Ether bridges

This polymer is vulcanised by heating with metalhc oxides, such as zinc oxide the reaction may involve the formation of ether bridges ... 

Stopping the polymer at this point requires the ratio of formaldehyde to phenol to be less than unity. Both methylene and ether bridges are known to be present. The reaction is either acid or base catalyzed, and branching is uncommon at this stage. The products are variously known as A stage resins, novolacs, or resole prepolymers. 

Fig. 4. Two 4,6-linked biflavonoid units linked to each other through a methylene ether bridge formed by reaction with formaldehyde.

Annotinine, C gHjjOjN, (1). M.p. 232° perchlorate, m.p. 267°. In a later paper (1947) Manske and Marion record the results of the action of alkali and of halogen acids on annotinine, and of the oxidation of the base and discuss the reaction products. They conclude that two of the oxygen atoms are present as a lactone group and that the third oxygen may form an ether bridge in a 5- or 6-membered ring. 

On the other hand the acetolysis of the 6y5,19-ether bridge is a useful reaction in the 6a-methyl-6j9,19-ether series ... 

The second step is the condensation reaction between the methylolphe-nols with the elimination of water and the formation of the polymer. Crosslinking occurs hy a reaction between the methylol groups and results in the formation of ether bridges. It occurs also by the reaction of the methylol groups and the aromatic ring, which forms methylene bridges. The formed polymer is a three-dimensional network thermoset ... 

By these two processes a network is built up that consists of both methylene and ether bridges in commercial materials there is some evidence that methylene groups predominate. 

Formation of asphaltenes during solubilization of low-rank bituminous coals has been attributed to cleavage of open ether-bridges (6). But while the presence of such configurations in high- and medium-rank bituminous coals is well established (7), their existence in less mature coals remains to be demonstrated. From reactions of low-rank bituminous coals with sodium in liquid ammonia or potassium in tetrahydrofuran, it has, in fact, been concluded that open ether-bonds are absent (8) or only present in negligible concentrations (9). 

It may be considered that the nacent hydrogen more selectively contributed to the cleavage of the ether bridge and that H2 was more selective than 00 + H2O regarding the cleavage of the CH2 bridge. 

The recognition of barium containing crown ether bridged chiral Schiff base zinc complex [44] with the rigid bidendate guest 1,4-diazobicyclo-[2,2,2]octane (DABCO) was studied by aH NMR titration.107... 

When rationalizing the significant difference of the hydrocarbon- and ether-bridged radical anions, the main aspect will certainly be the conformation of the oxyethylene chain, which brings the electrophores into closer contact. An additional aspect follows from the ability of the oxygen centres along the chain to chelate the counterion and thus to fix the cation between the electrophores. It is not possible from the available experimental evidence to discriminate between the two effects. The role of ion pairing and the relative position of the counterion and carbanion will be dealt with below. 

The triethylsilane reduction of the peroxy ethyl ether shown in Eq. 307 takes place at the C-0 bond of the methyl ether without reduction of either the iodide or the peroxide functionalities (Eq. 307).499In contrast, a bridged peroxy ether undergoes reduction of both C-0 bonds of the peroxide linkage rather than at the ether bridge (Eq. 308) 499... 

These data, together with the molecular formula, indicated that the oxygen must be present as an ether bridge. If the doublet at 77.7 ppm is due to an aminomethine, the second carbon of the ether bridge should give rise to the singlet at 80.1 ppm. The only biosynthetically reasonable structures that could explain this spectral behavior are 2,14-oxidopseudoaspidospermidine (306) (without stereochemistry) or the new structure 173. Structure 306 was easily... 

The only study available on metabolites of AE was performed by Crescenzi et al. [17]. The initial biodegradation of AE occurs by cleavage at the ether bridge between the alkyl and ethoxylate chain, resulting in polyethylene glycols (PEG) and alcohols. In consecutive oxidation steps, the PEG chains are shortened and mono- and dicarboxylated metabolites (MCPEG and DCPEG, respectively) are formed. 

Both reactions act to reduce hydrogen bonding within the coal structure which may have a direct positive impact on liquefaction reactivity. More indirectly, these reactions lower the concentration of OH species in coal-derived products and hence, reduce the extent of retrogressive condensation via ether bridge formation. Reducing production of THF-insoluble condensation products increases the net THF-soluble coal conversion observed during the liquefaction experiment. None of the spectra from coals pretreated with alkyl alcohols and HCl showed any significant evidence of alkylation at carbon sites in the coal. 

Oxidative caibon-carbon bond formation from laudanosine derivatives generally favours a 6-membered ring. Severe steric constraints result in exceptions to this rule. Oxidation of the bridged ether derivative 34 results in carbon-carbon bond formation to form a 5-membered ring product and this process has been used for one stage in the synthesis of erythrina alkaloids [138]. Some of the morphinadie-none system is also formed, in spite of the steric constraint imposed by the ether-bridge. 

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