Arylsulfonyl tetrazoles
FIELD OF THE INVENTION 
     This invention is directed to novel arylsulfonyl tetrazole compounds, their method of preparation, and their use as improved coupling agents in polynucleotide synthesis. 
     DESCRIPTION OF THE PRIOR ART 
     5-Arylthioalkyl-tetrazoles have been prepared by reaction of the arylthioalkyl nitrile with an azide compound, and 5-arylsulfonylalkyl-tetrazoles prepared by further reaction with an oxidizing agent (see U.S. Pat. No. 3,337,576 Buchanan et al). 5-Para-nitrobenzene-sulfonamido-tetrazoles have been prepared by reacting 5-amino-tetrazole monohydrate with para-nitrobenzene sulfonyl chloride (see U.S. Pat. No. 2,209,243 Winnek). Both of these types of 5-tetrazole derivatives had biological activity. 
     Certain arylsulfonyl-1,2,4-triazoles were prepared from arylsulfonyl chloride and 1,2,4-triazole, and were found to have biological activity (see U.S. Pat. No. 3,293,259 Wolf). 
     No references have been noted to arysuflonyl tetrazoles nor their utility as coupling agents. 
     In polynucleotide synthesis the coupling of nucleotides is carried out by condensing the free phosphate group of one nucleotide to the free hydroxyl group of a second nucleotide or nucleoside, and such couplings can be repeated many times. 
     The development of dicyclohexylcarbodiimide (DCC), mesitylenesulfonyl chloride (MS) and triisopropylbenzenesulfonyl chloride (TPS) as reasonably effective condensing or coupling reagents has played a significant role in the synthesis of polynucleotides by the diester method (see S. A. Narang, K. Itakura, C. P. Bahl and N. Katagiri -- J. Am. Chem. Soc. Vol. 96, page 7074, 1974). In the case of the triester synthetic approach (see K. Itakura, N. Katagiri, C. P. Bahl, R. H. Wightman and S. A. Narang -- J. Am. Chem. Soc. Vol. 97, page 7327, 1975), triisopropylbenzenesulfonyl chloride (TPS) has been used almost exclusively as the condensing reagent because dicyclohexylcarbodiimide (DCC) will not activate phosphodiester functions and mesitylenesulfonyl chloride (MS) causes extensive sulfonation of the primary 5&#39;-hydroxyl group of the nucleotide component thus blocking possible condensation. 
     The search for new condensing reagents was initiated because of our continued realization of low yields (ca. 20%) when attempting condensation with TPS of products containing purine bases, especially guanine. These low yields might be attributed to the liberation of hydrogen chloride from the triisopropylbenzenesulfonyl chloride (TPS) during the condensation reaction. 
     SUMMARY OF THE INVENTION 
     Arylsulfonyl tetrazoles have been prepared of the general formula ##STR2## where R 1 , R 2  and R 3  are selected from hydrogen, lower alkyl, and lower alkoxy groups, and these compounds have been found to be advantageous coupling agents. These 1-arylsulfonyl-tetrazoles can preferably have as aryl groups, unsubstituted phenyl or mono- or di-substituted-phenyl, or 2,4,6-trisubstituted-phenyl groups. The substituents can most suitably be lower alkyl or lower alkoxy groups having from 1 to 4 carbon atoms. Suitable aryl groups include phenyl, para-tolyl, para-ethylphenyl, paralower alkoxyphenyl (e.g. anisyl), mesityl, and 2,4,6-triisopropylphenyl. The phenyl compound tends to be more reactive and less stable than the substituted-phenyl compounds. 
     These arylsulfonyl tetrazoles can be prepared by reacting, in a non-polar organic solvent, an arylsulfonyl chloride with tetrazole, in the presence of a basic amine catalyst. Suitable solvents include dioxane, chloroform, carbon tetrachloride and benzene. Suitable basic amine catalysts include triethylamine, aniline and lutidine. Approximately equimolar amounts of the two reactants will normally be used. After the reaction is complete a precipitate will usually be present in the cooled reaction medium, and this precipitate should be removed and the liquor processed to recover the soluble arysulfonyl tetrazole. 
