Stereo Isomerism

Space isomerism or stereo-isomerism, due to differences only in the spatial orientation of the atoms in the molecule. 

FIGURE 7 8 Stereo isomeric 2 3 dihydroxybu tanoic acids Stereoisomers I and II are enantiomers Stereoisomers III and IV are enantiomers All other rela tionships are diastereomeric (see text)... 

FIGURE 7 10 Stereo isomeric 2 3 butanediols shown in their eclipsed con formations for convenience Stereoisomers (a) and (b) are enantiomers of each other Structure (c) is a diastereo mer of (a) and (b) and is achiral It is called meso 2 3 butanediol... 

In this section we shall consider three types of isomerism which are encountered in polymers. These are positional isomerism, stereo isomerism, and geometrical isomerism. We shall focus attention on synthetic polymers and shall, for the most part, be concerned with these types of isomerism occurring singly, rather than in combination. The synthetic and analytical aspects of stereo isomerism will be considered in Chap. 7. Our present concern is merely to introduce the possibilities of these isomers and some of the vocabulary associated with them. 

The second type of isomerism we discuss in this section is stereo isomerism. Again we consider the number of ways a singly substituted vinyl monomer can add to a growing polymer chain ... 

Assign configurations, using the sequence rule, to each chiral center of the stereo-isomeric isocitric acids and alloisocitric acids ... 

These compounds were not stuicturally but stereo-isomeric, the relation being similar to that which exists between fumaric, maleic or mesaconic and citraconic acids (p. 265), or again between the tw O diazotates of potassium (p. 283), and wdiich may be represented as follow s ... 

A substituted a,/3-unsaturated aldehyde, cinnamaldehyde, has been observed to undergo the same type of two-step 1,3-cycloaddition reaction with a cyclohexanone enamine as acrolein does, forming in this case a stereo-isomeric mixture of substituted bicycloaminoketones in excellent yield (29a,31a,31b). 

There exists in East Indian sandalwood oil an alcohol, of the formula CjHjgO, which has been named santelol, or santenone alcohol. It is closely allied to, and much resembles, the alcohol obtained by the hydration of the hydrocarbon, santene q.v.), and is probably stereo-isomeric with it. There is some difference of opinion as to the proper nomenclature of the two alcohols. According to Charabot, the naturally occurring alcohol, also obtainable by the reduction of santenone, is analogous to borneol, and should therefore be termed, if that analogy is... 

Disilenes readily add halogens14,66 and active hydrogen compounds (HX), such as hydrogen halides,63,66 alcohols, and water,27 63 as well as hydride reagents, such as tin hydride and lithium aluminum hydride.66 These reactions are summarized in Scheme 9. The reaction of the stereo-isomeric disilene (E)-3 with hydrogen chloride and alcohols led to a mixture of E- and Z-isomers, but the reaction with chlorine gave only one of the two possible stereoisomers, thus indicating that the former two reactions proceed stepwise while the latter occurs without Si—Si rotation. 

Horner and Roder examined the electrochemical reduction of the stereo-isomeric a, a -dimethylstilbenesf//2 and 113) at a mercury cathode 1,7>. The reduction mechanism was not investigated, although the suggestion... 

Es sind beide Stereo-Isomere in bezug auf C-16 entstanden ihre jeweilige Konfiguration ist unbekannt. 

In the study of substituent effect on the thermal electrocyclic ring-closing reactions of vinylallenes, two stereo-isomeric boryl-substituted vinylallenes 13 and 14 are synthesized by means of the palladium-catalyzed stannaboration of alkynes (Schemes 59 and 60).248... 

Thus two stereo-isomeric forms of 1 4-dihydroxycyclohexane (quinitol) are known ... 

In excellent agreement with the theory, benzil yields two stereo-isomeric monoximes and three dioximes ... 

