Puckering conformation

FIGURE 3 11 Nonplanar ( puckered ) conformation of cydobutane The nonplanar con formation avoids the eclipsing of bonds on adjacent carbons that characterizes the planar con formation... 

Cyclobutane adopts a puckered conformation in which substituents then occupy axial-like or equatorial-like positions. 1,3-Disubstituted cyclobutanes show small energy preferences for the cis isomer since this places both substituents in equatorial-like positions. The energy differences and the barrier to inversion are both smaller than in cyclohexane. 

Cyclobutane has less angle strain than cyclopropane and can reduce the torsional strain that goes with a planai geometry by adopting the nonplanai puckered conformation shown in Figure 3.11. 

Cyclic monomers with five- and six-membered ring atoms exist in strainless puckered conformations their heats of polymerization are either negative or have small positive values due to the repulsion of eclipsed hydrogen atoms. Because the nthalpy and entropy contributions are comparable, the free energies of polymerization are either positive or may become positive at high temperatures. 

The NMR spectra of the corresponding dimethyl sulfide and dimethyl sulfone consist of two singlets at 1.27, 2.92 and 1.43, 3.80 ppm, respectively (in CDCI3), most probably implying a rapid interconversion of puckered conformations . 

The cyclophosphazene ring assumes a highly puckered conformation in both the zinc and aluminum complexes, indicating its remarkable flexibility to adapt to the coordination requirement of the metals. 

Figure 9 Molecular structure of tetramethylenedithiodimethyltetrathiafulvalene (TMDTDM-TTF) showing puckered conformation in the neutral state (Reproduced from Konarev et al.91 with permission from Elsevier)...

These molecules also have large substituents, and it might seem surprising that the radialene rings avoid puckered conformations in these species. The nonbonded repulsions are, however, reduced in these molecules because of external ring closures (13) or because two of the exocylic CC double bonds involve cumulated double bond systems (14 and... 

Presumably, in these systems the energy cost of ring puckering is greater than the stabilization due to bishomoaromaticity (Bartlett and Rice, 1963). However, if the ring is already forced into a puckered conformation (e.g. by bridging between the 3 and 5 positions) then homoaromatic stabilization becomes effective. Solvolytic studies on the bridged systems [30] and [31]... 

Balasubramanian et concluded that the Pro ring has two typical puckered conformations (1) conformation A characterized by negative values of %1 (Cq-exo) (found in residues having a-helix-type torsion angles) and (2) conformation B characterized by positive (C7-endo) (found in residues having collagen-type torsion angles). 

Energy profiles in Figure 14 also reveal that planar furanose forms are often of lower energy than puckered conformers. For example, relative conformational energies determined for 7 with the 3-21G basis set indicate that the planar conformer is more stable than the Eq conformer in 8, the planar conformer is calculated to be more stable than. These observations suggest that the conformational dynamics of some furanose rings may not be completely described by pseudorotation in these cases,conformer interconversion may occur by both inversion and pseudorotational pathways, with the latter being the more preferred route. 

In a metal complex (102), five-membered chelate rings fonr d by 1,2-dia-mines exist in two puckered conformations, the enantiomeric X and 5 forms 36 and 37. For an achiral ligand in a bis or tris complex, steric interactions between rings are present, and the two conformations are no longer equivalent energetically. 

The Watson and Crick model for DNA as a double helix is only a generalized model to describe much more complex structures. Along with the typical double helix there exist structural elements such as supercoils, kinks, cruciforms, bends, loops, and triple strands as well as major and minor grooves. Each of these structural elements can vary in length, shape, location, and frequency. Even the simple DNA double helix can vary in pitch (number of bases per helical turn), sugar pucker conformation, and helical sense (whether the helix is left-or right-handed). 

Dioxathiolane 2,2-dioxide (21) adopts a puckered conformation in the solid state, as shown by x-ray diffraction (Section 4.15.3.1). In solution, the H NMR spectrum of (21) indicates that the compound undergoes a rapid pseudorotation between twist-envelope forms (Section 4.15.3.3.1) the O NMR spectra of simple derivatives also indicate a rapid conformational equilibrium by pscudorotation, although substitution may act as a barrier to complete ring inversion (Section 4.15.3.3.3). The solution-phase dipole moment of 1,3,2-dioxathiolane 2,2-dioxide is consistent with a nonplanar conformation (Section 4.15.3.8). 

This mixed sugar pucker conformation is associated with an extension of the -polynucleotide chain. 

The radical cation (19 " ) of the strained bicyclo[2.1.0]pentane also has a puckered conformation, supported by one strongly coupled (flagpole) proton (flsyn = 4.49 mT). Ab initio calculations indicate that the transannular bond retains some bonding and that the bridgehead carbons remain pyramidal. ... 

Problem 9.10 Depict the flexible puckered conformation of cyclobutane in a Newman projection. 

See Fig. 9-3. The circle represents the C s of any given ring C—C bond. The other C s of the ring bridge these two C s, one to the front C (heavy line) and the other to the back C (ordinary line). This Newman projection formula reveals that the H s on adjacent C s are skewed in the puckered conformation. 

Most stable puckered conformation of cyclopentane Most H atoms are nearly staggered... 

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