Oxygen substitution

The structure of guanine illustrates an important feature of substituted pyrimidines and purines Oxygen substitution on the ring favors the keto form rather than the enol Ammo substitution does not... 

The a-oxygen-substituted hydroperoxides and dialkyl peroxides comprise a great variety as shown in Figure 1. When discussing peroxides derived from ketones and hydrogen peroxide, (1) is often referred to as a ketone peroxide monomer and (2) as a ketone peroxide dimer. 

Secondary alcohols, such as isopropyl alcohol, j -butyl alcohol, 2-pentanol, 3-pentanol, cyclopentanol, and cyclohexanol, have been autoxidized to hydroxyaLkyl hydroperoxides (1, X = OH R = H) (10,44). These autoxidations usually are carried out at ca 20°C with uv radiation in the presence of a photosensitizer, eg, benzophenone. a-Oxygen-substituted dialkyl peroxides (2, X = Y = OH and X = Y = OOH), also are formed and sometimes they are the exclusive products (10). 

Hydroxyall l Hydroperoxyall l Peroxides. There is evidence that hydroxyalkyl hydroperoxyalkyl peroxides (2, X = OH, Y = OOH) exist in equihbrium with their corresponding carbonyl compounds and other a-oxygen-substituted peroxides. For example, reaction with acyl haUdes yields diperoxyesters. Dilute acid hydrolysis yields the corresponding ketone (44). Reduction with phosphines yields di(hydroxyalkyl) peroxides and dehydration results in formation of cycHc diperoxides (4). 

Polymeric OC-Oxygen-Substituted Peroxides. Polymeric peroxides (3) are formed from the following reactions ketone and aldehydes with hydrogen peroxide, ozonization of unsaturated compounds, and dehydration of a-hydroxyalkyl hydroperoxides consequendy, a variety of polymeric peroxides of this type exist. Polymeric peroxides are generally viscous Hquids or amorphous soHds, are difficult to characterize, and are prone to explosive decomp o sition. 

Isotopic labeling experiments have established that C—O bond rupture occurs between the carbonyl carbon and oxygen substitution at the alcohol C—O bond is not involved. 

Pyrrole, furan, and thiophene, on the other hand, have electron-rich aromatic rings and are extremely reactive toward electrophilic aromatic substitution— rnore like phenol and aniline than benzene. Like benzene they have six tt electrons, but these tt electrons are delocalized over five atoms, not six, and ar e not held as strongly as those of benzene. Even when the ring atom is as electronegative as oxygen, substitution takes place readily. 

Ab initio calculations suggested that the oxygen-substituted compounds 40 and 41 should be much better candidates for nucleophilic substitution reactions that follow pathway b than the halo salts belonging to the 7a-7i family (Scheme 9). Tire properties of the CCls-substituted cation of 43 should reveal more similarities with those of 7a than with those of 40 and 41. 

Since niobates and tantalates belong to the octahedral ferroelectric family, fluorine-oxygen substitution has a particular importance in managing ferroelectric properties. Thus, the variation in the Curie temperature of such compounds with the fluorine-oxygen substitution rate depends strongly on the crystalline network, the ferroelectric type and the mutual orientation of the spontaneous polarization vector, metal displacement direction and covalent bond orientation [47]. Hence, complex tantalum and niobium fluoride compounds seem to have potential also as new materials for modem electronic and optical applications. 

The second group is the group of oxyfluorides that are derived from ferroelectric oxides by means of fluorine-oxygen substitution. The basic oxides are usually perovskite, tetragonal tungsten bronze, pyrochlore, lithium tantalate etc. [400]. 

Oxygen-substituted allylchromium reagents are conveniently generated from vinyloxi-ranes by chromium(II) chloride in the presence of lithium iodide and aldehydes with high anti diastereoselectivity14. qh oh... 

If an oxygen substitutent is present in the tz-position at C-3, the diastereoselectivity is lost completely. 

Sulfones have been prepared by three principally different strategies One-component methods include various isomerizations, rearrangements under degradation, and hydrolysis of oxygen-substituted dialkyl (diaryl) sulfuranes(VI). 

Whereas tropones usually act as dienes in cycloaddition reactions (Section 5.4), tricarbonyl (tropone) iron 59 displays a reactivity that is almost identical to that of a normal enone. High pressure cycloadditions of 59 with 1-oxygen substituted dienes 60 gave the desired cycloadducts 61 in good to excellent yields (Equation 5.9). The subsequent decomplexation of the cycloadducts has been accomplished by treatment with CAN [20]. 

Fallis and coworkers studied Jt-facial selectivity in the reactions of series of 5-substituted 1,2,3,4,5-pentamethylcyclopentadienes Cp -X. They reported that the diene 5 (Cp -X X = SCH3) with maleic anhydride proceeded more slowly than that of the 5-oxygen substituted cyclopentadienes 6 and 7 (Cp -X X = OH, OCH3), where the HOMO of the diene 5 lies higher than those of 6 and 7 [7, 8] (Table 2). These results seemed to suggest that in the case of the reaction of 5 the NHOMO considerably contributed to the reactions. 

Lewis TA, RL Crawford (1995) Transformation of carbon tetrachloride via sulfur and oxygen substitution by Pseudomonas sp strain KC. J Bacteriol 177 2204-2208. 

The interesting work of Martin and coworkers " on oxygen-substituted sul-furanes(VI) lO-S-4 and 12-S-6 species made available for the first time quasi mono-and bis-acetals of sulfones (1 and 2). Proton-catalyzed fragmentation of lb led to the sulfone isomer 3 the corresponding fragmentation of 2a gave, depending on reaction conditions, the isomeric sulfone 4 or a mixture of the sulfone isomers 4 and 5 . 

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