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The neutral and radical-cationic decarbonylation of tropone and benzoannelated tropones is experimentally investigated by pyrolysis and electron-impact mass spectroscopy, respectively. The reaction mechanisms are discussed by means of MNDO calculations. From these results it is deduced that the thermal reaction proceeds in two steps, the first one being an electrocyclic ring closure while the second, rate-determining step, consists of the cheletropic CO extrusion. The cationic reaction also starts with the (rate-determining) ring closure. However, the norcaradienone ion opens to a benzoyl-type ion, which is the actual precursor of the CO loss. The effect of the benzoannelated substitution is different for the thermal and cationic reaction profile.Comparable mechanisms are postulated for the apparent similarity of the thermal-and electron impact-' induced CO loss of tropones, which, by ring fraction, preferentially yield aromatic systems. The reaction can be explained by a sequence 0 ( 3 ) Scheme 1.

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