Substituents trans

There is kinetic evidence that the migration step in these base-catalyzed rearrangements is concerted with ionization. Thus, in cyclopentane derivatives, the rate of reaction depends on the nature of the trans substituent R, which implies that the migration is part of the rate-determining step.26 27 28 29... 

Figure 5 Steric repulsion between ruthenium-carbene complex with the silyloxy group in trans-substituents.

In complexes of symmetrical fran-s-olefins where the positions of all four substituents have been determined, it is possible for one set of trans substituents to be bent back more than the other set. This twist of the CR2 groups about the C=C bond would not be obvious from an inspection of the a and /3 angles. This is true in compound XIX (Table I) where the CFs groups are bent back more than the F atoms 7 = 132(1)° and 120(1)° for F and CF3, respectively. It is difficult to ascribe this effect solely to either steric or electronic causes. It seems reasonable that certain groups bend back more for electronic reasons, but in this instance it is the bulkier group that is bent back more. It is not possible to predict on electronic grounds whether F or CF3 should bend back more. 

Figure 2-12B. The ring junction between rings A and B is trans in some steroids, cis in others. The junctions between B and C and C and D are normally trans. Substituents that lie above the plane, as drawn in Figure 2— 12C, are named p, those below the plane, a. The 3-OH group in cholesterol (Figure 2-12C) is the -configuration, and it is this group that may form ester linkages. The composition of the plant sterols is given in Figure 2-13. Part of the sterols in natural...

A ring flip converts two adjacent axial substituents to equatorial substituents, and vice versa. As in Problem 4.29, no two adjacent trans substituents can have an axial-equatorial relationship. 

A cis alkene gives an epoxide with cis substituents. A trans alkene gives an epoxide with trans substituents. 

The trans methyl groups in frans-2-butene. become trans substituents in the epoxide. Addition of an O atom from either side of the trigonal planar alkene yields an equal mixture of two enantiomers—a racemic mixture. 

Asymmetric intramolecular allylation of imine 39, with a chiral auxiliary on the imino nitrogen, was attempted with 2 equiv. Lewis acids (Eq. 17) [18]. When ZrCU was employed as a Lewis acid, of four possible diastereomers of /3-aminotetrahydro-furan (40), the only isomer obtained was that with trans substituents on the tetrahy-dropyran ring. 

If, for example, there are two trans substituents, as in 4, the molecule is chiral. The configurations of the stereogenic centres of this trans compound are (IR,2S) and 4 is therefore (lf ,2S)-2-chlorocyclopropanecar-boxylic acid. A trans cyclopropane in which both substituents are the same is also chiral, and an example is shown in 5. 

Caution is required in extrapolating the conclusions from one particular set of reactants and conditions to other closely related systems. The chimeric nature of CoH, reacting as a free-radical or an ionic species, can depend both on the nature of the equatorial ligand and on the trans substituents, particularly if migration of the hydrogen atom over the surface of the equatorial ligand plays a role in the insertion reaction. In this case, the structure of the equatorial ligand rather than the redox potential of the metal center may control the course of the reaction. 

Thionopeptides, with the —CSNH— group replacing one or more peptide bonds, closely resemble the related peptides. The —CSNH— group usually has trans substituents the major differences are the length of the C—N bond and the size of the sulphur atom. Thionopeptides are resistant to hydrolysis by proteinases. Despite... 

As illustrated with the above example, a notable feature of this annulation method is its stereoselectivity. As in the earlier version of the method, the overall annulation process can be viewed as accomplishing the effective suprafacial exo cycloaddition of a substituted carbene to the conjugated diene (Scheme 11). However, the generality of this stereochemical control does not appear to be as impressive as in the previous oxyanion-based strategy. Thus, whereas rearrangement of the (vinylcyclopropyl)methyl sul-fones derived from ( )-l,3-pentadiene affords almost exclusively the predicted trans-substitu cy-clopentene, annulation employing (Z)-l,3-pentadiene produces a mixture of cyclopentenes in which the expected cis-substituted isomer is the minor product (Scheme 13). 

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