Fragmentation path

Figure 2-33. Any bond of a ring can be broken without fragmenting the compound (path 1 on the left-hand side), whereas breaking a bond in a chain results in two fragments (path 2 on the right-hand side).

The mass spectra of methyl 3-deoxy-p-v-tkreo-pentopyrano-side, methyl 4-deoxy-j3-T>-thieo-pentopyranoside, and 5-deoxy-fi-D-xylo-furanoside are discussed and compared fragmentation paths are sufficiently different to allow identification on the basis of their mass spectra. On the other hand, the mass spectra of methyl 2- and 3-deoxy-5-O-methyl-f3-i>-erythro-pentofuranosides do not exhibit fragmentation differences. The mass spectra of 3-deoxy-l,2 5,6-di-O-isopropylidene -d-xylo - hexofuranose, 5- deoxy -1,2-0-isopropylidene-D-xy o-hexofuranose, and 6-deoxy-l,2-0-iso-propylidene-D-glucofuranose show prominent differences, even between the 5- and 6-deoxy isomers. The interpretation of the spectra was aided by metastable-ion peaks, mass spectra of DzO-exchanged analogs, and the mass spectrum of an O-isopropylidene derivative prepared with acetone-d6. 

In the hydration of compounds 2f and 2g, besides the expected ester, three other products (acetic acid, an alkene, and alcohol) were observed. These products were postulated to arise via a fragmentation of the intermediate vinyl cation, 6, as shown in Scheme II. The importance of the fragmentation path is presumably determined by the stability of the alkyl cation formed by the alkyl oxygen fission. 

The loss of NO, HNO, N02 and HN02 moieties is all a result of fragmentations common for nitroarenes. The importance of these fragmentation paths appears to be more... 

Several original papers must be mentioned that deal with mass spectrometric techniques which the numerous reviews do not comprise. Kaufmann and coworkers268,288 studied the mass spectrometric analysis of carotenoids and some of their fatty acid esters using matrix-assisted laser desorption/ionization (MALDI) mass spectrometry and its post-source-decay (PSD) variant. Some advantages concerning the thermal instability and limited solubility were discussed, but the fragmentation paths of the carotenoid cations were found to be essentially the same as those observed with conventional techniques. 

This often permits positive identification of a particular fragmentation path. 

SCHEME 13. Main homolytic fragmentation path of acetyl henzoyl peroxide undergoing CID with 107 Ag+ ions... 

SCHEME 14. One of the CID fragmentation paths of the primary complex of bis(t-butoxycarbonyl) peroxide (242) with NH4 +, involving McLafferty rearrangements and CO2 eliminations... 

Scheme 5.5 Alternative fragmentation path ways for hexane...

Remarkably, the stability of the molecular ion associated with the furan and benzofuran nuclei is preserved into the benzo[c]furan series. For the 1,3-diphenyl compound the fragmentation path is believed to be as shown in Scheme 12, the molecular ion being by far the most abundant (760MS258). 

For silatranes containing alkyl and aryl substituents at the skeleton carbons, additional fragmentation paths were found under FAB. They lead to formation of [MH — OCHR CH2]+ and [MH — OCH2CR2]+ ions as products of side chain elimination from the MH+ ions. An a-cleavage with elimination of benzaldehyde molecule from the molecular ions was observed in the spectra of 1-methyl- and 1-benzyl-3,7-diphenylsilatranes291. 

The type refers to the structural fragment (path, cluster, path/cluster or chain) used in computing the index. Only path indices are possible for orders less than 3. 

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