Conformation, representation

Fig. 1. Conformational representation of the piperidine ring of morphine (1) and analogues meperidine (7, R = H, R = COOC2H ) and alphaprodine (7, R = R = 0CC2H ). The chiral center of interest in stmcture (7) is starred (see text).

Draw clear conformational representations of the / -pyranose forms of each of the following carbohydrates ... 

Spherical polar coordinates are used for conformational representation of pyranose rings in the C-P system. Unlike the free pseudorotation of cyclopentane, the stable conformations of cyclohexane conformers are in deeper energy wells. Even simong the (less stable) equatorial (6 = 90 ) forms, pseudorotation is somewhat hindered. Substitutions of heteroatoms in the ring and additions of hydroxylic or other exocyclic substituents further stabilize or destabilize other conformers compared to cyclohexane. A conformational analysis of an iduronate ring has been reported based on variation of < ) and 0 (28), and a study of the glucopyranose ring... 

Alternate Conformational Representations. In some instances, the C-P formalism does not provide the most economical description, as exemplified by the six-atom ring of dihydropyran. Like cyclohexene, dihydropyran has a double bond that enforces essential coplanarity on four contiguous ring atoms. While the C-P puckering description for... 

Thus this model provides a very serviceable conformational representation for the interpretation of the unusual behavior of modified polyethylenimines. 

See for instance Carath6odory, C., Conformal Representation, Cambridge University Press, 1952, Chapters 10 and 11. 

TABLE 4. Decalones, conformational representations, octant projection diagrams and predicted and observed n — jr CD Cotton effects... 

Conformational representations of the two different forms of glucose are drawn in the usual way. An oxygen atom is present in the six-membered ring, and we are told in the problem that the ring exists in a chair conformation. 

Table I. (Also continued on next page) Decalones and their conformational representations, Octant Projection Diagrams, predicted and observed n - x CD Cotton effects.

The most definitive conformational representation of a calixarene is derived from X-ray crystallography, although even these pictures sometimes fail to... 

The yellow colour of allosecurinine in solution [u.v. (max) 342 nm in hexane] as well as in the crystal suggests the presence of the transannular interaction between the nitrogen lone pair and the conjugated diene in both states. Interestingly, available chemical and physical (including A-ray) evidence supports the conformational representation (33) for securinine (diastereomeric with allosecurinine), and this alkaloid also exhibits the transannular interaction [u.v. (max) 328 nm in hexane]. On the basis of the results on securinine and allosecurinine, it may be conjectured that this interaction is independent of conformation. Further work is required in particular, an A-ray structure determination of securinine as the free base would be welcome. 

The conformational drawing does not clearly show that attack will follow the author s predictions. This simple model suggests selectivity but it does not accurately reflect the real conformation, and its results are suspect. If the ketone is treated as part of a norbomyl-like system, exo attack is from the bottom and should be preferred, which is opposite to that predicted by the author. Both the 2D representation and the 3D drawing are shown with these comments. These drawings suggest that attack by the Grignard should be from the bottom of the molecule (as it is drawn). This emphasizes that it is important to draw the molecule in the correct conformational representation. 

These chains are depicted in their Fischer projections rather than as conformational representations for the specific reason that the planar zigzag orientation of the chain is clearly favored only in chains having the arabino stereochemistry ( ) For the other configurations, the conformational preference is for non-extended conformations that may be conformational mixtures separated by low energy-barriers. It is clearly naive to depict exact molecular orientations for putative transition states in such reactions. Nevertheless, the model depicted here for interpreting the course of the reaction, which is in accord with the general model proposed by Trost (10) for diastereofacial selectivity in additions to alkenes having an adjacent asymmetric center, has predictive utility in these reactions. 

Fig. 1 Various representations of the monosaccharide Glucose (a) extended Fischer projection (b) Haworth projection (c) pyranoside Cj chair representation (d) 3D conformational representation. The numbers indicate the standard designations of each carbon atom in the monosaccharide. This numbering is also used to identify the OH groups, OHn being the OH group bound to Cn...

Kober, H., Dictionary of Conformal Representations, Dover Publications, New York, 1957. 

In the simulation of viscoelastic flow, a significant numerical problem, the so-called High Weissenberg Number Problem (HWNP), often occurs with loss of convergence of numerical algorithms. In order to alleviate the problem, we have, as the first attempt, implemented the conformation tensor Positive Definiteness Preserving Scheme (PDFS) by Stewart et al. [28], and then adapted and implemented the Log-Conformation Representation (LCR) approach by Fattal and Kupferman [7] in the viscoelastic two-phase flow solver in FS3D. 

To simulate the viscoelastic flow, the Oldroyd-B model has been implemented in the VOF-code. Stabilization approaches, such as the Positive Definiteness Preserving Scheme and the Log-Conformation Representation approach have been adapted and implemented in the code to stabilize the simulations at high Weissenberg numbers. The collision of viscoelastic droplets behaves as an oscillation process. The amplitude of the oscillation increases and the oscillation frequency decreases when the Deborah number becomes larger. The phenomenon can be explained with the dilute solution theory with Hookean dumbbell models. An increase of the fluid relaxation time yields a decrease of the stiffness of the spring in the dumbbell and restrains the deformation of the droplets. In addition, with larger the viscosity ratio the collision process is more similar to the Newtonian one since the fluid has less portion of polymers. 

Fattal, R., Kupferman, R. (2005). Time-dependent simulation of viscoelastic flows at high Weissenberg number using the log-conformation representation. Journal of Non-Newtonian Fluid Mechanics, 726(1), 22-21. 

Another linear notation is the ROSDAL coding. It has very elaborate stereochemical syntax elements, which make it useful for the definition of organic as well as inorganic compounds. It covers even conformation representations. ROSDAL strings are extensively used in the Beilstein XFIRE system. 

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