Allenes chirality

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. 

Buynak et al. reported the synthesis of representative 7-vinylidenecephalosporine derivatives bearing an axial allene chirality (Scheme 4.5) [9]. A chiral allene 24 was prepared stereoselectively utilizing the reaction of an organocopper reagent with propargyl triflate 23, obtained by a diastereoselective ethynylation of the ketone 22 with ethynylmagnesium bromide. Terminally unsubstituted allene 26 was synthesized via bromination of the triflate 23 followed by reduction of the bromide 25 with a zinc-copper couple. 

For reviews of allene chirality, see Runge, in Landor The Chemistry of the Allenes. vol. 3 Academic Press New York. 1982, pp. 579-678, and in Patai The Chemistry ofKetenes. Allenes. and Related Compounds, pt. I Wiley New York. 1980, pp. 99-154 Rossi Diversi Synthesis 1973, 25-36. 

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 additions of chiral nonracemic allenylmetal reagents to chiral a-methyl propanal derivatives have been proven useful for the assembly of polypropionate fragments. These reagents rely on allene chirality to favor one of the two possible diastereomeric transition states in the addition and, thus, differ in a fundamental way from the aforementioned methods in which a chiral auxiliary or catalyst provides the control element. For example, a chiral allenylstannane 246 is added to a chiral aldehyde (S)-230, derived from the Roche ester, in the presence of various Lewis acid promoters to afford any of the four diastereo-mers with excellent diastereo- and enantioselectivity, depending on the reaction conditions. Representative results are depicted in Scheme 10.48. From the stereocontrol point of view, these transformations follow Cram-fike open transition state models without or with chelation, respectively. If InBr3, SnCLi, BuaSnCl, or other additives... 

Disubstituted allenes are stereogenic. Their employment in the Nazarov cyclization provides a means to transfer axial allene chirality to tetrahedral chirality within the five-membered ring. Homochiral allene 115 was combined with vinyllithium species 116 in THF at —78°C (Scheme 19.30). ° Workup with aqueous potassium dihydrogen phosphate led to cross-conjugated cyclopentenone... 

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 nonplanarity of allenes has an interesting stereochemical consequence 1 3 Disubstituted allenes are chiral they are not superimposable on their mirror images Even an allene as simple as 2 3 pentadiene (CH3CH=C=CHCH3) has been obtained as sep arate enantiomers... 

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)... 

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... 

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... 

Other approaches to (36) make use of (37, R = CH ) and reaction with a tributylstannyl allene (60) or 3-siloxypentadiene (61). A chemicoen2ymatic synthesis for both thienamycia (2) and 1 -methyl analogues starts from the chiral monoester (38), derived by enzymatic hydrolysis of the dimethyl ester, and proceeding by way of the P-lactam (39, R = H or CH ) (62,63). (3)-Methyl-3-hydroxy-2-methylpropanoate [80657-57-4] (40), C H qO, has also been used as starting material for (36) (64), whereas 1,3-dipolar cycloaddition of a chiral nitrone with a crotonate ester affords the oxa2ohdine (41) which again can be converted to a suitable P-lactam precursor (65). 

The property of chirality is determined by overall molecular topology, and there are many molecules that are chiral even though they do not possess an asymmetrically substituted atom. The examples in Scheme 2.2 include allenes (entries 1 and 2) and spiranes (entries 7 and 8). Entries 3 and 4 are examples of separable chiral atropisomers in which the barrier to rotation results from steric restriction of rotation of the bond between the aiyl rings. The chirality of -cyclooctene and Z, -cyclooctadiene is also dependent on restricted rotation. Manipulation of a molecular model will illustrate that each of these molecules can be converted into its enantiomer by a rotational process by which the ring is turned inside-out. ... 

As with i -substituted allyl alcohols, 2,i -substituted allyl alcohols are epoxidized in excellent enantioselectivity. Examples of AE reactions of this class of substrate are shown below. Epoxide 23 was utilized to prepare chiral allene oxides, which were ring opened with TBAF to provide chiral a-fluoroketones. Epoxide 24 was used to prepare 5,8-disubstituted indolizidines and epoxide 25 was utilized in the formal synthesis of macrosphelide A. Epoxide 26 represents an AE reaction on the very electron deficient 2-cyanoallylic alcohols and epoxide 27 was an intermediate in the total synthesis of (+)-varantmycin. 

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. 

Long before chiral allenes were known (Problem 9.72), the resolution of 4-methylcyclohexylideneacetic acid into two enantiomers had been carried out. Why is it chiral What geometric similarity does it have to allenes ... 

The reaction of propargylic chiral acetals with a catalytic copper reagent (RMgX/5% CuX) provides the expected alkoxy allenes in quantitative yield (Table 3)61. The diastereomeric excess is highly dependent on the size of the ring of the acetal and on the type of substituents it contains. The best diastereomeric excess is 85% with the acetal derived from cyclooctanediol. The use of lithium dimethylcuprate results in 1,2-addition lo the triple bond and the resulting lithium alkenyl cuprate bearing a cyclic acetal does not eliminate even at reflux temperature ( + 35°C). 

Conjugate addition of chiral amines to allenic and acetylenic sulfones has been reported73. The reaction 112 with (—)-ephedrine gives only one of the two possible diastereomeric oxazolines in high yield (equation 89). 

A chirality transfer from the allenic sulfone 154 to the Diels-Alder adducts is observed in the reaction of (+)-(S)-l-(p-tolylsulfonyl)buta-l,2-diene with cyclopentadiene (equation 112), in which the diastereomers 155 and 156 are formed predominantly107. 

Epoxidations of chiral allenamides lead to chiral nitrogen-stabilized oxyallyl catioins that undergo highly stereoselective (4 + 3) cycloaddition reactions with electron-rich dienes.6 These are the first examples of epoxidations of allenes, and the first examples of chiral nitrogen-stabilized oxyallyl cations. Further elaboration of the cycloadducts leads to interesting chiral amino alcohols that can be useful as ligands in asymmetric catalysis (Scheme 2). 

Arai Y., Koizumi T. Synthesis and Asymmetric Diels-Alder Reactions of Chiral. Alpha.,.Beta.-Unsaturated Sulfoxides Bearing a 2-Exo-Hydroxy-lO-Bornyl Group As an Efficient Ligand on the Sulfur Center Rev. Heteroat. Chem. 1992 6 202-217 Keywords allenic sulfoxide, a-sulfinylmaleate, a-sulfinylmaleimide, asymmetric synthesis, chiral unsaturated sulfoxides... 

Like biphenyls, allenes are chiral only if both sides are unsymmetrically substituted. For example. 

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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