Cyclopentane conformational analysis

Spherical polar coordinates are used for conformational representation of pyranose rings in the C-P system. Unlike the free pseudorotation of cyclopentane, the stable conformations of cyclohexane conformers are in deeper energy wells. Even simong the (less stable) equatorial (6 = 90 ) forms, pseudorotation is somewhat hindered. Substitutions of heteroatoms in the ring and additions of hydroxylic or other exocyclic substituents further stabilize or destabilize other conformers compared to cyclohexane. A conformational analysis of an iduronate ring has been reported based on variation of < ) and 0 (28), and a study of the glucopyranose ring... 

Conformational analysis of rings larger than cyclohexane is more complicated. These rings are also less common than cyclohexane, so we discuss their conformations only briefly. As can be seen from Table 6.1, the seven-membered ring compound cycloheptane has only a small amount of strain. Obviously, it is nonplanar to avoid angle strain. It does have some torsional strain, but the overall strain is comparable to that of cyclopentane. It is a fairly common ring system. 

The conformational analysis of monosaccharides, disaccharides, and ohgosaccharides is reviewed. Conformational terms are introduced through examination of the conformations of cyclohexane and cyclopentane then applied to the pyranose, furanose, and septanose rings. Concepts such as the anomeric effect are discussed. Topics of current interest, such as hydroxymethyl group and hydroxyl group rotation and disaccharide conformations are summarized. Physical methods for studying conformation are outlined. 

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 cyclopentane cyclitols have been characterized by nuclear magnetic resonance. The conformational analysis of cyclopentene and of poly-substituted cyclopentanes has been discussed. 

The pseudorotation concept has been introduced by K. S. Pitzer [49] to describe the continuous interconversions between an infinite number of indefinite puckered conformations of the cyclopentane ring. PseudorotatiOTi [50] allows cyclopentane to relieve the ring strain, which would be induced by a 120° bond angle and the torsional strain by an eclipsed methylene group, if it were to adopt a planar conformation. A barrier to the planarity of cyclopentane of 22 kJ/mol has been reported [51]. The concept of pseudorotation has been applied for the first time to sugar furanoses by Hall et al. [52] studying the conformational analysis of pentofuranosyl fluorides. 

Aspects of cycloalkane conformational analysis. A. The two puckered conformations of cyclobutane. Note how the substituents (X and Y) exchange positions when puckered forms interconvert. B. Left The envelope form of cyclopentane—the conformer contains a mirror plane in the plane of the page. Right The half-chair form of cyclopentane, looking down the two-fold rotation axis. 

This paper represents the most penetrating analysis yet performed of structures and spectra of small alkanes, and it is imperative for serious students of potential energy functions employed in conformational analysis. Here is a set of functions, with parameters, which reproduce very well observations on methane, cyclopentane, normal alkanes, and, mirabile dictu, tris-(tert-butyl)-methane. 

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. 

It is reasonable to assume that the simple concepts introduced for monocyclic rings such as cyclopentane, cyclohexane, cyclohexene, and cyclohexanone can be applied, with little change, to complex molecules containing those units. Throughout this book the analysis of simple rings will be used to understand the conformational bias and reactivity of larger and more difficult synthetic targets. 

An analysis of multipath transmission of spin-spin coupling in cyclic compounds has been made by Wu and Cremer with the help of partially spin-polarized orbital contributions. The calculations have been performed for cyclopropane, cyclopentane, bicydo[1.1.1]pentane and tetrahydrofuran. The authors have shown that the measured and calculated Vcc couplings of cyclopentane and tetrahydrofuran are averages over the pseudorotational motion of these ring molecules where each individual coupling of a conformation passed in the pseudorotation is the sum of different path contributions. 

Determination of Absolute Configuration and Solution Conformation. Abraham et have employed H and C lanthanide induced shifts produced by Yb(fod)3 as an experimental probe of the solution conformations of cyclopen-tanol and cis- and trans-cyclopentane-l,2-diol. Analysis of the LIS was undertaken in conjunction with molecular mechanics and ab initio calculations of structure. 

The pseudorotation concept was introduced to describe the continuous interconversion of puckered forms of the cyclopentane ring [104]. The same concept is applied to the furanose geometry where the Cl, 04, and C4 atoms lie in one plane and the C2 and C3 atoms lie above and below that plane. A statistical analysis of X-ray crystal structures of nucleosides and nucleotides has shown that North (N) 130 and South (S) 131 conformations are the most dominant forms, which has been the basis of the assumption of the two-state N — S pseudorotational equilibrium in solution (Fig. 2.44). 

Two specifically deuteriated methylcyclopentanes (473) and (474) have been subjected to complete iterative H n.m.r. analysis between -1-17 and — 100°C It was shown that in the presence of a methyl substituent the cyclopentane ring no longer pseudorotates freely but is rather restricted to conformations where methyl is equatorial. A diagrammatic representation of the pseudorotational circuit is presented. 

By this analysis, planar cyclopentane should be most stable, then cyclohexane, cyclobutane (which is about the same as cycloheptane), and finally cyclopropane. However, this is not the correct order for the inherent stability of cyclic alkanes. The energy inherent to each ring is shown in Table 8.1. The data in this table clearly show that cyclopropane is the highest in energy, but they also show that cyclohexane is lower in energy than cyclopentane. Indeed, cyclopentane and cyclohexane are the more stable (lowest energy) cyclic alkanes in the series 45-49 because cUc alkanes are not planar. The pseudorotation mentioned before leads to conformations that are lower in energy than... 

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