Chiral carbon atoms. See

For compounds containing a nitrogen or sulphur atom directly attached to a chiral carbon atom, see Chapter A. 

As you can see from their structures, a- and /3-glucose have several chiral carbon atoms. Both isomers are optically active they are not enantiomers (mirror images of one another) because they differ in configuration only at carbon atom 1. As it happens, both a- and /3-glucose rotate the plane of polarized light to the right (clockwise). 

The ligands discussed so far all contained C2 symmetry. An important new class of ligands having Ci symmetry was introduced by Togni [22] (see Figure 4.19). They can be easily made from an enantiomeric amine as the precursor in a few steps. Different substituents can be introduced at the phosphorus atoms. In addition to the chiral carbon atom the molecule has planar chirality as well. The chiral carbon atom is used to introduce the planar chirality, i.e. lithiation of the ferrocenyl amine takes place at a specific side of the amine at the ferrocene moiety. 

Fischer polypeptide synthesis org chem A synthesis of peptides in which a-amino acids or those peptides with a free amino group react with acid halides of a-haloacids, followed by amination with ammonia. fish-ar pal-e pep,tTd. sin tha sas ) Fischer projection orgchem) A method for representing the spatial arrangement of groups around chiral carbon atoms the four bonds to the chiral carbon are represented by a cross, with the assumption that the horizontal bonds project toward the viewer and the vertical bonds away from the viewer fish-ar pra.jek-shon) Fischer s salt See cobalt potassium nitrite. fish-3rz solt)... 

Lithium and alkyllithiums in aliphatic hydrocarbon solvents are also used to initiate anionic polymerization of 1,3-butadiene and isoprene.120,183-187 As 1,3-butadiene has conjugated double bonds, homopolymerization of this compound can lead to several polymer structures. 1,4 Addition can produce cis-1,4- or tram-1,4-polybutadiene (19, 20). 1,2 Addition results in a polymer backbone with vinyl groups attached to chiral carbon atoms (21). All three spatial arrangements (isotactic, syndiotactic, atactic) discussed for polypropylene (see Section 13.2.4) are possible when polymerization to 1,2-polybutadiene takes place. Besides producing these structures, isoprene can react via 3,4 addition (22) to yield polymers with the three possible tacticites ... 

The feature of 2-chlorobutane that makes it chiral is the presence of a carbon attached to four different groups. Such carbons are another type of stereocenter. The currently accepted term to describe such a carbon, or any other tetrahedral atom attached to four different groups, is chirality center. (Some older terms that you may encounter are chiral carbon atom or asymmetric carbon atom.) Any molecule with one chirality center as its only stereocenter is chiral. (As we shall see shortly, many, but not all, molecules with multiple chirality centers are also chiral.) So, another way to identify a chiral molecule is to look for a single chirality center, which requires some practice. It helps to remember that any carbon that is attached to two identical groups (this includes all doubly and triply bonded carbons) is not a chirality center. Consider these examples ... 

Mycomycin contains no chiral carbon atoms, yet is chiral. To see why, make a model of mycomycin. For simplicity, call -CH=CHCH=CHCH2C02H "A" and -OCC=CH MB". Remember from Chapter 6 that the carbon atoms of an allene have a linear relationship and that the n bonds formed are perpendicular to each other. Attach substituents at the spT carbons. 

Another interesting and more general methodology to a carbene with only one chiral carbon atom in the backbone was introduced by Hahn et al. [61] (see Figure 5.16). Here, an arylamine is treated with n-BuLi and carbon disulfide to the corresponding thiocarbonic acid amide. Second deprotonation with 5ec-BuLi and subsequent reaction with an asymmetrically substituted Schiff base affords the imidazolidinone that can be reduced with Na/K alloy to the respective carbene. Chirality is introduced via the prochiral N=C bond of the Schiff base and the prodnct is usually a racemate since no chiral discriminator is present in the formative step. 

For discussions of chiral organic molecules that do not contain asymmetric carbon atoms, see Wude, L. C., Jr. Organic Chemistry Prentice-Hall Englewood ClilTs, NJ, 1987 pp 354-356. Schlogl, K. Topics Ciirr. Chem. 1984, 125, 27. Laarhoven, W. H. Prinsen, W. J. C. Ibid. 1984, 125, 63. Meurer, K. P. Vogtle, F. Ibid. 1985, /27, 1. 

Allenes with three different substituents arranged Cah=C=Cae, e.g. the substituent ka that occurs twice must be present at both terminal carbon atoms (see Worked Problems for an example of an allene Caa=C=Cbc). Allenes of type . = =0 are also chiral, but now lack any symmetry axis. 

Table 5 reveals the products of substrates which have been deuterated in H20. In other cases deuterated enoates were reduced in H2O. The reductions in H20 lead to acylates with two chiral carbon atoms due to stereospecific deuteration of methylene groups. Most of these have been used in studies of biosynthetic pathways of natural products. For examples see (29,30). Reactions in H20 proceed usually slower than in H2O. The differences may be less than a factor 2 if optimal pH and p H values are determined and applied. The p H and pH value for optimal reaction rates are not the same (31). 

Products with 3 chiral carbon atoms synthesized from various (/ )-2-hydroxy-(3 )-en-carboxylates (i )- RCH=C R-CHOHCOOH (44). See notes a-c on the next page. 

The next part of the specific behavior of the cinchona alkaloids as modifiers resides in their stable conformational structure. The molecules of cinchona alkaloids, e.g. cinchonidine or cinchonine (see Scheme 5.20.), consists of two relatively rigid parts an aromatic quinoline ring and an aliphatic bicyclic quinuclidine ring, both connected through the hydroxyl-bearing chiral carbon atom, C9. 

Because the presence of a single chiral carbon atom gives rise to stereoisomerism, it is important to be able to recognize one when you see it. It is really not difficult. If a carbon atom is connected to four different groups, no matter how slight the differences, it is a chiral carbon. If any two groups are identical, it is not a chiral carbon. 

Carbon atom 3 is the sp hybrid allene carbon atom. Carbons 2 and 4 are both sp and planar, but their planes are perpendicular to each other. None of the carbon atoms is attached to four different atoms, so there is no chiral carbon atom. Nevertheless, 2,3-pentadiene is chiral, as you should see from your models and from the following drawings of the enantiomers. 

The NMR spectrum of the pure hydroxypropylated polystyrene showed peaks in the regions of d 21.5-26 ppm and d 64-67.5 ppm. Analysis by the attached proton test (APT) in conjunction with model compounds indicated that the region between 21.5 and 26 ppm corresponds to the methyl carbon resulting from attack of the polymeric organo-lithium at the least hindered carbon to form a secondary alcohol chain-end functional group (see (a) in eqn [7]). The area between 3 64 and 67.5 ppm was assigned to the carbon bonded to oxygen for two diastereomerically different products as shown by structure 1, where the chiral carbon atoms are labeled with asterisks. 

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