Stereoisomerization and

The bromination of 3-aroyloxycyclohexanes gives rise to a mixture of stereoisomeric and positionally isomeric addition products. The product composition for Ar = phenyl is shown. Account for the formation of each of the products and describe the factors which will affect the product ratio. 

BlaC32 Blair, C. M., Henze, H. R. The number of stereoisomeric and non-stereoisomeric mono-substitution products of the paraffins. . Amer. Chem. Soc. 54 (1932) 1098-1106. 

Carbon-carbon double bonds are present in most organic and biological molecules, so a good understanding of their behavior is needed. In this chapter, we ll look at some consequences of alkene stereoisomerism and then focus on the broadest and most general class of alkene reactions, the electrophilic addition reaction. 

I think that this problem can be answered satisfactorily only by direct evidence for a (Z)-a-complex 13.24. This may take some time, but I hope not as long as the 78 years between the discoveries of the first (Z)/(is)-stereoisomerism and that of the triazenes (See Wiberg and Pracht, 1972b Fanghanel et al., 1975a, 1975b see also Sec. 13.1.)... 

Scheme 1.4S. Stereoisomerization and skeletal rearrangement of alkenylalanes formed via Zr-catalyzed carboalumination.

Two reports deal with the E I mass spectra of some related, stereoisomeric and partly saturated 1,3- and 3,l-benzoxazino-l,3-benzoxazines (95RCM1035) and of some substituted stereoisomeric l,6,7,llb-tetrahydro-2//,4//[l,3]oxazino[4,3-fl]isoquinolines and l,6,7,llb-tetrahydro-2//[l,3]ox-azino[4,3-a]isoquinolin-4-ones (95RCM998, 95RCM1035). 

C Chemical Shifts in Aliphatic Molecular Systems, Substituent Effects on Dependence on Constitution and Stereochemistry (Duddeck) Chirality, On Factoring Stereoisomerism and (Hirschmann and... 

P. A. Lehman (1986). Stereoisomerism and drug action. Trends Pharmacol. Sci. 7 281-285. 

Some heteropolycycloalkanes, e.g. indolizidine, quinolizidine, tropane, and decahydro-quinoline, are parent skeletons of alkaloids. Carbon-13 shifts of these and other parent polycycloalkanes [408] (Tables 4.64 and 4.65) are predominantly determined by heteroatom electronegativity, ring size, stereoisomerism, and dynamic effects. Aza- and thiade-calins are representative examples (Table 4.64). As described for cis- and trans-decalin (Section 4.1.3), carbon nuclei in the cis isomers of aza- and thiadecalins are shielded... 

Thus, in cases where four different atoms or groups are attached to the same atom, it is possible to have two arrangements in space that cannot be made to coincide geometrically. This situation can be demonstrated by use of a special type of formula, shown in Fig. 2 for the two forms of the compound fluoroehlorobromomethane. This existence of two forms due to a difference in orientation in space is called stereoisomerism, and is discussed in the entry on Isomerism. It also follows that for compounds containing more than one atom bonded to four unlike groups, the number of different forms increases rapidly, as is shown by the three possible forms of tartaric acid, HOOC-CH-OH-CHOH-COOH. as portrayed by the three formulas shown in Fig. 3. 

Enoate 3-substitution and 3-disubstitution cause a decrease in the rate of the initial conjugate addition step of the reaction sequence that is directly related to the steric bulk of the substituent.103,105 Equation (24) provides a representative case in the a-alkylation of enoates by means of conjugate amination-enol-ate alkylation followed by dehydroamination.106 When 3-substitution results in stereoisomeric ( )- and (Z)-alkenoate substrates, tandem difunctionalization typically proceeds with greater facility for ( )-isomers.64103 Obviously, when the double bond of the ester is part of a medium-sized ring, an ( )-alke-noate geometry is mandated in such cases, tandem vicinal difunctionalization proceeds with uniformly excellent results (equation 25).25... 

Seven stereoisomeric and regioisomeric vinyloxetans (384 to 390) were obtained on photoaddition of acetaldehyde to ( )- or (Z)-penta-l,3-diene.336 Other instances of [2 -I- 2] photoaddition of ketones to conjugated dienes have been described.337 Iminooxetans are similarly prepared by addition of carbonyl-containing compounds to ketenimines.338... 

We have previously noted 9> that three-bladed propeller molecules with D3 skeletons exhibit stereochemical isomorphism with respect to stereoisomerism, and with respect to stereoisomerization when all pathways are considered. This fact is dramatically illustrated by the stereochemical correspondence between the Ar3Z systems and the transition metal tris-chelates. We shall first describe a model in the sense discussed above which may be used to analyze the stereochemistry of one, and therefore of both of these systems. In general, such a model will consist of two components a topographical reference structure, and a set of allowable operations on this structure. For example, let structure 4 represent the skeleton of the molecule, where the numbered vertices identify substituent sites connected in a pairwise fashion. The abstract model may then be defined as structure 4 together with the following operations reflection of the structure as a whole, and/or permutations among the substituent sites which maintain the pairwise connectivity of the structure. This model or similar ones have been employed for tris-chelates 12-I6) an(i for systems of the type Ar3Z. s)... 

We have seen that the concept of stereochemical correspondence is a useful analytical tool. If two or more systems are recognized to be stereochemically correspondent, an analysis of stereoisomerism and stereoisomerization in one system can be directly applied to the others, by way of the model abstracting the key features of both. 

Camacho A. D., Pierce H. D., Jr, and Borden J. H. (1994) Aggregation pheromones in Dryocoetes affaber (Mann.) (Coleoptera Scolytidae) Stereoisomerism and species specificity. J. Chem. Ecol. 20, 111-124. 

In 1899 Fischer turned his attention to the study of proteins, wishing to understand their chemical structures. It was known at that time that proteins were composed of amino acids, and thirteen naturally occurring ones were identified. Fischer was able to isolate via the hydrolysis of proteins three additional naturally occurring amino acids valine, proline, and hydroxyproline. Amino acids exhibit stereoisomerism, and Fischer was able to separate individual forms from mixtures of stereoisomers for several of these compounds. 

Stereochemistry is the discipline of chemistry that relates the observable chemical properties and behaviour of chemical compounds to the geometric features of their molecules. Stereochemistry is mostly concerned with stereoisomerism and its observable consequences. 

The purpose of the present paper is to review this field of research - a summary of this review appeared in (4). After a brief introduction (part 2) to the XPS technique itself, specially oriented to the study of the polymer valence bands, given in order to stress upon the inherent difficulties (and limitations) of the experiment, we shall present specific examples of the various types of informations that can be acquired on the polymer molecular and electronic structure (part 3) substitution effects in the valence bands (part 4) structural isomerism, stereoisomerism, and geometrical crystalline structure (part 5) other types of information. 

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