Uridine cytidine oxidation

To a suspension of 77.7 mg 4-amino-l-(3,4-dihydroxy-5-hydroxymethyltetra-hydrofuran-2-yl)-lH-pyrimidin-2-one 3-oxide (cytidine -oxide, 0.30 mmol) and 12.6 mg 95% pure lithium hydride (1.5 mmol) in 5 mL dry methanol was added 40 /xL 98% pure benzyl bromide (0.33 mmol) the mixture was stirred at 37°C for 1 day under an argon atmosphere. TLC analysis of the reaction mixtures with chloroform/methanol/acetic acid (16 6 3) and chloroform/methanol (10 1) as the developing solvents showed complete consumption of the starting material for almost quantitative conversion to a less polar compound. After being neutralized with 1 N HCl solution and subsequent removal of the solvent under reduced pressure, the resulting residue was subjected to a short silica gel column by eluting with chloroform/methanol (20 1) to isolate 99.5 mg l-(3,4-dihydroxy-5-hydroxymethyltetrahydrofuran-2-yl)-17/-pyrimidin-2,4-dione 4- 0-benzyl oxime (uridine 4-0-benzyloxime) as a colorless amorphous powder, in a yield of 95%, m.p. 123-125°C (from methanol). 

Figure 6.42 Oxidation of deoxy-5-methylcytosine to deoxythymidine and cytidine to uridine. This may be caused by reactive oxygen species or oxidizing agents such as nitrous acid. d5mC is present in methylated CpG sequences.

Such changes can occur spontaneously or from exposure to chemicals. Cytidine can be deaminated to uridine (Fig. 6.42) and adenine to hypoxanthine (Fig. 6.43) by exposure of DNA to the oxidizing agent nitrous acid. 

The pyrimidine nucleosides from ribonucleic acid (uridine and cytidine) present a more formidable problem, since they cannot be hydrolyzed by dilute mineral acid, and the use of concentrated acid usually destroys the sugar moiety. It was observed, however, that simultaneous bromination, hydrolysis, and oxidation of cytidine (with bromine-hydrobromic acid) yields 5-bromouracil plus D-ribonic acid.60 Since cytidine can be deaminated to uridine with nitrous acid,61 both of these pyrimidine nucleosides must... 

Oxidation of the naturally occurring nucleosides, uridine and cytidine,137 with metaperiodate showed an uptake of one mole of oxidant per mole of nucleoside without the liberation of formic acid, in agreement with and in confirmation of the furanoid ring structure which had been established long previously by Levene and Tipson.108-127 129... 

JCS(P1)1171] and Sung (81CC1089 82JOC3623) found that these 1,2,4-triazole derivatives of uridine can be readily prepared at room temperature with excess 1,2,4-triazole and phosphoryl chloride and triethylamine or with 0-chlorophenyldi-(l//-l,2,4-triazol-l-yl)phosphine oxide in acetonitrile. These stable, often crystalline 1,2,4-triazole derivatives react readily with aqueous ammonia and primary or secondary amines at room temperature to afford the corresponding cytidines in high yields. 

Reaction of aspartic acid (14) with carbamoyl phosphoric acid (17) in the presence of the allosteric enzyme aspartate carbamoyltransferase (aspartate transcar-bamoylase) gives N-carbamoyl aspartic acid (18), which is cyclised to L-dihy-droorotic acid (19) by dihydroorotase. Oxidation of L-dihydroorotic acid by flavoprotein, orotate reductase gives orotic acid (20), which reacts with 5-phosphori-bosy 1-1-pyrophosphate (PRPP) in the presence of orotate phosphoribosyl transferase to form orotidine 5 -monophosphate (OMP, 21). Decarboxylation of OMP by orotid-ine 5 -phosphate decarboxylase yields uridine 5 -monophosphate (UMP, 22), which acts as precursor for the cytidine nucleotides (CTP) (Chart 6). 

Hydrogenation of methylated uridylic acids with platinum oxide is rather sluggish. However, satisfactory reduction of the 2 (3 )-phosphates of uridine and cytidine was accomplished by Cohn and Doherty, who introduced the use of 5% rhodium-on-alumina in these hydrogenations it was from such studies that chemical proof of the position of the phosphoric group in the a and 6 isomers of uridylic and cytidylic acid (see... 

There are also metal interactions that do not involve coordination chemistry. For example, cytidine and uridine can form covalent bonds with Hg via C 5. Another metal-binding site is the 5,6-alkenic bond in thymidine, which can be oxidized by hexavalent Os or by heptava-lent Mn. ... 

A great deal of research has been carried out on the formation of various modified nucleotides following direct ionization or reaction with HO under aerobic and anaerobic conditions. Pyrimidine nucleosides yield the hydrates (78, 79) and glycols (Tg) via regjoisomeric HO adducts (76, 77) under anaerobic and aerobic conditions, respectively (Scheme 34). The cytidine molecules are unstable upon saturation of the Tt-bond and hydrolyze to the corresponding modified uridines. The reaction of dG is especially rich because the initial two-electron oxidized product, 8-oxodGuo is more reactive than the native nucleoside and produces a variety of DNA lesions. Many of the DNA lesions produced by oxida-... 

Pyrimidine biosynthesis commences with a reaction between carbamyl phosphate and aspartic acid to give carbamyl aspartic acid which then nndergoes ring closure and oxidation to orotic acid. A reaction then occurs between orotic acid and 5-phosphoribosyl pyrophosphate to give orotidine-5-phosphate which on decarboxylation yields uridine-5-phosphate (UMP). By means of two successive reactions with ATP, UMP can then be converted into UTP and this by reaction with ammonia can give rise to cytidine triphosphate, CTP (11.126). 

Other nucleoside triphosphates, which are energetically equivalent to ATP, are important in some metabolic reactions cytidine triphosphate in phospholipid biosynthesis, guanosine triphosphate in protein biosynthesis and oxidative decarboxylation of 2-oxoacids (see Thcarboxylic acid cycle), inosine triphosphate in certain carboxylations, uridine triphosphate in polysaccharide biosythesis. 

Preparation of Nvcleosides. Levene and Jacobs 83c) treated ribonucleic acid with ammonia in an autoclave (175 ) and isolated the crystalline purine and pyrimidine ribonucleosides, adenosine and guanosine (9-iV -j8-D ribofuranosyladenine and -guanine) and cytidine and uridine (9-iV-i8-D-ribofuranosylcytosine and -uracil) respectively. Improvements in the chemical hydrolysis have been made through the use of magnesium oxide... 

The carbamyl phosphate is condensed with a molecule of aspartate giving ureidosuccinic acid, from which orotic acid is formed by cyclization and oxidation. In the presence of PRPP and a pyrophosphorylase this acid forms a ribotide and decarboxylation yields uridine monophosphate. The decarboxylation of the product of amination of orotidine phosphate gives cytidine monophosphate (Fig. 75). It can be seen that the pentose intermediate in pyrimidine nucleotide biosynthesis is PRPP, the same as for purine nucleotide biosynthesis. 

In contrast to the case of uridine derivatives noted above, attempts to carry out a Corey-Winter fragmentation of N, 5 - protected cytidine 2 ,3 -thionocarbonates caused isomerisation to 2 -deoxy-2 -thiocytldine-2, 3 -carbonates (69). 121 The 3 -thiomethyl-, methylsulphinyl-, and methylsulphonyl derivatives of 3 -deoxythymidine have been prepared by treatment of the 5 -0-trltyl derivative of (63) with methanethiolate ion, followed by oxidation at sulphur and deprotection as appropriate. 122... 

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