Twist-chair conformation

Fully saturated seven-membered heterocycles with one or two heteroatoms are normally in mobile twist-chair conformations (Section 5.17.1.1, Chapter 5.18) (b-77SH(2)123). Annelation and the introduction of exocyclic double bonds can have profound effects oxepan-2-one, for example, is in a near chair conformation (67JA5646). 

As ring size increases, there are progressively more conformations that need to be considered. For cycloheptane, four conformations have been calculated to be particularly stable. NMR investigations indicate that the twist-chair is the most stable. Various cycloheptane derivatives adopt mainly twist-chair conformations. ... 

The total spread in energies calculated for the four conformations is only 2.7 kcal/mol. The individual twist-chair conformations interconvert rapidly by pseudorotation. 

Electron diffraction measurements of gaseous 1,2,4-trioxolane point to either a C2 symmetry, as depicted at the bottom of Appendix A, or a Cj symmetry, corresponding to an envelope conformation. However, consideration of steric interactions favors the twisted chair conformation (288c) for the molecule. Table 5 presents the structural parameters derived from electron diffraction, assuming that both C—O distances are equal and that the H atoms are symmetrically placed with respect of the neighboring atoms. Assumptions on the vibrational amplitudes of the ring bonds do not affect the calculated values of Table 5, except for the H—C—H angle . ... 

Among the possible conformations of oxepane the boat, twist-boat and chair forms were calculated to be of higher energy than the predominant twist-chair conformations (which were found to exist in approximately equal proportions). 

Further experimental confirmation of the preferred geometry of oxepane (1) has been obtained from its IR and Raman spectra. These vibrational and rotational spectra have been rationalized in terms of the lowest energy twist-chair conformations pseudorotating via a chair form (771A2866,2876). 

Exercise 12-22 If the twist-chair conformation 8 were rigid rather than flexible, how many different monochlorocycloheptanes would you expect (a) excluding mirror-image isomers and (b) including mirror image isomers ... 

The conformational analysis of oxepane conducted within the MM2 and MM3 force fields shows a preference for the twist-chair conformation, which can be explained based on nonbonding interactions between hydrogen atoms <1994JST247>. 

The absence of Bohlmann bands in the infrared (IR) spectrum indicates that the nitrogen lone pairs of the di-iV-nitroso-l,4-diazepines 23 are delocalized <1995JOC7461>. While the absence of an amide moiety in the ring confers torsional freedom, the m-2,7-di-Ph groups reduce ring flexibility and the 111 NMR data are consistent with major and minor families of four twist chair conformers that project the C-7 Ph and C-6 R -substituents equatorially These interconvert by a pseudorotation that allows the C-2 phenyl and C-3 R substituents to adopt a pseudoaxial orientation in the preferred conformer family. 

Ab initio calculations at HF/6-31+G level of theory for geometry optimization and MP2/6-31+G //HF/6-31+G level for single-point total energy calculations were reported for lenthionine (1,2,3,5,6-pentathiepane). The unsymmetrical twist-chair conformation is the most stable conformation of lenthionine. The unsymmetrical twist-boat form is 33.0 kj mol-1 less stable. Conformational racemization of twist-chair conformation can take place via plane symmetrical chair geometry as transition state and requires 50.7 kj mol 1 <2003JMT225>. 

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