Stereoisomerism, types

Alkaloids of the ajmalicine type, which differ from the tetrahydro-alstonine type only a trans D/E ring junction, show quite different spectra. Instead of the fragments of mass 251 [202] and 223 [203], they exhibit an intense peak at mass 225, thus differentiating these stereoisomeric types.49... 

Fig. 10.12 Five /f5-stereoisomeric types (Types I-V) for tetrahedral (pro)molecules (Fujita 2004c, 2009b). The symbols a, b, X, and Y represent atoms or achiral (pro)ligands. The symbols p,q,r, and s represents chiral (pro)ligands, while each symbol with an overbar represents the corresponding chiral (pro)ligand with the opposite chirality. Each promolecule surrounded by a box is a prochiral and/OT pro-/fS-stereogenic one...

Only a few of the known configurational types of these alkaloids are represented on pp. K8-K9. The yohimbane skeleton contains 3 asymmetric centres. All known yohimbane derivatives are homo-chirally analogous at C(i5), having the C(i5) configuration corresponding to (155) in yohimbane. There are thus 4 stereoisomeric types derived from the following 4 parent compounds ... 

The situation with alkaloids containing the dihydro-corynantheane skeleton is similar there are four stereoisomeric types, normal (e.g. corynantheine YAAA, pseudo, alio (e.g. corynantheidine K9.5) and epiallo. Their stereochemistry has been fully discussed [2]. On the basis of this work a corynantheine type alkaloid can be allotted to one of the four classes by spectroscopic measurement, making the same assumption about Cys) configuration [2]. 

In Chapter 3 another type of isomerism called stereoisomerism, will be introduced Stereoisomers have the same constitution but differ m the arrangement of atoms m space... 

In describing the stereochemical features of chemical reactions, we can distinguish between two types stereospecific reactions and stereoselective reactions. A stereospecific reaction is one in which stereoisomeric starting materials aflFord stereoisomerically different products under the same reaction conditions. A stereoselective reaction is one in which a single reactant has the capacity of forming two or more stereoisomeric products in a particular reaction but one is formed preferentially. 

Figure 3.7 shows some early examples of this type of analysis (39), illustrating the GC determination of the stereoisomeric composition of lactones in (a) a fruit drink (where the ratio is racemic, and the lactone is added artificially) and (b) a yoghurt, where the non-racemic ratio indicates no adulteration. Technically, this separation was enabled on a short 10 m slightly polar primary column coupled to a chiral selective cyclodextrin secondary column. Both columns were independently temperature controlled and the transfer cut performed by using a Deans switch, with a backflush of the primary column following the heart-cut. 

Isoporphycene is of special interest because this structural type should exist in two stereoisomeric forms (E)- and (Z)-15, respectively, which arc possible due to the thrcc-carbon bridge linking two of the pyrrole rings. 

Although they are built from the same numbers and kinds of atoms, structural isomers have different chemical formulas, because the formulas show how the atoms are grouped in or outside the coordination sphere. Stereoisomers, on the other hand, have the same formulas, because their atoms have the same partners in the coordination spheres only the spatial arrangement of the ligands differs. There are two types of stereoisomerism, geometrical and optical. 

Another type of stereoisomerism, called out-in isomerism, is found in salts of tricyclic diamines with nitrogen at the bridgeheads. In cases where k, , and m >6, the N—H bonds can be inside the molecular cavity or outside, giving rise to three isomers, as shown. Simmons and Park ° isolated several such isomers with k, I, and m varying from 6 to 10. In the 9,9,9 compound, the cavity of the in-in isomer is large enough to encapsulate a chloride ion that is hydrogen bonded to the two N—H... 

Let us examine the properties of molecules where a central atom M is surrounded by five ligands A, B, C, D, E. We assume that the ligands are at the vertices of a trigonal bipyramid. This assumption is adequate for most pentacoordinate complexes but we ougth to mention that the description of stereoisomerization we propose could be applied if another polytopal form—the tetragonal pyramid, for example—was the stable one. The same type of description has already been undertaken for hexacoordinate octaedral complexes . ... 

Now let s turn to a different type of stereoisomerism, one that we already discussed in the chapter on nomenclature. Let s look at double bonds. Recall we indicate the presence of a double bond using the term -en- ... 

Scott was able to leverage the same type of methodology in an impressive display in which /V-methyltryptamine was dimerized directly to afford chimonanthine (7) (Scheme 9.2b) [9c]. Deprotonation of the indole 1H proton with methyl Grignard followed by treatment with FeCl3 accomplished the singleelectron oxidation and dimerization of the indole moiety. The racemic and meso stereoisomeric products were obtained as a mixture in 19 % and 7 % yields, respectively. Takayama later found hypervalent iodine to be a superior oxidant, affording yields of 17 % and 30 %, respectively [9j]. In both cases, however, as in the case of Hendrickson s example, stereocontrol could not be achieved. 

The second type of stereoisomerism is optical isomerism, in which two molecules that are mirror images of each other are not superimposable on each other. Consider the compound 2-butanol, CH3CH(OH)CH2CH3. It has two optical isomers, because it is not superimposable on its mirror image. 

Geometrical isomerism is a type of stereoisomerism the compounds have the same molecular and structural formulae but the arrangement of their atoms in space is different. 

Figure 1.1 shows the typical representation proposed to distinguish easily between the different types of stereoisomerism present in vinyl polymers. Viewing the main chains as lying down on a plane, there are (i) isotactic sequence of configurations, (ii) syndiotactic sequence of configurations, and (iii) no order present, atactic sequence of configurations. 

The stereochemistry deals with the study of spatial structure of molecules and its effect on the physical and chemical properties of the compound. Until recently stereochemistry was thought to be purely a theoretical area of study but since it not only affects the properties but also controls the rate of reaction, it has assumed great practical importance. Now stereochemistry is applied to study physiological properties, biochemistry, molecular biology, pharmacy and even in medicine. So the scope of the subject has become enormous. Stereoisomerism is classified into two types. 

In general, organic molecules having a central carbon atom to which are attached four altogether different moieties, as C (WXYZ) thereby rendering the molecule asymmetric, are all optically active. Such types of molecules usually exist in two stereoisomeric forms as mirror images of each other. For example ... 

Figure 3. Some major types of centers (X) and lines (X—X) of stereoisomerism. Different degrees of ligand diversity are possible for most of these types.

The primacy given by the Prelog school to chirality over stereoisomerism is equally evident when wea examine a catalog (17c) that shows various types of ligands occupying the vertices of a regular tetrahedron. The ligands were either... 

A third type of configurational interdependence exists if two elements are so interrelated that a change in the configuration of one automatically alters that of the other. This characterization applies to the two centers of 1,4-cyclohexanediol of the type Cg+g hi (5,51). Consequently only two isomers exist and a single pair of descriptors suffices for their distinction. We can remove the mutual dependence of the two elements by waiving the requirement that a line of stereoisomerism be occupied by bonds. The H and OH ligands have different distributions in the isomers about the line between C(l) and C(4), and the usual terms cis and trans express this relationship. Undoubtedly this is the most convenient description and the only one now available, but should we go further and say that the proper element of stereoisomerism in this case is this achiral line of torsion, and that its further factorization into two graphochiral centers is unwarranted ... 

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