Clinal conformation

The same authors have also studied compounds 86 and 87 having a methoxy group (Fig. 15). These compounds could exist either in the chair form with an axial (88) or an equatorial methoxy group (89), or in the twist-boat 90. Conformer 88 is disfavored sterically but is favored electronically, whereas the twist boat 90 with a syn clinal methoxy group is favored electronically and does not suffer from severe steric hindrance. It was found... 

The vicinal coupling Jl 9 of 3-4-4-3 Hz in the spectra of adlumine [124], hydrastine [125], and narcotine [126] implies a syn clinal relationship between C(l)-H and C(9)-H. Considering conformation consonant with this, together with the chemical shifts of the aromatic protons which are related to the mutual orientation of the two aromatic rings, it is possible to assign the threo-configuration to adlumine and the... 

The variation ranges of all the torsion angles are very wide from 145 ° for T5-T8, through 172°-180° for T3, T4 up to 225 °-228 ° for T2 and Tl. This means that individual enamine fragments may adopt, in respect of particular torsion angles, any conformation from syn-c ina through sjw-periplanar to anti-clinal, obviously, some of these conformations are observed more frequently than others. 

The molecular structure of the parent compound was investigated in the vapor and in the solid phase using X-ray, XN and GED methods. The reported data are shown in Table i6. In both phases a clear bond length separation could be detected with a localized three-membered ring and its three adjacent double bonds. The symmetry-equivalent cyclopropane bonds are rather long in Csv-symmetric BUL (1.533-1,542 A), which can be explained by the common electron-withdrawing effect of the r-systems in a syn-clinal conformation. For comparison, the unaffected bonds in unsubstituted cyclopropane are 1.499 A in the crystal and 1.5l0 A in the gas phase. Therefore, the bond lengths in BUL... 

Restricting considerations to cases in which the leaving group has an axial conformation, n/ifi-elimination from the cis isomer (85) is diaxial and coplanar whereas elimination from the Irons isomer (86) is syn-clinal and is axial-equatorial. In the latter case it is necessary for the two larger groups to occupy... 

Such a mechanism requires that the major product, 1,2,4-trichlorobenzene should contain deuterium when the reaction is performed in a deuterated medium , but the amount of label found was no greater than predicted from the rate of exchange of the product under the reaction conditions . Presumably fairly extensive double-bond character is exhibited in the transition state for bimolecular elimination from the benzene hexachlorides and the abnormally high activation energy for the beta isomer is most plausibly attributed to the lack of coplanarity in the iy -c//nfl/-elimination or the high energy and low population of the boat conformer required for jy/i-elimination. Alternatively, a sparsely populated conformation for a concerted syn-clinal-... 

In the rigid 2,3-dihalogenonorbornane system, the only possible eliminating conformations in the substrate are planar syn and anti-clinal. Rate studies on the dibromo and dichloro substrates , the use of deuterium tracers "... 

The pyrolyses of esters and xanthates involve six-membered cyclic transition states and these are presumably almost strainless and chair-shaped rather than planar and eclipsed. Thus, they are more realistically regarded as occurring from syn-clinal rather than a syn conformation of the eliminating fragments in the substrate. 

For four- or five-membered cyclic transition states, pyrolysis only leads to beta-elimination if the C -H and C -X bonds can eclipse so as to ensure the necessary proximity for reaction. The syn nature of the amine oxide decomposition has been demonstrated by elimination from the diastereoisomeric pairs of the 1,2-diphenylpropyl system. In the cyclohexyl series, an eclipsed transition state is only achieved if elimination proceeds through the boat conformer and consequently the pyrolysis of 1-methylcyclohexylamine oxide gives an almost quantitative yield of the less stable exo-olefin. This orientation contrasts markedly with that of the six-membered cyclic transition states of the corresponding esters and xanthates which yield predominantly the endo-olefin, syn-clinal stereochemistry being preferred. For all the other ring... 

The importance of coplanarity of the eliminating fragments is emphasised by the large difference in the Arrhenius activation energies for elimination from cw-4-r-butylcyclohexanol (E (cis) = 21 kcal.mole" ) and rra/iJ-4-r-butylcyclo-hexanol (Epitrans) = 38 kcal.mole ) . In the trans isomer, elimination must occur from a diequatorial syn-clinal conformation, or a diaxial conformation with the bulky r-butyl substituent in an unfavourable axial conformation, or an initial epimerisation to the cis isomer, or via a boat conformation, all possibilities which involve much higher energy intermediates than the diaxial elimination from the cis isomer. The products of elimination from the four stereoisomers of 1-decalol are also consistent with anti stereospecificity ... 

H-NMR spectroscopic analysis of purine-like C-nucleosides revealed the presence of a hydrogen bond between N-1 and OH-5, an indicator of syn- over a/i//-conformational preference, in compound 6, but not in 1 The bicyclic deoxynucleoside derivatives 8 have been subjected to conformational analysis by H-NMR spectroscopy, to show that a r-exo/2 -emfo-arrangement is preferred and that the acetoxy group at C-5 is axially disposed, ie., the torsion angle y is in the unusual jjn-clinal range. ... 

It is useful to discuss the stereochemistry of bimolecular elimination reactions in terms of the H—C-C—L dihedral angle (Figure 10.18). Inan anti-periplanar conformation, the dihedral angle H-C 3-Cq,-L is near 180°, while it is near 0° in a syn-periplanar conformation. If dihedral angle is exactly 180°, the conformation is anti-coplanar, while it is syn-coplanar if the dihedral is exactly 0°. An anti-clinal conformation has a dihedral angle of approximately 120°, while a syn-clinal conformation has a dihedral of about 60°. 

Representing the C -L and C -H bonds as shown in Figure 10.19 makes it apparent that both the anti-coplanar and syn-coplanar conformations allow the development of n bonding without the necessity for rotation about the carbon-carbon double bond as the elimination occurs. With the anti-clinal... 

Similarly, the rate constant for dehydrochlorination of 9, which can react by syn-coplanar elimination, was found to be about eight times greater than the rate constant for 10, which can only react from an anti-clinal conformation (Figure 10.20). ... 

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