Phosphite triesters

Because coupling is not always quantitative, the non-reacted terminal deoxynucteoside must be excluded from the following synthesis cycles. Otherwise deletion sequences will render the isolation of the pure final product difficult. Therefore a capping step (step 3) follows, e.g., acetylation with acetic anhydride and N,N-dimethyl-4-pyridinamine (DMAP) in dioxane. Capping times should be as short as possible, especially with 2-cyanoethyl phosphite triesters, which are sensitive to bases such as DMAP. 

The newly formed phosphite triester linkage is unstable to acids and bases and is immediately oxidized to a stable phosphate triester (step 4). A solution of iodine, water, 2,6-dimethylpyridine, and tetrahydrofuran is commonly used. The oxidation is usually complete within 30 seconds. 

Phosphite triesters, P(OR)3, form donor complexes with transition metals and other acceptors and are oxidized to the respective phosphates under appropriate conditions. 

The phosphite triesters are also useful ia esterilying carboxyUc acids. The triesters are readily oxidized to the respective phosphates. 

The disadvantage of this method is that the dichloridites and monochloridites are sensitive to water and thus could not be used readily in automated oligonucleotide synthesis. This problem was overcome by Beaucage and Caruthers, who developed the phosphoramidite approach. In this method, derivatives of the form R 0P(NR2)2 react with one equivalent of an alcohol (catalyzed by species such as l//-tetrazole) to form diesters, R OP(OR")NR2, which usually are stable, easily handled solids. These phosphoroamidites are easily converted to phosphite triesters by reaction with a second alcohol (catalyzed by l//-tetrazole). Here, again, oxidation of the phosphite triester with aqueous iodine affords the phosphate triester. Over the years, numerous protective groups and amines have been examined for use in this approach. Much of the work has been reviewed. ... 

In the phosphite triester approach to the synthesis of nucleotides, morpholinophos-phorous ditetrazolide is used as the phosphitylating agent. The procedure generally consists of three steps ... 

Unreacted support-bound nucleoside hydroxy groups can be blocked with diethoxy-triazolylphosphine. Oxidation of the phosphite triester to phosphate triester was achieved by I2. Yields in the condensation exceeded 95%. Tetrazolide as phosphitylating reagent is superior to a 4-nitroimidazolide, a triazolide, or even a chloride.11061... 

In this context, it is interesting to note that the first synthesis of 2, 3 -0,0-cyclic phosphorothioate 22a was reported by Eckstein in 1968 [25], He also isolated pure Rp diastereomer by fractional crystallization of the triethylammonium salts [26] and used it as reference to determine the absolute configurations of the other phosphorothioate analogues [27], 2, 3 -0,0-Cyclic H-phosphonate 20a was used as a key substrate for the synthesis of uridine 2, 3 -0,0-cyclic boranophosphate 27. Silylation of H-phosphate 20a gave the phosphite triester 25 (two diastereomers). Its boronation, with simultaneous removal of the trimethylsilyl group, was achieved by its reaction with borane-A.A-diisopropylethylamine complex (DIPEA-BH3). 

Phosphite triesters, 19 37 Phosphobetaine monomers, 20 480 Phosphobetaines, 20 482 Phosphoboranes, 4 170, 204 Phosphocreatine, 17 671 Phosphodiesterase inhibitors, 5 181t, 186 Phosphodiester backbones, modified, 17 628-629... 

An innovative approach towards glycosyl phosphoramidites, which can be subsequently converted into the corresponding phosphite triesters,... 

A major development in global phosphorylation was realized through the application of highly reactive diorganyl dialkylphosphoramidites for the simple phosphite-triester phosphorylation of target sites. 76 While this phosphorylation procedure has been success-... 

Scheme 23 Acid-Mediated Conversion of Di-tert-butyl Phosphite Triester into the H-Phosphonatel1351...

The chemical basis for the phosphite triester approach is the observation, that dialkyl phos-phorochloridites such as (CjHjOJjPCI react very rapidly at the 3 -OH of nucleosides in pyridine even at low temperatures. In contrast, the reactions of analogous chloridates, e.g. (C2HjO)2POCIi require several hours at room temperature. It was later found that phosphite esters can be oxidized quantitatively to the phosphates by using iodine in water and that clean condensation of phosphorochloridites with nucleosides can be achieved in THF at -78 °C. To develop this chemistry into a useful synthetic procedure it was necessary to establish which... 

Polystyrene loaded with cyanoethoxy N,N-diisopropylamine phosphine 35 has turned out to be a versatile and mild phosphitylating agent (Scheme 15) [41]. The intermediate phosphite triester 36 was oxidized and the cyanoethoxy group was removed using DBU followed... 

Several chemical methods are available for the phosphorylation of suitably protected saccharides. The following approaches, classified according to the initial phosphorylation product, can be distinguished phosphotriester, phosphite triester and //-phosphonate methods. 

An important advantage of the //-phosphonatc method is that at no stage of the synthetic route, is a labile phosphotriester prepared.1036 1041 1 8 The reaction of salicylchlorophosphite (Scheme 3.19) with a glucosamine derivative furnished a phosphite triester which was subsequently hydrolysed to the //-phosphonate monoester. The latter compound was coupled... 

Not surprisingly the procedures described in Section 3.7.2 for the phosphorylation of nonanomeric hydroxyl groups, that is, the phospho-triester, phosphite triester, and 77-phosphonate approaches, have been applied to the preparation of anomeric phosphates.103d,e 110... 

Phospholipids. This phosphite in combination with ethyldiisopropylamine can be used to prepare phosphite triesters, which on oxidation (H202) and deprotection... 

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). 

The reaction results in a phosphite triester which is oxidized to a phosphodiester by flushing the column with iodine. Subsequently the DMT group is removed from the second nucleotide with 80% acetic add (detritylation) in preparation for reaction with the next DPA-activated monomer. This cycle continues until the oligonucleotide is complete, and it is capable of generating sequences of up to 150 residues. 

The 2-(methyldiphenylsilyl)ethyl (DPSME) group was used to protect a phosphate in a synthesis of Lysobisphosphatidic Acid [Scheme 7.40].82 The phosphate protection was introduced by a phosphite triester approach to give the symmetrical phosphate 40 2. The two isopropylidene groups were removed by acid hydrolysis without harm to the phosphate and the resultant diols selectively protected on their less hindered termini with fert-butyldiphenylsilyl groups. Acylation of the remaining secondary hydroxyls with oleic anhydride afforded the diester 403, whereupon the silyl protecting groups were removed with... 

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