Molecular fragmentation patterns Diels-Alder

Molecular orbital calculations indicate that cyclo C-18 carbyne should be relatively stable and experimental evidence for cyclocarbynes has been found [25], Fig. 3B. Diederich et al [25] synthesised a precursor of cyclo C-18 and showed by laser flash heating and time-of flight mass spectrometry that a series of retro Diels-Alder reactions occurred leading to cyclo C-18 as the predominant fragmentation pattern. Diederich has also presented a fascinating review of possible cyclic all-carbon molecules and other carbon-rich nanometre-sized carbon networks that may be susceptible to synthesis using organic chemical techniques [26]. 

The fragmentation pattern of l,5-dimethyl-7-methoxy-l,2,3,4,5,10-hexahydro-4fl//-pyridazino[l,6-h]isoquinoline (22) has been studied (73JHC999). The base peak at m/e 148 might be derived through the retro Diels-Alder fission of the molecular ion. The 7-hydroxy derivative gives the same fragmentation pattern. 

The mass spectra ofbromacil and terbacil and their A-methyl derivatives show only weak molecular ions, but they have the isotope patterns expected from bromine and chlorine compounds, respectively. The major ions arise from [M—55] and [M—56] and also reflect the halogen substituent. Less abundant, but highly characteristic ions, corresponding to [M-99Y and [M-98], are formed by retro Diels-Alder fragmentations. 

The mass spectra of cycloalkenes show quite distinct molecular ion peaks. For many cycloalkenes, migration of bonds gives virtually identical mass spectra. Consequently, it may be impossible to locate the position of the double bond in a cycloalkene, particnlarly a cyclopentene or a cycloheptene. Cyclohexenes do have a characteristic fragmentation pattern that corresponds to a retro Diels-Alder reaction (Fig. 4.4). In the mass spectrum of the monoterpene Umonene (Fig. 4.16), the intense peak at m z = 68 corresponds to the diene fragment arising from the retro Diels-Alder fragmentation. 

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