Asymmetric centers multiple

All of these functions are made possible by the characteristic chemical features of carbohydrates (1) the existence of at least one and often two or more asymmetric centers, (2) the ability to exist either in linear or ring structures, (3) the capacity to form polymeric structures via glyeosidie bonds, and (4) the potential to form multiple hydrogen bonds with water or other molecules in their environment. 

In summary, asymmetric cycloadditions are powerful methods for the synthesis of complex chiral molecules because multiple asymmetric centers can be constructed in one-step transformations. Among them, reactions using chiral catalysts are the most effective and promising, and fruitful results have been reported in asymmetric Diels-Alder reactions. 

Only tethered terminal olefins are reactive, and ring junctures are always formed by coupling to the internal carbon of the multiple bond. If an asymmetric center is present in the tether, the reaction proceeds with high diasteroselectivity. Alkenes with substituents a to the double bond favor trans product formation, whereas fi substituents lead to cis products. 

The biochemical rationale for incorporating fluorine in the carbohydrate residue is that replacement of a hydroxyl by fluorine would cause only a very minor steric perturbation of the structure or conformation while at the same time would have a profound electronic effect on neighboring groups. The substitution is possible while retaining the capacity of the position as an acceptor in hydrogen bonding. Yet these same attributes make the synthesis of fluorinated carbohydrates difficult. The synthesis of fluorinated carbohydrates offers a particularly fruitful field for the combination of modem chemical and enzymatic synthetic techniques. Total synthesis would be difficult because of the stereochemical control required at the multiple adjacent asymmetric centers of a fluorinated carbohydrate (105). 

Remarkably, the reversibility of fhe reaction is conserved until the stage of the monohydride intermediate 159 (Scheme 1.39). Reversibility of all stages (and even of the stage in which the optical center is being created) of the catalytic cycle preceding the irreversible release of the product and regeneration of the catalyst effectively levels the effect of the multiple reaction pathways and excludes the possibility of a racemization of the already created asymmetric center. 

Isomers differing by the spatial arrangement of their functional groups not being mirror image of each other. They may contain multiple asymmetric centers. Diastereoisomers may or may not be optically active. 

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 ... 

Recently, the transition-metal-catalyzed addition of active methylene C-H bonds to electron-deficient olefins having a carbonyl, a nitrile, or a sulfonyl group has been extensively studied by several research groups. In particular, the asymmetric version of this type of catalytic reaction provides a new route to the enantioselective construction of quaternary carbon centers [88]. Another topic of recent interest is the catalytic addition of active methylene C-H bonds to acetylenes, allenes, conjugate ene-ynes, and nitrile C-N triple bonds. In this section, the ruthenium-catalyzed addition of C-H bonds in active methylene compounds to carbonyl groups and C-C multiple bonds is described. 

Efficient and elegant syntheses of complex organic molecules with multiple stereogenic centers continue to be important in both academic and industrial laboratories (Nicolaou et al. 2003, 2006). In particular, catalytic asymmetric multicomponent domino reactions, used in the course total syntheses of natural products and synthetic building blocks, are highly desirable (Nicolaou et al. 2003, 2006 Tietze and Beifuss 1993 Tietze 1996 Tietze and Haunert 2000 Wasilke et al. 2005 Ramon and Yus 2005 Guo and Ma 2006 Pellissier 2006 Pellisier 2006 Tietze... 

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