Uridine dihydro

If polyribonucleotides are treated simultaneously with methoxylamine and bisulphite, cytidine residues are converted into 5,6-dihydro-7V4-methoxycytidine-6-sulphonate,154 and uridine into 5,6-dihydrouridine-6-sulphonate.155 Treatment with dilute ammonia regenerates the uridine residues, leaving the dihydrocytidine derivatives unaffected. When only the cytidine residues have been derivatized, pancreatic ribonuclease becomes uridyl ribonuclease, since it is unable to cleave the chain on the 3 -side of the modified cytidine.154 This allows the isolation of blocks of modified cytidine residues. T2 ribonuclease may also be used. Alternatively, a ribonuclease from Physarum polycephalum has been found to hydrolyse CpX links very slowly, allowing the isolation of cytidine blocks.156 If both uridine and cytidine residues are modified, T2 ribonuclease acts as puryl ribonuclease, allowing the isolation of cumulative blocks of pyrimidines.155 This ability to alter the specificity of nuclease cleavage is a useful tool in sequence analysis. 

On the other hand, the presence of a glycosyl group attached to the nucleoside pyrophosphate has been found to influence the reactivity of the heterocyclic base. Thus, the catalytic hydrogenation of uridine and 6-azauridine 5 -(a-D-glucopyranosyl pyrophosphates) to the 5,6-dihydro derivatives proceeds more slowly than that of the respective nucleoside 5 -phosphates or 5 -pyrophosphates.338,339 Such differences have not been observed in comparisons of analogous derivatives of 2 -deoxyuridine, N3-methyluridine, and cytidine. 

A similar effect occurs in the reaction of some pyrimidine nucleoside derivatives with hydroxylamine. Studies of the mechanism of this reaction with uridine derivatives340 shows that the initial point for nucleophilic attack is at C-6, and the resultant 5,6-dihydro-6-(hydroxy-amino)uridine derivative (86) is an intermediate in the conversion of the uridine derivative into the ribosylurea derivative 87 and 2-isoxazolin-5-one (88), as shown in Scheme 8. 

The 5,6-double bond in uracil, 5-fluorouracil, /V-alkyluracils, thiouracils, and uridines adds sodium sulfite or bisulfite to give the corresponding 5,6-dihydro-6-sulfonic acid salts. Bisulfite addition to cytosines and cytidine may be succeeded by a second reaction involving nucleophilic replacement of the amino group, for example, by water. 

Reactions of nucleophiles. A number of nucleophilic reagents add reversibly at the 6 position of pyrimidines. Thus, bisulfite adds to uridine (Eq. 5-10).528 Hydroxylamine (HONH2) adds in a similar fashion to give a compound with -HNOH in the 6 positions.528 Sodium borohydride (NaBH4), which can be viewed as a donor of a hydride ion (H ), reduces uridine to the 5,6-dihydro derivative. This presumably occurs by attack of the hydride ion at position 6 in a manner analogous to the reaction of bisulfite in Eq. 5-10. 

Reaction between DNA/RNA and Mn04 or 0s04 , where the formal metal oxidation states are Mn + and Os +, results in base-specific modification of thymidine/uridine residues. These anionic metal species can oxidize the pyrimidine C5-C6 double bond, which results in formation of m-5,6-dihydroxy-5,6-dihydro-pyrimidine. This reaction is followed by opening of the pyrimidine ring and its subsequent removal from the polynucleotide chain. Permanganate can also oxidize and remove guanine residues (but not adenine) while osmium tetroxide acts much more specifically on pyrimidine residues with the following kinetic preferences (relative rates shown in parenthesis) T (45) U(4.5) > dU(2.8) >... 

Levene and LaForge also made the remarkable discovery that on hydrogenation the uniquely stable bond between base and sugar is weakened and the resulting dihydro derivative is susceptible to hydrolysis under conditions comparable with those employed for the hydrolysis of the ribosylpurines. Thus, hydrogenation of uridine (for example) gives dihydro-uridine which is readily hydrolyzed to dihydro-uracil and D-ribose. 

