Conformations cyclopentane

A new method for assessing the minimum energy, in a molecular mechanics context, of a molecule in a given conformation has been devised. This relatively simple approach can be readily adapted to impose constraints e.g. symmetry) on the molecule in question. It will also differentiate true minima from secondary minima in finding the most stable conformation. Cyclopentane and cyclohexane are considered explicitly. 

Synthetically useful stereoselective reductions have been possible with cyclic carbonyl compounds of rigid conformation. Reduction of substituted cyclohexanone and cyclopentan-one rings by hydrides of moderate activity, e.g. NaBH (J.-L. Luche, 1978), leads to alcohols via hydride addition to the less hindered side of the carbonyl group. Hydrides with bulky substituents 3IQ especially useful for such regio- and stereoselective reductions, e.g. lithium hydrotri-t-butoxyaluminate (C.H. Kuo, 1968) and lithium or potassium tri-sec-butylhydro-borates or hydrotri-sec-isoamylborates (=L-, K-, LS- and KS-Selectrides ) (H.C. Brown, 1972 B C.A. Brown, 1973 S. Krishnamurthy, 1976). 

The furanose rings of the deoxyribose units of DNA are conformationally labile. All flexible forms of cyclopentane and related rings are of nearly constant strain and pseudorotations take place by a fast wave-like motion around the ring The flexibility of the furanose rings (M, Levitt, 1978) is presumably responsible for the partial unraveling of the DNA double helix in biological processes. 

FIGURE 3 12 The (a) planar (b) envelope and (c) half chair conformations of cyclopentane... 

Cyclopentane has two nonplanar conformations that are of similar sta bility the envelope and the half chair... 

Envelope (Section 3 6) One of the two most stable conforma tions of cyclopentane Four of the carbons in the envelope conformation are coplanar the fifth carbon lies above or be low this plane... 

Cyclopentane is nonplanar, and the two minimum-energy geometries are the envelope and half-chair. In the envelope conformation, one carbon atom is displaced from the plane of the other four. In the half-chair conformation, three carbons are coplanar, vdth one of the remaining two being above the plane and the other below. The energy differences between the conformers are very small, and interconversion is rapid. All of the carbon atoms r idly move through planar and nonplanar positions. The process is called pseudorotation. 

Identify the lowest-energy conformer from among those provided cyclopropane, planar and puckered cyclobutane, planar and puckered cyclopentane and chair, half-chair, boat and twist-boat cyclohexane. (If... 

Figure 4.6 The conformation of cyclopentane. Carbons 1, 2, 3, and 4 are nearly planar, but carbon 5 is out of the plane. Part (c) is a Newman projection along the C1-C2 bond, showing that neighboring C-H bonds are nearly staggered.

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... 

Fig. 8 Superimposition of inhibitors and key active site residues in influenza A virus sialidase cyclopentane-based inhibitor peramivir 34 (brown carbons, PDB - 117f), Neu5Ac2en 4 (green carbons, PDB - lf8b). Note the overlap of the carboxyl and acetamido-methyl groups of the inhibitors, and the alternative conformations of the side-chain of Glu276. To the right is shown peramivir 34 oriented as in the crystal structure...

The ring templates can be further used to construct larger, multicyclic systems as illustrated in Fig. 7.8. For norbornane, two fitting conformahons of cyclopentane in the envelope conformation can be joined in order to construct the complete 3D structure of norbornane. In this case, this is the only low-energy conformahon known for norbornane due to its rigidity. 

A conformational effect was detected for the H-transfer reactions from cycloalkanes to a series of attacking radicals. The data of Table 6 show that cyclopentane is generally a better H-donor than cyclohexane. The rate ratio is generally largest for the least reactive radicals because the change in hybridization at transition state... 

The six-membered rings in these T.S.s are more flexible than the five-membered T.S.—(81) above—and need not be planar (cf cyclohexanes v. cyclopentanes). Elimination may thus proceed, in part at least, from conformations other than the syn-periplanar, with the result that the degree of SYN stereoselectivity in these eliminations may sometimes be lower than that observed in the Cope reaction. Both reactions require higher temperatures than for the Cope reaction, carboxylic esters particularly so. 

In a similar way the potential constant method as described here allows the simultaneous vibrational analysis of systems which differ in other strain factors. Furthermore, conformations and enthalpies (and other properties see Section 6.5. for examples) may be calculated with the same force field. For instance, vibrational, conformational, and energetic properties of cyclopentane, cyclohexane and cyclodecane can be analysed simultaneously with a single common force field, despite the fact that these cycloalkanes involve different distributions of angle and torsional strain, and of nonbonded interactions 8, 17). This is not possible by means of conventional vibrational spectroscopic calculations. 

Figure 4.11 (a) The folded or bent conformation of cyclobutane, (b) The bent or envelop form of cyclopentane. In this structure the front carbon atom is bent upward. In actuality, the molecule is flexible and shifts conformations constantly... 

Cyclopentane assumes a slightly bent conformation => relieves some of the torsional strain. 

Cyclopentane is flexible and shifts rapidly form one conformation to another. 

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