Conformational analysis chair

The various conformations of cyclohexane are m rapid equilibrium with one another but at any moment almost all of the molecules exist m the chair conformation Not more than one or two molecules per thousand are present m the skew boat confer matron Thus the discussion of cyclohexane conformational analysis that follows focuses exclusively on the chair conformation... 

The principles involved in the conformational analysis of six-membered rings containing one or two trigonal atoms, for example, cyclohexanone and cyclohexene are similar. The barrier to interconversion in cyclohexane has been calculated to be 8.4-12.1 kcal mol . Cyclohexanone derivatives also assume a chair conformation. Substituents at C2 can assume an axial or equatorial position depending on steric and electronic influences. The proportion of the conformation with an axial X group is shown in Table 4.4 for a variety of substituents (X) in 2-substituted cyclohexanones. 

An interesting case of conformational analysis comes to play when we consider a six-membered ring (cyclohexane). There are many conformations that this compound can adopt. You will see them all in your textbook the chair, the boat, the twist-boat. The most stable conformation of cyclohexane is the chair. We call it a chair, because when you draw it, it looks like a chair ... 

The conformational behaviour in solution of a dermatan-derived tetra-saccharide has been explored by means of NMR spectroscopy, especially by NOE-based conformational analysis. RDCs were also measured for the tetrasaccharide in a phage solution and interpreted in combination with restrained MD simulations. The RDC-derived data substantially confirmed the validity of the conformer distribution resulting from the NOE-derived simulations, but allowed an improved definition of the conformational behaviour of the oligosaccharides in solution, which show a moderate flexibility at the central glycosidic linkage. Differences in the shapes of the different species with the IdoA in skew and in chair conformations and in the distribution of the sulphate groups were also highlighted.28... 

The partially hydrogenated ring of dihydtocorannulene constitutes a 1,3-cyclo-hexadiene ring, a system that has been well-studied with respect to its geometry and the conformational preferences of substituents. However, the curvature of the corannulene surface introduces an additional stereochemical factor that makes the conformational analysis especially interesting. 1,3-Cyclohexadiene (23) and 9,10-dihydrophenanthrene (24) serve as models they are structurally similar systems, and their stereochemistry and conformational preferences are well documented in the literature. In both cases, the reduced ring adopts a nonplanar, semi-chair conformation of symmetry. 

Functionalization of the methyl groups in the photolysis of the nitrites of 6-hydroxy-4,4-dimethyl steroids is used as a probe for conformational analysis. It is confirmed that in the 4,4-dimethyl-19-nor-series ring A has the normal chair... 

Ab initio calculations at the MP2 level of theory of a collection of substituted 1,3-dioxanes, 1,3-oxathianes, and 1,3-dithianes have been employed to study both the position of the conformational equilibria and the validity of the Perlin effect <2005T7349>. The 7h,c coupling constant proved to be a valuable tool in conformational analysis both twist conformers, in addition to the chair and alternative chair forms, could be readily identified simply by comparing experimental Vh,c coupling constants to the corresponding calculated values in the particular forms. In addition, the Perlin and reversed-Perlin effects of the C2-H fragments, c in 1,3-dioxanes,... 

The semi-empirical theoretical study (AMI, PM3, and force field) of 2,2,5-trimethyl- 2,2-dimethyl-5- o-propyl-, and 5- T7-butyl-2,2-dimethyl-l,3-oxathiane 47-49 afforded the correct conformational equilibria as obtained by FI NMR spectroscopic conformational analysis <2001RJC1487>. The chair conformation is adopted while the substituent at position 5 is in an equatorial position. Furthermore, there was no indication for the presence of twist conformers. 

The first precise evaluation (2A, 25) of both the anomeric and the exo-anomeric effects was obtained by studying 1,7-dioxaspiro[5.5]undecane (9) (Fig. 2). With this system, conformational analysis by low temperature nmr spectroscopy was possible because each conformational change involves a chair inversion which has a relatively high energy barrier. The steric effect could also be easily evaluated, and by adding appropriate alkyl substituents, it was theoretically possible to isolate isomeric compounds which would exist in different conformations. 

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>. 

Conformational analysis of 4-oxo-6,7,8,9-tetrahydro-4//-pyrido[l,2-a]pyrimidine-3-acetates and -3-carboxylates 30 (R = H) and their mono-methylated (R = Me, R1 = H) and 6,9-, 7,9-, and 8,9-dimethylated derivatives were carried out by H and l3C NMR spectroscopy (86JOC394). At ambient temperature the 6-methyl derivatives predominantly adopt the energetically most favorable half-chair conformation with a pseudoaxial methyl group. In the other half-chair conformation a serious 1-3 allylic strain exists between the pseudoequatorial methyl group and the adjacent carbonyl group. At the 7- and 8-methyl derivatives the half-chair conformations with equatorial methyl group occur almost exclusively, but the 9-... 

Conformational analysis of some 9-chloro- and 9-bromo-6,7,8,9-tetrahy-dro-4//-pyrido[ 1,2-a]pyrimidin-4-ones 31 and their 9,9-dichloro and 9,9-dibromo derivatives was also carried out by I3C NMR spectroscopy (83JHC619). The halogen atoms in the 9-chloro and 9-bromo derivatives 31 (R = H) in the predominantly half-chair conformation occupy the pseudoaxial position. This conformer is probably stabilized by a favorable orbital interaction, while the other one, with a presudoequatorial halogen atom, is destabilized by the unfavorable dipole-dipole interaction between the 9-halogen and C(9a)=N(l) bonds. The methyl group in the 6-methyl derivatives in predominantly half-chair conformations is in the pseudoaxial position (83JHC619). 

The studies of cyclohexane and its derivatives by Hassel and co-workers in the late thirties and early forties using mainly the electron diffraction method laid the foundation of conformational analysis. In 1943 Hassel295 summarized that cyclohexane exists mainly in the chair conformation as distinct from any other possible conformation. The chair conformation will have distinguishable axial, a, and equatorial, e, substituents. (See Fig. 13). The equatorial position is the energetically favored one. Furthermore, Hassel stated that there is a rapid inversion of the ring with an associated low barrier. This motion interchanges the a and e positions with the result that a and e conformers cannot be isolated. 

The spirobis[2,4]-benzodithiepine (90) has been prepared from l,2-bis(chloromethyl)-4,5-dimethylbenzene and sodium trithiocarbonate prepared in situ and a conformational analysis has been carried out using H- and l3C-NMR, X-ray analysis and semi-empirical calculations <95JOC6335). In the solid state it exists in a chair-twisted boat conformations whereas in solution it undergoes two dynamic processes, namely pseudorotation at lower temperatures and ring inversion at higher temperatures. 

In 1937 Isbell published an important paper on the conformational analysis of aldopyranoses, in which several of the forms were depicted (see Fig. 2). They comprise a 4C, chair (I), afl03 boat (II), two half-chairs (III and IV), and a coplanar pyranose (V). He correctly favored the chair form (I) and predicted that, in the case of saccharides, the chair would tend to assume a somewhat flatter conformation than that of carbocycles such as cyclohexane because of the smaller bond angles of oxygen as compared to those of carbon (105° instead of 109.5°). We now know that such coplanarity is strongly avoided because of the strain that would be produced and because of the repulsive forces between the substituents. 

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