     In polynucleotide synthesis, an appropriate N-protected nucleotide or oligonucleotide phosphate and an appropriate N-protected nucleotide or oligonucleotide containing a free 5&#39;-hydroxyl group are mixed in an appropriate aprotic organic solvent medium and treated with the arysulfonic tetrazole coupling agent. From about 2 to about 3 molar equivalents of coupling agent based on the 5&#39;-protected component are suitable. The coupling or condensing reaction will normally take about 1-2 hours at room temperature. The reaction mixture is then inactivated and processed to recover the phosphoester product. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The following Examples illustrate the various aspects of the invention, and are not to be construed in a limiting sense. Alternative components and conditions will be apparent to those skilled in the art. 
     EXAMPLE 1 
     Preparation of Arylsulfonyltetrazoles 
     To a solution of dioxane (2 ml) containing triethylamine (10.1 mmoles) was added the appropriate arysulfonyl chloride (10 mmoles) and tetrazole (10 mmoles) with cooling. After 2 hours, the precipitate was filtered off, discarded, and the liquor evaporated to dryness. The solid residue was dissolved in chloroform (˜50 ml) and washed with water (2 × 20 ml). The chloroform solution was dried over anhydrous sodium sulfate and after evaporation of solvent, the residue was recrystallized from an appropriate organic solvent as stated below. The isolated yield was 70-80%. Each of the compounds was characterized by its melting point, elemental analysis and NMR spectra as detailed below. 
     1-(Benzenesulfonyl) -tetrazole (BS-tetr.): White crystals m.p. 86-92° (Recrystallized from benzene) Elem Anal. Calcd. for C 7  H 6  N 4  O 2  S: C, 36.36; H, 3.05; N, 28.27. Found: C, 36.50; H, 3.00; N, 28.24. NMR (CDCL 3 , ppm from trimethyl silicane TMS) 9.25 (1H, S, CH in tetrazole); 8.0 (5H, m. aromatic ring). 
     1-(Mesitylenesulfonyl)-tetrazole (MS-tetr.): White crystals m.p. 108-119° (Recrystallized from benzene) Elem. Anal. Calcd. C 10  H 12  N 4  O 2  S: C, 47.61; H, 4.79; N, 22.21. Found: C, 47.69; H, 4.84; N, 22.30. NMR (CDCL 3  ppm from TMS) 9.21 (1H, S, CH in tetrazole), 7.5 (2H, S, aromatic ring); 2.7 (6H, S, CH 3 ) ortho, 2.36 (3H, S, CH 3  para). 
     1-(2,4,6-Triisopropylbenzenesulfonyl)-Tetrazole (TPS-tetr.): White crystals m.p. 95°-97° (Recrystallized from benzenepetroleum ether), Elem. Anal. Calcd. C 16  H 24  N 4  O 2  S: C, 57.12; H, 7.19; N, 16.65 . Found: C, 57.25; H, 7.20; N, 16.69. NMR (CDCL 3 , ppm from TMS) 9.28 (1H, S, CH in tetrazole); 7.4 (2H, S, aromatic ring); 4.15 (2H, M, CH in ortho); 3.14 (1H, M, CH in para); 1.28 (18H, pseudoquartet, CH 3 ). 
     EXAMPLE 2 
     Synthesis of Polynucleotides of Defined Sequences 
     A 5&#39;-O-dimethoxytrityl-N-protected oligonucleotide-3&#39;-p-chlorophenyl phosphate (1 molar equiv. ) and an appropriate N-protected oligonucleotide containing free 5&#39;-hydroxyl (1.2 molar equiv.) were mixed in anhydrous pyridine (5ml per g) and treated with arylsulfonyl tetrazole (3 molar equiv. based on 5&#39;-protected component) at room temperature. When the reaction was over (after 1-2 hours) as checked by tlc on silica-gel, the reaction mixture was decomposed with water (10 ml per g) with cooling followed by extraction with chloroform (100 ml per g). The chloroform layer was washed with 0.1 M triethylammonium bicarbonate (50 ml × 3) followed by water, dried over anhydrous sodium sulfate and evaporated to a gum in the presence of excess of toluene under reduced pressure. The gum was dissolved in choroform and purified by silica-gel column chromatography. The column was monitored by checking an aliquot from every second fraction collector tube on silica-gel tlc plate using chloroform-methanol (1-10% v/v) as eluent. 
     The following Table 1 gives comparisons of the yields of various di- and trinucleotide products using triisopropylbenzenesulfonyl chloride (TPS), a prior art coupling agent, and two arylsufonyl tetrazole coupling agents according to the invention. The reaction time was 0.5 hour in each case. The yields are seen to be significantly improved using the coupling agents of the invention. For the di- or trinucleotides as in Table 1, a reaction time of 0.5 hour was sufficient, but for higher polynucleotides a longer reaction time will normally be required. 
