Chiral: axis

The enantiomers shown are related as a right hand and left hand screw respectively Chiral allenes are examples of a small group of molecules that are chiral but don t have a chirality center What they do have is a chirality axis, which m the case of 2 3 pen tadiene is a line passing through the three carbons of the allene unit (carbons 2 3 and 4)... 

Propadiene (H2C=C=CH2) also called allene, is the simplest cumulated diene The two tt bonds m 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 chirality axis and are chiral... 

Chirality and Optical Activity. A compound is chiral (the term dissymmetric was formerly used) if it is not superimposable on its mirror image. A chiral compound does not have a plane of symmetry. Each chiral compound possesses one (or more) of three types of chiral element, namely, a chiral center, a chiral axis, or a chiral plane. 

A chiral axis is present in chiral biaryl derivatives. When bulky groups are located at the ortho positions of each aromatic ring in biphenyl, free rotation about the single bond connecting the two rings is inhibited because of torsional strain associated with twisting rotation about the central single bond. Interconversion of enantiomers is prevented (see Fig. 1.16). 

FIGURE 1.19 Viewing angle as a means of designating the absolute configuration of compounds with a chiral axis, (a) (R )-2-Butanol (sequence clockwise) (b) (fi)-2-butanol (sequence counterclockwise). 

The Cahn-Ingold-Prelog R-S notation has been extended to chiral allenes and other molecules that have a chirality axis. Such compounds are so infrequently encountered, however, we will not cover the rules for specifying their stereochemistry in this text. 

Chirality 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. 

Introduction of a double bond between the triple bond and the leaving group leads to enyne electrophiles 45, which would give access to vinylallenes 46 if the attack of the nucleophile takes place at the triple bond in an SN2" (1,5) substitution reaction (Scheme 2.16). In addition to the regioselectivity, two types of stereoselectivity also have to be considered in this transformation, i.e. the configuration of the olefinic double bond of the vinylallene and the (relative or absolute) configuration of the allenic chirality axis. 

Cyclononadiene is the smallest isolable cyclic allene with a chirality axis. Heating the racemic mixture without solvent at 125 °C afforded an essentially quantitative yield of a mixture of three isomers 4-6 [4]. The combination of (S)-allene with (S)-allene [or (R) with (R)] gives 4, while the cross-combination of (S)-allene with (R)-allene gives 5 and 6 (cf. Chapter 6). 

ReiSig and Hausherr synthesized (-)-preussin in ten steps starting from L-fructose. The chiral alkoxy allene 143 was formed as a mixture of diastereoisomers which differed in the configuration of the chiral axis [77]. 

The use of diazodicarboxylates has been recently explored in Cinchona alkaloid catalyzed asymmetric reactions. Jprgensen [50] reported the synthesis of non-biaryl atropisomers via dihydroquinine (DHQ) catalyzed asymmetric Friedel-Crafts ami-nation. Atropisomers are compounds where the chirality is attributed to restricted rotation along a chiral axis rather than stereogenic centers. They are useful key moieties in chiral ligands but syntheses of these substrates are tedious. 

CHIRALITY ANALYSIS OF ENZYME REACTIONS Chirality axis,... 

The asymmetric Horner-Wadsworth-Emmons (HWE) reaction of l,3-dioxan-5-ones with phosphonate 184 and a chiral diamine was reported. With the /i r/-butyl-substituted l,3-dioxan-5-one, the product possesses a chiral axis. It was obtained in good yield and with 80% ee (Scheme 53) <2002TL281>. The HWE reaction with similar heterocyclic substrates was used to provide conformationally restricted arachidonic acid derivatives <1999TA139>. 

In a tour de force in 1956, Cahn, Ingold and Prelog introduced the terms chirality axis (descriptors aR/aS) and chirality plane (descriptors pRjpS) in order to deal with compounds such as allenes, biaryls and cyclophanes. Rules for assigning the chirality sense were devised ad hoc. In 1966 the helieity concept was introduced and it was recognized that its use allows the corresponding models to be treated in an alternative way. The specific proposals, as illustrated in Table 1, however, were only published in 19821. 

Fortunately, the original assignment rule for the chirality plane is identical to the helieity assignment defined above with descriptors aRjaS and PjM, respectively, corresponding. However, the rule for the chirality axis was based on an elongated tetrahedron as the stereogenic unit and the descriptors aR and P or aS and M are not equivalent ... 

In addition to this nonequivalence, the imprecise definition of the chirality axis and difficulties in dealing with stereogenic centers in polycyclic structures led to confusion in the case of certain adamantanes, cyclohexanes and spiranes (see Section 1.1.5.3.3.). These problems were solved in the late 1970s and it was recommended1 that the PjM rather than the obsolete aR/aS description be used. This use is recommended as it leads to a considerable simplification of static Stereochemistry. 

Table 7. Slcreomodels with Stereogenic Centers That Were Described by a Chirality Axis"...

Spiranes have caused many problems. The stereomodel 3, of general type 3a, is a good example of where the obvious necessity of specifying stereogenic centers was circumvented by the chirality axis despite too low a symmetry of the skeleton (C2v). 

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