Conformer population change

The indirect influence of the solvent on the optical rotation due to the change in the conformational equilibrium upon solvation was studied by Polavarapu et al. [63] for (R)-epichlorhydrin. No solvent model was used, and the conformer populations in... 

The conformational principles developed earlier appear to control septanose conformations. In both solution and in crystals, TC conformations are preferred that have hydroxymethyl, alkoxy groups, and hydroxyl groups in axial or isoclinal positions [307,317,318,319,320,321,322,323, 324]. The favored TC conformations have oxepane conformations of type TCb or TCc. The equilibria between conformers are often closely balanced as indicated by the significant change in conformers populated when 0-5 of methyl /3-D- /ycero-D-guloseptanoside was methylated [319]. 

In the remainder of this paper the solvent-dependent CD of a particular complex will be considered in some detail. The example has been chosen because the variation in CD with solvent is related to a solvent-induced change in the conformer populations of a chiral ligand. 

The conformational effect of the aglycon in alkyl glycopyranosides has been examined with the methyl, ethyl, isopropyl, and tert-butyl tri-0-acetyl-/3-D-ribopyranosides, and the corresponding benzoates. In each series, there was a negligible change in conformational population as between the methyl, ethyl, and isopropyl derivatives, but the terf-butyl derivatives contained somewhat less of that con-former [IC(d)] having the aglycon axially attached, presumably because there is loss of some rotational freedom about the C-1-0-1 bond in the tert-butyl derivatives as a result of steric hindrance. 

Conformational behavior of 1-alkyl-l,2,3,4-tetrahydroazocin-2-ones was studied by using dynamic H NMR. The (—60 to 40°C) and AGf (10.3-10.9 kcal mol ) values were reasonably explained as being due to the ring inversion associated with rotation of the N—C a-bond. The temperature-dependent chemical shifts were due to population changes in the chiral azacinone ring systems, where steric hindrance due to the 3-substituent plays a major role <82JOCl564>. 

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