Chiral compounds allenes

Axial Chirality. For a system with four groups arranged out of the plane in pairs about an axis, the system is asymmetric when the groups on each side of the axis are different. Such a system is referred to as an axial chiral system. This structure can be considered a variant of central chirality. Some axial chiral molecules are allenes, alkylidene cyclohexanes, spiranes, and biaryls (along with their respective isomorphs). For example, compound 7a (binaphthol), which belongs to the class of biaryl-type axial chiral compounds, is extensively used in asymmetric synthesis. Examples of axial chiral compounds are given in Figure 1-5. 

Hg(OAc)2 easily reacts with allene to yield methoxymercuration products, i.e. vinylmercury compound 41, with the trans isomer being the major product. The chirality of allene was transferred into the final products, indicating the intermediacy of a cr-bridged mercurinium ion [23-26], The stereoselectivity of this reaction was determined by the relative stability of intermediates 40 and 42 and steric hindrance for the incoming methoxy group. 

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... 

Chiral separations are concerned with separating molecules that can exist as nonsuperimposable mirror images. Examples of these types of molecules, called enantiomers or optical isomers, are illustrated in Figure 1. Although chirality is often associated with compounds containing a tetrahedral carbon with four different substituents, other atoms, such as phosphorus or sulfur, may also be chiral. In addition, molecules containing a center of asymmetry, such as hexahelicene, tetrasubstituted adamantanes, and substituted allenes or molecules with hindered rotation, such as some 2,2 disubstituted binaphthyls, may also be chiral. Compounds exhibiting a center of asymmetry are called atropisomers. An extensive review of stereochemistry may be found under Pharmaceuticals, Chiral. 

A limited number of chiral compounds having no stereogenic centers exist. For example, although A is achiral, constitutional isomer B is chiral. Make models and explain this observation. Compounds containing two double bonds that share a single carbon atom are called allenes. 

Molecules are chiral if they have no plane of symmetry. This may arise from a tetrahedral atom with four different substituents or from a molecule that is forced to adopt a shape that lacks a plane of symmetry. Examples include spiro compounds, axial chirality in allenes, chiral C, P, S, etc. You should give definite examples in this part of the answer, which are different from those given in the question. Ask someone to check if yours are all right. 

The alkaloids have also been used for the resolution of various phosphoric and carboxylic adds 4—10, including axially chiral compounds, such as biaryl derivatives 4, 5 and 7 [13], allenic acids 9, and planar chiral naphthalenophane carboxylic acids 10 (Figure 13.5). 

Fructose 1,6 bisphosphate (FBP) is an allosteric activator of the thermostable l-2-hydroxyacid dehydrogenase from B. stearothermophilus, which might be useful for the asymmetric synthesis of chiral compounds. Since FBP is quite expensive, Allen and Holbrook wished to create an FBP-independent variant1208. Three rounds of shuffling and screening produced a mutant L-2-hydroxyacid dehydrogenase with three amino acid substitutions that is almost as active in the absence of FBP as the wild-type is in its presence. 

Allenes are a class of compounds with unique reactivity due to the presence of the cumulated diene unit, which also makes the chirality of allenes [1]. For a long period of time, this class of compounds has been considered as very unstable [2]. In reality allenes are not so unstable recent observations have shown that allenes possess good reactivity and selectivity in organic synthesis [3]. Since some of the most recent advances in the chemistry of allenes have been summarized in several reviews and accounts, this chapter will summarize the typical advances in Pd-catalyzed two- or three-component cyclization of functionalized allenes, which have not been included in these reviews and accounts [3]. 

Interactive chiral compounds without stereogenic centres— allene ... 

For a review of the chirality of allenes, see Runge, W. in Patai, S., Ed. The Chemistry of Ketenes, Allenes and Related Compounds, Part 1 Wiley-lnterscience Chichester, England, 1980 pp. 99-154. 

The two most important classes of axially chiral compounds which will be considered here are the allenes and hindered biaryls. 

The same group reported an intermolecular version that gives a straightforward access to functionalized, monocyclic, eight-membered ring compounds. In the same way, from chiral 4-allenal and alkyne a high chiral transfer has been observed (Scheme 8.27). It is noteworthy that gaseous acetylene can be utilized as partner in these cyclizations [46]. 

The axial chirality of allenes makes them attractive building blocks for chiral macrocydes, yet very little has been done in this area. Krause and coworkers reported the first example of this new class of molecule, the allenophanes, in 1999 [69]. Their multistep synthesis of 181 was not stereoselective and ultimately afforded a mixture (presumably) of four diastereomers. All attempts to estabhsh the presence of the four diastereomers via HPLC were thwarted by the low solubility of the compound. 

FIGURE 1.24. Samples of axial chirality in allenes, spiro- and cyclic compounds, as well as biaryl-derivatives (BINOL, BINAP, etc.). 

The copper-catalysed y-selective coupling between propargylic phosphate (83) and alkylboron compound (84) afforded chiral multisubstituted allenes (85) with various functional groups (Scheme 20). ... 

In certain crystals, e.g. in quartz, there is chirality in the crystal structure. Molecular chirality is possible in compounds which have no chiral carbon atoms and yet possess non-superimposable mirror image structures. Restricted rotation about the C=C = C bonds in an allene abC = C = Cba causes chirality and the existence of two optically active forms (i)... 

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

Allenes are compounds with adjacent carbon-carbon double bonds. Many allenes are chiral, even though they don t contain chirality centers. Mycomycin, for example, a naturally occurring antibiotic isolated from the bacterium Nocardia acidophilus, is chiral and has = -130. Explain why mycomycin is chiral. Making a molecular model should be helpful. 

These cases are completely different from the cis-trans isomerism of compounds with one double bond (p. 157). In the latter cases, the four groups are all in one plane, the isomers are not enantiomers, and neither isomer is chiral, while in allenes, the groups are in two perpendicular planes and the isomers are a pair of optically active enantiomers. 

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