Uracil fluorination

All of the direct fluorinations reported appear to be addition-elimination processes with solvent involvement (Scheme 4). A study of the mechanism and stereochemistry of uracil fluorination using F2 and AcOF has implicated a radical-cation mechanism [28]. The effect of acetate ion on the products proved to be important. In its absence both cis- 16 and /rans-isomers 15 were observed in the reaction nuxture, but only trans-15 in its presence. NMR studies have revealed that acetate originated from the solution containing acetate ion, rather than the residue from acetyl hypofluorite, binds to the 6-position of uracil to form the intermediates 15 and 16 (Solv=OAc). Acetate is a sufficiently strong base to induce /ran -elimination of acetic acid from the cix-isomer 16 [29, 30]. 

Fluorinated Heterocyclic Compounds. HeterocycHc compounds containing the CF group are prepared by methods similar to those used in the fluorination of aHphatic compounds. The direct action of fluorine on uracil yields the cancer chemotherapy agent, 5-fluorouracil [51-21-8] as one special example of a selective fluorination on a commercial scale (25). 

Fluoropyrknidine derivatives are of tremendous importance in cancer chemotherapy, eg, 5-fluorouracil [51-21-8] (5-FU). The original 5-fluorouracil process featured a multistep low yield route based on ethyl fluoroacetate (451). Direct fluorination (fluorine) of uracil [66-22-8] gives high yields of 5-FU (452—455). This process has now been commercialized. 

Uracil is used more effectively, in nucleic acid synthesis within a rat hepatoma than in normal liver. This observation appears to have stimulated the synthesis of 5-fluorouracil (1027) as an antimetabolite mainly because the introduction of a fluorine atom involves a minimal increase in size. In the event, 5-fluorouracil did prove to have antineoplastic activity and it is now a valuable drug for treatment of tumors of the breast, colon or rectum, and to a lesser extent, gastric, hepatic, pancreatic, uterine, ovarian and bladder carcinomas. As with other drugs which interfere with DNA synthesis, the therapeutic index is quite low and great care is required during treatment (69MI21301). 

Fluorination of pyridine (90TL775), uracil (90T3093), and octaethyl-porphyrin [88JCS(P1)1735] has been described using cesium fluorooxysulfate. The outcome of the former was strongly dependent on the solvent. For example, with pyridine in methanol no fluorination was observed and 2-methoxypyridine was obtained. 2-Fluoropyridine was isolated when cyclohexane was the solvent (90TL775). 

Fluorouracil (5-fluorouracil, 5-FU, Fig. 5) represents an early example of rational drag design in that it originated from the observation that tumor cells, especially from gut, incorporate radiolabeled uracil more efficiently into DNA than normal cells. 5-FU is a fluorinated pyrimidine analog that must be activated metabolically. In the cells 5-FU is converted to 5-fluoro-2>deoxyuridine-monophosphate (FdUMP). This metabolite inhibits thymidilate synthase which catalyses the conversion of uridylate (dUMP) to thymidilate (dTMP) whereby methylenetetrahydrofo-late plays the role of the carbon-donating cofactor. The reduced folate cofactor occupies an allosteric site of... 

Different kinds of nucleosides (778) were prepared by condensation of compound 400 (see Section 11,3) with trimethylsilylated uracils under Lewis acid catalysis, and removal of one fluorine atom at C-2 of the sugars. Uridine 5 -(2-acetamido-2,4-dideoxy-4-fluoro-a-D-galactopyranosyl di-... 

Photo-induced conversions are also important synthetically. The photocycloaddition of 5-substituted uracils 69 with ethylene has been used for the synthesis of 2-aminocyclobutanecarboxylic acids. The addition reaction worked well with carbon and fluorine substituents and also with a Cbz-protected amino. However, uracils with other 5-nitrogen substituents (NH2, NHBn and NO2) failed, only starting material being recovered. The sequence also worked for the 6-isomers but somewhat less consistently <06SL1394>. 

One interesting application that involves XeF2 as a fluorinating agent involves the fluorination of uracil, which has the structure... 

An interesting dinically useful prodrug is 5-fluorouracil, which is converted in vivo to 5-fluoro-2 -deoxyuridine 5 -monophosphate, a potent irreversible inactivator of thymidylate synthase It is sometimes charaderized as a dead end inactivator rather than a suicide substrate since no electrophile is unmasked during attempted catalytic turnover. Rathei since a fluorine atom replaces the proton found on the normal substrate enzyme-catalyzed deprotonation at the 5 -position of uracil cannot occur. The enzyme-inactivator covalent addud (analogous to the normal enzyme-substrate covalent intermediate) therefore cannot break down and has reached a dead end (R. R. Rando, Mechanism-Based Enzyme Inadivators , Pharm. Rev. 1984,36,111-142). 

