Hydrolysis phosphoramidates

A few organophosphorus insecticides are also phosphoramidates, hydrolysis of the P-N bond being considered a route of detoxification. This is exemplified by the metabolism of acephate (9.82, Fig. 9.15), whose mechanisms of activation and detoxification have recently been re-examined in mice to better understand the relative innocuity of the compound in mammals and its selective toxicity in insects [156],... 

Another example of reagent-induced asymmetric synthesis is the enantioselective preparation of phosphoramides 6 by addition of dialkylzine reagents to A-diphenylphosphinoylimincs 4 in the presence of the enantiomerically pure 1,2-amino alcohols 5a or 5 b (diethylzinc does not add to A-silyl- or A-phenylimines)12. Phosphoramides 6 (crystalline solids) are obtained in moderate to good yield and good enantioselectivity. The latter can be enhanced by recrystallization. Acidic hydrolysis with dilute 3 M hydrochloric acid/tetrahydrofuran provides the corresponding amines 7 without any racemization. 

Phosphoramidate analogues of dideoxyribonucleoside phosphates (26) and trideoxyribonucleoside phosphates are acid labile and can be hydrolysed enzymically. Snake venom phosphodiesterase cleaves (26) to thymidine and 5 -deoxy-5 -aminothymidine (27 R = H). The latter presumably arises by spontaneous decomposition of the phosphoramidate (27 R = PO3H2) and P—O fission must have occurred during the initial hydrolysis. With acid or spleen phosphodiesterase, (26) gave Tp and (27 R = H), i.e. P—N fission occurred. 

Carbodiimides are used to mediate the formation of amide or phosphoramidate linkages between a carboxylate and an amine or a phosphate and an amine, respectively (Hoare and Koshland, 1966 Chu et al., 1986 Ghosh et al., 1990). Regardless of the type of carbodiimide, the reaction proceeds by the formation of an intermediate o-acylisourea that is highly reactive and short-lived in aqueous environments. The attack of an amine nucleophile on the carbonyl group of this ester results in the loss an isourea derivative and formation of an amide bond (see Reactions 11 and 12). The major competing reaction in water is hydrolysis. 

The formation of a phosphorimidazolide intermediate provides better reactivity toward amine nucleophiles than the EDC phosphodiester intermediate if EDC is used without added imidazole. The EDC phosphodiester intermediate also has a shorter half-life in aqueous conditions due to hydrolysis than the phosphorimidazolide. Although EDC alone will create nucleotide phosphoramidate conjugates with amine-containing molecules (Shabarova, 1988), the result of forming the secondary phosphorimidazolide-activated species is increased derivatiza-tion yield over carbodiimide-only reactions. 

Phosphoramidates continue to attract much attention, not only with regard to their interest in organophosphorus chemistry but also in the context of general organic synthesis. Acid hydrolysis of the phosphoramidates (123) and (124) with... 

Mechanistically, the compounds (162 X = O or S) and (163) represent two extremes in the base hydrolysis of phosphoramidates and phosphoramidothioates the former hydrolyse in an elimination-addition (EA) process whereas for the latter the sequence is one of addition followed by elimination AE). Further exploration of... 

Relatively low molecular weight polymers include R1R2P(0)(N=PR1Ra) n-PRxR2OH (R1, R2 included alkyl, aryl, and alkoxy)175 and phosphoramidic acids from the hydrolysis of (NPC12)W,178 the latter of which form hydrophobic materials. 

A more complex pathway of activation is seen in N-amino acid derivative of phosphoramidic acid diesters of antiviral nucleosides, as exemplified by prodrugs of stavudine (9.79, Fig. 9.14) [153 -155], The activation begins with a carboxylesterase-mediated hydrolysis of the terminal carboxylate. This is followed by a spontaneous nucleophilic cyclization-elimination, which forms a mixed-anhydride pentacycle (9.80, Fig. 9.14). The latter hydrolyzes spontaneously and rapidly to the corresponding phosphoramidic acid monoester (9.81, Fig. 9.14), which can then be processed by phosphodiesterase to the nucleoside 5 -monophosphate, and by possible further hydrolysis to the nucleoside. 

The first observed product is the hydroxo-N-bound phosphoramidate complex, although there are almost certainly other intermediates. Both ester hydrolysis (to nitrophenolate ion) and transfer of a phosphate residue from O to N occur. An acceleration of at least 10 fold can be assessed for both processes, compared with the reaction of the uncoordinated ester with NH3 or 0H ion. The O to N transfer is a general biochemical occurrence, e.g. creatine kinase uses and creatine to transform ATP to ADP and form creatine... 

