Stereochemistry stereoisomers, definition

The lUPAC 1974 Recommendations, Section E, Fundamental Stereochemistry, give definitions for most of the terms used in this chapter, as well as rules for naming the various kinds of stereoisomers. They can be found in Pure Appl. Chem., 1976, 45,... 

Since the stereochemistry of the triene system of LTB4 had not been determined prior to synthesis, a number of stereoisomers of LTB4 were prepared for purposes of definitive comparison of physical properties and bioactivity with biologically produced LTB4. The various stereoisomers of LTB4 were much less active biologically than LTB4 itself. 

The butadiene polymers represent another cornerstone of macromolecular stereochemistry. Butadiene gives rise to four different types of stereoregular polymers two with 1,2 linkage and two with 1,4. The first two, isotactic (62) and syndiotactic (25), conform to the definitions given for vinyl polymers, while the latter have, for eveiy monomer unit, a disubstituted double bond that can exist in the two different, cis and trans, configurations (these terms are defined with reference to the polymer chain). If the monomer units all have the same cis or trans configuration the polymers are called cis- or trans-tactic (30 and 31). The first examples of these stereoisomers were cited in the patent literature as early as 1955-1956 (63). Structural and mechanistic studies in the field have been made by Natta, Porri, Corradini, and associates (65-68). 

From 1970 to 1984 Dugundji, Ugi et al. [9, 10] analyzed the logical structure of chemistry. They found that chemistry consists of two parts with distinct logical structures, constitutional chemistry and stereochemistry. This division is already implied by the nongeometric definition of stereoisomers [11] (see Sect. 1.3). Accordingly, Dugundji and Ugi formulated a qualitative mathematical model of the logical structure of chemistry that consists of two distinct parts. 

Unfortunately, our carbon 13 and proton data (Table II) taken together with other data for the model compounds, do not allow an unambiguous assignment of the 3 stereoisomers which we have obtained. For the present, then, we must be content to leave the relative stereochemistry of the 5-membered ring model compounds in question. For now, we can only say that the fine structure in the copolymer spectra are certainly well within isomers shifts seen for the model compound. However, based on the fact that the relative stereochemistry of the tetrahydrofuran models are unknown and that we have only one pyran isomer, we are unable to make a definite assignment as to the relative stereochemistry of ring junctions in the original copolymer. 

An atactic polymer is a regular polymer with macromolecules composed of a certain number of statistically distributed configurational units. The constitutional unit is a type of atom or group of atoms composing the macromolecule (e. g. —[CH2—CHPhJ- or —[CHPh— in polystyrene). The configurational unit is a constitutional unit with one or several stereoisomer-ic centres. These definitions would require a more detailed explanation. In this volume they will only rarely be used, the stereochemistry of jjolymers is a special branch of macromolecular chemistry. More information can be found in the original literature [2]. 

Stereochemistry is not primarily germane to the subject of this book, and it is not intended to discuss the subject in any detail. Nevertheless, the basic concepts, definitions and conventions, currently used in stereochemistry, will be considered to help those less familiar with the subject to understand the separation technology that will be introduced and described in the subsequent chapters. Stereochemistry is the study of the three-dimensional structure of chemical compounds. Isomers of the same substance, that only differ in the spatial arrangement of their atoms are called stereoisomers. Certain stereoisomers that diff er only in their capacity for rotating the plane of polarized light passed through them, are termed optically active, or chiral, and the isomers are called enantiomers. It follows, that as the subject of this book pertains to the separation of chiral substances, the method used for measuring optical activity needs to be briefly described. 

Chapter 5 Using the Mislow and Siegel definition (/ Am. Chem. Soc. 1984, I06y 3319), I introduce the popular (but often incorrectly defined) term stereocenter and explain the differences between this term and the lUPAC terms chirality center and asymmetric carbon atom (or chiral carbon atom). The term stereocenter is much broader than the more precise term asymmetric carbon atom, and it assumes that one already knows the stereochemical properties of the molecule (to know which bonds will give rise to stereoisomers upon their interchange). Therefore, I have continued to encourage students to identify the (immediately apparent) asymmetric carbon atoms to use as tools in examining a molecule to determine its stereochemistry. 

The total synthesis of cytostatin (58), an antitumor agent belonging to the fostriecin family of natural products has been described in detail. The definitive assignment of the relative and absolute stereochemistry of cytostatin and that of each of its Cio-Cn stereoisomers were established. It was found that each element of... 

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