Condensation agents, nucleoside

In the 1,3,2-dioxaphosphole method a bis(2-butene-2,3-diyl) pyrophosphate is used as the condensing agent. It allows two successive esterifications of one phosphate group to be performed without additional activation. First a 5 -O-protected nucleoside is added in methylene chloride in the second reaction an unprotected nucleoside can be used, since only the 3 OH group is able to attack the cyclic enediol 3 -nucleosidyl phosphotriester. Protected dinucleoside triesters are obtained in 80% yield. Removals of protective groups, methoxytrityl by means of trifluoroacetic acid in methylene chloride and 1-methylacetonyl by aqueous triethyl-amine, also give about 80% yield (F. Ramirez, 1975, 1977). 

Urea is another prebiotic condensing agent for the phosphorylation of nucleosides. Condensed phosphates have been obtained from ammonium phosphate and urea [126], while alcohols are easily phosphorylated at 150 °C by urea and phosphoric acid [127]. When an equimolar mixture of uridine 48 and Na2HP04 was heated at 100 °C in the presence of a ten-fold molar excess of urea and ammonium chloride, 97% of the phosphate was found to be incorporated into the nucleoside moiety mainly as uridine 5 -monophosphate 49 and uridine 2, 3 - cyclic phosphate 50 (Scheme 23). 

Although there are many methods available, there are two main routes for the synthesis of nucleoside monophosphates. These involve either the reaction of the nucleoside with 2-cyanoethylphosphate in the presence of a condensing agent such as dicyclohexyl carbodiimide (DCC)2,3 or phosphorylation of the nucleoside... 

Esterification of phosphonic acids by hydroxy nucleosides using a condensing agent [108] ... 

Use Enzyme cross-linking agent, condensing agent for nucleoside triphosphate synthesis. 

Triethylammonium salts of aryl H-phosphonates have been coupled to the 3 -OH of deoxynucleosides using pivaloyl chloride as condensing agent subsequent hydrolysis by aqueous pyridine gives nucleoside H-phosphonate building blocks... 

Transesterification of nucleoside H-phosphonates with various diols in pyridine in the presence of 2-chloro-5,5-dimethyl-l,3,2-dioxaphosphorinane (a mild condensing agent) furnished rapid and clean formation of the corresponding hydroxyalkyl nucleoside H-phosphonate [193]. With ethylene glycol (n = 2), the desired hydroxyethyl H-phosphonate was formed as a minor product, only 10% (5p = 8.6 ppm), while the main phosphorus-containing species was a compound resonating at 5p = 23.1 ppm. The presence... 

Notice that the actual phosphodiester bond formation did not involve the reaction of a phosphorochloridate with the 5 -hydroxyl of the nucleoside. This approach has been seldom used for diester synthesis as these reactions tend to be sluggish and can be complicated by comparison with other methods. More favorable is the reaction of a nucleoside phosphorochloridite with a second molecule of nucleoside, as shall be discussed shortly. Alternatively, phosphodiester formation may be accomplished via in situ activation of a nucleotide with a condensing agent. Some of these reagents are described below. 

Trichloroethanol has been used as a protecting group in the triester approach for the synthesis of deoxyribo- [29] as well as ribo-oligonucleotides [30]. It can be removed selectively by reduction with Zn or Zn/Cu, as previously discussed. The triesters may be synthesized either by stepwise addition of the two properly protected nucleosides to, , -trichloroethyl phos-phorodichloridate (Fig. 6.11a) or by condensation of a/3, 3, 3-tri-chloroethyl ester of a nucleotide with a nucleoside using an arylsulphonyl chloride as a condensing agent (Fig. 6.11b). 

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