Cyclopentanes conformation

Figure 2 depicts the relevant cyclopentane conformations for either di-astereomeric alcohol 48. Conformations A and C would lead to the desired cis-anti-cis tricyclodecane skeleton of kelsoene whereas conformations B and D would be responsible for the formation of the undesired cis-syn-cis product. We were convinced that the influence of the stereogenic center to which the hydroxymethyl group is attached (" ) was marginal as compared to... 

Conformations of Cyclobutane and Cyclopentane Conformations of Cyclohexane 127 Axial and Equatorial Bonds in Cyclohexane 129 Conformational Mobility of Cyclohexane 131 Conformations of Monosubstituted Cyclohexanes Conformational Analysis of Disubstituted Cyclohexanes Boat Cyclohexane 140 Conformations of Polycyclic Molecules 141... 

The most frequent cyclopentane conformations show carbon atoms that carry two substituents equally disposed above and below the ring plane (Fig. 1.2.7). These are called isoclinal (i). Substituted cyclopentanes usually do not show significant conformational preferences. cw-l,3-Dimethylcyclopentane, for example, is only 0.5 kcal/mol more stable than the trans isomer, compared with an approximate 2 kcal/mol difference between the 1,3-dimethyl-cyclohexanes. 

The most stable cyclopentane conformations, according to calculations based on thermodynamic and spectroscopic measurements, are a half-chair (symmetry point group C2) and an envelope (C.v), shown in Figure 9, with the... 

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...

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