Mesoionic ground-state species

The presentation of the rearranging intermediates as mesoionic ground-state species (196) and (199) has proved an adequate tool for the prediction of structural changes in this field. The steric factors which direct the partly selective rearrangement paths of the bicyclohexenone and dienone photoisomers of (174) and their corresponding alkylated homologs have been extensively discussed in a recent review by this author. ... 

The bicyclohexenone-dienone isomerization 33a - -34a + 35a (Chart 5) has also been observed in the dark by acetic and formic acid catalysis and clearly involves the cationic intermediate 110 (Chart 16)." Further, under more drastic acidic nonphotolytic conditions, photoketones of the general formula 111 (R = H or alkyl) give hydroxy-ketones 113 (R = alkyl) and 114 (R = H), respectively. In these transformations, cationic intermediates of type 112 are obviously formed. Mixtures of hydroxyketones such as 113 and 114 are also produced upon irradiating the corresponding dienones (cf. Chart 7) in aqueous acetic acid. Av, is, 27,30 Here, participation of intermediates that are either identical with or structurally related to the ground-state species 112 seems indicated Ai Both intermediates, 112 and 110, represent conjugate acids of the proposed mesoionic intermediates (general formula 102 and 106, respectively) in the photoisomerizations. 

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