Stereochemical analyses

Nesselrodt D R, Potts A R and Baer T 1995 Stereochemical analysis of methyl-substituted cyclohexanes using 2+1 resonance enhanced multiphoton ionization Anal. Chem. 67 4322-9... 

The stereochemical analysis of chiral structures starts with the identification of stereogenic units [101], Those units consist of an atom or a skeleton with distinct ligands. By permutation of the ligands, stcrcoisomcric structures arc obtained. The three basic stereogenic units arc a center of chirality (c.g., a chiral tctravalcnt... 

Stereochemical analysis can add detail to the mechanistic picture of the Sj l substitution reaction. The ionization mechanism results in foimation of a caibocation intermediate which is planar because of its hybridization. If the caibocation is sufficiently long-lived under the reaction conditions to diffirse away from the leaving group, it becomes symmetrically solvated and gives racemic product. If this condition is not met, the solvation is dissymmetric, and product with net retention or inversion of configuration may be obtained, even though an achiral caibocation is formed. The extent of inversion or retention depends upon the details of the system. Examples of this effect will be discussed in later sections of the chapter. 

Stereochemical analysis shows that the adducts of entries 1, 3 and 6 in Table 4.14 are derived by an exo-approach of 69 with their respective dieno-philes. 

Nagasawa S, R Kikuchi, Y Nagata, M Takagi, M Matsuo (1993a) Stereochemical analysis of y-HCH degradation by Pseudomonas paucimobilis UT26. Chemosphere 26 1187-1201. 

Gourtieu, J. Deuterium NMR stereochemical analysis of threo-erythro isomers bearing remote stereogenic centres in racemic and non-racemic liquid crystalline solvents. Tetrahedron Asymmetry 2000, 11,1911-1918. 

M. N. James, and A. R. Sielecki, Stereochemical analysis of peptide bond hydrolysis catalyzed by the aspartic proteinase penicillopepsin, Biochemistry 24 3701 (1985). 

Stereochemical analysis of the benzylic alcohols formed from both (R)- and (S)-phcnylcthanc-1 -d after incubation with CYP2B4 established a strong cross-over component... 

Vogel P. in "Stereochemistry and Reactivity of Sytems Containing n-Electrons", Watson, W.H., Ed. Verlag Chemie International Deerfield Beach, FL, 1983 "Methods in Stereochemical Analysis", pp 147-195. 

One of the most sophisticated methods is the use of chiral gas chromatographic capillary columns for the direct separation of volatile enantiomers. Complexation gas chromatography with enantioselec-tive transition metal fl-ketoenolates permits the stereochemical analysis of volatile oxygenated compounds in the nanogram range with high 44,45... 

Weber R, Schurig V, Complexation gas chromatography— a valuable tool for the stereochemical analysis ofpheromones, Naturwissenschafien,7 AQi-A 5,1984. 

Konig WA, Krebber R, Evers P, Bruhn G, Stereochemical analysis of constituents of essential oils and flavor compounds by enantioselective capillary gas chromatography HRC 13 328—332, 1990. 

The first point of the stereochemical analysis is in the recognition of the sequence to which a given nucleus is sensitive the problem seems rather obvious for vinyl or vinylidene polymers where the sequence must extend equally from the two sides of the nucleus in question but for diene polymers or those containing heteroatoms, the problem is not so simple. In the present case, the methylene protons are sensitive to the structure of the even sequences, dyads and tetrads, whereas the methyl protons are sensitive to the odd sequences, triads, and pentads. 

Stereochemical analysis of H and C NMR has been accomplished for the polymers of butadiene, isoprene, and other dienes, for polymers containing heteroatoms in the main chain, and for vinyl and diene copolymers lack of space precludes discussion of these interesting studies (11, 118). 

In recent years new NMR techniques offering broad applications in stereochemical analysis have come into use. A prominent example is 2D-NMR (both 2D-resolved and 2D-correlated spectroscopy), which has been extensively applied to biopolymers (149-151). Its use with synthetic polymers has, until now, been limited to but a few cases (152, 153). A further technique, cross-polarization magic-angle spinning spectroscopy (CP-MAS NMR) will be discussed in the section on conformational analysis of solid polymers. 

The first stereochemical analysis of the polymerization mechanism of unsaturated monomers was proposed by Arcus 30 years ago (58, 269). The diaste-reomeric relationships within a single chain may be connected to monomer configuration by two factors the type of addition to the double bond (cis or trans) and the way in which successive monomer molecules approach the growing chain. For a CHA=CHB olefin the relationship between the stmcture of the monomer and that of the adding monomeric unit is illustrated in Scheme 22 (31), where cis or trans addition indicates that the new bonds are formed in the same half-space or in the two opposite half-spaces, provided there is no rotation aroimd the central C—C single bond. 

As for carboxylate-hydrogen bond donor interactions, carboxylate-metal ion interactions are expected to be most favorable with syn stereochemistry. In a survey of the Cambridge Structural Database, Carrell et al. (1988) presented a stereochemical analysis of carboxylate—metal ion interactions which supports this expectation (Scheme 1). These investigators found that the syn-oriented lone electron pair of the carboxylate... 

Fig. 34. Glu-72- Zn interactions in native carboxypeptidase A and in carboxypep-tidase A-inhibitor complexes (inhibitors have been reviewed by Christianson and Lipscomb, 1989). When substrates or inhibitors bind to the enzyme active site and interact with the zinc ion, the interaction of the metal with Glu-72 tends from bidentate toward uniden-tate coordination. The flexibility of protein-zinc coordination may be an important aspect of catalysis in this system, and the Glu-72->Zn - coordination stereochemistry observed here is consistent with the stereochemical analysis of carboxylate-zinc interactions from the Cambridge Structural Database (Carrell et al., 1988 see Fig. 4).

B-1995MI201 E. Kleinpeter in Methods in Stereochemical Analysis Conformational Analysis of Six-Membered Sulfur-Containing... 

Empirically, y-gauche effects are extremely useful in stereochemical analysis. For example, the cis- and trans-fusion of six-membered rings can be easily differentiated, since, in contrast to the trans-isomer of bicyclo[4.4.0]decane, there are gauche-butane fragments in the m-isomer (emphasized bonds) causing upheld signal shifts of the carbons involved (numbers refer to I3C chemical shifts in ppm)49-52. 

J. K. Whitcscll, M. A. Minton, Stereochemical Analysis of Alicyclic Compounds by C-13 NMR Spectroscopy, Chapman Hall, London 1987. 

W. G. Bentrude. W. N. Setzer in Phosphorus-31 NMR Spectroscopy in Stereochemical Analysis,... 

Lanthanide Shift Reagents in Stereochemical Analysis, T. C. Morrili, Ed., VCH, Weinheim 1986. 

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