Sofa conformation structure

We also performed a single-crystal X-ray structure analysis of this lead compound. The solid state structure of this compound depicted in Fig. 3-15 shows a half-boat-like ( sofa ) conformation with the 9-phenanthryl group in a quasi-axial or r/Mf/.v/-flagpole position, and the a, 3-unsaturated exocyclic ester in a s-cis conformation. This cleft-like conformation is advantageous for the creation of centers with a high recognition ability, since one enantiomer fits in better than the other thus leading to selectivity. 

Structural characterization of many quinolizidine derivatives has been established by X-ray diffraction. For example, this technique, in combination with spectroscopic methods, showed that (+)-2-thionosparteine 21 and (+)-2,17-dithionospartine 22 are conformationally rigid and have their lactam and thiolactam groups close to planarity, with the exception of the lactam group in 21, and that rings A and C adopt distorted sofa conformations <2005JST75>. 

The gas-phase structure of 1,3,5-trisilacyclohexane was determined by gas electron diffraction (01JMS245) and compared with the results of quantum chemical calculations (98JMS(T)91). The chair conformation, the strongly preferred conformer, is more flattened than cyclohexane due to the intrinsically large Si-C-Si bond angles. By the quantum chemical calculations also the twist and boat conformers were identified to be much less stable than the chair conformation. The chair-to-chair interconversion barrier is 5.5 kcal/mol the transition state was identified as a sofa conformation with an approximately Cs symmetry, similar to the transition state in 173 (cf. Scheme 57). 

The detailed structure of the clathrate compound formed between 2,5,5-trimethylhex-3-yn-2-ol and 4-p hydroxyphenyl-2,2,4-trimethylthio> chroman has been determined by A -ray crystallography. tra/i5 -3,4-Dibromothiochroman and analogous dibromo-naphthothio-pyrans, prepared by addition of bromine to the appropriate A -thio-chromenes, may exist in sofa conformations. The dibromo-compounds undergo ring-contraction reactions with alkalis, yielding benzo[ ]thiophen derivatives. ... 

Figure 6. Analysis of molecular motions of the columnar phase of a tetrabenzocyclophane derivative by 2D exchange NMR. Top molecular structure and schematic diagram of the motion. The first sofa con-former is converted by pseudorotation into a second sofa conformer, this first step being followed by a proper rotation of the molecule. Bottom left deuter-on 2D exchange spectrum of the phenyl-ring deuter-ated compound showing the ridge pattern characteristic of 90 jumps. Bottom right stacking of the molecules in a column. For details see [97].

Bisquinolizidine alkaloids have also been widely studied by this technique. For instance, the crystal structure of (—)-A16(17 ,-dehydrolupaninium perchlorate 23 was obtained from sealed-tube and synchroton X-ray diffraction data, and showed that the A, B, C, and D rings assume distorted half-chair, chair, distorted sofa, and chair conformations, respectively it was also used to determine the most precise dimensions so far known for the iminium group <1999JST245>. The crystal structure of quinolizinium hexafluorophosphate has also been studied <2001CSC174>. 

The structure of 8-bromotetra-0-methyl-(+)-catechin (161) has been determined. The conformation of the heterocyclic ring in the crystal is between a C-2, C-3 half-chair and a C-2 sofa arrangement. The 2R,1R absolute configuration of (-l-)-catechin is confirmed (78CC695). 

The X-ray structures of two anthraquinone derivates of 1,3-dioxane were published. In both 1 -methoxy-4-(2-methylprop-2-enyloxy)-2-[(2i ,6i )-4,4,6-trimethyl-1,3-dioxan-2-yl]-anthraquinone and 4-hydroxy-3-(2-methyl prop-2-enyloxy)-2-[(27 ,67 )-4,4,6-trimethyl-l,3-dioxan-2-yl]anthraquinone the 1,3-dioxane ring adopts the chair conformation and the substituents in positions 2 and 6 are in equatorial conformations (99AX(C)436). Finally, Freeman et al. (02JMS(T)43), employing both ab initio theory and density function theory, calculated the energies of chair, half-chair, sofa, twist, and boat conformers of 1,3-dioxane. 

The structures and relative energies for the basic conformations of silacyclohexane 173 were calculated using HF, MP2, RI-DFT and MM3 methods (00ZAAC853). All predict the chair form to be the dominant conformation (boat and twist forms, also found, are less stable) the transition state for the chair-to-chair interconversion consists of a sofa-like half-chair conformation (cf. Scheme 57). The steric energy of the transition state is 6.57 kcal/mol higher than that of the chair conformation. 

Figure 3.2.3 Two hundred and thirty-seven crystal structures of 4-en-3-one steroids are known, 191 of which have conformations ranging between the la,2(3 half-chair and the la sofa. The remaining 46 steroids have an inverted half-chair conformation.

---