Conformations acyclic molecules

A particular advantage of the low-mode search is that it can be applied to botli cyclic ajic acyclic molecules without any need for special ring closure treatments. As the low-mod> search proceeds a series of conformations is generated which themselves can act as starting points for normal mode analysis and deformation. In a sense, the approach is a system ati( one, bounded by the number of low-frequency modes that are selected. An extension of th( technique involves searching random mixtures of the low-frequency eigenvectors using Monte Carlo procedure. 

An acyclic molecule has more entropy than a similar cyclic molecule because there are more conformations (cf. hexane and cyclohexane). Ring opening therefore means a gain in entropy and ring closing a loss. 

P2X Z = 2 Dx = 1.59 R = 0.11 for 881 intensities. The acyclic molecule has the planar, zigzag conformation. The terminal OH and N-OH groups are +syn and ap, respectively. There is an interesting system of O-H N hydrogen-bonds which forms a spiral along the screw-axis. Only seven of the hydrogen atoms were located. 

Table 8-1 Conformational Energy Differences in Acyclic Molecules... 

Determination of relative configurations in acyclic molecules or acyclic regions of complex molecules258 by NMR methods is a difficult task, since it requires knowledge of the preferred conformation or of conformational equilibria. It is, therefore, common practice to prepare cyclic (very often 5- or 6-membered) derivatives with restricted conformational mobility and then determine the relative configuration by means of appropriate NMR methods. On the basis of the functional groups present in the compound under investigation, it must first be decided which type of cyclic derivative is appropriate. 

For reviews of the cyclization of acyclic molecules, sec Nakagaki Sakuragi Mutai J. Phys. Org. Chem. 1989 2, 187-204 Mandolini Adv. Phys. Org. Chem. 1986, 22. Mil. For a review of the cyclization and conformation of hydrocarbon chains, see Winnik Chem. Rev. 1981,5/. 491-524. Fora review of steric and electronic effects in heterolytic ring closures, sec Valters Russ. Chem. Rev. 1982 51, 788-801. 

Eliel E, Wilen SH (1994) Stereochemistry of organic compounds. Conformation of acyclic molecules. Wiley, New York, pp 597-664, chap 10... 

These two reactions are not nearly as diastereoselective as most of the reactions of cyclic compounds you met in the last chapter. But we do now need to explain why they are diastereoselective at all, given the free rotation possible in an acyclic molecule. The key, as much with acyclic as with cyclic molecules, is conformation. 

The conformational behavior of cyclic molecules is conveniently discussed separately from that of acyclic molecules. Ring closure indeed imposes conformational constraints and limitations not existing in linear systems. 

S(XSO)2 is a planar acyclic molecule with C2, symmetry (32). The stabihty of the observed molecular conformation can be attributed to an electrostatic interaction between the negatively charged oxygen centers and the positively charged central sulfur atom. The central S-X distances are ca. 165 pm, while the 8 X distances in the XSO group are ca. 153 pm. 

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