Fluorouracil (5FU) is 5-fluoropyridrimidine-2,4(1/7, 3H)-dione. Its structure is illustrated in Figure 11. The hydrogen in the naturally occurring pyrimidine, uracil, is substituted by fluorine in the 5 position. The presence of the heteroatoms in the structure imparts hydrophilicity to the compound as they are capable of hydrogen bonding. 

New formulations are regularly commercialised. Studies on the action mechanism of 5-FU had a large influence on the development of other anti-tumour drugs derived from pyrimidine and purine. The 5-FU is industrially prepared at important tonnage by fluorination of uracil with elemental fluorine. 

Nucleosides Derived from Fluorinated Bases Many nucleosides have been synthesized from different fluoropyrimidines (uracil, thymidine, cytosine) or fluoropurine (adenine, guanine) bases, which are generally prepared by electro-... 

The fluorine atom can be present in position 5 in uracil derivatives, or in position 1 in that of purine, as in fludarabine, which is used in the treatment of some leukemias (Figure 6.14 cf. Chapter 8). Nucleoside derivatives of fluorouracil (e.g., capecitabine) are prodrugs that allow the oral administration of 5-FU in cancer chemotherapy. The mechanism of action of these nucleosides is detailed in Chapters 7 and 8. Nucleosides having a trifluoromethylated base have been described for example, trifluridine is active on herpesvirus (Figure 6.14). 

Fluorouracil is a fluorinated pyrimidine antimetabolite that resembles uracil, with a fluorine atom substituted for the 5-methyl group. Its systemic pharmacology is described in Chapter 54. Fluorouracil is used topically for the treatment of multiple actinic keratoses. 

Fluorouracil has been used for some time for cancer treatment. Its preparation using fluorine is operated commercially (RC.R. Inc) and has been the focus of numerous studies [12, 150] in the past. In a more recent study, the products obtained upon fluorination of uracil, usually carried out in acetic acid solvent, indicate that the reactions proceed via an addition-elimination process involving radical cation intermediates (Fig. 63) [156]. 

It is interesting to note that a number of researchers have attempted, more or less successfully, to construct reliable mathematical models which combine structural elements and experimental data to predict the toxicities of new substances not yet experimentally tested for humans and other species.110152 Furthermore pharmacology, toxicity, metabolism and enzyme-inhibiting effects of fluorine-containing aromatic systems (e.g., anilines, ben-zothiadiazines, butyrophenones, corticoids, phenothiazines, steroids, uracils) have been discussed in depth in the literature.153-156... 

The fluorination of heterocyclic compounds with xenon difluoride has received much less attention. Pyridine reacts readily with xenon difluoridc in dichloromethane without a catalyst to form 2-fluoropyridine (35 %), 3-fluoropyridine (20 %) and 2,6-difluoropyridine (11 %).80 8-Hydroxyquinoline treated with xenon difluoride forms 5-fluoro-8-hydroxyquinoline at room temperature in 35% yield.80 Uracil reacts with xenon difluoride to give 5-fluorouracil in 10% yield.81 Only one example exists of a highly substituted pyrrole fluorinated with xenon difluoride in acetonitrile in 35% yield.82... 

Fluorine addition with trifluoromethyl hypofluoritc to various uracil and cytosine bases and nucleosides has been extensively studied. Vicinal fluoro trifluoromethoxy or fluoro methoxy adducts, formed mainly by syn addition, are usually hydrolyzed thus resulting in fluoro-sub-stituted derivatives,42 48 e.g. formation of 24 and 25. 

Various uracil, cytosine and pyrimidine nucleosides, e.g. 22, have been successfully fluorinated by acetyl hypofluorite and also some labeled I8F derivatives synthesized.16- 18 34... 

An example of the introduction of one fluorine into a nitrogen-containing compound, with nickel electrodes in anhydrous hydrogen fluoride, is the fluorination of uracil to give 5-fluoro-uracil (1).< 2... 

The formation of a solid, isolated by bubbling fluorine diluted with nitrogen into a solution of pyridine in trichlorofluoromethane at — 80 °C followed by filtering at low temperature, has been noted.30 The solid is considered to be a pyridine/fluorine compound with polar character however, spectral or other data on the structure of the pyridine/fluorine compound are presently not available, probably due to its explosive nature. This compound has been used for the fluorination of uracil and some chloroalkenes,30,31 but the adduct is of little use because of its violent decomposition above — 2CC. 

Direct 5-fluorination can be effected on activated pyrimidines for example, F2 in HOAc or anhydrous HF have been used for the preparation of 5-fluoro-2(lH)-pyrimidinones (77CCC2694), and for 5,5-difluorination of 6-O-cyclouridines (83TL1055). Uracil and cytosine are 5-fluorinated by the same procedure (80TL4605,82CPB887). 

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