With the aid of isotopic labelling (D and I*O) and the metastable defocusing technique the mass spectra of xanthone, the monohydroxyxanthones and the monomethoxyxanthones have been recorded and interpreted. The relative position of the hydroxy group exerts no direct effect on the fragmentation, whereas the decomposition of the methoxy compounds is clearly influenced by the position of the substituent. The 1-methoxy-group exhibits an 'ortho-eEect'. The presence and significance of doubly-charged ions in the spectra are discussed.ALTHOUGH the mass spectra of several classes of oxygen heterocycles have been extensively studied (4. a recent comprehensive review1) little attention has been directed towards the xanthones. The mass spectrum of xanthone itself has been reported,, but apart from a discussion of the mass spectra of some lichen xanthone~,~ no systematic investigation of the electron-impact-induced fragmentation of xanthones appears to have been made.In this and later papers the results of a mass spectrometric study of a number of xanthones with hydroxy and methoxy substituents will be reported. The different positions of these substituents in the nucleus constitute major structural variations in the naturally-occurring xanthones, so that their influence on the fragmentation patterns is of considerable analytical interest.
RESULTS A N D DISCUSSION
XanthoneThe fragmentation of xanthone2 upon electron-impact ( Fig. 1) will be presented again here together with additional details now available. The dominating fragmentation route (Scheme l) passes through the successive loss of two molecules of CO, giving ions formulated as a and b, after which the loss of a hydrogen atom leaves an ion which may be formulated with the benzdehydrotropylium structure (c), in analogy with the dehydrotropylium structure proposed for the [C,H,]+ ion in the mass spectrum of 2-formylben~ofuran.~ Use of the metastable defocusing technique has now revealed that a process leading directly from a to c through the loss of CHO or CO + He is also operative. The further decomposition of c gives rise to minor ions at m/e 114 and 113, corresponding to the loss of .C,H and C,H,, respectively.Features from doubly-charged ions corresponding to [MI++ and [M -CO]+f, as well as a metastable peak arising from the transition between them, are present in 667
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