Stereoisomerism conformational isomers

The interconversion of isomers in the case of optical or geometric pairs, if structurally feasible, may only take place by the breaking of a- or 7c-bonds. However, there is a further area of stereoisomerism wherein the isomers are interconvertible by rotation about a single (conformational isomers or conformers. 

Stereoisomers have the same order of atom attachments but different arrangements of the atoms in space. Cis-trans isomerism is one kind of stereoisomerism. For example, two substituents on a cycloalkane can be on either the same (c/s) or opposite (trans) sides of the mean ring plane. Stereoisomers can be divided into two groups, conformational isomers (interconvertible by bond rotation) and configurational isomers (not interconvertible by bond rotation). Cis-trans isomers belong to the latter class. 

Conformational isomers (conformers) are stereoiso-meric forms characterized by different relative spatial arrangements of atoms that result from rotation about sigma bonds. Thus, unlike configurational isomers, conformers are interconverting stereochemical forms of a single compound. The nature of conformational and configurational stereoisomerism, as well as the role of stereoisomerism in drug activity is the subject of this article. 

Here we introduce a further category of isomerism known as stereoisomerism, in which the molecules concerned have the same molecular formula and structural formula, but their atoms are arranged differently in space. There are three types of stereoisomerism — conformational isomerism, cis— trans isomerism and optical isomerism (Figure 20.46). Configurational isomers have permanent Figure 20.46 The different types of isomerism differences in their structural geometry and cannot... 

Stereoisomerism describes isomers whose atoms are connected in the same order but differ in their spatial arrangement. Examples of stereoisomers include the relatively stable and isolable cis-trans isomers and the rapidly equilibrating (and usually not isolable) conformational ones (Sections 2-5 through 2-8 and 4-1). 

Also, 2,2,3,6-tetra-Me-5-Cl-l,3-dioxan, due to 1,3-diaxial interactions, prefers the 2,5-twist-boat form (76BSF563) the same conformation was reported for the stereoisomeric 2-Ph-4-(2 -furyl)-5-N02-6-Me-l,3-dioxanes and 2,2,6-tri-Me-4-(2 -furyl)-5-N02-l,3-dioxanes (75MI2), for 2-Alk-2,4,4-tri-Me-l,3-dioxane derivatives (78KGS1172) and for the cis isomers of 2-OR-4-Me-l,3-dioxane (R = Et, nPr, /Pr, nBu, n-CsHn) (81DOK116). The corresponding trans isomers adopt the chair conformation with di-eq substitution. The isomeric 2-OR-4,4-di-Me-l,3-dioxanes also prefer the 1,4-twisted-boat conformer (81DOK116). 

As shown in Scheme 2, the reaction of 1 with a-terpinene leads to the stereoisomeric E/Z [2+2] cycloadducts 3 in competition with the formation of the Diels-Alder products exo/endo-4 (ratio E/Z-3 exo/endo-4 = 39 41 8 12). This result conforms with investigations on the cycloadditon between 1 and 1,3-cyclohexadiene [7], An analogous rearrangement of the [2+2] isomer A to the more stable bicyclic 2-silaoctene B (Scheme 1) via a dipolar intermediate could not be observed for the homologous terpinene cycloadducts. 

Isomers are molecules which have the same empirical formulae, i.e. they have the same total composition in terms of constituent atoms, but are different in the way the atoms are bonded together or arranged in space. In this chapter we will ignore functional isomers (e.g. allylic alcohol versus aldehyde) and tautomers (e.g. ketone versus enol) and will concentrate on those forms of isomerism relating to the spatial arrangement of atoms. There are five principal forms of such isomerism in organic molecules structural, positional, geometrical, conformational and stereo. In fact, most of the discussion will concern stereoisomerism. 

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