Oxygen exocyclic

Figure 7. Water oxygen-exocyclic methylene carbon pair distribution function, calculated from a molecular dynamics simulation of a-D-glucopyranose in aqueous solution, giving the normalized probability of finding a water oxygen atom a given distance r from the C6 carbon atom. (Reproduced from Ref. 32. Copyright 1989 American Chemical Society.)...

Acyl-, 4-alkoxycarbonyl- and 4-phenylazo-pyrazolin-5-ones present the possibility of a fourth tautomer with an exocyclic double bond and a chelated structure. The molecular structure of (138) has been determined by X-ray crystallography (Table 5). It was shown that the hydroxy group participates in an intramolecular hydrogen bond with the carbonyl oxygen atom of the ethoxycarbonyl group at position 4 (8OCSCII21). On the other hand, the fourth isomer is the most stable in 4-phenylazopyrazolones (139), a chelated phenyl-hydrazone structure. 

The mesomeric anion (157 Scheme 10) reacts readily with electrophilic reagents such as alkyl or acyl halides at N-2, C-4 and the exocyclic oxygen atom. The percentages of the different products formed are controlled by the HSAB principle. The acylium ion (hard) attacks preferentially at oxygen (hard), whilst the softer alkylating agents attack the nitrogen atom. 

The puzzling discovery of Collie and Tickle in 1899 that 2,6-dimethyl-4-pyrone (8) affords crystalline salts (9) with acids, which were the first monocyclic pj rylinm salts to be isolated, was interpreted by a formula (21) with tetracovalent oxygen.An active period of research followed. The methosulfate or methiodide of 2,6-dimethylpyrone was converted by ammonia into 4-methoxy-2,6-lutidine, therefore, the exocyclic oxygen of the pyrone must be involved in the salt formation. Thus, formula 21 was disproved and formula 22 was demonstrated for these salts. 

Dimethylpyrazine 1-oxide (52) does not appear to react at the exocyclic oxygen with methyl iodide or benzyl chloride but instead reacts at N-4 to form the quaternary salt 53. ... 

Positional changes of the exocyclic oxygen atom in 157b and 158b discriminate between the reactivity of these compounds. Thus, 158b readily reacts with the potassium salt of thiophenol to give sulfide 177 in 85% yield (83JHC783),... 

Shaw concluded that hydrogenation of 3-alkyl-4-aminomethylene isoxazol-5-ones (184) in the presence of palladium catalyst resulted in the saturation of either the endocyclic double carbon-nitrogen bond or the exocyclic double C—C bond with the retention of the heterocyclic nitrogen-oxygen bond. Recent data reported by Kochetkov et al. on the properties, and in particular on hydrogenation, of isoxazolid-5-ones - indicate, however, that Shaw had probably ob-... 

The difference of reactivity between 2,4-dinitrochlorobenzene and the iron complexes is explained by the fact that the additional electronic density is localized on the oxygen atoms in P-exocyclic position in the case of the nitro derivatives and on the ring in the case of the iron complexes [91]. 

From 95 g of SOCI2 and 183 g of H2S , both dissolved in CS2 and added simultaneously but slowly (within 8 h) to a mixture of 0.75 1 CS2 and 0.35 1 Me20, ca. 4 g of pure SgO were obtained after repeated recrystallization. SgO forms needle-shaped yellow crystals of m.p. 78 °C (dec.). The molecules consists of a crown-shaped Sg rings with exocyclic oxygen atoms in axial positions [89]. 

The Raman spectra of solid a- and PS O show characteristic differences but both spectra can be assigned assuming a chair-like six-membered homocycle with an exocyclic oxygen atom (molecular symmetry Cj see Scheme 4). 

The density of 87O has been determined as 2.15 g cm at 25 °C and calculated from the lattice constants as 2.179 g cm at —110 °C, measured by a single-crystal X-ray diffraction analysis [1, 64, 65]. The 87O molecules are of Cl symmetry and consist of chair-hke seven-membered homocycles with the exocyclic oxygen atom in an axial position see Fig. 2. Most remarkably are the two almost planar groups 0-8-8-8 (torsion angle r=2.9°) and 8-S-8-8 (r=6.3- ). 

Attraction of the exocyclic tailing oxygen atom with X steers the oxygen atom to the same side of the double bond [92], Lone pairs (Scheme 18a) on X and aromatic rings (Scheme 18b) can attract the tailing oxygen. The reactions can take place with X or the substituent on the same side of the double bond rather than with those (R , R ) on the opposite side. 

Okada and Mukai [48] showed a preference for a contrasteric approach of singlet oxygen to anti face of 7-isopropylidene double bond in photooxidation of 7-isopropylidenenorbomene followed by reduction with dimethyl sulfide (Scheme 32). They explained the stereoselectivity by applying the orbital mixing rules (Scheme 33). The r orbital of the exocyclic double bond enlarges its extention in the anti face. 

Alkyl sulfonates are very effective cationic initiators of e-caprolactone, although only the more reactive methyl triflate and methyl fluorosulfate result in a high conversion. The mechanism of polymerization in the presence of these initiators is believed to involve methylation of the exocyclic carbonyl oxygen, followed by partial ring opening of the activated lactone by the counteranion (Fig. 

Details of the hydrolytic process are somewhat more complicated because the acid-catalyzed hydrolysis proceeds via the initial protonation of an alkoxy oxygen followed by bond cleavage. Because the protonation can involve the exocyclic or endocyclic alkoxy group, two different sets of initial products are possible. However, in both cases the ultimate degradation products remain the same. These two possible reaction paths are shown on page 130. 

Z = 8 D, = 1.84 R = 0.059 for 2,587 intensities. In the asymmetrical unit, there are two molecules that have similar conformations. The glycosyl dispositions are anti (45.8°, 40.9°) in both molecules. The D-ribosyl conformation is 2Tj (157.6°, 32.1°) in one molecule, and 2T3 (165.1°, 31.9°) in the other. The exocyclic, C-4 -C-5 bond torsion-angles are gauche+ (51.9°, 53.9°) in both molecules, and the C-5 -0-5 bond torsion-angles are trans (172.5°, 176.6°). The two UDP molecules form a dimer coordinated by three K+ ions. There is no metal-ion or water bridge between the pyrophosphate chain and the uracil base of the same molecule. The three K+ ions are coordinated by oxygen atoms of... 

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