Abstract-The mass spectra of enol derivatives of B-diketones, such as enol ethers and enamines are discussed. Their behaviour under electron-impact is in accordance with the fragmentation we suggested for an acyclic /3-diketone in the enol form.Rearrangement of the enol functional group is observed in the spectra. This process is a general one, as it shows not only migration of oxygen and nitrogen in the enol ethers and enamines, but also migration of sulphur and chlorine in the thio-ether and in 4-chloro-3-pentene-2-one.Enol derivatives are suitable compounds for determining the branching of alkyl chains in Bdiketones.THE DEGRADATION under electron-impact of @-diketones depends largely on the tautomeric e q~i l i b r i u m .~*~*~ In previous papers an attempt was made to differentiate typical fragmentations of both enol-and diketo-molecular ions.lS2 In this context the fragmentation of enol derivatives like enol ethers and enamines of several 1,3-diketones was investigated. En01 ethersStructure A.The mass spectrum OR, 0of 4-methoxy-3-pentene-2-one (I: R, = R, = R, = Me;. .

thyl enol ethers of 1,3-cyclohexanediones and 1,3-cyclopentanediones is studied. Six membered rino; compounds behave quite straithforwardly and in accordance with the fragmentation suggested f o r 1,3-cyclohexanediones in the enol form. Some fragmentations are supported by deuterium labelling. tuted isomers display identical spectra. This isomerisation is studied by metastable transitions, low energy spectra and deuterium labelling; a possible mechanism is advanced.The influence of alkyl substituents on the mass spectral behaviour of meIn the case of enol ethers of 1,3-~yclopentanediones the 4-or 5-substi-The mass spectral behaviour of acyclic 1,3-diones1 to 4, 1,3-~yclohexane-diones5 to and 1,3-cycl~pentanediones~ has already been discussed. It could be proven that both tautomeric forms display different fragmentation patterns.It is therefore of interest to investigate corresponding enol ethers. The fragmentation behaviour of some enol derivatives of dirnedone'l and acyclic 1,3-diketones" has been published.ethers of alkyl substituted 1,3-cyclohexanediones and 1,3-cyclopentanediones.It appears, as well from the published data as from the present work that the most important ions are formed by initial cleavage 0 to the carbonyl function.The fragmentation of the cyclic enol ethers is discussed in function of the length and the position of the alkyl substituent; the mechanisms are in all cases, supported by high resolution measurements.We now present the fragmentation of methyl en01 I; R = H 11; R = Et 111; R = n.Pr IV; R = n.Bu).Holder of a bursary of V; R = Et VIII; R = Me VI; R = n.Pr IX; R = n.Pr VII; R = n.Bu X; R = n.Bu C. Van

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Examination of isomeric 3-propyl-and 3-butyl-2,4-pentanediones demonstrates that the electron impact induced degradation of a /?-diketone molecule is different when it is enolised or not.(1) N. SCHAMP and M. VANDEWALLE, Bull. SOC. Chim. Belg., 75, 539 (1966).

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