500 g / ml . the comparison of the antiviral activity of the products with commercial reference leads to the following conclusions : ( a ) the antiviral activity is affected by the type of the substituents present in the compound .
the compounds containing 2-fluorophenyl or 4-fluorophenyl group showed better anti - cmv activity compared with those derived from other groups .
in particular , n - substituted benzamides 5o ( r= o - f , r= p - cl ) and 5c ( r= p - f , r= m - cl ) displayed moderate curative rates ( 48.7% and 42.2% , respectively ) against cmv at the concentration of 500 g / ml .
these values were comparable to the curative rate ( 53.4% ) shown by the commercial reference ningnanmycin , and superior to other compounds bearing different substituents .
( b ) substituents have a certain influence on the activity . compared with the compounds
5o and 5c bearing suitably substituted aryl substituents , the compounds bearing an unsubstituted phenyl ring , such as 5a and 5n , showed lower inhibitory activities .
( c ) the structural modification caused by changing the substituents ( rand r ) in the phenyl ring have a wide impact on anti - viral activity of the prepared compounds .
bioactivity of various compounds having the same substituents at different positions of the phenyl ring is various .
thus , amongst the compounds 5b , 5c , 5o and 5p containing the same substituents at different positions of the phenyl ring , the compound 5o carrying 4-chloro and 2-fluoro groups in their respective phenyl rings exhibited better bioactivity than others . although these compounds , in general , exhibited slightly lower activity in comparison with the commercial reference ningnanmycin at the concentration of 500 g / ml , some suitably substituted n - substituted benzamides bearing 1,3,4-oxadiazol moiety showed favourable antiviral activity in the preliminary studies . subtle structural variation might lead to enhancement of activity and should be the direction of future research .
curative effect of the title compounds 5 against cmv in vivo   all antiviral tests were carried out at the concentration of 500 g / ml .
the melting points of the products were determined on an xt-4 binocular microscope ( beijing tech instrument co. , china ) and were not corrected .
the ir spectra were recorded on a bruker vector 22 spectrometer in a kbr disk .
h nmr ( 500 mhz ) , c nmr ( 125 mhz ) and f nmr ( 470 mhz ) spectral analyses were performed on a jeol - ecx 500 nmr spectrometer at room temperature using tms as an internal standard and cdcl3 as the solvent .
intermediate 1 , intermediate 2 and 5-subsititued phenyl-1,3,4-oxadiazole-2-thiol 3 were prepared according to the reported methods   and used without further purifications [ additional file 2 ] .
the leaves of nicotiana tabacum l. inoculated with cmv were selected and grinded in phosphate buffer and filtered through double - layer pledget .
the filtrate was centrifuged at 8000 g and the supernatant liquid was the crude extract of virus .
absorbance values were estimated at 260 nm using an ultraviolet spectro - photometer . tested compounds and 2% ningnanmycin aqua used as a reference antiviral agent were first dissolved in minimum volume of n , n - dimethylformamide ( dmf ) and then diluted with distilled water containing 1% tween 20 at 500 g / ml concentration .
crude extracts of cmv were dipped and inoculated with a brush on the whole leaves , which were previously scattered with silicon carbide .
the compound solution was smeared on the left side of leaves , and the solvent was smeared on the right side for control .
the inhibition rate of the compound was calculated according to the following formula ( av denotes average ) :
the melting points of the products were determined on an xt-4 binocular microscope ( beijing tech instrument co. , china ) and were not corrected .
the ir spectra were recorded on a bruker vector 22 spectrometer in a kbr disk .
h nmr ( 500 mhz ) , c nmr ( 125 mhz ) and f nmr ( 470 mhz ) spectral analyses were performed on a jeol - ecx 500 nmr spectrometer at room temperature using tms as an internal standard and cdcl3 as the solvent .
intermediate 1 , intermediate 2 and 5-subsititued phenyl-1,3,4-oxadiazole-2-thiol 3 were prepared according to the reported methods   and used without further purifications [ additional file 2 ] .
