Stereochemistry, definitions

WL Interactive versions of these problems are assignable in OWL. Stereochemistry Definitions and Stereogenic Centers 5.26 Define or describe the following terms. 

Clearly, the next step is the handling of a molecule as a real object with a spatial extension in 3D space. Quite often this is also a mandatory step, because in most cases the 3D structure of a molecule is closely related to a large variety of physical, chemical, and biological properties. In addition, the fundamental importance of an unambiguous definition of stereochemistry becomes obvious, if the 3D structure of a molecule needs to be derived from its chemical graph. The moleofles of stereoisomeric compounds differ in their spatial features and often exhibit quite different properties. Therefore, stereochemical information should always be taken into ac-count if chiral atom centers are present in a chemical structure. 

Chemistry in three dimensions is known as stereochemistry At its most fundamental level stereochemistry deals with molecular structure at another level it is concerned with chemical reactivity Table 7 2 summarizes some basic definitions relating to molec ular structure and stereochemistry... 

Many [2 + 2] photocycloadditions have not been assigned a definitive mechanism, but they serve well as synthetic methods. Thiones add vinyl ethers to give thietanes in very good yields (Section 5.14.4.1.2), and interesting wavelength-stereochemistry relations were found in the photoaddition of 2-adamantanone to dicyanoethylene (Section 5.14.4.1.2). Diheterocyclobutanes can also be prepared by [2 + 2] photocycloadditions (Section 5.13.3.3). 

Retrosynthetic analysis of antheridic acid produced a totally different plan of synthesis from that which had been employed for the structurally related target gibberellic acid. The synthesis of antheridic acid, which included a number of novel steps, allowed definitive assignment of structure and revised stereochemistry at C(3). 

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. 

Protonation of the a-carbanion (50), which is formed both in the reduction of enones and ketol acetates, probably first affords the neutral enol and is followed by its ketonization. Zimmerman has discussed the stereochemistry of the ketonization of enols and has shown that in eertain cases steric factors may lead to kinetically controlled formation of the thermodynamically less stable ketone isomer. Steroidal unsaturated ketones and ketol acetates that could form epimeric products at the a-carbon atom appear to yield the thermodynamically stable isomers. In most of the cases reported, however, equilibration might have occurred during isolation of the products so that definitive conclusions are not possible. 

It was anticipated all along that the vinylsilane residue could serve as a vinyl iodide surrogate. After protection of the C-14 secondary hydroxyl in 180 in the form of a triisopropylsilyl ether, the vinyltrimethylsilyl function can indeed be converted to the requisite vinyl iodide with AModosuccinimide (NIS) (see 180—>181, Scheme 43). Vinyl iodide 181 is produced stereospecifically with retention of the A17,18 double bond geometry. This transformation is stereospecific since the stereochemistry of the starting vinylsilane and the vinyl iodide product bear a definite relationship to each other.67b 75... 

The study of optical isomers has shown a similar development. First it was shown that the reduction potentials of several meso and racemic isomers were different (Elving et al., 1965 Feokstistov, 1968 Zavada et al., 1963) and later, studies have been made of the ratio of dljmeso compound isolated from electrolyses which form products capable of showing optical activity. Thus the conformation of the products from the pinacolization of ketones, the reduction of double bonds, the reduction of onium ions and the oxidation of carboxylic acids have been reported by several workers (reviewed by Feokstistov, 1968). Unfortunately, in many of these studies the electrolysis conditions were not controlled and it is therefore too early to draw definite conclusions about the stereochemistry of electrode processes and the possibilities for asymmetric syntheses. 

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,... 

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). 

Sodium borohydride reduction of aknadinine gave a pair of epimeric alcohols, one of which was found to be identical to natural epihernandolinol and the other identical to the known alkaloid hemandolinol (10) (28). As the structure of hemandolinol (10) had been proposed without resolution of the stereochemistry (28), the stereochemistry of epihernandolinol (9) was not definitely established (27). 

Biogenetic considerations [71], and a correlation of all polypropionates from pulmonate molluscs led to revision of the relative stereochemistry of pectinatone (20) and nor-pectinatone (21), metabolites of the pulmonate Siphonaria pectinata [72, 73] the point had already been questioned by Oppolzer s synthesis [74] and was definitively confirmed by two independent X-ray diffraction studies [71, 75] of pectinatone (20). 

The most famous mechanism, namely Cossets mechanism, in which the alkene inserts itself directly into the metal-carbon bond (Eq. 5), has been proposed, based on the kinetic study [134-136], This mechanism involves the intermediacy of ethylene coordinated to a metal-alkyl center and the following insertion of ethylene into the metal-carbon bond via a four-centered transition state. The olefin coordination to such a catalytically active metal center in this intermediate must be weak so that the olefin can readily insert itself into the M-C bond without forming any meta-stable intermediate. Similar alkyl-olefin complexes such as Cp2NbR( /2-ethylene) have been easily isolated and found not to be the active catalyst precursor of polymerization [31-33, 137]. In support of this, theoretical calculations recently showed the presence of a weakly ethylene-coordinated intermediate (vide infra) [12,13]. The stereochemistry of ethylene insertion was definitely shown to be cis by the evidence that the polymerization of cis- and trans-dideutero-ethylene afforded stereoselectively deuterated polyethylenes [138]. 

For many chemists LCs are mysterious and complex materials, their very definition defying a simple understanding. The basic idea behind this discussion of stereochemistry in LCs is that molecules and LCs represent the same phenomenon. Liquid crystals are supermolecules in a different way than are supramolecular assemblies. Indeed, LCs can be composed of supramolecular... 

The field of stereochemistry serves as a unifying theme for the expanded definition and diversification of chemistry. The consequences of molecular and macromolecular shape and topology are central to issues of chemical reactivity, physical properties, and biological function. With that view, the importance of stereochemistry had never been greater, and it is hoped that this series will provide a forum for documentation of significant advances in all of these subdisciplines of chemistry. 

The Topics in Stereochemistry series has set itself apart by maintaining a remarkable balance of chapters that are both definitive, standing the test of time, and current, addressing the impact of stereochemistry at the most exciting frontiers. As a student and researcher, I have often turned to chapters in Topics in Stereochemistry for the foundations and the state of the art in new areas of interest. It is my hope that the series continue to enjoy that level of confidence in the chemistry community and that it retain, in this second incarnation, the esteem that the founders have worked so hard to establish. 

SOME COMMON DEFINITIONS IN ASYMMETRIC SYNTHESIS AND STEREOCHEMISTRY... 

In this chapter a number of common terms in the field of stereochemistry have been introduced. These terms appear repeatedly throughout this book. Therefore, it is essential that we establish common definitions for these frequently used terms. 

B uilding on the original proposal by Yates, the mechanism of this reaction is believed to involve the formation of copper carbenoids as intermediates, Scheme 1. Beyond the fact that copper, its ligands, the carbenoid fragment, and alkene are involved in the stereochemistry-determining event, as evidenced by Noyori et al. (2) and later by Moser (11, 12), little definitive mechanistic information has been acquired for this process. The basics of the mechanism will be discussed in this section. In subsequent sections detailing enantioselective variants, specific factors that have added to the understanding of this reaction will be addressed as will the models used to rationalize the observed stereochemistry. 

---