The ring structure of the ribose residue was ascertained" in the same general manner as for adenosine and guanosine. Triacetyl-dihydrouridine was prepared by the hydrogenation of triacetyl-uridine. On simultaneous deacetylation and methylation this was transformed to the fully methylated dihydro-uridine. By simultaneous hydrolysis and oxidation of this product, with hydrobromic acid and bromine, trimethyl -D-ribonolactone was formed, its identity being confirmed by oxidation to meso-dimethoxy-succinic acid. It follows that the ribose component has the furanose ring structure, and that uridine is 3 -D-ribofuranosyl-uracil. 

It should therefore be possible to design chemical structures, modeled after known or putative reaction intermediates that resemble postulated transition states. They may exhibit high affinities for the reactive sites of enzymes and therefore function as effective, but reversible, inhibitors. A successful example is the potent and specific cytidine deaminase inhibitor 3,4,5,6-tetrahydrouridine, which effectively blocks the conversion of cytidine to uridine. It was similarly demonstrated that 1,6-dihydro-6-hydroxymethylpurine effectively blocked the deamination of adenine to hypoxanthine by adenine deaminase (Eq. 2.13). 

There are five dihydropyrimidines. The 1,4- and the 1,6-dihydropyrimidines can readily interconvert by tautomerism because of the mobile NH proton. The common metals to effect hydrogenation can be used . Platinum has been the catalyst of choice for the reduction of the 5,6-double bond of uracils, for example, in the addition of deuterium to uracil to produce [5,6- H2]5,6-dihydrouracil. But in the reduction of 2(l/f)-pyrimidinone and its iV-methyl derivative it is the 1,6-dihydro derivatives which are formed. The addition of hydrogen to the 5,6-bond of thymidine and other 5-substituted uridines is stereospecific with rhodium-on-alumina as catalyst. Rhodium-on-charcoal has been useful for hydrogenation of the 5,6-double bond in uracils, uridine, and isocytosine. Raney nickel readily promotes saturation of the 5,6-double bond. Thio derivatives may either be dethiated or taken further to reduced forms by Raney nickel catalysts . 

Nucleosides prepared conventionally from deoxy-sugars have included the antiviral nucleoside dihydro-5-azathymidine (12), the tritiated analogue (13), and the 6-substituted derivatives (14) (Scheme 2), [ C-2] and [ C-4]-2-deoxy-uridine, and 2-deoxy-ribonucleosides from 5-azapyrimidine derivatives, ... 

The key step in a synthesis of cyclo-5,6-dihydro-2 -deoxyuridine, a major product of gamma irradiation of deoxygenated aqueous solutions of deoxycytidine, is the cyclization of aldehyde 33 using BuaSnH and AIBN, to give 34 of stereochemistry as indicated. The method was previously used for the thymidine analogue (J. Chem. Soc., Perkin Trans. 1,1999,1257). Both the uridine and thymidine analogues were incorporated into oligodeoxynucleotides, where they acted as blocks for DNA polymerases. ... 

The sequence of nucleotides in yeast phenylalanine transfer RNA. D, 1, and Y represent the nucleotides 4,5-dihydro-uridine, pseudouridine, and dimethyl-guanosine, respectively. 

Treatment of the 5,6-dihydro-6-methoxyuridine derivative (597) with acidified methanol below 10 °C afforded 5, 0 -cyclo-5,6-dihydro-2, 3 -0-isopropylidene-uridine (598)(Scheme 90), which reverted to 2, 3 -0-isopropyIideneuridine upon warming. Deuterium labelling showed that the first step of the conversion of (597) into (598), presumably involving an immonium species, is reversible. Methyl-ation of (598) with methyl iodide-sodium hydride in THF gave 2, 3 -0-isopropyl-... 

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