Stereoisomerism configurational, absolute

A further hetero-Diels-Alder reaction with inverse electron demand between o-QM 3 as the dienophile and either of the two diastereomers of spiro dimer 9 as the diene provided the spiro trimers 31 and 32 (Fig. 6.25). The absolute configuration was derived from NMR experiments. It was moreover shown that only two of the four possible stereoisomeric trimers were formed in the hetero-Diels-Alder reaction the attack of the o-QM 3 occurred only from the side syn to the spiro ring oxygen.28... 

It has been shown that CD measurement is a proper tool to determine the absolute configuration of the C-3 stereo center in corynantheine and yohimbine alkaloids (300). The chiroptical properties of stereoisomeric yohimbanes and 17-ketoyohimbanes also have been studied. Cotton effects due to aromatic and ketone absorptions have been considered in terms of the appropriate sector and... 

Configurational sequence in which the relative or absolute configuration is defined at all sites of stereoisomerism in the main chain of a polymer molecule. 

Assessing stereoselectivity in terms of descriptors is very simple as it merely involves the CIP specification of absolute and/or relative configurations of the stereoisomeric products of a reaction. This is just the task for which the CIP system was developed. The addendum of extraction of relative configuration by the like/ unlike (// ) description1 is explained in Section 1.1.8.1. 

Pyrrolizidine derivatives with at least one substituent, and particularly the pyrrolizidine alkaloid components, have one or more asymmetric carbon atoms. The stereochemistry of pyrrolizidine was clarified for the most part in the course of investigation of the naturally occurring pyrrolizidine alcohols. Here, the problems of relative and absolute configuration and of stereoisomeric transformations will be considered. 

Now we do know. X-ray crystallographic studies in 1951 confirmed that the levorotatory and dextrorotatory forms of tartaric acid are mirror images of each other at the molecular level and established the absolute configuration of each (Fig. 1). The same approach has been used to demonstrate that although the amino acid alanine has two stereoisomeric forms (designated d and l), alanine in proteins exists exclusively in one form (the l isomer see Chapter 3). 

The H-NMR spectra of the S,S and S,R 11-11 biphenyl and 8-7 aryl ether-linked dimers show subtle differences that allow both stereoisomeric series to be differentiated. Most obvious is the higher, broader range of chemical shifts of the aryl protons (6.4-7.6 ppm) observed in the spectra of the S,S bases as compared with the corresponding range (6.3-7.3 ppm) found for the S,R substances. The CD spectra of these alkaloids are complex, but a positive extremum can always be observed near 220 nm for the S,S dimers and a negative one for their S,R counterparts. A more readily accessible criterion is provided by the magnitude of the specific rotation of these compounds in chloroform, which is around 40° for the S,S and about 190° for the S,R alkaloids. Application of these rules to a number of other bases of this type allowed their absolute configurations to be established (31). 

Standardized and consistent representations of stereoisomers and stereoisomeric mixtures are similarly important for the unique representations of distinct compounds. Recent file formats such as SDF v3000 and ChemAxon Extended SMILES provide clear definition and representation of complex relative and absolute stereochemical configurations. In practice these are not widely used because many commercially available files are represented by established v2000 or SMILES formats and also because HTS compounds are mostly relatively simple low molecular weight structures. 

The molecular helices and propellers discussed above contain no center of chirality, and the P and M nomenclature is thus the only way of describing their absolute configuration. This nomenclature, however, is also applicable to some series of chiral compounds which display several centers of chirality. As will be discussed in Section 6, the presence in a molecule of two or more centers of chirality usually implies the existence of several stereoisomers, but steric reasons may reduce down to two the possible number of stereoisomeric forms. Thus, 2,3-epoxycyclohexanone contains two asymmetric carbon atoms, but for steric reasons only two stereoisomers, namely the (2S 3S)-(—)- and the (2/ 3/J)-( + )-enantiomer, exist the former is depicted in diagram XL [49]. 

Chiral cages are such examples of molecules containing several centers of chirality but existing only in two stereoisomeric (enantiomeric) forms the absolute configuration of which can be described according to helicity rules. Thus, the two enantiomers of 4,9-twistadiene are the (lS 3S 6S 8S)-( + )- and (l/ 3/J 6/J 87 )-(—)-isomer,... 

The enantioselective synthesis of phthalide 227 (the (3 )-isomer), and other substituted phthalides, and the determination of their absolute configuration has been reported <2005CH218>. In a different approach to the same compounds, 2-alkylbenzoic acids were fed to microorganisms known to affect asymmetric hydroxylation. Lactonization of the resulting alcohols yielded the phthalides, used as scents in cosmetics and soaps <1997JPP10243794>. There is sufficient interest in these optically pure compounds for a chiral gas chromatography (GC) stationary phase to have been developed to quantify stereoisomeric mixtures. A silylated /3-CD was employed... 

Fischer Projections Stereoisomerism in Cyclic Compounds Methods of Determining Absolute Configuration Asymmetric Synthesis... 

Most commonly, chiral molecules have point chirality, centering around a single atom, usually carbon, which has four different substituents. The two enantiomers of such compounds are said to have different absolute configurations at this center. This center is thus stereogenic (i.e., a grouping within a molecular entity that may be considered a focus of stereoisomerism), and is exemplified by the a-carbon of amino acids. 

The 20 amino acids commonly found as residues in proteins contain an a-carboxyl group, an a-amino group, and a distinctive R group substituted on the a-carbon atom. The a-carbon atom of all amino acids except glycine is asymmetric, and thus amino acids can exist in at least two stereoisomeric forms. Only the L stereoisomers, with a configuration related to the absolute configuration of the reference molecule L-glyceraldehyde, are found in proteins. 

The absolute configuration, i.e. S,S), of a tetrahydropyran (17) that is present in civet Viverra civetta) has been determined, using a chiral shift reagent [Eu(hfc)3] and 360 MHz n.m.r. spectroscopy in comparison with a synthetic sample of (+)-(5,5)-(17) and its methyl ester.X-Ray analysis was applied to the determination of the conformation of the violet form of cunaniol acetate (18), which was shown to have an undistorted chair form with both substituents equatorial. An e.p.r. spectral study has shown that the radical which is formed from several stereoisomeric 2,4-disubstituted tetrahydropyrans is the same, namely the cis-2-alkoxy-4-methyltetrahydropyran-2-yl radical. 

Very recently, detailed studies on the stereoisomerism of the Ni(CTH) system have confirmed the existence of some eight isomers and the results of physical measurements assist in the assignment of structures to these isomers. A number of significant stereochemical relationships can be deduced from the relative stabilities of the various possible isomeric structures (40 in all) since the complicated array of possible structures include many stereochemical features, e.g., ring conformations, absolute and relative configurations of asymmetric centers, and axial and equatorial distinctions between bulky groups. 

It would be absolutely wrong to say that the product will always be the tram isomer. The product obtained depends on the configuration of the starting alkyl hahde. The only way to predict the configuration of the resulting alkene is to analyze the substtate carefully, draw a Newman projection, and then determine which stereoisomeric product is obtained. The E2 reaction is said to be stereospecific, because the stereoisomerism of the product is dependent on the stereoisomerism of the substrate. The stereospecificity of an E2 reaction is only relevant when the P position has only one proton ... 

Cholesterol is one of the most widely occurring steroids, present in most animal tissues. While cholesterol was isolated as early as 1770, the last configurational details of its structure were not known until 1955. Much of this difficulty in assigning an absolute strucutre is that cholesterol contains eight different chiral centers, giving 2, or 256 possible stereoisomeric forms for the basic structural unit, only one of which is cholesterol. 

---