Ring contraction reactions oxidative rearrangements

The oxidative rearrangement of cyclic alkenes and ketones often leads to ring expansion or ring contraction reactions. The reagents generally used for this purpose are hypervalent main group oxidants such as thallium(III), lead(lV), iodine(III) and selenium(IV), alAough palladium(II) has been used as well. 

Oxidative rearrangement takes place in the oxidation of the 1-vinyl-l-cyclo-butanol 31, yielding the cyclopentenone derivative 32[84], Ring contraction to cyclopropyl methyl ketone (34) is observed by the oxidation of 1-methylcyclo-butene (33)[85], and ring expansion to cyclopentanone takes place by the reaction of the methylenecyclobutane 35. [86,87]... 

Kubota and co-workers describe a novel oxidative rearrangement of the diosphenol (58) of 17iS-hydroxyandrost-4-ene-2,3-dione to the A-nor-A -1,2-diketone (59) in 33 % yield by the action of specially prep d manganese dioxide in boiling acetone. The rate of ring contraction is very sensitive to the source of the oxidant, and a trace of dilute sulfuric acid in the reaction mixture causes oxidative fission of ring A. 

An electrochemical reaction on the azepine carbamates 19 resulted in a ring contraction to give the N-substituted anilines 20 in moderate yield (Equation 1). Mechanistically, initial oxidation of the carbamate to give the radical cation 21, followed by electrocyclic rearrangement to 22, C-N cleavage, and then reduction was proposed <1998H(48)1151>. 

With some polycyclic substrates, a tandem ring expansion and ring contraction can take place under conditions of oxidative rearrangement. The 11-oxolanostanyl acetate (48 equation 46) undergoes such a reaction, in which ring c is contracted and ring d is expand and aromatized. The yield is poor though, and such a transformation would seem to have limited synthetic potential. 

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