Elcb elimination 838 Subject

The methoxy-substituted acetal C is isolated and is subjected to another two reactions (1) the acetal function is hydrolyzed under acidic conditions to provide the methoxylated aldehyde E. (2) With a catalytic amount of sodium methoxide, the methoxylated aldehyde E reacts via an Elcb elimination, which leads to the formation of the poly-unsaturated aldehyde D. The substrates A and B of the three-step reaction sequence in Figure 12.23 derive from crotonic aldehyde (E13C-CE[=CE[-CH=0) the respective methods of preparation are outlined in the figure. The final product, which is the highly unsaturated aldehyde D, displays the structure of an aldol condensation product, namely of the crotonic aldehyde mentioned above. 

But the mechanism is not a simple E2 reaction (Fig. 19.70). Removal of the a hydrogen is relatively easy, as this process gives a resonance-stabilized enolate anion. In a second step, hydroxide is lost and the a,P-unsaturated compound is formed. This kind of elimination mechanism, called the ElcB reaction, was the subject of discussion in Chapter 7 (p. 309). 

In Scheme 9.6, previously reported mechanisms of the thermal degradation of PHB and PHAs are illustrated. Even though many approaches have been taken on the subject of PHB thermal degradation, a clear explanation for the wide variation in value has yet to be provided. Despite this uncertainty, the random jS-elimination scission has been widely held as the exclusive degradation mechanism of PHB based on typical structures of pyrolysis products, i.e., crotonic acid and oligomers with a crotonate end-group [68, 80]. In 2007, an ElcB mechanism proceeding via a-deprotonation by a carboxylate anion to produce the same products was proposed by Kawalec et al. [81]. 

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