Beta-methyl elimination

If one now considers 16, the diastereoisomer of 17, it is evident that, due to the syn relationship between the Pd and the methyl group, syn beta-hydride elimination can only proceed in two directions, one to generate 11 (via 15) and one to generate 12. Here, then, was solution to the apparent paradox mechanism IV need merely be adapted so that intramolecular addition of the Pd-H to the alkene in intermediate 15 generates 16, and thus 12. 

Elimination from 2-benzyl-1,1,1-trifluoropropane with potassium /-butoxide undoubtedly occurs through a carbanion-like E2 transition state. The structure of this substrate (14, Table 3, p. 192) is closely related to the pentahaloethanes known to react by the carbanion mechanism. The lesser tendency of the beta methyl and beta benzyl groups to withdraw electrons than the beta halogens accounts for a reduction in carbanion stability and a change to a concerted elimination. 

When the original methyl D-fructofuranoside sirup was fermented with yeast, the unstable beta isomer was selectively eliminated and the residue yielded a crystalline methyl D-fructoside melting at 81° and with [a] D +93° in water. The ring structure of this new isomer was proved to be furan by methylation to the liquid tetramethyl derivative, of [a] °D +129.4°, and subsequent hydrolysis to 1,3,4,6-tetramethyl-D-fructofuranose (structure IX) with the correct specific rotation of +29.8° in water. Both the methyl D-fructoside and its fully methylated derivative were therefore of the alpha configuration, since the latter was more dextrorotatory than the tetramethyl-D-fructose and also since the former was more dextrorotatory than the isomer, of [a] D —51°, unstable to invertase. Similar work with the benzyl D-fructofuranoside sirup produced the crystalline alpha isomer, melting point 89°, [a] D +45.7° in water, the liquid tetramethyl derivative, [a] D +83.3° in chloroform and, after acid hydrolysis of the latter, 1,3,4,6-tetramethyl-D-fructofuranose. 

The beta-tritium secondary isotope effect for elimination from the propyl-trimethylammonium ion (8, Table 7) seems most probably explained in terms of reduced acidity of the beta hydrogen caused by the greater inductive effect of the bond to the heavier isotope. A hyperconjugative effect also fits the observed data as in the transition state the developing double bond would be more effectively stabilised by the methyl than the monotritiomethyl group. However, the latter explanation seems less likely as the elimination from the propyl compound is slightly slower than that from the ethyl derivative . 

Figure 1.1 Beta-elimination at a residue of galacturonic acid methyl ester in a pectin. The reaction will not occur if the carboxyl group is free. Breakage of the glycosidic bond occurs under alkaline conditions and both fragments can be labelled with tritiated borohydride. Similar reactions occur with alkali-labile glycopeptides.

The research by Beerman also demonstrated the relative stability of the Ti-methyl bond as compared to the relatively less stable Ti-ethyl bond that contains a hydrogen on the beta carbon and can, therefore, undergo beta-hydride transfer to the titanium metal and eliminate an ethylene molecule. This early research eventually lead in the 1970s to the identification of transition metal carbene complexes (M=CH2), which when reacted with olefins provide metallacyclobutanes [29]. 

The first task was to prepare the chiral sulfoxide. The synthesis began with the conversion of methyl propionate (144) to keto-sulfide 145. Enzymatic reduction of the ketone using Baker s Yeast gave 146 with decent enantiose-lectivity. A directed oxidation of the sulfide provided an unequal mixture of sulfoxides 147 and 148 (and presumably minor amounts of material derived from the 4-5% of ent- 46 present in the starting material) from which 148 could be isolated in 50% yield. Dehydration of the alcohol provided 149 (along with some of the Z isomer). Notice that Mori decided to place the alcohol beta to the sulfoxide in the precursor of 149. There might be a number of reasons for this, but one is that it facilitated the elimination reaction (dehydration) because of the electron-withdrawing properties of the sulfoxide. 

The iabiie beta acids, present in hops, are sensitive to oxidation reactions, initiated by air (auto-oxidation). Thus, the beta acids are converted partiaiiy in the hop plant and to a greater extent during storage of the hops. The oxidation is almost quantitative in the brewing process. A very complex reaction mixture is formed. The experimental data show that auto-oxidation occurs via radical mechanisms, which transform the polyfunctional beta acids to a large number of oxidized compounds. In particular the three 3-methyl-2-butenyl side chains are very sensitive to oxidation, either at the double bonds or in the allylic positions. The native compounds are usually oxidized further or transformed by hydration and/or elimination. These reactions are responsible for the complexity. Reaction between two side chains leads to bicyclic and tricyclic derivatives. It is remarkable that in all known oxidized compounds derived from the beta acids, at least one 3-methyl-2-butenyl group remains unchanged. 

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