Stereo-center carbon atom

If the a carbon atom of an aldehyde or a ketone is a chnality center its stereo chemical integrity is lost on enolization Enolization of optically active sec butyl phenyl ketone leads to its racemization by way of the achiral enol form... 

As demonstrated in Section I, numerous representatives of this type of compounds occur naturally. Such natural substances are commonly characterized by two special features, (a) The C-5 carbon atom of the pyrrolone ring is a stereo center that can be provided by natural derivatives of amino acids. (b) The acyl substituent at C-3 often includes a complex... 

One of the synthetic routes for the anticancer drug Taxol, which has twelve stereo centers, involves a homogeneous C-C coupling reaction. The industrial production of a protease inhibitor that has stereospecific arrangements of amino and hydroxyl groups on two adjacent carbon atoms also involves homogeneous catalysis. From Table 1.1 identify the possible reaction types that are used in these two syntheses. 

Fig. 3. Stereo drawing of a-carbon atoms of L. easel DHFR enzyme with bound inhibitor (methotrexate right center of view) and cofactor (NADPH left center). Stereo viewers are recommended, but many people can see the 3-D image by placing an index card between the Images and staring, one eye on each Image, until the images coalesce. The bound substrate and cofactor have oxygen atoms shaded and nitrogen atoms blackened, respectively. Every fifth residue is numbered (from Ref. 75 with permission).

SAMPLE SOLUTION (a) Glycine is achiral, and so Gly-Ala has only one stereo-genic center, the a-carbon atom of the L-alanine residue. When the carbon chain is drawn in an extended zigzag fashion and L-alanine is the C terminus, its structure is as shown ... 

Asymmetric carbon atom van t Hoffs definition for a carbon atom having four different ligands (i.e. Cabcd). See also stereo genic center, stereogenic element. 

The synthesis of gm-diallyl derivatives can be achieved by double alkylation of active methylene groups. We realized that installation of gem-diallyl functionality on a carbon atom, not activated by any electron-withdrawing group, is a difficult proposition. The problem becomes insurmountable on carbohydrate precursors because base-catalyzed reactions lead to tandem elimination of water molecules, resulting in the formation of complex mixtures, We observed interesting reactions with carbohydrate cyclopropyl precursors. For example, the radical-mediated cyclopropyl scission of the spirocyclopropyl bromide (86) with n-BusSnH gave the C-aUyl derivative (87) in a stereo-controlled fashion. On the other hand, hydrogenation of the cyclopropylaldehyde derivative (88) over Pd/C provided 89 with a quaternary chiral center (Scheme 30.14). 

PLA is a chiral polymer containing asymmetric carbon atoms with a helical conformation. Polymers that have stereo centers in the repeating unit can exhibit two structures of maximum order, that... 

In several studies concerning the synthesis of brassinolides such as compound 343 described by Takatsuto [77, 78], the Claisen-Johnson rearrangement afforded stereoselectively functionalized side chains. As previously, formation of the center in of ester group was stereoselective. A mixture of epimers at the a-stereocenter was formed during the process, but since this carbon atom was no longer stereo-genic in compound 343, this lack of stereoselectivity was of no consequence (Scheme 6.56). 

Among organic molecules, chirality is mainly introduced by a so-called stereo-center carbon atom that has four nonequivalent groups bonded to it. The simplest examples to illustrate the differences between the achiral and chiral molecules are propanol, which is achiral, and butanol, which is a chiral molecule. Their simplified structures are illustrated in Figure 1.9. 

Illustration of the chirality of organic molecules containing stereo-center carbon, (a) 2-propanol, which is an achiral molecule, and (b) 2-butanol, which is a chiral molecule. The sketches are oversimplified in reality a carbon atom is tetrahedrally bonded to four atoms and chemical groups. 

Even isotopes qualify as different substituents at a chirality center The stereo chemistry of biological oxidation of a derivative of ethane that is chiral because of deu terium (D = H) and tritium (T = H) atoms at carbon has been studied and shown to... 

---