Conformational considerations

The parent fulvene, 5-methylene-1,3-cyclopentadiene, was the subject of numerous calculations and conformational considerations. Both structures derived from microwave... 

The behaviour of the CD of (3R,3 R)-zeaxanthin at low temperature can also be explained on the basis of simple conformational considerations. On lowering the temperature, the most stable form becomes more populated and gives a predominant contribution to the spectrum. Obviously there may be instances where the CD as a function of temperature is much more complicated than that just discussed, one example being (3S, S S j-astaxanthin. However, the low-temperature spectra can be interpreted even here by... 

The next three sections (Sections 7.7.1, 7.7.2, and 7.7.3) cover fluorescence spectroscopy, I15-18 infrared, and circular dichroism, three powerful approaches to characterize the structure and conformational considerations of synthetic peptides. Section 7.7.1 deals with the use of fluorophores and broad aspects of fluorescence spectroscopy to characterize conformational aspects of peptide structure. In a similar manner, Section 7.7.2 covers a broad aspect of the uses of infrared (IR) techniques to study peptide conformations 19-22 Many IR techniques are discussed, as are approaches for the study of specific peptidic structures including amyloid, p-turn, and membrane peptides. Finally, there is a section on circular dichroism (Section 7.7.3) that covers the major issues of concern for peptide synthetic chemists such as the assignments of a-helix, 310-helix, -sheets and P-turns, and polyproline helices 23-25 There is also a brief description of cyclic peptides. 

Hence, it was concluded that epimerization of oxindole alkaloids with a trans D/E ring junction results in only C-7 epimers, whereas that of oxindole alkaloids with a cis D/E ring junction can result in C-7 and C-3 epimers [47]. The conclusion was based on steric and conformational considerations. 

Alkyl spacers thus offer the opportunity to finely tune the interplay between the antagonist sites of ambiphilic compounds. The proximity of the phosphine and borane moieties, and thus the possible formation of intramolecular P-B interaction depend on the number of carbon atoms present in the backbone and on the Lewis acidity of the borane. Conformational considerations may also play an important role as for the ethylene-bridged systems. 

A situation analogous to that of the pyrroline derivatives also exists, according to spectroscopic data, with N-unsubstituted piperideine compounds. There is little experimental data because A 2-piperideines have not been studied as extensively as the analogous pyrrolines. The A structure has been established for some aliphatically substituted piperideines, e.g., J1(8)-hexahydropyrindene,12,13 J1(10)-octahydro-quinoline,13 and the alkaloid y-coniceine.14,15 According to conformational considerations, structures other than A piperideine could be expected more frequently in the piperideine series. The thia analog16 3 occurs in the amino form as shown by infrared spectral data and the estimation of active hydrogen. 

The conformational considerations lead us to expect that the monolayer thickness differs among the three classes that is accessible experimentally by the reflectivity techniques [94], For instance, fatty acid monolayers have thicknesses on the order of 20 A [95,96], whereas the vinyl polymer mono-layers of the first kind can have thicknesses of less than 10 A [97]. On the other hand, the other class of vinyl polymers gives rise to thicker mono-... 

Analgesics of the 4-arylpiperidine class mimic the geometry of a large portion of morphine when in the axial phenyl chair conformation 33a and were thus earlier believed capable of accommodation at the proposed receptor. This view later fell into disrepute as a result of conformational considerations and particularly the radically different structure-activity relationships in regard to N-substituents and the influence of phenolic hydroxyl between the 4-arylpiperidine and morphine groups. There is evidence that reversed esters of pethidine and other 4-phenylpiperidines with C-4 oxygen functions bind in the equatorial 4-phenyl chair conformation 33b, and receptor modifications necessary to allow uptake of both ax and eq 4-phenyl chairs are discussed... 

These and other conformational considerations discussed in this article are of obvious practical importance in the preservation of biological strue -tures at low temperatures (Smith, 1954). 

Julian, R. R., May, J. A., Stoltz, B. M., Beauchamp, J. L. Gas-Phase Synthesis of Charged Copper and Silver Fischer Carbenes from DIazomalonates Mechanistic and Conformational Considerations In Metal-Mediated Wolff Rearrangements. J. Am. Chem. Soc. 2003, 125, 4478 486. 

Conformational considerations are of special importance in the reduction of flexible monocyclic ketones. For substituted cyclohexanones, the preference of the equatorial position over the axial orientation is reported in table form148. The effect is approximately additive and conformational equilibria can be calculated from the individual free energy values of conformation... 

---