Cubane derivative, Favorskii rearrangement

Within the cubane synthesis the initially produced cyclobutadiene moiety (see p. 329) is only stable as an iron(O) complex (M. Avram, 1964 G.F. Emerson, 1965 M.P. Cava, 1967). When this complex is destroyed by oxidation with cerium(lV) in the presence of a dienophilic quinone derivative, the cycloaddition takes place immediately. Irradiation leads to a further cyclobutane ring closure. The cubane synthesis also exemplifies another general approach to cyclobutane derivatives. This starts with cyclopentanone or cyclohexane-dione derivatives which are brominated and treated with strong base. A Favorskii rearrangement then leads to ring contraction (J.C. Barborak, 1966). 

No Favorskii rearrangement products are produced when a-halocyclopentanones are treated with base, and products of aldol, substitution and dehydrohalogenation reactions are produced. However, when an a-halocyclopentanone is part of certain condensed systems, cubane derivatives may be obtained (see Section 3.7.3.5). 

At this point, it is interesting to compare the absence of reactivity of a-halocyclopentanones under Favorskii rearrangement conditions, with the ring contraction observed when such a-halocyclopentanones are included in cage structures used as precursors of cubane derivatives. We will show in what follows that this marked difference in reactivity can be explained by the equatorial stereochemistry of the halogen in the cage a-cyclentanone derivatives. 

SCHEME 18.10. Cubane derivative through Favorskii rearrangement. 

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