Stereochemistry asymmetric atoms

This time, there were no polemics and no fight over priority (neither of the principals had Keknle s driving ambition to be the first and only inventor of the theory) - in fact, in his snbseqnent papers on stereochemistry, van t Hoff was careful to point out Le Bel s contributions, and may have saved them from obscurity. In both 1874 papers, the authors asserted that the physical properties of organic compounds, especially their optical activity, could be accounted for by specifying that the molecules contained an asymmetric atom corresponding to a tetrahedron surrounded by four different groups. While van t Hoff s paper concentrated on the tetrahedron as the basis for optical activity, Le Bel s paper was more wide-ranging, and allowed other chiral shapes to be considered. 

The first four terms of the function are commonly found in molecular mechanics strain energy functions, and they are modified Hooke s law functions. The last term has been added to insure the proper stereochemistry about asymmetric atoms. A model is refined by minimizing the highly nonlinear strain energy function with respect to the atomic coordinates. An adaptive pattern search routine is used for the strain energy minimization because it does not require analytical derivatives. The time necessary to obtain good molecular models depends on the number of atoms in the molecule, the flexibility of the structure, and the quality of the starting model. 

Atom-transfer addition of primary and secondary bromide oxazolidinones to alkenes in the presence of Lewis acids has been investigated and the effects of solvent, temperature, and catalyst were determined. The best Lewis acids were found to be Sc(OTf)3 and Yb(OTf)3 and control was possible using chiral auxiliary oxazolidinones. Tertiary bromides did not react (Scheme 37). Stereochemistry of reduction of the cw-mesityl-alkene (53) with BusSnH proceeds to give the ( )-alkene (54) as the major product ( Z = 9 1). Theoretical calculations at the BLYP/6-31G level were undertaken to rationalize the stereochemistry. Asymmetric hydroxylation of the benzylic position of a range of substrates can be achieved by using a chiral dioxomthenium(VI) porphyrin (55). The oxidation proceeds via a rate-limiting H-abstraction to produce a benzylic radical intermediate. ... 

The BC rings of physostigmine are probably cis fused (cf. chimon-anthine, p. 46) since treatment of eserethole methine with acid regenerated eserethole methiodide this means that the solution to the absolute stereochemistry will be given once the configuration of any one of the asymmetric atoms is known. 

A useful catalyst for asymmetric aldol additions is prepared in situ from mono-0> 2,6-diisopropoxybenzoyl)tartaric acid and BH3 -THF complex in propionitrile solution at 0 C. Aldol reactions of ketone enol silyl ethers with aldehydes were promoted by 20 mol % of this catalyst solution. The relative stereochemistry of the major adducts was assigned as Fischer- /ir o, and predominant /i -face attack of enol ethers at the aldehyde carbonyl carbon atom was found with the (/ ,/ ) nantiomer of the tartaric acid catalyst (K. Furuta, 1991). 

Note 2. In the last four examples, new asymmetric centres have been introduced at the carbonyl carbon atom of the aldehyde or ketone that has reacted with the saccharide. When known, the stereochemistry at such a new centre is indicated by use of the appropriate R or S symbol ([13], Section E) placed in parentheses, immediately before the locants of the relevant prefix. 

The presence of asymmetric carbon atoms also confers optical activity on the compound. When a beam of plane-polarized hght is passed through a solution of an optical isomer, it will be rotated either to the right, dextrorotatory (+) or to the left, levorotatory (—). The direction of rotation is independent of the stereochemistry of the sugar, so it may be designated d(—), d(+), l(—), or l(+). For example, the naturally occurring form of fructose is the d(—) isomer. 

This chapter, however, does not deal with above-mentioned reactions of sulfoxides. Rather it is limited to asymmetric synthesis using a-sulfinyl carbanions and -unsaturated sulfoxides, specifically in which the stereogenic sulfoxide sulfur atom is enantiomerically pure. Therefore reactions of racemic sulfoxides are for the most part excluded from this review. For more general discussions, the reader is referred to other chapters in this volume and to other reviews on the chemistry of sulfoxides. Especially useful are the reviews by Johnson and Sharp and by Mislow in the late 1960s and by Oae and by Nudelman as well as a book by Block . A review by Cinquini, Cozzi and Montanari" through mid-1983 summarizes the chemistry and stereochemistry of optically active sulfoxides. This chapter emphasizes results reported from 1984 through mid-1986. 

