Boat-chair

For cyclooctane, a total of 11 conformations have been suggested for consideration and their relative energies calculated. The boat-chair was calculated to be the most stable conformation. This prediction was confirmed by analysis of the temperature dependence of the F-NMR spectra of fluorocyclooctanes. The activation energy for interconversion of conformers is 5-8 kcal/mol. A few of the most stable conformations are shown below. 

Studies of cyclodecane derivatives by X-ray crystallographic methods have demonstrated that the boat-chair-boat conformation is adopted in the solid state. (Notice that boat is used here in a different sense than for cyclohexane.) As was indicated in Table 3.7 (p. 146), cyclodecane is significantly more strained than cyclohexane. Examination of the boat-chair-boat conformation reveals that the source of most of this strain is the close van der Waals contacts between two sets of three hydrogens on either side of the molecule. 

Fig. 3.7. Equivalent diamond-lattice conformations of cyClodecane (boat-chair-boat).

In contrast to the 1,4-dithiocin system, 1,4-dioxocin (1) is well-known and has been characterized as an olefinic compound by its spectra as well as its chemical behavior.5-6 The reason why 1,4-dioxocin in contrast to 1.4-dihydro-1.4-diazocine (see Section 1.4.) and 4//-l,4-oxazocinc (sec Section 1.12.), does not qualify as a 107r-aromatic species, is the less pronounced tendency of oxygen atoms for 7t-electron delocalization. An X-ray analysis of the 6-substituted 1,4-dioxocin 2 confirms the presumed nonplanar conformation of the 1,4-dioxocin structural element.9 The eight-membered ring exhibits a twisted boat-chair confirmation. 

P212121 Z = 4 D = 1.25 R = 0.069 number of intensities not reported. This analysis was to confirm a configuration derived from n.m.r. data. The pyranoid rings are slightly distorted 4Q, due to fusion to the trioxacyclooctane ring, which has an almost ideal, boat-chair conformation. The overall molecule has a convex, sickle conformation. There is an error in the atomic coordinates reported, which do not correspond to the structure given in the paper. 

A cascade of alkene addition reactions begin through a chair-boat-chair conformation transition state. 

At one time it was believed that cyclooctane occurs in the extended crown form and the saddle conformation as shown below but on the basis of calculations of minimum energy strain, Hendrickson (1964) and Wiberg (1965) suggested that neither of the above two forms is the correct picture. R. Srinivasav and T. Srikrishnan (Tetrahedron 27, 5, 1009-1012, 1971) showed that the molecule exists as the boat-chair form in a number of crystalline derivatives. 

This boat-chair difference is not immediately obtainable from experiment, e.g. the high-temperature conformational isomerization—and then quenching—of cyclohexane does not result in a chair — boat interconversion, but rather chair — twist-boat M. Squillacote, R. S. Sheridan, O. L. Chapman and F. A. L. Anet, 7. Am. Chem. Soc., 97, 3244 (1975). 

Introduction of a-substituents into the 3- or 7-position of 12a leads to a shift of the prevailing conformation from chair-chair (cc) to boat-chair (be), and finally to twist double boat (16) as judged by H NMR spectra (121). The changes in the H H coupling constants in these derivatives can be interpreted with skewed cc conformations as well. MM calculations, however, ruled out the latter possibility (121). The results of acetolysis of exo- and endobicyclo[3.3.1]nonan-3-yl tosylates were also interpreted with the aid of MM calculations (122). 

Twist (skew) boat boat-chair chair almost planar... 

MM calculations are in agreement with the boat-chair conformation of symmetry being slightly more stable than the crown conformation [5]. 

At this stage of the synthesis, it was necessary to resort to a second conformational control element (shifting the B ring from a chair-chair to a boat-chair conformation), which ensures the generation of the C(l)-C(2) enolate of a C(2) ketone and allows the epimerisation of the substituent at C(3)a (which will return the B ring to the chair-chair conformation). 

J. T. Groves and J. R. Olson, Inorg. Chem. 24, 2715 (1985) and previous references. Metal coordination forces the boat-chair conformation in the bicyclic azalactam portion of the tridentate ligand, alleviating severe steric interactions present in the chair-chair conformation of the free ligand. 

In the tetra-bridged phosphocavitands, the preorganized structure is imposed by the fixed boat-chair conformation of the four fused eight-mem-bered rings. Inwards (i) and outwards (o) configurations are defined relatively to the endo and exo orientations of the P=X bonds (X=0, S, electron pair), and six different stereoisomers arise from the equatorial or axial orientation of the substituents on the phosphorus atoms (Scheme 3). 

Among quinolizidine alkaloids, sparteine and its stereoisomers have been studied in detail by X-ray analysis (42-50). It was demonstrated that proper conformation was not reorganized in monohydrates (42), diperchlorates (43), or methyliodides of a-isosparteine (11) (53). Unlike in the case of a-isosparteine, in spareteine diperchlorate rings C/D appear to have a boat-chair conformation (44-46). On the basis of spectroscopy data a cis conformation for sparteine methyliodide (12) was proposed (57,52). However, radiographic examination (53) of this compound showed it to have the trans conformation (13). 

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