Cycloheptane twist chair

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. ... 

X-ray analysis of the 1,6-diol derivative (15) of the bicydo[4.4.1 ]-undecane ring system revealed the often calculated (9a,81) but rarely observed twist-chair cycloheptane ring. MM calculations confirmed that the observed conformation of 15 is the global minimum conformation (119). 

Possible conformations for cycloheptane include the comfortable appearing chair form, 7. However, this form has eclipsed hydrogens at C4 and C5 as well as nonbonded interactions between the axial-like hydrogens on C3 and C6. The best compromise conformation is achieved by a 30°-40° rotation around the C4-C5 bond to relieve the eclipsing of the hydrogens. This spreads the interfering hydrogens at C3 and C6 and results in a somewhat less strained conformation called the twist chair. The twist chair, 8, is very flexible and... 

After a short qualitative introduction to the principles of dynamic nuclear magnetic resonance spectroscopy , the proposed interconversion processes for cycloheptanes and cycloheptenes are explained in detail. According to calculations, the most favourable conformation for cycloheptanes seems to be the twist chair cycloheptenes prefer the chair form. Possible conformational processes for chair and boat forms are discussed and illustrated. 

P2i2j2 Z = 4 D, = 1.20 R = 0.041 for 1,929 intensities. The septan-oside has a distorted, twist-chair conformation, TCe,o. with the symmetry axis for the corresponding cycloheptane conformation passing through C-4. The distortion is toward the Ci conformation, similar to that observed in 5-0-(chloroacetyI)-l,2 3,4-di-0-isopropylidene-a-D-glucoseptanose. The two dioxolane rings have envelope conformations. 

Semi-quantitative potential-energy curves for pseudorotation within chair and boat forms of 1,1-dimethyl- and 1,1,2,2-, 1,1,3,3-, and 1,1,4,4-tetramethyl-cycloheptanes have been constructed interconversion of the preferred twist-chair forms of the latter is calculated to proceed via the twist-boat form rather than by pseudorotation within the chair family. 

Cycloheptane is found in two main groups of conformers the chair form (including the half-chair) and the boat form (including the twist boat), with the chair form lower in energy by about 7 kcal/mol (ap-... 

Cycloheptane and cyclooctane data on the thermal properties are also given in Table 3.1 They show little change from the cyclopentane and cyclohexane properties. Again, there is no indication of increasing amounts of conformational entropy in the transition entropies. For cyclooctane in solution H and NMR could prove ring-inversions and pseudorotation among the boat-chair conformations through the twist-boat-chair intermediate to very low temperatures (100 K). Only about 6% of the cydooctane could be found at about 300 K in the other three crown-family... 

This flattening is due to the presence of an odd niimber of carbons in the ring and it means that there will be some torsion strain due to eclipsing bonds and atoms in this form of cycloheptane. Some twisting of the ring can occur to relieve this strain, but such pseudorotation may increase strain elsewhere in the molecule. This increase in strain makes conformations 49A and 49B for cycloheptane higher in energy than the chair conformations of cyclohexane. 

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