Carbon-Oxygen Cleavage

Using a binary sensitizing system (phenanthrene P/DCNB /7-dicyanobenzene) in acetonitrile solution, O-aryl glycosides are transacetalized with alcohols after generation of aromatic radical cations [23], According to kinetic anomeric effects, the a-side attack of nucleophiles to cyclic oxocarbenium ions follows scheme 9. 

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BF3 is also used for ether carbon-oxygen cleavage in the presence of sulfide or thiol (Scheme 62). 

An experimental demonstration that the Nef mechanism for the initial conversion of a sugar by alkali to the a-dicarbonyl structure is not acceptable was provided by a study of the action of alkali on 2 hydroxy-3-meth-oxy-3-phenylpropiophenone. Nicolet observed that the products in this case are 2,3-diphenyllactic acid and methanol. This conversion, which is completely analogous to the formation of a saccharinic acid from a reducing sugar, demonstrated clearly that carbon-oxygen cleavage occurs at the /3-carbon atom, rather than at the a-carbon atom, with respect to the carbonyl group. 

Ethers, in general, are quite stable to hydrogenolysis even over Raney nickel at temperatures as high as 250°C. However, ethyl ether slowly reacts above this temperature over nickel to give ethane and ethyl alcohol. If the ether contains an aromatic nucleus, complete hydrogenation of the latter will occur at a lower temperature than is required for carbon-oxygen cleavage. 

C-acylation with - 26, 804 -, addition with carbon-oxygen cleavage 27, 314... 

Whereas lithium aluminium hydride-aluminium trichloride reduction of monothioacetals effects carbon-oxygen cleavage to give hydroxythioethers. 

In the majority of substitution and elimination reactions, the sulfonate moiety functions as a leaving group i.e. reaction as shown by Equation 17 involving carbon-oxygen cleavage). 

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