Chirality centers defined

The neighborhoods of the atoms directly bonded to tbe chiral center must be defined. The neighborhood of an atom A. dircetly bonded to the ehiral eenter, is dc-fned as the set of atoms whose distance (in number of bonds) to A is less than their distance to any of the other three atoms bonded to the chiral center (Figure 8-9. In cyclic structures different neighborhoods can overlap. 

R,S convention (Section 9.5) A method for defining the absolute configuration at chirality centers using the Cahn-IngoId-Prelog sequence rules. 

Concerted cycloaddition reactions provide the most powerful way to stereospecific creations of new chiral centers in organic molecules. In a manner similar to the Diels-Alder reaction, a pair of diastereoisomers, the endo and exo isomers, can be formed (Eq. 8.45). The endo selectivity in the Diels-Alder arises from secondary 7I-orbital interactions, but this interaction is small in 1,3-dipolar cycloaddition. If alkenes, or 1,3-dipoles, contain a chiral center(s), the approach toward one of the faces of the alkene or the 1,3-dipole can be discriminated. Such selectivity is defined as diastereomeric excess (de). 

Theoretically, with 6 chiral centers within the molecule (at 6, 9, 2, 5, 11 and 12 positions) 64 diastereomeric forms are possible. However, bromocriptine is sterically well defined at all of these positions, as it is derived from the naturally occurring a-ergocryptine. 

Another mechanism of chiral amplification that extends over an even larger scale has been reported by Huck et al. [119] The molecule 12-(9 H-thioxantbene-9 -yli-dene-12H-benzo[a]xanthene (Fig. 11.6), which has no chiral center, nevertheless exists, like the helicenes, in two chiral forms defined by their enantiomeric configurations. Consistent with the discussion in Section 11.2.3, a small net handedness (ca. 0.7 %) could be induced in racemic solutions of this molecule by use of ultraviolet CPL. However, introducing 20 wt% of this molecule, which contained a 1.5% chiral excess of one roto-enantiomer, into a nematic phase of liquid crystals produced macroscopic (100 pm) regions of a chiral cholesteric liquid crystal phase. The... 

Abstract 1,3-Dipolar cycloaddition reactions (DCR) are atom-economic processes that permit the construction of heterocycles. Their enantioselective versions allow for the creation of up to four adjacent chiral centers in a concerted fashion. In particular, well-defined half-sandwich iridium (111) catalysts have been applied to the DCR between enals or methacrylonitrile with nitrones. Excellent yield and stereoselectivities have been achieved. Support for mechanistic proposals stems from the isolation and characterization of the tme catalysts. 

The degeneracy of the non-chiral complexes can be removed by incorporating chiral centers, usually as resolved amino acids, into the arms at close vicinity to the hydroxamate iron binding sites. Thus, only one of the energetically non-equivalent diastereomers predominates, leading to pure enantiomeric iron(III) complexes with defined hehcity that allows assessing stereospecific recognition by the ferrichrome receptor. 

Terms in bold are defined i amino acids 75 R group 76 chiral center 76 enantiomers 76 absolute... 

As mentioned earlier, alkylation of sugar enolates is a method of choice to introduce two different carbon chains in a defined sequence to construct a quaternary chiral center with the desired configuration [11] (see Scheme 39). Several other methods have been proposed, such as the ring opening of spirocyclopropanes [182] or the 1,4-addition of dimethylcuprate on a p-methyl-substituted enone [183]. 

Often (e.g. in asymmetric synthesis) one is interested in the fact that in certain molecules, such as propionic acid (2, Fig. 1), an achiral center (here C ) can be transformed into a chiral center by replacement of one or other of two apparently identical1 ligands2 by a different one. Thus the replacement of HA at C in propionic add (Fig. 1) by OH generates the chiral center of (lactic acid. C in propionic acid is therefore called a prochiral center 4) HA and HB are called heterotopic ligands 5 7) (from Greek heteros = different and topos" = place — see also below). Prochiral axes and planes may similarly be defined in relation to chiral axes and planes (see below)... 

The synthesis of CHDTC02H is, in principle, straightforward and was first accomplished in two laboratories in 1969 150). A modified version of the Comforth synthesis151) is shown in Fig. 67. Most of the steps are self-evident. The doubly labeled phenyl methyl carbinol is, of course, a dl pair in the first instance the absolute stereochemistry at the CHDTX chiral center is RS but the relative stereochemistry of the two chiral centers is defined as IRS,2RS, i.e. only one of the two possible diastereomers is obtained. The resolution at the methyl group occurs automatically when the carbinol is resolved by classical methods. 

Both the cis- and the trans-disubstituted spiranes resulted, in different ratios, depending on the reaction conditions. Clearly, the trans spiranes are chiral. The first conjugate addition to the Michael acceptors 75a-c is intermolecular in nature and defines the sense of chirality at the first chiral center. Subsequent intramolecular ring closure to the spiranes 76 defines the cis or trans configuration of the product. When cyclohexane-1,3-dione (74a) was reacted with dibenzalacetone (75a) in the presence of ca 5 mol% (—)-quinine (3a, Scheme 4.3), a 2.5 1 trans/cis mixture resulted, with the trans isomer 76 having optical purity of ca 30% (Scheme 4.37) [61] (the absolute configuration of the predominant enantiomer was not assigned). 

Figure 6.10 a Example of a chiral molecule, (R)-4-chloro-4-(dimethylamino)heptan-3-one, indicating b the four atoms directly bound to the chiral center and c their neighborhood (the neighborhood of atom A is defined as the set of atoms which are closer, i.e., on a shorter path, to A than to B, C, and D). 

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