A-ether bonds

Fig. 7-6. Behavior of /3-aryl ether and open a-ether structures during acid sulfite pulping (Gellerstedt and Gierer, 1971). R = H, alkyl, or aryl group. The first reaction step involves cleavage of the a-ether bond with formation of a resonance-stabilized carbonium ion which is then sulfonated. Note that both the phenolic and nonphenolic structures are sulfonated, while the /3-aryl ether bonds are stable.

Structures Containing a-Ether Bonds The a-ether bonds in phenolic phenylcoumaran (Fig. 7-27) and pinoresinol structures are readily cleaved by hydroxide ions, usually followed by the release of formaldehyde. Only in the case of open a-aryl ether structures does this reaction result in the fragmentation of lignin. In contrast, the a-ether bonds are stable in all etherified structures. 

Selective cleavage of ether bonds is useful to determine the contribution of carbon-carbon bonds for polymer lignin. Pivaloyl iodide [90] is known to cleave a-ether bonds selectively and trimethylsilyl iodide [91,92] can cleave a- and P-ether bonds quite effectively under the proper reaction conditions. Because of the very small amount of sample required, pyrolysis GC-MS may be applied for the analysis of a specific morphologi region of a cell wall. 

Prodrugs with a ether bond) a Protease b 1,6 elimination. 

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