Olefins chiral

Another example is a chiral olefinic alcohol, which is disconnected at the double bond by a refro-Wittig transform. In the resulting 4-hydroxypentanal we recognize again glutamic acid, if methods are available to convert regio- and stereoselectively... 

The isomerization of an allylic amine to an enamine by means of a formal 1,3-hydrogen shift constitutes a relatively small structural change. However, this transformation could be extremely valuable if it could be rendered stereoselective. In important early studies, Otsuka and Tani showed that a chiral cobalt catalyst, prepared in situ from a Co(ii) salt, a chiral phosphine, and diisobutylaluminum hydride (Dibal-H), can bring about the conversion of certain pro-chiral olefins to chiral, isomeric olefins by double bond migra-... 

Relative insensitivity to preexisting chiral centers In allylic alcohols with preexisting chiral centers, the diastereofacial preference of the chiral titanium-tartrate catalyst is often strong enough to override diastereofacial preferences inherent in the chiral olefinic substrate. 

In addition to providing a novel approach to the preparation of chiral compounds, this type of chemistry may allow one to inquire into the subtle stereochemical details of some crystal-state reactions. For example, what are the approach geometry and the preferred side of attack in the addition of bromine to a chiral olefin (259) What can be learned of the geometry of the labile electronically excited species involved in (2 + 2) photocycloaddition reactions (260) ... 

The only operative directing effect for chiral olefins withont heteroatoms at the stere-ogenic centre is 1,3-aUylic strain. In this case, a moderate preference for erythro diastere-oselectivity is observed (Scheme 37). For example, photooxidation of both 118 and 119 afforded mainly the erythro diastereomer (71/29 and 81/19, respectively). 

SCHEME 37. Threo/erythro diastereoselectivity in the photooxygenation of chiral olefins without heteroatoms at the stereogenic centre... 

From the 1980s on, many efforts were directed toward asymmetric induction of nitrile oxide cycloadditions to give pure (dia)stereoisomeric isoxazolines, and acyclic products derived from them (17,18,20-23). The need to obtain optically active cycloaddition products for use in the synthesis of natural products was first served by using chiral olefins, relying on 1,2-asymmetric induction, and then with optically active aldehydes or nitro compounds for the nitrile oxide part. In the latter case, insufficient induction occurs using chiral nitrile oxides, a problem still unsolved today. Finally, in the last 5 years, the first cases of successful asymmetric catalysis were found (29), which will certainly constitute a major area of study in the coming decade. 

A large number of nitrile oxide cycloadditions to chiral olefins were reported from several groups in the early 1980s (16,18,20). A clear rationale of regioselec-tivity was developed only when the traditional view, which focused on ground-state conformers, was abandoned and calculational approaches to identify transition states and transition state conformations were refined (158,159). Calculations... 

Use of Intermediates from Cycloadditions of Chiral Nitrile Oxides to Chiral Olefins... 

Thus, in the example of the chiral olefin 3, there is simple diastereoselectivity of (m4a + m4b)/(m4c + m4d) and induced diastereoselectivities of m4a/m4b and m4c/m4d. It is not strictly necessary, but conversely there is no harm, in applying the term simple diastcrcosclcc-tivity to the first case, i.e., to a diastereoselective reaction of achiral reactants. In this volume the presentation of a given reaction type always begins with simple diastereoselectivity of achiral reactants. 

A quite unique approach is also the complexation of chiral olefins by a ligand exchange type reaction with the chiral platinum(lV) complex (Table 1, entry 57). It is an equilibrium... 

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