Stereogenic elements

Due to the inherent unsymmetric arene substitution pattern the benzannulation reaction creates a plane of chirality in the resulting tricarbonyl chromium complex, and - under achiral conditions - produces a racemic mixture of arene Cr(CO)3 complexes. Since the resolution of planar chiral arene chromium complexes can be rather tedious, diastereoselective benzannulation approaches towards optically pure planar chiral products appear highly attractive. This strategy requires the incorporation of chiral information into the starting materials which may be based on one of three options a stereogenic element can be introduced in the alkyne side chain, in the carbene carbon side chain or - most general and most attractive - in the heteroatom carbene side chain (Scheme 20). 

The second option involves the incorporation of either chiral amines or chiral alcohols into the heteroatom-carbene side chain (R ), which represents the most versatile approach to diastereoselective benzannulation. The optically pure (2R,3R)-butane-2,3-diol was used to tether the biscarbene complex 37. The double intramolecular benzannulation reaction with diphenylbutadiyne allowed introduction of an additional stereogenic element in terms of an axis... 

Fig. 1. Introduction of one or more than one stereogenic elements (center, axis, plane or helix) leads to different types of chiral dendrimers...

Stereogenic Elements in SmCP Phases SmCP Supermolecular Diastereomers... 

The first consideration when investigating HPLC method development protocols is the chemical structure of the analyte, in particular, the presence of functional groups capable of interacting with the stationary phase and containing or in the vicinity of the stereogenic elements [79]. Since the natural target of macrocyclic antibiotics is the A-acyl-D-alanyl-D-alanine terminus (see Section 2.1), the early choice of suitable substrates for this kind of CSPs was that of amino acids [45]. However, it turned out that the macrocyclic CSPs were very successful not only in amino acids enantioresolution, but also in the separation of a wide variety of different structures. 

Yet, indirect information on enantioselective complexation by silica-bound antibiotics in HPLC can be extracted from the analysis of retention data of several ligands whose structure is systematically varied to explore chemical diversity in terms of functional groups, stereogenic elements, molecular complexity, and rigidity-flexibility. 

Depending on the substrate, matched/mismatched combinations between the stereogenic elements at BINOL and the sulfoximine were observed. In the conver-... 

Consequently, matched/mismatched cases [25] can result, and indeed our investigations on cooperative effects of stereogenic elements in such systems revealed 9 be the matched case and 23 (which is also easily prepared by following a directed deprotonation-silylation-deprotonation-trapping-desilylation sequence [11]) to be the mismatched case in diethylzinc additions to aldehydes [26]. Later, these investigations were extended to more complex systems such as 24 [27], but ferrocene 9 still remains superior to all other compounds. 

Most organic molecules incorporate at least one stereogenic element (centre, plane or axis) [26] and may, therefore, exist in more than one stereoisomeric form. These stereoisomers, as far as mechanistic argument is concerned, are distinct species, and the kinetics and... 

Stereochemical integrity may also be lost when the reaction of a stereoisomer occurs via an intermediate which retains a stereogenic element, but whose bonding permits interconversion of stereoisomers faster than its conversion of stereoisomeric products. [2 + 21-Cycloaddition of TCNE and cis-propenyl methyl ether [30] yields cis- and trans-adducts, 22 in Scheme 9.13, in ratios which depend on the solvent (84 16 in favour of the cis-adduct in acetonitrile). The dipolar 23 was proposed as an intermediate. The initial bonding destroys the double bond character between Cl and C2 of the enol ether reactant, and the much... 

The 191 problems in this book cover most of the area of stereochemistry, including nomenclature, stereogenic elements (centers, axes, planes) and their descriptors, symmetry, inorganic stereochemistry, determination of enantiomer excess, conformation of acyclic and cyclic compounds, and more. The answers, in addition to providing solutions to the problems, frequently include additional explanations of the underlying principles. The problems are ordered more or less in order of increasing difficulty. (I had a hard time with some of the problems toward the end myself )... 

Biaryls merit special interest due to their axial element of chirality and are among the most widely used ligands in enantioselective synthesis and catalysis. Their coordination by a tricarbonyl chromium fragment following benzannulation provides an additional stereogenic element in terms of a chiral plane to the molecule [68]. Biaryl quinones are similarly relevant to natural product synthesis and enantioselective catalysis. 

Further oxidation of 43 leads to bis- or tris-spirodienone derivatives of calixarenes. They possess with the directionality of each spirodienone moiety an additional, independent stereogenic element. Thus, for a calix[4]arene derived bis(spirodienone) 45 six isomeric forms are possible, two meso forms and two pairs of C2-symmetric enantiomers, see Figure 8. The number of possible isomers increases with increasing of the number of rings in the macrocycle.92... 

Siegel, J. Biali,S.E. Mislow, K. J. Am. Chem. Soc. 1987,109,3397. In the present case, however, the directionality of the hydrogen bonds is the only stereogenic element and consequently all inherently chiral calixarenes would be cycloenantiomers. The expression cycloenantiomers was also used for instance for rotaxanes composed of wheel and an axle, which both are achiral by themselves but have a structural directionality Yamamoto, C. Okamoto, Y. Schmidt, T. Jager, R. Vogtle, V. J. Am. Chem. Soc. 1997,119, 10547-10548. 

Higher Fullerene Derivatives with Stereogenic Elements Located Exclusively in the Addends... 

Figure 1.6. Schlegel diagrams of two stereoisomers of tetrakis[( )-l-phenylbutyl] l,2 56,57-bis (methano)[70]fullerene-71,71J2,72-tetracarboxylate.35 54 The two types of stereogenic elements (inherently chiral addition pattern and stereogenic centers in the ester groups) are specified independently of each other. As can be seen from the descriptors (S,S,S,S, C) and (S fA), the depicted molecules are diastereoisomers.

In this section we present a number of C60 derivatives that have been prepared by addition of a chiral reagent to C60 or by coupling a chiral residue to a C60 derivative without creation of new stereogenic elements. Whereas most of... 

The formal complex of an imine RCH=NR with a zirconocene of C2v symmetry will be formed as a racemic mixture, with equal amounts of two enantiomers. If a zirconaaziridine possesses another stereogenic element, it will have two diastereomers of unequal energy, and they may be generated in a kinetic ratio that is not equal to their thermodynamic ratio. 

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