Triesters phosphate

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. 

Phosphate triesters (18) are iatermediates ia both the phosphotriester and phosphoramidite methods, and under appropriate conditions for deprotection of the bases and cleavage of the support, can be obtained directiy by usiag these approaches. The ethyl and isopropyl esters have been obtained directiy by usiag the phosphoramidite method because these are stable duting the normal deprotection procedure (62). By changing the oxidizing agent to Sg, both amidate and triester thiolates can be obtained. 

Both phosphoramidate and phosphate triester derivatives have been used as linkers to attach reporter groups to oligonucleotides. These derivatives are not entirely resistant to nucleases and they possess a chiral center. They have not been widely iavestigated as antisense dmgs. 

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. ... 

The 4-nitrobenzyl group, used in the synthesis of phosphorylated serine, is introduced by the phosphoramidite method and can be cleaved with TFMSA/ MTB/m-cresol/l,2-ethanedithiol/TFA, 4 h, 0° to rt. 7V-Methylmorpholine at 80° also cleaves a 4-nitrobenzyl phosphate triester. 

LeBel GL, Williams DT. 1983. Determination of organic phosphate triesters in human adipose tissue. J Assoc Off Anal Chem 66 691-699. 

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... 

An early attempt to achieve metal-metal cooperation on the nonen-zymatic hydrolysis of a phosphate triester failed, probably due to the long and flexible linker connecting two imidazole-containing metal-binding sites [41]. 

Fig. 9 A hypothetical More-O Ferrall Jencks diagram for the attack of methoxide on O-aryl phosphate triesters (20) and 5-aryl phosphorothioates (21). Note that the diagram for attack of a metal-coordinated methoxide would be similar, but Mx +-coordination would push the TS toward the S-corner, possibly stabilizing the pentacoordinated intermediate to the point that the reaction occurs stepwise with the likely rate-limiting step being breakdown.

Scheme 6 A proposed mechanism for the concerted La + ( OCH3)2-catalyzed methanolysis reaction of phosphate triesters with XAr leaving groups.

The use of a lipophilic zinc(II) macrocycle complex, 1-hexadecyl-1,4,7,10-tetraazacyclododecane, to catalyze hydrolysis of lipophilic esters, both phosphate and carboxy (425), links this Section to the previous Section. Here, and in studies of the catalysis of hydrolysis of 4-nitrophenyl acetate by the Zn2+ and Co2+ complexes of tris(4,5-di-n-propyl-2 -imidazolyl)phosphine (426) and of a phosphate triester, a phos-phonate diester, and O-isopropyl methylfluorophosphonate (Sarin) by [Cu(A(A(A/,-trimethyl-A/,-tetradecylethylenediamine)l (427), various micellar effects have been brought into play. Catalysis of carboxylic ester hydrolysis is more effectively catalyzed by A"-methylimidazole-functionalized gold nanoparticles than by micellar catalysis (428). Other reports on mechanisms of metal-assisted carboxy ester hydrolyses deal with copper(II) (429), zinc(II) (430,431), and palladium(II) (432). 

Fig. 19 Bond length-reactivity plots for phosphate triesters and for monoester dianions. Note the increased slope for the monoesters, which undergo SNl(P)-type P-O cleavage in solution. Reprinted with permission from Jones and Kirby (1984). Copyright 1984 American Chemical Society.

This now allows the calculation of EM for A.5.7 and A.5.8 using a correction factor of 10 for the decrease in reactivity expected when one spectator aryloxy group of a phosphate triester is replaced by an alkoxy group (Bromilow el al., 1972)... 

Parathion is one of a class of phosphorothionate triesters widely used as insecticides. These compounds exert their toxic effects in insects and mammals by inhibiting the enzyme acetylcholinesterase. The phosphorothionates, in general, are relatively poor inhibitors of acetylcholinesterase but are converted by the cytochrome P-450-containing monooxygenase enzyme systems in insects and mammals to the corresponding phosphate triesters that are potent inhibitors of this enzyme. 

Chemical modification may also simply be achieved by complex formation with an optically active agent191. For example, the correlation of the configuration of chiral non-racemic phosphate triesters, such as 11 (see p 417)11, with the relative (when compared with ent- ) change in H chemical shift of the methoxy doublet induced by the addition of Eu(hfc)3192 has been used for the assignment of absolute configuration of optically active phosphate triesters (chiral at phosphorus), which were obtained by asymmetric synthesis as indicated. 