the leaves of nicotiana tabacum l. inoculated with cmv were selected and grinded in phosphate buffer and filtered through double - layer pledget .
the filtrate was centrifuged at 8000 g and the supernatant liquid was the crude extract of virus .
tested compounds and 2% ningnanmycin aqua used as a reference antiviral agent were first dissolved in minimum volume of n , n - dimethylformamide ( dmf ) and then diluted with distilled water containing 1% tween 20 at 500 g / ml concentration .
crude extracts of cmv were dipped and inoculated with a brush on the whole leaves , which were previously scattered with silicon carbide .
the compound solution was smeared on the left side of leaves , and the solvent was smeared on the right side for control .
the inhibition rate of the compound was calculated according to the following formula ( av denotes average ) :
we have demonstrated a general and practical route for the synthesis of the n - substituted benzamides bearing 1,3,4-oxadiazol moiety in the presence of an inorganic base as the catalyst .
the reaction of 5-subsititued phenyl-1,3,4-oxadiazole-2-thiol which serves as the source of n - nucleophile with in situ generated protected imine from n - benzoyl--amido sulfones provides a ready access to a series of structurally diverse n , n - aminals .
the antiviral tests indicated that some of the synthesized compounds possessed of moderately high curative activity against cmv .
the structure of the target products needs to be optimized to enhance their antiviral activity .
further studies on mechanistic aspects , enantioselectivities and asymmetric variants of catalysts for this reaction are currently being investigated in our group .
the current study is an outcome of constructive discussion with bas , sy , lhj and dyh who offered necessary guidance to yj to carry out his synthesis and characterization experiments .
xw performed the antiviral tests ; hxq carried out the h nmr , c nmr and f nmr spectral analyses .
yield and elemental analyses data for title compounds 5a - u . which contains the table about structure , yield and elemental analyses data for title compounds 5a - u .
experimental details and data of title compounds 5a - u . which includes the experimental procedure , spectroscopic data of intermediate 3 , intermediate 4 , title compounds 5a - u , copies of h nmr and c nmr .
we gratefully acknowledge the generous financial support received from the national key project for basic research ( 2010cb126105 , 2010cb134504 ) and the national natural science foundation of china ( 20872021 ) .
 backgroundheteronucleophiles as well as carbanionic reagents can be used to react with -amido sulfones , thus giving the opportunity to prepare a large array of amino derivatives . since , 
 novel 1,3,4-oxadiazole-2-thiol derivatives can serve as potent nucleophiles , we employed 5-subsititued phenyl-1,3,4-oxadiazole-2-thiols as the nucleophilic source of nitrogen in the reaction with -amido sulfones.resultsa series of n - substituted benzamides bearing 1,3,4-oxadiazol unit were prepared for the first time by the reaction of in situ generated protected imine from -amido sulfones with 5-subsititued phenyl-1,3,4-oxadiazole-2-thiols as the source of nitrogen nucleophile . 
 some of the synthesized products displayed favourable antiviral activity against cucumber mosaic virus ( cmv ) in preliminary antiviral activity tests . 
 the title compounds 5c , 5o and 5r revealed curative activity of 42.2% , 48.7% and 40.5% , respectively against cmv ( inhibitory rate ) compared to the commercial standard ningnanmycin ( 53.4% ) at 500 g / ml.conclusiona practical synthetic route to n - benzoyl--amido sulfones by the reaction of 5-subsititued phenyl-1,3,4-oxadiazole-2-thiols as the source of nitrogen nucleophiles with in situ generated protected imine from n - benzoyl--amido sulfones is presented . 
 the reaction catalyzed by an inorganic base has considerable significance to exploit the potential of -amido sulfones in organic synthesis . 
Background
Results and Discussion
Antiviral activity and structure-activity relationship
Experimental
Chemistry
Antiviral biological assay
Preparation of medicaments
Curative effect of compounds against CMV in 
Conclusion
Competing interests
Authors' contributions
Supplementary Material
Acknowledgements