Cyclopentanone, conformation

Cyclopentane, angle strain in. 116 conformation of, 116 molecular model of, 116 strain energy of, 114 torsional strain in, 116 Cyclopentanone. TR spectroscopy of, 731... 

Similar to cyclohexanones, substituted cyclopentanones also adopt a conformation with the substituents in a sterically favorable position. In the case of 2-substituted cyclopentanones 1 the substituent occupies a pseudoequatorial position and the diastereoselectivity of nucleophilic addition reactions to 1 is determined by the relative importance of the interactions leading to predominant fra s(equatorial) or cw(axial) attack of the nucleophile. When the nucleophile approaches from the cis side, steric interaction with the substituent at C-2 is encountered. On the other hand, according to Felkin, significant torsional strain between the pseudoaxial C-2—H bond and the incipient bond occurs if the nucleophile approaches the carbonyl group from the trans side. 

Bis(4-hydroxyphenyl) sulphoxides butadienyl - see Butadienyl sulphoxides carbamoyl - see Carbamoyl sulphoxides chiral - see Chiral sulphoxides complexes of 567-573 conformational analysis of 84-89 cyclic - see Cyclic sulphoxides cyclopentanone - see Cyclopentanone sulphoxides... 

Figure 4. Contribution of the enantiomeric ring conformers to the cyclopentanone n-tr Cotton effect.

Steric control elements are also important for the diastereoselectivity in alkylations of mono-cyclic cyclohexanone enolates. However, electronic control becomes more evident in these systems compared to monocyclic cyclopentanone enolates The flexibility of the six-membered ring system, and the large number of possible ring conformations, makes predictions of the diastereoselectivity difficult. In general, one may conclude that the diastereoselectivity in alkylations of enolates derived from monocyclic cyclohexanones is not as high as in alkylations of cyclopentanone enolates. The syntheses of compounds 21-27 demonstrate the effect of substitution in each position of the six-membered ring49,61 -7°. 

The C2 conformation has been found for cyclopentanone (7< )268, while succinic anhydride (19) is planar269. However, in tetramethylsuccinic anhydride270 and tetrafluorosuccinic anhydride271 conformations with non-planar rings were observed. 

Mn(II)-catalyzed oxidation of cyclic ketones with lead tetraacetate is zero order in the oxidant. The order of reactivity is cyclohexanone > cyclooctanone > cycloheptanone cyclopentanone. The reactivity has been analysed in terms of the conformation.80... 

Peterfy, K. and Garcia-Garibay, M.A. (1998) Generation and reactivity of a triplet 1,4-biradical conformationally trapped in a crystalline cyclopentanone. Journal of the American Chemical Society, 120, 4540 1541. 

On the other hand, lithium enolates derived from substituted endocyclic ketones have largely been exploited in the synthesis of steroids since the regioselectivity of their deprotonation can be controlled and high levels of 1,2- and 1,3-stereoselection occur9,418. The control is steric rather than electronic, with the attack directed to the less substituted ji-face of the enolate for conformationally rigid cyclopentanones, whereas stereoelectronic control becomes significant for the more flexible cyclohexanones. Finally, an asymmetric variant of the formation of a-branched ketones by hydration of camphor-derived alkynes followed by sequential alkylation with reactive alkyl halides of the resulting ketones was recently reported (Scheme 87)419. 

It can be seen from model observations that this hairpin conformation is only then optimally attained, if both side chains are bonded to a cyclopentane, cyclo-pentene, cyclopentanone or cyclopentenone. Therefore the chances to obtain a final molecule showing good biological activity seemed to be optimal, when the prostanoid ring system is kept intact, while the ring substitution is varied. 

The rate of the ring-opening reaction of 5, " and other substrates have been determined using an indirect method for the calibration of fast radical reactions, applicable for radicals with lifetimes as short as 1 ps/ This radical clock method is based on the use of Barton s use of pyridine-2-thione-Al-oxycarbonyl esters as radical precursors and radical trapping by the highly reactive thiophenol and benzeneselenol/ A number of radical clock substrates are known/ Other radical clock processes include racemization of radicals with chiral conformations, one-carbon ring expansion in cyclopentanones, norcarane and sprro[2,5]octane, a-and p-thujone radical rearrangements, and cyclopropylcarbinyl radicals or... 

Cyclopentene oxide is very unreactive (k < l( itcyclohexene oxide), an observation clearly not in accord with expected Sn2 reactivity it gives cyclopentanone as the only observed product. This ketone cannot be formed by antiperiplanar hydride displacement of bromide from the tra/is-bromohydrin, whereas the c/5-bromohydrin could easily adopt the necessary conformation for hydride migration. Almost certainly the latter is formed by slow, perhaps rate-determining 5n2 attack by bromide on the initially formed trans-bromohydrin salt, as outlined in equation (121). This secondary reaction may be more important with Lil than with LiBr, although this has not been proven (see also equation 83 and discussion of MgX2-catalyzed reactions). 

The complete absence of the cyclopentanone product (103) in the reaction above suggests that, in the intermediate rhodium carbenoid, there is a significant preference for the conformation (104), in which the ketone carbonyl and the rhodium center are syn to one another (equation 36). 

Fig. 4.30. Potential energy contour diagram for cyclopentanone. The contour in the third and fourth quadrants is the mirror image of that in the first two quadrants. The interconversion of the two equivalent C2 conformers occurs via the planar conformation.

FIR, MIR, R, MW Two-dimensional potential function determined. The two half chair (C2) conformers interconvert via the planar con formation. Barrier = 1454 cm (See cyclopentanone CjHgO) 179-181) 1... 

The results of the Bucherer-Bergs and Read reactions on cyclopentanones such as cis-bicyclo[3.3.0]octan-3-one and cis-3,4-dimethylcyclopentanone show a stereochemical course related to the preferred conformation of the cyclopentane rings.67 The stereochemical preferences of biased cyclopentanones are much less pronounced than those of biased cyclohexanones, but Edward s rule indicates trends. 

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