Cis/trans relationship

Evidence for the configurations, i.e. the cis/trans relationship of a substituent with respect to the bridgehead hydrogen atoms, has also been presented. An a priori estimation of the relative stabilities of isomeric substituted indolizidines, assuming a trans junction between the rings, led to the conclusion that equatorial substituents in the six-membered ring and pseudo equatorial substituents in the five-membered ring are thermodynamically favoured. 

Figure 20-6 The structure of chlorophyll a and chlorophyll b, showing cis-trans relationships of the substituents...

Substituent effects (electronegativity, configuration) influence these coupling constants in four-, five- and seven-membered ring systems, sometimes reversing the cis-trans relationship 2 3, so that other NMR methods of structure elucidation, e.g. NOE difference spectra (see Section 2.3.5), are needed to provide conclusive results. However, the coupling constants of vicinal protons in cyclohexane and its heterocyclic analogues (pyranoses, piperidines) and also in alkenes (Table 2.10) are particularly informative. 

The cis/trans relationship is also used more generally, when the attached groups are not identical but are located on opposite sides of the same double bond in different isomers in this sense it describes the overall shape of the carbon skeleton of otherwise identical isomers (see Fig. 2.4). 

A classic prototype of these reactions is the conversion of oxiranes into thiiranes by thiocyanate ion . Inversion at both ring carbons makes the reaction stereospecific with respect to the cis/trans relationship of the substituents on the oxirane carbons (Scheme 50). 

Name D and indicate the cis/trans relationship of the substituents. Draw both possible chair conformations, and calculate the energy difference between them. 

Recently,3 the stereochemical definitions of the addition of carbenes to C-C double bonds have been summarized. The term stereoselectivity refers to the degree of selectivity for the formation of cyclopropane products having endo vs. exo or, alternatively, syn vs. anti orientation of the substituents in the carbene species relative to substituents in the alkene substrate. The term stereospecificity refers to the stereochemistry of vicinal cyclopropane substituents originating as double-bond substituents in the starting alkene, i. e. a cyclopropane-forming reaction is stereospecific if the cis/trans relationship of the double-bond substituents is retained in the cyclopropane product. Diastereofacial selectivity refers to the face of the alkene to which addition occurs relative to other substituents in the alkene substrate. Finally, enantioselectivity refers to the formation of a specific enantiomer of the cyclopropane product. 

Three classes of [2 -t- 2] cycloadditions do proceed under thermal conditions. Ketenes (R2C=C=0) undergo concerted cycloadditions to alkenes under thermal conditions because the ketene can react antarafacially with an alkene that reacts suprafacially. The two termini of the C=C tt bond of the ketene react from opposite faces of the tt bond, creating positive overlap between the orbitals at both termini of the two tt systems. The antarafacial nature of the ketene does not have any stereochemical consequences, as there is no cis-trans relationship in the ketene to preserve in the product. The alkene component of the [2 -I- 2] cycloaddition with ketenes, however, reacts suprafacially, and its stereochemistry is preserved in the product. 

The oxidation of 1,4-dihydropyridines with dimethyldioxirane provides dimeric tetrahydropyridines fused as a 1,4-dioxane. These products can be converted to useful iminium ion precursors and 2-substituted-3-hydroxy-l,2,3,4-tetrahydropyridines <97CC213>. The cycloaddition of 1,4-dihydro- and 1,4,5,6-tetrahydro-nicotinates with cyanoalkenes gives a mixture of regioisomers. No isomerization of the cis/trans relationship is observed with Z- or -but-2-enenitrile indicating a consorted cycloaddition mechanism. Low diastereoselectivity in... 

C(3a)/C(4) relationship arises from the dipolarophile approach proximal to C(4) substituent on the corresponding nitronate (Scheme 16.32). Only two- and three-atom tethered substrates have been studied in these cycloadditions. The two-atom tether leads to the fra/i5-C(3)/C(3a) relationship because it can only fold endo- during the [3 + 2] cycloaddition. However, the three-atom tether is flexible enough to react via the ejto-transition structure and to provide the cis-C (3)/C(3a) relationship. As always, the dipolarophile configuration is preserved as the relationship between C(2) and C(3) in the nitroso acetal. Unlike the double intermolecular cycloadditions or the spiro mode, the trans—trans or cis—trans relationship is established between the substiments at C(3),... 

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