Anionic pinacol rearrangement

The stereoeontrolled construction of hydroazulenones is achieved by sequential anionic oxy-Cope rearrangement/SN allylic ether displacement of Cope precursors 5 and 7y71. Optically active compounds 5-8 are prepared from (-)-(lR,6S)-A4(10)-caren-rrons-3-ol via intermediates 3 and 4 by individual exposure of 3 and 4 to 3 equivalents of vinylmagnesium bromide. This reagent promotes initial pinacol rearrangement with liberation of a cyclohexanone unit which is captured by a second equivalent of the organometallic. The divinyl alcohol pairs 5/6 and 7/8 are isolated in 2 1 ratios (43-82% combined yields after MPLC). 

In reactions that resemble the pinacol rearrangement, thiopyranoside sulphonate esters undergo ring contraction and expansion reactions via the intermediacy of sulphonium ions, e.g. equations 37 and 38102. Similarly, cyclic sulphonates are able to trap oxime anions at the carbon atom (equation 39)103. Such carbon alkylation of oxime ions is rare, and by suitable choice of substrate both ring-contraction and ring-expansion reactions are possible103. 

Reaction of c/5-l,2-cyclooctanediol with dichlorocarbene yields cyclooctanone, cyclooctene, and the dichlorocarbene adduct of cyclooctene according to equation 3.6 [10]. The formation of cyclooctanone is expected from a pinacol rearrangement of the a-hydroxy carbonium ion formed by decomposition of the a-hydroxyalkoxy-chlorocarbene adduct. On the other hand, reaction of the alkoxychlorocarbene with an adjacent alkoxide anion yields a cyclic dialkoxycarbene which decomposes with loss of carbon dioxide and formation of cyclooctene. Addition of dichlorocarbene to cyclooctene yields 2,2-dichlorobicyclo[8.1.0]-nonane. 

Rearrangements may also proceed via intermediates that are essentially cations, anions, or radicals, though those involving carbocations, or other electron-deficient species, are by far the most common. They may involve a major rearrangement of the carbon skeleton of a compound, as during the conversion of 2,3-dimethylbutan-2,3-diol (pinacol, 42) into 2,2-dimethylbutan-3-one (pinacolone, 43, cf. p. 113) ... 

Diphenylcyclopropenone undergoes bimolecular reduction by the action of aluminium amalgam or magnesium-magnesium iodide the product is not, however, the expected pinacol but tetraphenyl-resorcinol [116], It was suggested that a radical anion is the first reduction product and that this then dimerises, as in normal pinacol reductions, but that the dimerisation product rearranges, first to a prismane derivative, then to a bicyclohexadiene derivative, and finally into the resorcinol derivative. 

Pinacol-pinacolone rearrangements proceed faster and more selectively in solid state. A 1 3 molar ratio of the pinacol and p-toluene sulphonic acid (p-TsOH) (powered mixture) on keeping at 60° C give the products (A) and (B). It is found that the hydride migrates more easily than the phenyl anion and the yield of A is higher than that of B in all the reactions (Scheme 22). 

---