The mass spectra of 1,3-cyclohexanedione and its 2, 4 and 5 substituted derivatives are discussed.Most fragmentation patterns can be rationalised starting from the diketo form, through initial 1.2 and 1.6 bond cleavages. Evidence is advanced that ionised enol-molecules mainly yield retro Diels-Alder type degradations. Ring bond cleavage processes are strongly dependent on the position and size of the substituents.The mass-spectral behaviour of acyclic 1,3-diketones has been discussed recently (293). We have now studied the fragmentation upon electron impact of 1,3-cyclohexanediones.The only mass spectra of the latter class of compounds which have been discussed are those of dimedone (IX) and 2-ethyldimedone ( 5, (XI).For the interpretation of the mass spectra, the two different initial a cleavages (1,2 and 1,6 bonds) have to be taken in account; each leading to two distinct groups of fragmentation patterns. The relative importance of the two different cleavages is dependent on additional substitution in the 2-or 6-positions, resulting in variation in stability of the two bonds involved. However, like in 1,3 acylic diketones (3) enol-forms give a different fragmentation than the diketo-forms, although this influence of keto-enol tautomerism is less straightforward for the cyclic 1,3-diketones under
In 2-propyl-and 2-butyl-l,3-~yclohexanediones, fragmentations in the side chain carry most of the total ionisation. Exact mass measurements and deuterium labeling led to mechanistic interpretations for the formation of the principal fragment ions.
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