## 2 Chemical Synthesis of Nucleoside Di- and Triphosphates
### Synthesis of Nucleoside Di- and Triphosphates via Phosphobetaines  
Further developing the phosphoramidate strategy, [PERSON] and colleagues proposed a new way to activate the phosphorus center in the form of a highly reactive pyrrolinium intermediate **22** (Scheme 14). The pyrrolinium group in this betaine can be effectively substituted by phosphate nucleophiles, leading to the formation of nucleoside 5\({}^{\prime}\)-di- and triphosphates, as well as disubstituted 5\({}^{\prime}\)-diphosphates bearing sugar residues on the P\({}_{\beta}\) phosphates (Scheme 14) [83; 84]. The reaction times were rather short; for example, 1 h for the reaction with carbohydrate phosphates or 10 min for the formation of nucleoside triphosphates. For carbohydrate-nucleoside diphosphates, the formation of small amounts of nucleoside monophosphates was observed as a side product, presumably due to the presence of traces of water in the reaction system [83]. Despite the promising good yields, this method is not frequently used, probably due to the time-consuming and rather laborious synthesis of the starting phosphoramidate diester **23**[32].  
Another type of a phosphobetaine, which potentially can be used in the synthesis of di- and triphosphates, is nucleoside \(N\)-pyridiniumphosphonate **25** (Scheme 15). The formation of such compounds (which may be considered as a pyridine adduct to nucleoside metaphosphate) was postulated by [PERSON] in 1960s during studies on the activation of phosphate monoesters [85], but for many years they did not find a practical application in the synthesis of phosphate anhydrides, mainly due to the lack of an effective method for the generation of this type of zwitterions. At the end of the 1990s, [PERSON] and co-workers developed an efficient and simple method for the formation of phosphobetaine of type **25** from H-phosphonate monoesters **26**, and showed its high reactivity towards different nucleophiles, e.g., alcohols, amines, and fluorides [86; 87; 88]. Drawing on these findings, in 2008, [PERSON] et al. used phosphobetaine **25** for the preparation of NTPs by reacting it with a tetrabutylammonium salt of pyrophosphate (Scheme 15). While the yields of isolated products were moderate (26-41%), the conversion of H-phosphonates **26** into NTPs was simple and rapid, as was the purification of the products [89].  
The same research group took advantage of the ease of the in situ generation of \(N\)-pyridiniumphosphonate **25** and its high susceptibility to nucleophilic substitution in the synthesis of symmetric dinucleoside diphosphates. To this end, the generated zwitterion **25** was reacted with 1 equiv. of water to furnish in good yields (70-75%) ribo-homodinucleotides (**27a**, Figure 3) as well as homodinucleotides containing nucleosides of known antiviral activity (AZT and d4T derivatives **27b**; Figure 3). [90; 91]  
Figure 14: Synthesis of nucleoside di- and triphosphates using zwitterion intermediate, \(N\)-methylpyrrolidiniumphosphonate **22**[83; 84].  
Recently, [PERSON] et al. described the use of orthophosphoric acid as a nucleophile in the reaction with \(N\)-pyridiniumphosphonate **25** as a simple and convenient method for the preparation of various ribo- and deoxyribonucleoside diphosphates and their analogs. The use of orthophosphoric acid instead of lipophilic phosphate salts (e.g., tetrabutylammonium salts) greatly simplified the purification process of the NDPs' synthesis [92].