Chiral compounds cyclooctene

On the basis of this definition (Fig. 1) the following classes of chiral compounds will be treated in this article cyclophanes, bridged anulenes and [8]anulenes, and ( )-cyclooctene and related structures. As mentioned above, metallocenes will be excluded. 

Introduction of the allene structure into cycloalkanes such as in 1,2-cyclononadiene (727) provides another approach to chiral cycloalkenes of sufficient enantiomeric stability. Although 127 has to be classified as an axial chiral compound like other C2-allenes it is included in this survey because of its obvious relation to ( )-cyclooctene as also can be seen from chemical correlations vide infra). Racemic 127 was resolved either through diastereomeric platinum complexes 143) or by ring enlargement via the dibromocarbene adduct 128 of optically active (J3)-cyclooctene (see 4.2) with methyllithium 143) — a method already used for the preparation of racemic 127. The first method afforded a product of 44 % enantiomeric purity whereas 127 obtained from ( )-cyclooctene had a rotation [a]D of 170-175°. The chirality of 127 was established by correlation with (+)(S)-( )-cyclooctene which in a stereoselective reaction with dibromocarbene afforded (—)-dibromo-trans-bicyclo[6.1 0]nonane 128) 144). Its absolute stereochemistry was determined by the Thyvoet-method as (1R, 87 ) and served as a key intermediate for the correlation with 727 ring expansion induced... 

Sensitization with Chiral Aromatic Amides, Phosphoryl Esters. Besides the above-mentioned studies employing aromatic carboxylic esters as sensitizers and naturally occurring alcohols as chiral auxiliaries, some attempts have been made to use other types of sensitizers, such as aromatic amides [43] and phosphoryl esters [44] with (— )-menthyl, as well as synthetic C2-symmetric chiral auxiliaries, shown in Scheme 8. These chiral compounds can efficiently sensitize the Z-E photoisomerization of cyclooctene 47 to give moderate E Z ratios of up to 0.17 and 028 and low-to-moderate ees of up to 5% and 14% for the aromatic amides and phosphoryl esters, respectively [43,44]. 

One of the most interesting developments in the stereochemistry of organic compounds in recent years has been the demonstration that trans-cyclooctene (but not the cis isomer) can be resolved into stable chiral isomers (enantiomers, Section 5-IB). In general, a Wa/w-cycloalkene would not be expected to be resolvable because of the possibility for formation of achiral conformations with a plane of symmetry. Any conformation with all of the carbons in a plane is such an achiral conformation (Figure 12-20a). However, when the chain connecting the ends of the double bond is short, as in trans-cyclooctene, steric hindrance and steric strain prevent easy formation of planar conformations, and both mirror-image forms (Figure 12-20b) are stable and thus resolvable. 

The molecule that has been investigated more than any other olefin is the trans -cyclooctene. This molecule, which is a twisted olefin, served as a model compound for the olefinic chromophore, the main reason being that chromophores in general can be divided into two types those which are asymmetric (inherently asymmetric) by nature and those which become asymmetric as a result of chiral centers in their vicinity. 

Cyclodextrin Esters. - CDs with mono-6-benzoate or monosubstituted 6-benzoates and related compounds with benzoylamino-deoxy groups in C-6 positions have been used for enantio-differentiating photoisomerization of (Z)-cyclooctene to the chiral (E)-isomer. ... 

There are many chiral molecules for which enantiomeric forms can be interconverted by a rotation about a single bond. The enantiomeric conformations of gauche butane provide an example, where rapid rotation interconverts the two under most conditions. If the rotation that interconverts a pair of such enantiomers is slow at ambient temperature, however, the two enantiomers can be separated and used. Recall from our first introduction of isomer terminology (Section 6.1) that stereoisomers that can be interconverted by rotation about single bonds, and for which the barrier to rotation about the bond is so large that the stereoisomers do not interconvert readily at room temperature and can be separated, are called atropisomers. One example is the binaphthol derivative shown in the margin. It is a more sterically crowded derivative of the biphenyl compound discussed previously as an example of a chiral molecule with no "chiral center". A second example is frans-cyclooctene, where the hydrocarbon chain must loop over either face of the double bond (Eq. 6.4). This creates a chiral structure, and the enantiomers interconvert by moving the loop to the other side of the double bond. 

Other examples of chiral helical stmctures are provided by tra 5-cyclooctene and suitably substituted heptalenes (Figure 1.28). Heptalene is not planar, and its twisted structure results in chirality. Although these conformations generally interconvert rapidly, bulky substituents may sufficiently slow this process to resolve and separate these compounds optically. The earlier example of the chirality of ( )-cyclo-octene is also an example for the planar chirality. 

Alkenes or polyenes with isolated or coupled double bonds that are devoid of chiral atoms are not optically active. Such activity occurs in cyclic alkenes where double bonds occur in the trans form, such as in trans-cyclooctene (Figure 2.18). Another group of alkenes that includes representatives having optical activity is that of cumulenes. The name refers to cumulation of double bonds in such molecules. The best-characterized group of these compounds is that of allenes in which two double bonds occur next to each other [54]. Compounds of this type have a so-called chirality axis determined by cumulated double bonds. Besides allenes, higher optically active cumulenes are also known. An example of optically active cyclic allene is provided by 1,2-cyclononadiene, which was synthesized in 1972 [55]. 

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