H-phosphonate approach

Application of H-phosphonate approach in the preparation of glycosyl 1-phosphates... 

The H-phosphonate approach entails phosphitylation of a hydroxylic component with an activated unprotected derivative of phosphonic acid to furnish H-phosphonate (a stable tetra-coordinate form of the unprotected monophosphite) followed by oxidation on phosphorus. The H-phosphonate approach is not generally used for the synthesis of monophosphates due to the difficulty of oxidation of the H-phosphonate monoesters and the necessity either to render the phosphorus atom of H-phosphonate into the three-coordinate form by silylation (to form disilylphosphite) or to introduce a protecting group (to form H-phosphonate diester) prior to oxidation. Nevertheless, in rare cases, the H-phosphonate procedure turns out to be a method of choice if, for instance, the simultaneous oxidation of functional groups other than H-phosphonate is required.51... 

The chemical synthesis of RNA has the distinct advantage of allowing for site-selective incorporation of modified nucleosides into the oligonucleotide chain. The early phosphodi-ester method developed by Khorana (30) was followed by the H-phosphonate and phosphoramidite approaches (31). Although the H-phosphonate approach has distinct advantages over phosphoramidite synthesis, it is not as widely used. Therefore, the focus of this section will be on the more commonly employed... 

An additional reason for the inferior performance of the H-phosphonate method compared to the phosphoramidite lies in its mechanism, which makes the synthesis very sensitive to pre-activation of the H-phosphonate monoesters with the conventional coupling agents, which must be avoided [138], However, the main appeal of the H-phosphonate approach lies in the ease it offers for the synthesis of backbone analogues modified at the phosphorus atom phosphorothioates [139], phos-photriesters [126] and especially phosphoramidates [126, 140, 141]. Now it is rarely used for routine oligonucleotide synthesis but very often for the preparation of variously modified analogues. Further developments in the H-phosphonate method have been made in the last decade by Polish [142] and Japanese researchers [143]. 

Dinucleoside phosphorothioates are chiral analogues of phosphodiesters in which one of the nonbridging oxygens has been replaced by sulfur. These compounds are important research tools in stereochemical investigations, in mechanistic studies of various enzymatic reactions, and other biochemical stndies [210-212]. There have been only a few methods for the preparation of ribonncleoside phosphorothioates [213-216]. The H-phosphonate approach is the best method for the preparation of phosphorothioate diesters [217-219]. 

The H-phosphonate approach for the preparation of 3 -5 - intemucleotidic bonds shows the following advantages protection of phosphate is not required the oxidation reaction is performed by I2 solution at the end of the synthetic cycles the coupling reaction is much faster than the phosphoramidate approach the H-phosphonate units are more stable than... 

Iyel996 Iyer, R.P., Devlin, T., Habus, I., Ho, N.-H., Yu, D. and Agrawal, S., N-Pent-4-enoyl Nucleosides Application in the Synthesis of Support-bound and Free Oligonucleotide Analogs by the H-phosphonate Approach, Tetrahedron Lett., 37 (1996) 1539-1542. 

Agrawal, S. and Tang, J -Y. (1990) Efficient synthesis of oligoribonucleotide and its phosphorothioate analogue using H-phosphonate approach. Tetrahedron Lett. 31, 7541-7544... 

Stawinski, J, Stromberg, R, Thelin, M., and Westman, E (1988) Studies on the /-butyldimethylsilyl group as 2 -C -protection in oligoribonucleotide synthesis via the H-phosphonate approach. Nucl. Acids Res. 16,9285-9298. 

At the end of oligonucleotide chain assembly following phosphor-amiditeor H-phosphonate approach, the desired synthetic DN A is obtained as an either fully protected or partially protected form attached to the solid supports used for carrying out the synthesis. In order to obtain synthetic DN A as a fully deprotected form, the chain has to be released... 

The first chapter of Protocols for Oligonucleotides and Analogs provides a sense of the history of the development of oligonucleotide synthesis. This is followed by three chapters (Chapters 2-4) describing three main approaches to the synthesis of unmodified oligonucleotides. The phosphoramidite and H-phosphonate approaches have become the... 

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