Multiple Stereogenic Centers

The relative configuration of adjacent substituents in a Fischer projection formula are designated erythro if they are on the same side and threo if they are on the opposite side. The stereochemistry of adjacent stereocenters can also be usefully represented 

Because the Fischer projection formulas represent an eclipsed conformation of the carbon chain, the relative orientation of two adjacent substituents is opposite from the extended staggered representation. Adjacent substituents that are anti in an extended representation are on the same side of a Fischer projection formula, whereas adjacent substituents that are syn in an extended representation are on opposite sides in a Fischer projection. As with extended representations, an enantiomeric pair represented by Fischer projection formulas has the opposite configuration at all stereogenic centers (depicted as left or right.) 

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In the synthesis of polymers it is very important to control the configuration of the multiple stereogenic centers but free radical methods generally fail to give significant stereochemical control (96T(52)4181). To compare the effects of several chiral and achiral auxiliary groups, acrylamides of type 110 were studied. 

Whatever the nomenclamre, the concept of an enantioselective desymmetrization can be a powerful strategy when executed efficiently. The ability of the reaction to simultaneously control multiple stereogenic centers in a single transformation is attractive. The caveat that must be applied is that the substrates are not always readily available and the success of the reaction may be greatly tempered by the difficulty of accessing the starting material. ... 

Novel norephedrine-based chiral ligands with multiple stereogenic centers have been conveniently prepared from norephedrine and. -substituted pyrrole.112 These novel chiral ligands have been used to catalyse the enantioselective addition of diethylzinc to aldehydes and to chalcone in high yields and with good to high enantioselectivities. 

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... 

Two minor processes sometimes operate competitively with that illustrated in the scheme. One of these involves 1,4-addition of the second vinyl anion to give a reactive intermediate that differs structurally from 1, but is capable of setting into motion a closely related sequence of chemical events leading to an isomeric diquinane.4 This is the route followed to produce the minor product characterized here. The other option consists of cis-t, 2-addition, an event that is followed by a dianionic oxy-Cope rearrangement via a boat-like transition state.4 When sufficient substitution is present to allow the installation of multiple stereogenic centers, the adoption of this pathway is easily distinguished from the electrocyclic alternative since a cis relationship between relevant substituents is in place, instead of the trans arrangement required by the electrocyclization cascade. 

The letters D and L are used to label all monosaccharides, even those with multiple stereogenic centers. The configuration of the stereogenic center farthest from the carbonyl group determines whether a monosaccharide is D- or L-. 

Enyne titanium complexes are synthesized when enynes are treated with Ti(OPr1)2( 72-propene). These species add aldehydes, ketones, and chiral imines to generate multiple stereogenic centers in an acyclic system. Numerous methods for the stereoselective construction of two stereogenic centers are known, although those for more than three in one asymmetric procedure are less common.195... 

For the separation of anionic analytes, the positively charged macrocyclic antibiotic vancomycin is one of the most commonly used chiral selectors. It was introduced by Armstrong and coworkers as a new class of chiral selector in CE, HPLC and thin-layer chromatography for the separation of a wide range of enantiomers [29, 47-52]. It contains multiple stereogenic centers and a variety of functional groups. 

The development of efficient methods to access complex molecules vdth multiple stereogenic centers continues to be a formidable challenge in both academe and industry. One approach is the use of catalytic enantioselective cascade reactions [68] that have emerged as powerful tools to rapidly increase molecular complexity from simple and readily available starting materials, thus producing enantioenriched compounds in a single operation. 

Terada et al. successfully applied the aza ene type reaction to the cascade trans formation by taking advantage ofthe formation of imine products (Scheme 3.30) [69]. They employed monosubstituted enecarbamates [70] instead of the disubstituted versions and as a result, piperidine derivatives with multiple stereogenic centers were obtained in high stereoselectivities. The acid catalyzed aza ene type reaction of the initial aldimines with monosubstituted enecarbamates afforded aza ene type... 

The best examples of substances with multiple stereogenic centers are the carbohydrates (Chapter 25). One class of carbohydrates, called hexoses, has the constitution... 

Steroids are another class of natural products with multiple stereogenic centers. One such compound is cholic acid, which can be obtained from bile. Its structural formula is given in Figure 7.13. Cholic acid has 11 stereogenic centers, and so there are a total (including cholic acid) of 2 , or 2048, stereoisomers that have this constitution. Of... 

Because the configuration at C-2 is lost on enolization, the enediol intermediate can revert either to o-glucose or to o-mannose. Two stereoisomers that have multiple stereogenic centers but differ in configuration at only one of them are referred to as epimers. Glucose and mannose are epimeric at C-2. Under these conditions epimerization occurs only at C-2 because it alone is a to the carbonyl group. 

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