     
                                           TABLE 1                                 
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5&#39;-Protected  3&#39;-Protected                                                
                         Product % Yield (using 2 molar equiv.)           
component     component  based on 5&#39;-protected component                  
(1 molar equiv.)                                                          
              (1.2 molar equiv.)         TPS MS-tetr.                     
                                                   TPS-tetr.              
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                         Fully protected dinucleotides                    
[(MeO).sub.2 Tr]dbzA-ClPh                                                 
              dbzAτCE                                                 
                         [(MeO).sub.2 Tr]dbzAτbzAτCE              
                                         30  75    78                     
[(MeO).sub.2 Tr]T-ClPh                                                    
              dacGτCE                                                 
                         [(MeO).sub.2 Tr]dTτacGτCE                
                                         34  65    68                     
[(MeO).sub.2 Tr]dacG-ClPh                                                 
              dbzAτCE                                                 
                         [(MeO).sub.2 Tr]dacGτbzAτCE              
                                         22  66    52                     
[(MeO).sub.2 Tr]dbzC-ClPh                                                 
              dacGτCE                                                 
                         [(MeO).sub.2 Tr]dbzCτacGτCE              
                                         38  70    80                     
[(MeO).sub.2 Tr]dbzA-ClPh                                                 
              dTτCE  [(MeO).sub.2 Tr]dbzAτTτCE                
                                         52  73    81                     
[(MeO).sub.2 Tr]dbzA-ClPh                                                 
              dbzCτCE                                                 
                         [(MeO).sub.2 Tr]dbzAτbzCτCE              
                                         68  74    70                     
[(MeO).sub.2 Tr]T-ClPh                                                    
              dbzAτCE                                                 
                         [(MeO).sub.2 Tr]dTτbzAτCE                
                                         62  72    78                     
[(MeO).sub.2 Tr]dbzC-ClPh                                                 
              dbzCτCE                                                 
                         [(MeO).sub.2 Tr]dbzCτbzCτCE              
                                         71  83    80                     
[(MeO).sub.2 Tr]dbzC-ClPh                                                 
              TτCE   [(MeO).sub.2 Tr]dbzCτTτCE                
                                         68  79    82                     
                         Fully protected trinucleotides                   
[(MeO).sub.2 Tr]dbzAτbzA-ClPh                                         
              TτCE   [(MeO).sub.2 Tr]dbzAτbzAτTτCE        
                                         45  74    71                     
[(MeO).sub.2 Tr]dbzAτT-ClPh                                           
              dbzAτCE                                                 
                         [(MeO).sub.2 Tr]dbzAτTτbzAτCE        
                                         51  76    78                     
[(MeO).sub.2 Tr]dbzAτT-ClPh                                           
              T(oAc)     [(MeO).sub.2 Tr]dbzAτTτT(oAc)            
                                         59  82    78                     
[(MeO).sub.2 Tr]dbzAτbzC-ClPh                                         
              dbzAτCE                                                 
                         [(MeO).sub.2 Tr]dbzAτbzCτbzAτCE      
                                         52  70    72                     
[(MeO).sub.2 Tr]dTτacG-ClPh                                           
              TτCE   [(MeO).sub.2 Tr]dTτacGτTτCE          
                                         32  64    52                     
[(MeO).sub.2 Tr]TτbzA-ClPh                                            
              TτCE   [(MeO).sub.2 Tr]dTτbzAτTτCE          
                                         55  72    70                     
[(MeO).sub.2 Tr]dacGτbzA-ClPh                                         
              dacGτCE                                                 
                         [(MeO).sub.2 Tr]dacGτbzAτacGτCE      
                                         18  58    40                     
[(MeO).sub.2 Tr]dbzC-ClPh                                                 
              dacGτacG(oAc)                                           
                         [(MeO).sub.2 Tr]dbzCτacGτacG(oAc)        
                                         20  68    59                     
[(MeO).sub.2 Tr]dbzCτbzC-ClPh                                         
              dacGτCE                                                 
                         [(MeO).sub.2 Tr]dbzCτbzCτacGτCE      
                                         32  61    58                     
[(MeO).sub.2 Tr]dbzCτT-ClPh                                           
              dbzCτCE                                                 
                         [(MeO).sub.2 Tr]dbzCτTτbzCτCE        
                                         65  76    80                     
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