Enhancer Fragmentation

It should be noted that the observed bicycle motion is one of two a priori possibilties as shown in Scheme 10, which includes the alternative, pivot process. Interestingly, an even number of pivot steps would give rise to the same stereochemistry as the bicycle process, although an odd number of pivot steps affords the reverse stereochemistry relative to the bicycle mechanism. The differentiation between these possibilities resulted from the observation that the endo-aryl reactants of the type in Scheme 11 show a marked enhancement of the 1,4- (2,3) fragmentation at the end of an odd number of bicycle steps. The exo stereoisomers proceed nicely onward to the spiro bicycle photoproduct. This is understood on the basis of van der Waals repulsions of the endo-aryl groups, which leads to enhanced fragmentation due to relief of these repulsions. This, then identifies the aryl groups as endo after an odd number of steps. 

Substituted cyclopentadienones have been generated via rDA thermolyses. Thermolysis of adduct (108) unmasks dienone (109) as shown in equation (51). The generation of (109) was followed by trapping with cyclopentadiene to produce the tricyclic enones (110), which are used as synthons for sar-komycin derivatives. The presence of the alkyl group at the 6-position in ester (108) enhances fragmentation via a rDA process, while in its absence (R = H) fragmentation occurs by decarbonylation. 

The low activation energies with which these concerted reactions can take place 35 5 kcal) make them very important in all pyrolysis systems in which olefins are major products. They lead to an enhanced fragmentation of large monoolefins and have probably been responsible for much misidentification of cracking rate parameters. 

K. In turn, this more efficient cooling of the interstellar gas influences cloud fragmentation because low temperatures yield low pressures and thus less mass is needed for gravitational collapse to ensue. This metal enhanced fragmentation... 

Multiple branching on the same carbon enhances fragmentation at that point. The mass spectrum of 7-methyl-7-heptadecanol (Figure 2.19), for example, contains strong fragment ions corresponding to loss of CHj, CgHij, and CioHj, (Equation 2.36). 

Figure 8.8 Enhanced ion charging of peptic peptides during HX-ETD analysis. MS spectra of microglobulin peptide 1-9 (IQRTPKIQV) (left) and EED spectra (right). HX-ETD was performed in the absence (top) and in the presence of 0.05% m-NBA (bottom). A shift from the doubly to the triply charged precursor Is apparent resulting in enhanced fragmentation efficiency of the triply charged ion. Reproduced with permission from [47] 2009 American Chemical Society. (See Insert for color representation of the figure.)...

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