Stereochemistry representation

Chemical Abstracts Service Information System Nomenclature Automatic Generation and Conversion Online Databases in Chemistry Polymers Structural Representation Stereochemistry Representation and Manipulation. 

A number of special features of chemical structures, and certain types of structure, cause special problems, particularly for computer representation. Stereochemistry is one of the most important of these (see Stereochemistry Representation and Manipulation). 

The two structures in our example are identical and are rotated by only 1 20 h Clearly, rotation of a molecule docs not change its stereochemistry, Thus, the permutation descriptor of both representations should be (+ I). On this basis, we can define an equation where the number of transpositions is correlated with the permutation descriptors in an exponential term (Eq. (9)). 

Problem 25.2 Convert the following Fischer projections into tetrahedral representations, and, assign R or S stereochemistry to each ... 

In this representation of a monosaccharide, the carbon chain is written vertically, with the lowest numbered carbon atom at the top. To define the stereochemistry, each carbon atom is considered in turn and placed in the plane of the paper. Neighbouring carbon atoms are below, and the H and OH groups above the plane of the paper (see below). 

Schaffer CE (1968) A Perturbation Representation of Weak Covalent Bonding. 5 68-95 Schaffer CE (1973) Two Symmetry Parameterizations of the Angular-Overlap Model of the Ligand-Field. Relation to the Crystal-Field Model. 14 69-110 Scheldt WR, Lee YJ (1987) Recent Advances in the Stereochemistry of Metallotetrapyrroles. 64 1-70... 

Mechanistic studies have been designed to determine if the concerted cyclic TS provides a good representation of the reaction. A systematic study of all the E- and Z-decene isomers with maleic anhydride showed that the stereochemistry of the reaction could be accounted for by a concerted cyclic mechanism.19 The reaction is only moderately sensitive to electronic effects or solvent polarity. The p value for reaction of diethyl oxomalonate with a series of 1-arylcyclopentenes is —1.2, which would indicate that there is little charge development in the TS.20 The reaction shows a primary kinetic isotope effect indicative of C—H bond breaking in the rate-determining step.21 There is good agreement between measured isotope effects and those calculated on the basis of TS structure.22 These observations are consistent with a concerted process. 

The overall results of substituent effects are observed in the products of a reaction, their rates of formation, and their stereochemistries. The purpose of this article is to apply very simple theoretical techniques to correlations and predictions of the rate and stereoselectivity effects of substituents in [2+2] photocycloadditions. The theoretical methods that will be used are perturbational molecular orbital (PMO) theory and its pictorial representation, the interaction diagram. Only an outline of the theory will be given below, since several more detailed descriptions are available. 4,18-34)... 

The Morgan algorithm 5> is a device used by the Chemical Abstracts Service for assigning indices to the nodes of constitutional formulas whose H-atoms have been omitted, i. e. their reduced graphs. If the indices of all atoms are needed, e.g. for the representation of stereochemistry, an additional procedure is needed for establishing those for the H-atoms. Constitutional symmetry is not indicated directly by Morgan indices. 

We have discussed in this chapter the thermal pyrolyses of a number of strained ring compounds. In most of the cases considered there is good evidence that the processes are unimolecular. Where possible we have tried to suggest plausible transition complexes, and reaction paths, based on a consideration of such factors as the kinetic parameters, stereochemistry of the reaction and effect of substituents. In reactions of this type, the description of the transition complex is fraught with difficulties, since the absence of such things as solvent effects (which can be so helpfrd in bimolecular reactions) limit the criteria on which such descriptions may be based. Often two types of transition complex may be equally good at accounting for the observed data. Sometimes one complex will explain some of the data while another is better able to account for the remainder. It is probable that in many cases our representation... 

A special attention is given to stereochemistry as some compounds are published without proper chirality representation even though the information is available, for example, for natural compounds and their derivatives. Furthermore, as illustrated in Figure 9.6, compounds published in medicinal chemistry literature are often depicted... 

A representation of all of the elementary reactions that lead to the overall chemical change being investigated. This representation would include a detailed analysis of the kinetics, thermodynamics, stereochemistry, solvent and electrostatic effects, and, when possible, the quantum mechanical considerations of the system under study. Among many items, this representation should be consistent with the reaction rate s dependence on concentration, the overall stoichiometry, the stereochemical course, presence and structure of intermediate, the structure of the transition state, effect of temperature and other variables, etc. See Chemical Kinetics... 

AND CINACHYROLIDE A (3), SHOWING THAT NO STEREOCHEMISTRY WAS DETAILED IN THE ORIGINAL REPRESENTATION OF 1, THE PARTIAL STEREOCHEMISTRY PROPOSED FOR 3 CONFLICTED WITH THAT FOR 2 AND LATER REPRESENTATIONS OF 1 BY PETTIT. 

Another important aspect of the Tsuji-Trost reaction is the stereochemistry. The representation of the r -allylpalladium complex of formula 4 is an oversimplification. In reality, i73-allylpalladium complexes are square-planar, as indicated by formulas 10 and 11 in Scheme 4, the two auxiliary ligands L being placed approximately near the two vertical parallel planes... 

Finally, B is related to the stereochemistry of the reaction and the stereochemistry can be reasoned most simply using Rule 3, based on the FO-VB representations of R and R ... 

Exercise 6.10 The ground and promoted states of the reactants, R and R are shown in Fig. 6.Ans.8 below using the FO—VB representation, which is the simplest one for making predictions on stereochemistry. The electronic structure of R displays an electron transfer from the nucleophile to the C C bond. A transfer to the a-orbital is not relevant, as this would generate a closed-shell cyclopropane that cannot form a bond with the oxidized nucleophile. It is therefore the a orbital that accepts the transferred electron, thus generating the triplet oo- configuration of the C-C bond. Now, since R and R differ by one-electron shift from Nu to the a orbital of the cation radical, the corresponding... 

We are going to deduce a mechanistic model for molecular energy from the SLG-based QM method described in Section 2.4. We shall perform transformations and approximations, following me line mentioned above and arrive naturally at a tetrahedral representation of heavy atoms consistent with facts known from stereochemistry and usually interpreted in the VSEPR framework. The mechanistic model of PES will be derived in terms of these objects. 

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