Retro-Diels-Alder fragmentation in mass

Keywords retro-Diels-Alder fragmentation occurring in gas-phase, fragmentation in mass spectrometry 

Pyrido[l,2-a][l,3,5]triazines (54) formed by cyclodimerization of pyridyl heterocumulenes, gave typically retro Diels-Alder fragmentations in their mass spectra <81CCC2557,82ZC140,83JPR463). The molecular ions could also be identified in most cases. 

Dixon, J.S. Midgley, I. Djerassi, C. Mass Spectrometry in Stractural and Stereochemical Problems. 248. Stereochemical Effects in Electron Impact Induced Retro-Diels-Alder Fragmentations. J. Am. Chem. Soc. 1977,99,3432-3441. 

Bridged polycyclic chlorinated hydrocarbon insecticides include the dimethanonaphthalenes and methanoindenes. Their spectra are dominated by retro Diels-Alder fragmentation reactions which are accompanied, or preceded, by expulsion of Cl and/ or HC 1. A typical dimethanonaphthalene is aldrin, the mass spectrum of which is shown in Fig. 4. The corresponding epoxide of this compound, dieldrin, shows similar features. 

The spectroscopic distinction between thiocoumarins and thiochromones is best made by observing the low-field 5-proton in the NMR spectrum of the latter and also the characteristic retro-Diels-Alder fragmentation always observed in the mass spectrum of thiochromones. Thiochromones also show distinctive, strong ultraviolet absorptions at 250-270 nm.  

The spectrum of norbomene (Figure 2.40), for example, contrasts sharply with that of the saturated analog norbomane (Figure 2.41). The retro Diels-Alder fragmentation leads to the most intense peak in the norbomene spectrum at miz 66. The spectrum of norbomane, on the other hand, is much more like that of a typical hydrocarbon, with even-electron (odd-mass) ions dominant. 

An independent determination of the isotope effect could be derived from the mass spectrum of the 3-trimethylsiloxycyclohexene-retro Diels-Alder fragmentation (m/e = 142). 

Also the mirror image of the structure I, correctly denoted as cyro-3,10-dihydroxy-3,5,8,10-tetra-methyltricyclo[6.2.2.02 7]dodeca-5,ll-diene-4,9-dione, would be possible since enantiomers are not differentiated by NMR. A retro-Diels-Alder fragmentation of I to Cg//yo04 explains why the molecular ion corresponding to the molecular formula is not detected in the mass spec- 

---