Properties of the Retro-Diels-Alder Reaction

Note The McLafferty rearrangement and the RDA reaction have several features in common i) both belong to the rearrangement type of fragmentations, although the name conceals this fact in case of the latter, ii) both represent pathways for alkene loss from molecular ions, and iii) both are highly versatile in structure elucidation. 

Provided the double bond does not migrate prior to RDA reaction, the alkene loss is perfectly regiospecific, and it does not really suffer from extensive substitution at the cyclohexene unit. Thus, substitution patterns can be revealed from the mass spectrum (a). The RDA reaction also proceeds independent of whether the six-membered ring contains heteroatoms or not (b)  

Obviously, the RDA reaction has the potential needed for a widespread mass spectral fragmentation, and consequently, its analytical and mechanistical aspects have repeatedly been reviewed. [107,109,110] 

Note Almost any molecular ion having a six-membered ring that contains one double bond can undergo the RDA reaction to eliminate a (substituted) alkene or a corresponding heteroanalog. 

The RDA reaction is often observed from steroid molecular ions, and it can be very indicative of steroidal structure [97,100-102]. The extent of the RDA reaction depends on whether the central ring junction is cis or trans. The mass spectra of A -steroidal olefins, for example, showed a marked dependence upon the stereochemistry of the A/B ring juncture, in accordance with orbital symmetry rules for a thermal concerted process. In the trans isomer the RDA is much reduced as compared to the cis isomer. Moreoever, the differentiating effect increases at 12 eV. This represents the first example of apparent symmetry control in olefinic hydrocarbons [102]. 

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