Triester method

The rate of oligonucleotide synthesis by the triester method using mesitylenesul-fonyl chloride was increased five- to tenfold when this group was used as a protective group during intemucleotide bond formation. It was removed with coned. NH4OH at 60° for 12 h or by the oximate method. ... 

Diester methods have been used to synthesize analogues of the initiator codon ApUpG in which the adenine residue has a fixed torsion angle, as for instance in (78),132 and triester methods have been used to prepare dinucleoside phosphates and codon analogues133 134 containing the hydroxyalkyl nucleosides 9-(4 -hydroxybutyl)-adenine (80), 9-(3 -hydroxypropyl)adenine (81), and 1 -(3 -hydroxypropyl)uracil (82), with a view to determining the effect of the achiral residues on the c.d. spectra. 

Carboxylic acids can be protected as 2-haloethyl derivatives. These are reducible at different potentials depending on the kind and the number of halogen substituents at C2 (Fig. 40) [178]. This kind of protecting group has been employed in a nucleotide synthesis using the triester method ]179]. 

Figure 16.10. Phosphoric ester formation in the triester method [109].

A common method for DNA synthesis is the phosphite triester method (Fig. 7-6). A single-stranded oligonucleotide is formed by the sequential creation of diester bonds between the 5 -hydroxyl of one residue and the 3 -phosphate of the next. The 3 -phosphate is activated by substitution by dialkyl phosphoamidite (DPA) and reacts readily with the free 5 -OH of the first nucleotide. To prevent the formation of unwanted linkages the first nucleotide is linked, by the 3 -OH, to a solid support (often silica gel) in a column or funnel the 5 -OH of the second nucleotide is prevented from reacting by a dimethoxytrityl group (DMT). 

In the Werstiuk-Neilson approach to the synthesis of oligonucleotides by the triester method,60 N-benzoyl protection has been ap-... 

A method for the synthesis of pApUp that uses the triester method has appeared,324 and the concept of phosphorylating preformed oligonucleotides has been extended to the synthesis of 5 -phosphates of di(ribonucleoside) monophosphates by way of phosphoric triester intermediates 325 The terminal 3 -hydroxyl group was protected with the... 

Figure 6.7. Solid-Phase Synthesis of a DNA Chain by the Phosphite Triester Method. The activated monomer added to the growing chain is a deoxyribonucleoside 3 -phosphoramidite containing a DMT protecting group on its 5 oxygen atom, a p -cyanoethyl ( p CE) protecting group on its 3 phosphoryl oxygen, and a protecting group on the base.

Figure 5.6 Solid-phase synthesis of a DNA chain, by the phosphite triester method.

The phosphotriester approach solves some of the problems of the phosphodi-ester method. More recently, the phosphite-triester method has been introduced this makes use of the extreme reactivity of the phosphite reagent, i.e., the two building blocks are joined in a few minutes compared with the hours required by the phosphodiester approach. 

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