In organic stereochemistry the terms center of chirality or center of asymmetry are often used usually they refer to an asymmetrically substituted C atom. These terms should be avoided since they are contradictions in themselves a chiral object by definition has no center (the only kind of center existing in symmetry is the inversion center). 

Reactions.—Alkaline Hydrolysis. The first total resolution of a heterocyclic phosphonium salt containing an asymmetric phosphorus atom (128) has been reported, providing ready access to optically active phospholan derivatives of value for studies of the stereochemistry of nucleophilic displacement at phosphorus.124 Alkaline hydrolysis of (128) proceeds with retention of configuration at phosphorus to form the oxide (129). Stereochemical studies in the phospholan series have also been facilitated by the X-ray investigation125 of an isomer of l-iodomethyl-l-phenyl-3-methylphospholanium iodide, which is shown to have the structure (130). 

J. Jacques, C. Gros and S. Brourcier, in Stereochemistry, Vol. 4, Absolute Configurations of 6000 Selected Compounds with One Asymmetric Carbon Atom (Ed. H. B. Kagen), Georg Thieme, Stuttgart, 1977. 

The dissection of a molecular model into those components that are deemed to be essential for the understanding of the stereochemistry of the whole may be termed factorization (9). The first and most important step toward this goal was taken by van t Hoff and Le Bel when they introduced the concept of the asymmetric carbon atom (10a, 1 la) and discussed the achiral stereoisomerism of the olefins (10b,lib). We need such factorization not only for the enumeration and description of possible stereoisomers, important as these objectives are, but also, as we have seen, for the understanding of stereoselective reactions. More subtle differences also giving rise to differences in reactivity with chiral reagents, but referable to products of a different factorization, will be taken up in Sect. IX. 

The oxidative addition of alkyl halides can proceed in different ways, although the result is usually atrans addition independent of the mechanism. In certain cases the reaction proceeds as an SN2 reaction as in organic chemistry. That is to say that the electron-rich metal nucleophile attacks the carbon atom of the alkyl halide, the halide being the leaving group. This process leads to inversion of the stereochemistry of the carbon atom (only when the carbon atom is asymmetric can this be observed). There are also examples in which racemisation occurs. This has been explained on the basis of a radical chain... 

Durst and co-workers (309) investigated in detail the stereochemistry of deuteration and methylation of carbanions derived from optically active (+)-(5)-benzyl methyl sulfoxide 34 and (+)-(i )-benzyl t-butyl sulfoxide 290. The reactions, which allowed the extent of asymmetric induction on the a-carbon atom as well as the stereochemistry of deuteration and methylation to be determined, are summarized in Schemes 30 and 31. 

There s a whole area of chemistry dealing with the spatial configurations of organic molecules called stereochemistry. To get into this area, you have to have molecules that have an asymmetrical carbon atom. That s one that has four dissimilar atoms or groups attached to it. PP has that condition on a repeating basis—the methyl groups on every ocher backbone carbon. Such a polymer can be stereoregular or stereospecific. 

Staphyloferrin A (Fig. 20, 65) is a second siderophore of Staphylococcus spp. (226). D-Omithine coimects the two citric acid parts. Due to the unsymmetrical link the central C-atoms of the citric acid units are chiral, but their stereochemistry has not been determined. Another consequence of the asymmetric structure is that two mono- and one di-dehydration products are observed. Staphyloferrin A forms a 1 1 Fe -to-ligand complex, which is preferentially A-configured. For steric considerations only cis-(SR ) or cis-(RS ) arrangements can be considered. Uptake experiments with Fe showed that it is a true siderophore (193). 

SCHEME 16. The observed stereochemistry of the reported reactions by the use of (a) chiral ketones and (b) Grignard reagents in Grignard reactions. Asymmetric carbon atoms are denoted by i or 5... 

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. 

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