Cyclopentanone pinacol

Scheme XXIII. Synthesis of dl-perhydrohistrionicotoxim (72). The 2-epimer was the major product if the free alcohol was used in reaction E. A i)Mg, TiCU (Formation of cyclopentanone pinacol), ii) H2SO4, heat. B i) n-ButylLi, ii) SOCI2, pyridine, iii) a. Diborane, b. H2O2, OH , iv) CINO. C i) hv, ii) Tosyl chloride, pyridine. D i) UAIH4, ii) (CH3)2t-BuSiCl, iii) N-Bromosuccinimide, iv) t-AmylOK. E i) n-AmylLi, ii) Tetrabutylammonium fluoride...

Pinacol rearrangement driven by the release of the ring strain of a cyclobutane ring has been employed in an extremely efficient manner to form cyclopentanone derivatives. Experimentally. the Lewis acid mediated aldol condensation of benzaldehyde with l,2-bis(trimethyl-siloxy)cyclobutcne at —78 C gave the pinacol 1 in its silylated form.35,36 Subsequent treatment of this pinacol with trifluoroacetic acid at room temperature afforded 2-phenyl-cyclopentane-l,3-dione (2) in 97% yield.35,36... 

Table 4. Cyclopentanones by Pinacol-Type Rearrangement of Cyclobutanones with oc-Lithio Selenoxides...

The easiest way to do the disconnections is to reverse the rearrangement and there are two ways to do this 42a and 42b. Diol 44 can be made by pinacol dimerisation of cyclopentanone 43 while diol 45 would be the product of dihydroxylation of the alkene 46. 

The first step is straightforward—-just like the one you have just met. The pinacol dimer from cyclobutanone rearranges with the expansion of one of the rings to give a cyclopentanone fused spiro to the remaining fou(-membered ring. Reduction of the ketone then gives an... 

Tetraalkyl- and tetraatyl-ethylene glycols (pinacols) are made by reduction of ketones with active metals such as sodium, magnesium, and aluminum. The reaction is only fair for aliphatic and alicyclic ketones. Acetone, methyl ethyl ketone, cyclopentanone, and cyclohexanone all give less than 0% yields of pinacols. Mixtures of ketones are reduced to unsymmetrical pinacols. An active zinc-copper couple has been employed in the reduction of several simple olefinic aldehydes to dieodiols, e.g., crotonaldehyde to dipropenyl glycol,... 

The stereochemistry of the reduction of substituted cyclopentanones has not been studied in great detail and it is not possible to make detailed generalizations regarding their course. Based on limited experimental data, it appears that metal-alcohol and metal-NHs-proton donor reductions of alkyl-substituted cyclopentanones will usually afford a greater than equilibrium ratio of the thermodynamically more stable alcohol. In the absence of an added proton donor, pinacol formation may be a problem and different metals may give different ratios of epimeric alcohols. 

The rate of oxidation of diols with lead tetraacetate depends strongly on their configurations cis diols react 200-3000 times faster than trans diols [1154], and the racemates of certain diols react about 15 times faster than the meso forms [1154]. The rates of oxidation of pinacols prepared from cyclopentanone, cyclohexanone, and cycloheptanone are in the ratio... 

In contrast, when the X group is oxygen, the reaction appears to follow the cationic alkene cylization-pinacol-like rearrangement pathway. In this case, reaction of an optically pure acyclic oxygen-containing analog of the system in Scheme 71 leads to a product with preservation of optical activity.This reaction protocol, which accomplishes an overall ring expansion with a tetrahydrofiiran annulation, has been examined for ot-hydroxy-cyclopentanones and -cyclohexanones (Scheme 73 and equations 29-31). ... 

The symmetrical diols derived from cyclopentanone and cyclohexanone can similarly be converted to ring-expanded ketones in good yield. The diols from reductive coupling of cycloheptanone and cy-clooctanone give mainly the corresponding dienes in aqueous acid, especially when heated, but Chris-tol found that pinacol rearrangement is strongly favored even for these materials when cold concentrated sulfuric acid is used as the solvent. 

The situation is more complicated in the case of the pinacolic rearrangement of the isomeric cyclopentane-1,2-diols (Bunton and Carr, 1963b). In aqueous perchloric acid, cis-1, 2-dimethylcyclopentane-l,2-diol (19) is converted into a mixture of the polymeric cyclopentadiene). No O18 is found in the unrearranged cis-diol. The trans-1,2-dimethylcyclopentane-1,2-diol (20), on the other... 

Ghosh, S. and Patra, D., A convenient route to vicinally substituted cyclopentanones via pinacol type rearrangement of cyclobutanes. Tetrahedron Lett., 34, 4565, 1993. 

Patra, D. and Ghosh, S., Regioselectivity and stereospecificity in a contrastereoelectronically controlled pinacol rearrangement of alkoxycyclobutane derivatives. A novel route to vicinally substituted cyclopentanones,/. Org. Chem., 60, 2526, 1995. 

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