Various esterases exist in mammalian tissues, hydrolyzing different types of esters. They have been classified as type A, B, or C on the basis of activity toward phosphate triesters. A-esterases, which include arylesterases, are not inhibited by phosphotriesters and will metabolize them by hydrolysis. Paraoxonase is a type A esterase (an organophosphatase). B-esterases are inhibited by paraoxon and have a serine group in the active site (see chap. 7). Within this group are carboxylesterases, cholinesterases, and arylamidases. C-esterases are also not inhibited by paraoxon, and the preferred substrates are acetyl esters, hence these are acetylesterases. Carboxythioesters are also hydrolyzed by esterases. Other enzymes such as trypsin and chymotrypsin may also hydrolyze certain carboxyl esters. 

This selectivity permits use of a methyl protecting group in phosphate triester synthesis of nucleotides. Removal is effected by heating solutions of the protected nucleotide in f-hutylaminc at reflux (46°) for 15 hours. ... 

Macroreticular resins, particularly the Amberlite XAD series, have been used extensively to isolate and concentrate trace organic compounds from drinking water (1-8). We have previously reported the use of an XAD cartridge for this purpose and have evaluated the system for the analysis of organophosphorus pesticides (OPs) (4), polynuclear aromatic hydrocarbons (PAHs) (5), phosphate triesters (TAAPs) (6), or-... 

Other kinetically allowed mechanistic models, i.e. hydroxide ion attack on the monoanion, can be rejected on the grounds that the required rate coefficients far exceed that found for alkaline hydrolysis of phosphate triesters. At pH > 9 two new reactions appear, one yielding a 1,6-a.nhydro sugar by nucleophilic attack through a five-membered transition state of the 1-alkoxide ion upon C-6 with expulsion of phosphate trianion. The second is apparently general-base catalysis by 1-alkoxide of water attack on C-6 or phosphorus through greater than six-membered cyclic transition states. 

In view of the large number of esterases in many tissues and subcellular fractions, as well as the large number of substrates hydrolyzed by them, it is difficult to derive a meaningful classification scheme. The division into A-, B-, and C- esterases on the basis of their behavior toward such phosphate triesters as paraoxon, first devized by Aldridge, is still of some value, although not entirely satisfactory. 

The first term, representing acid-"catalyzed" hydrolysis, is important in reactions of carboxylic acid esters but is relatively unimportant in loss of phosphate triesters and is totally absent for the halogenated alkanes and alkenes. Alkaline hydrolysis, the mechanism indicated by the third term in Equation (2), dominates degradation of pentachloroethane and 1,1,2,2-tetrachloroethane, even at pH 7. Carbon tetrachloride, TCA, 2,2-dichloropropane, and other "gem" haloalkanes hydrolyze only by the neutral mechanism (Fells and Molewyn-Hughes, 1958 Molewyn-Hughes, 1953). Monohaloalkanes show alkaline hydrolysis only in basic solutions as concentrated as 0.01-1.0 molar OH- (Mabey and Mill, 1978). In fact, the terms in Equation(2) can be even more complex both elimination and substitution pathways can operate, leading to different products, and a true unimolecular process can result from initial bond breaking in the reactant molecule. 

The phosphate triester reacts with a range of sulfur nucleophiles that add to the carbon a to the ketone and open the six-membered phosphate ring. Thiourea adds not only to the a-carbon, but also to the ketone carbon, resulting in elimination of the ketone oxygen as water, a reaction sequence that mimics the conversion of the Moco precursor to a 1,2-enedithiolate (Eq. 7). 

The stereochemical analysis depends on converting a chiral[160,170,180]phosphate monoester into two diastereoisomeric conformationally locked six membered cyclic phosphate triesters (2 ). In the cyclization step any one of the peripheral oxygen isotopes will be lost with equal probability, and the residual oxygen isotopes will take up axial and equatorial positions. Methylation of the isotopically labelled cyclic phosphate... 

More direct evidence for the formation of o-quinones in light-induced yellowing was presented by Lebo et al. (14). They found that trimethyl phosphite reacts with o-quinones to produce oxyphosphoranes, which in turn, react with water to form cyclic phosphate triesters, as shown in Scheme 3. Treatment of yellowed refiner mechanical pulp with trimethyl phosphite decreased the light absorption at 437 nm, as seen in Table I. Similar decreases in specific absorption were observed at 413 and 320 nm. Solid state 31P nmr analysis of pulp treated with trimethyl phosphite was consistent with the formation of the cyclic phosphate triesters. 

Triallylphosphate is the phosphate triester of allyl alcohol and contains unsaturated C=C bonds in its structure. This compound is a liquid (fp, -50°C). It is regarded as having a high toxicity and produces abnormal tissue growth when administered subcutaneously. It has been known to explode during distillation. 

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