31. SYNTHESIS OF POLYNUCLEOTIDES 111 



compounds has been proposed as shown below and the reaction of these 

 derivatives with phosphoric acid esters leads to the formation of tertiary 

 phosphate esters. 14 The approach remains to be investigated in the nucleo- 

 tide field but it would appear to be much less attractive than the methods 

 described below. 



c. The Use of Phosphorochloridates and Other Mixed Anhydrides 



In analogy with the method of phosphorylation of hydroxy 1 groups using 

 dialkylphosphorochloridates, the aim of this approach has been to (1) con- 

 vert a suitably protected nucleoside to the corresponding nucleoside phos- 

 phorochloridate (VI) via the nucleoside phosphite (V) and (2) subject it to 

 reaction with a second suitably protected nucleoside. The approach was 

 used in the first synthesis of a dinucleoside phosphate [thymidylyl-(3' — > 

 5')-thymidine, (IX)] 16 containing the naturally occurring internucleotide 



O O 



II I 



ROP— H > ROP— CI 



OR' OR' 



(V) (VI) 



(V), (VI), R = nucleoside, R' = benzyl 



linkage. The steps are shown in Scheme 1. Worthy of note is the preparation 

 of nucleoside-3 '-phosphite (VII) and its subsequent conversion to the phos- 

 phorochloridate (VIII). 



Although the approach has successfully been used in two further syn- 

 theses (see below), the yields were uniformly low. The difficulties en- 

 countered in the preparation and purification of the neutral intermediates 

 in this approach make it unlikely that the method could be extended to the 

 synthesis of higher oligonucleotides. 



Another approach consists in the preparation of an anhydride (Xa) of a 

 mononucleotide ester (X) and a stronger acid such as diphenyl hydrogen 

 phosphate, as shown, and its use in the phosphorylation of a second nu- 

 cleoside. 16 



This procedure was very satisfactory in the synthesis of the Cb'-C b ' 

 linked substance adenylyl-(5' — ■» 5')-uridine but was ineffective in the 

 synthesis of C 3 '-C 5 ' internucleotide bonds. 15 The same approach has, how- 

 ever, been used for the activation of ribonucleoside-2',3'-cyclic phosphates 



14 H. G. Khorana, Can. J. Chem. 32, 227 (1954). 



15 A. M. Michelson and A. R. Todd, J. Chem. Soc. p. 2632 (1955). 



)6 R. H. Hall, A. R. Todd, and R. F. Webb, J. Chem. Soc. p. 3291 (1957). 



