Stereogenic axis

Molecules with a single helical unit, such as in hydrogen peroxide, are rare. More commonly found are bundles of two to nine units with coincident central bonds (see Table 1). The combination of two, e.g., in cyclophanes, and four, e.g., in biaryls and allenes, helical units are particularly important. Such compounds have a stereogenic axis, as stereoisomers may be... 

A stereogenic axis is defined by two or more atoms that are connected by a linear array of consecutive bonds. 

A particular atom can be both a stereogenic center and one of the atoms defining a stereogenic axis. In such cases the stereogenic center must first be specified. 

I.I.5.3.3. Specification of the Heh city of a Stereogenic Axis, Distinguishing of Centers... 

Examples of compound classes with members which contain a stereogenic axis are ... 

A review of the older literature on compounds with a stereogenic axis is available22, as are reviews on planar chiral molecular structures 23, on the stereochemistry of twisted double bond systems 24, on helical molecules in organic chemistry 25, and on the synthesis and stereochemistry of chiral organic molecules with high symmetry 26. 

Compounds with at least one stereogenic center and one stereogenic axis (this also includes anti-Bredt-olefins that exist in two stable atropisomeric forms). 

The zinc(II) chloride promoted intramolecular cycloaddition of l-[6-(3-isopropenyl-4-methyl-3-pentenyl)-2-methoxyphenyl]-2-propen-l-one yields a racemic cycloadduct93 that contains two chiral units a center and a stereogenic axis. On heating, an equilibrium mixture of the two diastereomeric atropisomers 8 and 9 is established94. 

An example is the addition of hydrogen chloride to 4-methylcyclohexylidene acetic acid, a chiral compound with a stereogenic axis. The configuration of the products was deduced from NOE-difference data29. 

The intramolecular Ullmann coupling of (/ )-36 in a completely diastereoselective reaction gives 37. The relative configuration at the newly introduced stereogenic axis had to be determined (see p 479)48. 

The conversion of 7, which has a stereogenic center, into 8 which has a stereogenic axis, by DDQ oxidation (for assignment, see p437, see also p424)147. 

DDQ oxidation of 5 furnishes 6 where the relative configuration around the stereogenic axis was determined by X-ray analysis. Note the difference in the problem of configurational assignment when compared to a similar reaction (see Section 4.3.3.2.4.1) where the absolute configuration needed to be determined (also see p 437). 

Assignment of the absolute configuration of a chiral compound containing a stereogenic axis, rather than a stereogenic center, by chemical means is very often achieved by correlation with a centrochiral compound in one of the following ways ... 

Chiral allenes are examples of a small group of molecules that are chiral, but don t have a stereogenic center. What they do have is a stereogenic axis, also called a chiral axis, which in the case of 2,3-pentadiene is a line passing through the three carbons of the allene nnit (carbons 2, 3, and 4). 

Propadiene (CH2=C=CH2), also called allene, is the simplest cumulated diene. The two tt bonds in an allene share an sp-hybridized carbon and are at right angles to each other. Certain allenes such as 2,3-pentadiene (CH3CH=C=CHCH3) possess a stereogenic axis and are chiral. 

Stereogenic axis (Section 10.8) Line drawn through a molecule that is analogous to the long axis of a right-handed or left-handed screw or helix. 

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. 

Stereogenic axis The descriptors may be modified to Rq, Sq when applied to a stereogenic axis [85], although it is usually more convenient to use the P, M system to specify the configuration of stereogenic axes and planes [86]. 

Chiral, definition of, 260 Chiral axis. See Stereogenic axis Chiral center. See Stereogenic center Chiral drugs, 273 Chiral molecules, 259—263, 290 absolute configuration, 267, 292 Fischer projection formulas, 271-272, 278, 280, 292-293... 

Axial axis Chirality is the result of arrangement of molecules about a stereogenic axis. 

Subsequently in 2004, the Baeh group presented a novel axially chiral A -aryl thiazolium salt and showed its eatalytie ability in intramolecular Stetter reactions (one example, 75% yield and 50% ee). Upon treatment with base, the diastereomerieally pure thiazolium salt was converted into a mixture of atropisomers with a diastereomerie ratio of 75 25. The authors considered the faet that in the earbene intermediate, the rotation around the stereogenic axis is not suffieiently restrieted to suppress a racemization or epimerization, which probably caused relatively low enantioseleetivity in the intramolecular Stetter reaetion. 

An / 5-permutation operated on 7 is shown in the horizontal direction of Fig. 10.4 (S-axis f 5-stereogenicity-axis), where its 5-diastereomer 7 (= 7)... 

---