Stereoisomeric pairs

Some stereospecific reactions are listed in Scheme 2.9. Examples of stereoselective reactions are presented in Scheme 2.10. As can be seen in Scheme 2.9, the starting materials in these stereospecific processes are stereoisomeric pairs, and the products are stereoisomeric with respect to each other. Each reaction proceeds to give a single stereoisomer without contamination by the alternative stereoisomer. The stereochemical relationships between reactants and products are determined by the reaction mechanism. Detailed discussion of the mechanisms of these reactions will be deferred until later chapters, but some comments can be made here to illustrate the concept of stereospecificity. 

Methylmospiiimethines. The principal characters of these derivatives of the four known morphines have been given already (p. 218). Below are shown formulae for the two stereoisomeric pairs of morphines and the three stereoisomeric pairs of methylmorphimethines. -Methyl-morphimethine is also neopinemethine (p. 219), and for that reason the formula of neopine is added. 

The stereoisomeric pair, morphine and a-womorphine, on methylation of the phenolic hydroxyl at C, give rise to codeine and isocodeine respectively, and similarly and y-i omorphines produce [Pg.250]

The relationship of psychotrinc and 0-methylpsychotrine to the two pairs of stereoisomerides a) cephaeline -j- tsocephaeline, and (b) emetine + isoemetine respectively, implies that in each of these reductions one ethylenic linkage is saturated and produces one new centre of asymmetry. This ethylenic linkage is assumed to he at to C , so that on this basis the components of each of the stereoisomeric pairs a) and (b) just referred to must he epimeric pairs about C. ... 

By assuming that the heat of combustion of the cis isomer was larger than the trans, structural assignments were made many years ago for the stereoisomeric 2-, 3-, and 4-methylcyclohexanols. This assumption is valid for two of the stereoisomeric pairs but is incorrect for the other. For which pair of stereoisomers is the assumption incorrect Why ... 

The 1,3-allylic rearrangement has also been observed with several cyclic sulfoxides. For example, the interesting thermal interconversion of the bicyclic stereoisomeric pair 141... 

Oxygen abstraction from S0C12-H20 by [NbCl2(RCp)(R Cp)] led to a series of new [NbOCl(Cp)(Cp )] compounds.417 This reaction, which allowed, the synthesis of dia-stereoisomeric pairs of optically active species, could be reversed upon heating (Scheme 4). 

Two structures that constitute a stereoisomeric pair are referred to as enantiomers. The two enantiomers for alanine are illustrated in figure 3.8. These two isomers are called L-alanine and D-alanine, according to the way in which the substituents are arranged about the asymmetric carbon atom. The naming by D and l (for dextrorotatory and levorotatory see chapter 6) refers to a convention established by Emil Fischer many years ago. According to this convention all amino acids found in proteins are of the l form. Some D-amino acids are found in bacterial cell walls and certain antibiotics. 

It was now important to examine the question of a possible stereochemical influence on diperoxide formation. We have approached this problem initially by ozonizing olefins of type 2. When either cis- or trans-3,4-dimethyl-3-hexene are ozonized, presumably a single stereoisomeric pair of diperoxides can be formed. In fact, this case is complicated by the possibility of two trans-diperoxide conformers being produced. The cis-diperoxide conformers are identical. Ozonolysis of cis-3,4-dimethyl-3-hexene, 8, for example, could give the diperoxides, cis-l,3-dimethyl-l,3-diethyl-2,3,5,6-tetraoxacyclohexane, 11a, and rans-l,3-dimethyl-l,3-di-ethyl-2,3,5,6-tetraoxacyclohexane, lib, with the latter capable of existing as conformers lib and lib with trans-diaxial methyl and trans-diaxial ethyl substituents, respectively. 

We hope to work with a stereoisomeric pair of olefins of type 2 where the substituent sizes are sufficiently different to permit an evaluation of the influence of olefin stereochemistry on the reaction. Similar considerations will have to be kept in mind when choosing a more complex olefin of type 3. 

Finally, the earlier literature lists (33) the preparation of [CotetraenCl] CI2, where tetraen = tetraethylenepentamine or 1, 4, 7, 10,13-pentaazatri-decane. It was noted at that time that the compound obtained appeared to be an intractable mixture of two components. This is not surprising in view of the fact that foui, optically-active, octahedral stereoisomeric pairs are theoretically predictable—i.e. ... 

Table 1. NMR data (ppm) of stereoisomeric pairs of educts and products.

Extensive studies by X-rays, IR-, and NMR-spectroscopy (65, 66, 68) on the stereoisomeric pairs of cyclic quaternary ammonium salts showed a preferable axial approach in the quaternization of the iV-alkyl cam-phidine, 2-methylpyrrolidine, and piperidine series while for tropanes the equatorial approach has been substantiated. The increase of a band takes place in the higher region (885-900 cm i) when the iV-methyl is equatorial while the same change occurs in the lower region (840-885 cm i) when iV-methyl is axial (65). The NMR-spectra of a series of iV-methyl and iV-alkyl nortropanium salts show the signal at 7.10 r for the equatorial position and at 6.97 t for the axial one (66). 

R =Me) cyclisation is the major route yielding stereoisomeric pairs... 

Usually, the reaction leads predominantly to the more stable alcohol of a stereoisomeric pair because the reaction conditions promote equilibration by the hydride-transfer process discussed in Section 4.1. 

Databases such as the NIST Mass Spectral Library (107,216 organic compounds, 5,943 duplicates or stereoisomeric pairs detected in the 1998 version) or the Maybridge Combinatorial Chemistry Database (MayDec02CCeus, 13,410 compounds with 19 duplicates) were processed by MOLGEN-CID. 

The stereoisomeric pairs of ricinoleate, methyl isoricinoleate, and methyl esters of three additional bis-homoallylic hydroxy fatty acids were separated as their a-naphthylethyl isocyanate derivatives [696]. A silica column and a 95/5/1 hexane/ ethyl acetate/THF mobile phase generated the separation. Elution was generally complete in <30 min and resolution was excellent in all cases. 

The observations made above for the bicyclo[2.2.1]heptane systems are not unique. It is generally observed that carbon absorptions are shifted upfield by steric interactions with 7 substituents as can be seen by comparision of the A8 values for the stereoisomeric pairs of 6- and 2-methylbicyclo[3.2.1]octanes (Figure 1-2). 

Some stereospecific reactions are listed in Scheme 2.4. Examples of stereoselective reactions are presented in Scheme 2.5. As can be seen in Scheme 2.4, the starting materials in these stereospecific processes are stereoisomeric pairs and the products... 

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