In an effort to identify possible sources of the 16 alkaloids found in the skin of the Panamanian poison frog Dendrobates auratus, ants from a total of 61 terrestrial nests were analyzed [124]. The alate queens of one species of myr-micine ants (Solenopsis (Diplorhoptrum) sp.) collected at Cerro Ancon were found to contain the decahydroquinoline (-)-ds-195A (112) which was also present as a minor alkaloid in the skin of the microsympatric population of D. auratus. Moreover, from wingless ants of two nests collected at Isla Taboga and identified as Megalomyrmex silvestrU the same workers isolated the stereo-isomeric 3,5-disubstituted pyrrolizidines rfs-251 K (117) and trans-251 K (118) in the same ratio 3 1 that was present in the skin of a microsympatric population of D. auratus (Fig. 20) [124]. 

FDA Policy, Statement for the Development of New Stereo-isomeric Drugs, Washington, DC, May 1992. 

Until now the discussion has centered on the addition polymers obtained fiom unsaturated monomers by reaction of the C=C or C=0 double bond. However, polymers obtained by other methods (ring-opening polymerization, polycondensation, etc.) offer interesting stereochemical phenomena also. As a rule, in these classes of macromolecular compounds the monomer units are clearly defined, the direction of the chain is often distinguishable and the stereo-isomeric elements present in the chain already preexist in the monomer. There are, however, numerous exceptions and further clarification is called for. 

Each of the scaffolds reported in Scheme 24 can be used for the production of a stereo-isomeric sublibrary based on the appropriate peptide sequence. For example, with the sequence A-B-C-D-E and scaffold (1) two types of stereoisomeric sublibraries can be prepared. One type includes the sublibraries A and B of Scheme 26 in which within a given sequence the configuration of each residue is successively inverted thus, retaining the identical connectivity as in the parent linear peptide. In the second type 265 of sublibraries C and D (Scheme 26) the direction of the amide bond is inverted and hence the connectivity is not maintained. In most members of these sublibraries the overall conformation of the scaffold is maintained and therefore these components constitute stereoisomeric sublibraries of the parent library. Conversely, by introducing amide bond surrogates such as reduced amide bonds1465 or thioamide bonds 260,466 the conformation of the scaffolds are changed and their conformational flexibility enhanced. 

Alkylation of 68 with triethyloxonium tetrafluorborate leads to three stereo-isomeric 4.4 -pero3Q -phosphinic acid ethyl esters 69a-c. One 69a) could be isolated as a pure substance, m.p. 178—180 C the existence of the two others, m.p. 145—152 °C, is proved by H-NMR, although they could not be fully separated. 

Dithiocyanato-diethylenediamino-chromic Salts.— These compounds occur in two stereo-isomeric forms, namely, the cis- and the trans-salts. The cis-series, corresponding to the violet dichloro-salts,... 

These salts resemble closely the salts just described. The chloride of the series exists, like dichloro-aquo-triammino-cobaltic chloride, in two stereo-isomeric forms. 

The diol-compounds are readily decomposed by concentrated hydrochloric and hydrobromic acids, yielding a mixture of m-diaquo-tetram-mino-cobaltic and cis-diacido-tetrammino-cobaltic salts, a reaction which has proved useful in determining the configuration of stereo-isomeric cobalt-ammines. 

Tetrammino-platinous salts are known containing pyridine in place of ammonia. Thus, diammino-dipyridino-platinous salts have been prepared. These salts are interesting, as they exist in isomeric forms, and through them the configuration of the isomeric diacido-diammino-platinous compounds have been determined. It is well known that diacido-diammino-platinum exists in stereo-isomeric forms. These isomers react with two molecules of pyridine in such a manner that pyridine replaces acidic radicles in the complex with formation of diammino-dipyTidino-platinous salts. Depending upon which isomer is treated with pyridine, two different diammino-dipyridino-platinous salts are produced. Cis- or unsymmetrical dichloro-diammino-platinum is... 

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