The study of the monoanions of a third class of phosphorus-containing monoesters, the phosphoramidates, has been restricted for the present to p- and o-substituted aryl phosphoramidates, and phosphoramidate and its O-methyl derivatives13-16. Two striking differences from the behavior of O- and S-phos-phate monoester monoanions are noted in the experimental criteria compiled in order to postulate mechanism (a) solvolysis in mixed organic solvents, particularly 50% v/v dioxan-water, results in a 50% decrease in the rate of hydrolysis of o- and p-substituted aryl phosphoramidate monoanions and all ionic forms of phosphoramidate and its O-methyl derivatives excepting the parent monoanion, and (b) partitioning of the aryl phosphoramidates and phos-... 

An obvious difficulty arises with this rather elaborate rationale when phosphoramidate and aryl phosphoramidate monoanions are compared for example, the dissimilarity of the dioxan effect yet the identity of product distribution observed in methanol-water competition experiments. Preliminary studies in the author s laboratory have revealed striking differences in the hydrolytic behavior between a series of phosphoramidafes derived from primary aliphatic amines and the above aryl systems. No linear structure-reactivity relationship between the logarithmic rate of hydrolysis of the monoanion species and the pKa of the amine is observed19. Moreover, the rate of hydrolysis of phosphoramidate monoanions derived from aliphatic amines is at least 104 times slower than those formed from aryl amines. In contrast, only a thirtyfold decrease in rate is observed for the corresponding ApKa in the O-phos-phate monoester series. The suspicion that mechanism (1), even with the above proposed modification, is not an accurate description of phosphoramidate monoanion hydrolysis derives some further support from the observation that the monoanion is subject to nucleophilic attack by substituted pyridines al-... 

Tabic 7.7 Product ratios in the hydrolysis of phosphate esters and phosphoramidates by alkaline phosphatase"... 

The application of fluorogenic labeling to the determination of some organophosphate insecticides has been attempted [178,179]. Fenthion (0,0-dimethyl 0-[(4-metiiylthio)-m-tolyl] phosphorothioate), Ruelene (0-2-chloro-4-ferf.-butylphenyl O-methyl methyl-phosphoramidate), GC 6506 [dimethyl p-(methylthio)phenyl phosphate] and several other compounds which yield phenols on hydrolysis have been examined. The limits of detection for some of these labeled derivatives have been reported to be in the low nanogram range. The organophosphate Proban [0,0-dimethyl 0-(p-sulphamoylphenyl) phosphorothioate] has been determined directly without hydrolysis by dansylation of the free amino group of the molecule [180]. The derivative exhibited blue fluorescence, as compared to yellow for phenol and alkylamine dansyl derivatives. 

The experiments to utilize proteinoid catalysts for efficient direct photophosphorylation of ADP to ATP in water solution are still in a preliminary stage of development. For example, the following reports might help further investigation. When adenyl-5 -yl phosphoramidate is incubated with orthophosphate in aqueous solution at pH 3 for 48 hrs, 40% of adenyl-5 -yl phosphoramidate is converted to ATP16). The diverse set of enzymes that build up and break down ATP may work according to the same mechanism. All of these enzymes can exist in two conformational forms, which catalyze ATP synthesis and hydrolysis 17). 

The basic hydrolysis of acyclic phosphoramidates occurs with a mixture of C-0 and P-0 bond cleavage (l, 2) however similar treatment of l,3>2-oxazaphospholidines can result in substantial P-N cleavage ( 1, 3). 1,3,2-Oxazaphospholidines derived from (-)-ephedrine react with solutions of alkoxides in alcohol to give only the products of P-N cleavage and with inversion of configuration at phosphorus. Scheme I ( +, 5). ... 

A related reaction is the addition of isonitriles 75 to aldehydes 1 (the Passerini reaction). Denmark has demonstrated that SiCU, upon activation by a chiral Lewis base, which increased the Lewis acidity of the silicon (vide supra Scheme 7.14), can mediate this reaction to produce a-hydroxy amides 77 after aqueous work-up (Scheme 7.16). Phosphoramide 60 was employed as the chiral Lewis-basic catalyst [74]. Modification of the procedure for hydrolysis of 76 gives rise to the corresponding methyl ester (rather than the amide 77) [74]. (For experimental details see Chapter 14.5.5). 

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