Chirality 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...

In ketone 26, the chiral control elements are close to the reacting carbonyl, thus enhancing the stereochemical communications between the catalyst and the substrate. The fused ring or quaternary centers are placed at the a-position to the carbonyl group, which minimizes potential epimerization of the stereogenic centers. Electron-withdrawing oxygen substituents inductively activate the carbonyl. 

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. 

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. 

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... 

Chiral atoms are stereogenic, but not all stereogenic centres are chiral atoms. For example, in an alkene abC=Cab the double bond is a stereogenic element. 

Asymmetric synthesis is a reaction or reaction sequence that selectively creates one configuration of one or more new stereogenic elements by the action of a chiral reagent or auxiliary, acting on heterotopic faces, atoms, or groups of a substrate. The stereoselectivity is primarily influenced by the chiral catalyst, reagent, or auxiliary, despite any stereogenic elements that may be present in the substrate. 

Absolute configuration The arrangement in space of the ligands of a stereogenic unit, which may be specified by a stereochemical descriptor such as R or S, D or L, P or M. See also chirality sense, chirality element, stereogenic element... 

Axis of chirality See chirality element, stereogenic axis, stereogenic element. 

Chirality element, element of chirality A stereogenic axis, center, or plane that is reflection variant. See also stereogenic element. 

Chirality transfer Asymmetric induction in which one stereogenic element is sacrificed as another is created. 

Stereogenic plane A planar structural fragment that, because of restricted rotation or structural requirements, cannot lie in a symmetry plane. If the stereogenic plane is reflection variant, the element may be called a chirality plane. For example with a monosubstituted paracyclophane, the stereogenic plane includes the plane of the benzene ring. For a 1,2-disubstituted ferrocene, the disub-stituted cyclopentadiene lies in a chirality plane. The absolute configuration may be specified by either R, S or P, M. See also stereogenic element. 

The possible intermediates were organized into an energy graph shown in Fig. (7), which indicates also the relative enthalpies of formation (in kJ/mol) estimated on simple model reactions. The arrows show the direction of the decrease of the free enthalpy of formation. As each structure has two stereogenic elements (unsymmetrically substituted double bond and/or center of chirality), each formula represents four stereoisomers. 

Four type of coupling reactions were carried out, which are shown in Fig. (11) and Fig. (13). (The cyclic skeletons were numbered according to the biogenetic numbering [40]). For the description of the stereogenic elements of the products the following system was used configuration of the new center of chirality (C-3 in tryptamine and oxotryptamine derivatives, C-l in dopamine and histamine derivatives) R or S type of conformation around C-l4 (in dopamine derivatives C-ll, in histamine derivatives C-10) 11,. .. 33, (see Fig. (14)) conformation of the... 

Similarly, those reactions carried out on an enantiomerically pure compound in which no new stereogenic element is created (e.g. the NaBH4 reduction of a chiral non-racemic aldehyde, the SN2 type conversion of a chiral non-racemic tosylate into the corresponding bromide, the conversion of a stereogenic element in a new one) will also be disregarded. 

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