31. SYNTHESIS OF POLYNUCLEOTIDES 113 



enesulfonyl chloride. 18 The diester of phosphoric acid (XII) is thus 

 obtained directly and the yields are consistently high. The mechanism of the 



O 



RO— P— OH + R'OH + C 6 H 11 N=C=NC 6 H 1I -» 



OH 



(XI) (DCC) 



O O 



ROP-OR' + CsHmNHCNHCbH,, 



OH 



(XII) 



reaction appears to be complex, 8 ' 19 but presumably involves the activation 

 of the monoester to poly- or metaphosphates. In the nucleotide field, the 

 reaction was first applied to the synthesis of thymidylyl-(3' — > 5')-thymi- 

 dine, (XVIa) by the reaction of 3 '-O-acetylthymidine-5 '-phosphate (XlVa) 

 and 5'-0-tritylthymidine (XIII) (Scheme 2) and the desired product was 

 obtained in about 65% yield when equimolar amounts of the two compo- 

 nents were used. When a 100% excess of one component over the second is 

 used, the yield of the product is quantitative with respect to the latter. The 

 same method has been applied to syntheses of two more dinucleoside phos- 

 phates (XVIb and XVIc) by reacting 5'-0-tritylthymidine (XIII) with 

 (XlVb) and (XIVc), respectively. Both compounds are obtained in satis- 

 factory yields. 8 The method is, thus, readily and generally applicable in the 

 deoxyribonucleotide field. 



2. Stepwise Synthesis of Deoxyribo-oligonucleotides 



The work in this field is still in its infancy and only limited progress has 

 been reported. The requirements in the stepwise synthesis of oligonucleo- 

 tides, higher than dinucleotides and containing different mononucleotide 

 units, are, first, selectively to expose an hydroxyl function at one end of the 

 fully protected dinucleoside phosphate (XV) and, then, to treat it with an- 

 other suitably protected nucleotide. 



a. Thymidylyl-{3' -> 5')-thymidylyl-{3' -> 5') -thymidine (XVIII) 20 



Mild alkaline treatment of (XVa) gave (XVIIa), which was permitted 

 to react with two molar equivalents of 3'-0-acetylthymidine-5'-phosphate 



18 H. G. Khorana, G. M. Tener, J. G. Moffatt, and E. H. Pol, Chem. & Ind. p. 1523 

 (1956). 



19 M. Smith, J. G. Moffatt, and H. G. Khorana, J. Am. Chem. Soc. 80, 6204 (1958). 



20 P. T. Gilham and H. G. Khorana, J. Am. Chem. Soc. 81, 4647 (1959). 



