Halogen derivatives fluoro

Dihydroquinazolines are normally stable compounds but they deteriorate on long standing. Some examples are known to oxidize to the corresponding 4(3H)quinazolinones. 3-Methyl-3,4-dihydroquin-azoline is converted to the 4-oxo compound after three recrystallizations from light petroleum. The most remarkable example is 3,4-dihydro-6-fluoro-3(p-fluorophenyl)quinazoline [(44), R = F] which oxidizes at its melting point (137°-I38°C), Other halogenated derivatives of (44) are more stable. ... 

Certain polar substituents at the 4-position of 2,5-DMA render the eompounds inaetive for example, the 4-COOH and 4-OH derivatives do not produee DOM-like effeets. On the other hand, 4-halogenated compounds result in relatively potent derivatives. The 4-fluoro derivative DOF is 4 times more potent than 2,5-DMA. whereas the 4-chloro (DOC) and 4-iodo (DOI) analogs are about 20 times more potent than 2,5-DMA. The most potent halogenated derivative is the 4-bromo analog DOB, which is about 40 times as potent as 2,5-DMA. 

The effect of the halogen substituent (fluoro, chloro, bromo and iodo) on the yield and mechanism of 4-halophenol photolysis was investigated by Durand et al. [24], Transient spectroscopy in aerated aqueous solutions indicated the formation of p-benzoquinone O-oxide from each derivative except 4-iodophenol for which no transients were detected p-benzoquinone and hydroquinone were found as photoproducts for all four compounds. It was concluded that the carbene mechanism was valid for the whole series. Under continuous irradiation, the 4-halophenol degradation quantum yields were determined to be

fluorescence lifetimes decreased in the same order, from 2.1 ns for 4-fluorophenol to 0.4 ns for 4-chlorophenol and < 0.1 ns for 4-bromophenol. 

Steric aspects The fluorine atom is considerably smaller than the rest of the halogen atoms. Seen from the steric point of view it resembles more hydrogen than chlorine (Table 15.17). Effectively fluoro-derivatives differ from the other halogenated derivatives because fluorine forms with carbon particularly stable bonds and, in contrast to other halogens, is only rarely ionized or displaced. Because it is both chemically inert and of small size organic fluorine is often compared to hydrogen. 

Similarly, the 6-(tetraethyl phosphorodiamide) of l,2 3,4-di-0-iso-propylidene-a-D-galactopyranose gives the corresponding halogen derivative with methyl iodide, benzyl bromide, benzyl chloride, and ethyl fluoro-acetate. ... 

So far we have steered clear of the reactions of halogenated derivatives of benzene. Before we explain their reactions, have a look at the table, which shows the rates of nitration of fluoro, chloro, bromo, and iodobenzene relative to benzene itself, and also gives an indication of the products formed in each case. ----... 

This enzyme is found in various animal tissues and that from sheep liver has been purified 280-fold 11). Both cytidine and deoxycytidine are deaminated, as are the 5-halogen derivatives (e.g., 5-fluoro-deoxycytidine). Cytosine, cytidylate, and deoi i idylate are not deaminated by this enzyme. Cytosine arabinoside (iHS-D-arabinofurano lcytosine), which has an important use in the treatment of human neoplastic disease, is a substrate for cytidine deaminase in the human, this compound is rapidly deaminated to yield the inactive uracil arabinoside 12). Cytidine deaminase is very active in E. ccli and S. typhimwrium, but is absent from yeasts and lactobacilli (7). The E. coli enzyme has been partly purified IS) and has a specificity similar to that of the animal enzyme. The E. coli enzyme deaminates 5-methyldeoxycytidine, enabling this compound to support the growth of a thymine-requiring E. coli mutant 11. ... 

This topic has been reviewed [2, pp 94, 100-111, 130-134] All of the standard approaches to the synthesis of a compound like methyl 2-fluorostearate from methyl 2-bromostearate result mall yield of the 2-fluoro ester and the unsaturated esters. Although silver fluoride is not a new reagent, its use moist in wet acetonitrile to convert methyl 2-bromostearate to its fluoro ester is a departure from the traditional set of anhydrous conditions (Procedure 6, p 194) [71] In contrast, silver tetrafluoroborate converts a-chloroketones to their respective fluoroketones under anhydrous conditions. The displacement of less activated halogen groups by silver tetrafluoroborate to form their respective fluorides is novel Although silver tetrafluoroborate could not be used to convert an aliphatic terminal dichloromethyl or trichloromethyl group to its corresponding fluoro derivative, it is an effective fluorine source in other situations [72] (Table 8)... 

The reaction of the carbanion derived from diethyl methylphosphonate with perhalogenated aromatics may result in substitution of halide to yield perhaloaryl(hetaryl)methylphosphonates, which can be converted into tris- or bis-(perhaloaryl)methanes. Displacement of fluoride ion has been reported in the reaction of dimethoxycarbene with l-fluoro-2,4-dinitrobenzene and with hexafluorobenzene. The hydrodehalogenation of halogenated aryl ketones may be facilitated using hydrogen over a Pt/C catalyst. " ... 

From the lithium chemistry point of view, the carbon-fluorine bond cannot be considered as a carbon-halogen bond because alkyl or aryl fluorides are not adequate starting materials in lithiation processes, due to the fact that this bond is the strongest that carbon can form . On the other hand, a possible reductive defluorination process could be important from an environmental point of view due to the difficult degradation of fluoro derivatives in nature . 

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