Ring contraction reactions cationic intermediates

Analysis of the 2-chlorocyclohexanone recovered at half-reaction time showed that there was no isomerization of the halogen from the a to the a carbon. The same isotopic distribution was found in the two ring-contracted esters. Consequently, Loftfield assumed that a cyclopropanone was the common intermediate. It should be noticed that a 2-oxyallyl cation or a zwitterion (Table 1) would lead to the same experimental results. 

A tandem polycyclization-Pinacol process has been developed using a similar strategy as described above. As shown in Scheme 29, the polyfunctionahzed phenol 100 is transformed into the compound 104 when treated with [bis(trifluoroacetoxy) iodo]benzene in a mixture of hexafluoroisopropanol and dichloromethane [25]. The reaction occurs through a succession of chair-Uke transition states, in which the phenoxenium ion 101 is first trapped by the endocychc alkene to deliver the intermediate 102, which then reacts with the alkyne to produce cation 103. The process concludes with a pinacol-like ring contraction. The presence of a chlorine... 

The photooxygenation of masked o-benzoquinones, such as 173, in methanol may generate a cationic intermediate, like 175, that gives functionalized cyclopentenones, such as 177, through a ring contraction (Scheme 18.41). On the other hand, when the reaction is performed in chloroform, endoperoxides are isolated. The required quinones 173 are readily prepared from 2-methoxyphenols 172 using the iodine(III) reagent PhI(OAc)2. ... 

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