Carbohydrates chirality

General Three Carbon Chiral Synthons from Carbohydrates Chiral Pool and Chiral Auxiliary Approaches... 

Despite the greater awareness of carbohydrate synthons in recent years, the full potential of the carbohydrate chiral pool is still not... 

Attachment of chiral substituents to calixarene skeleton is the first, the most straightforward and still the most popular way to constmct chiral calixarenes. In 1979 Gutsche reported the synthesis of the first chiral calixarene by attaching camphorosulfonyl group to p-tBu-calix[8]arene [1]. Nowadays, attachment of virtually any chiral moiety at the selected position is synthetically feasible. However, some chiral groups, due to their availability and versatility, have been particularly widely exploited, for example amino acid derivatives, peptides, carbohydrates, chiral amines and axially chiral groups. 

Carbohydrates Chiral Molecules Fischer Projections of Monosaccharides Haworth Structures of Monosaccharides Chemical Properties of Monosaccharides Disaccharides Polysaccharides... 

A Syntheses Using Carbohydrates as Chiral Educts ( Chirons )... 

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

Glyceraldehyde can be considered to be the simplest chiral carbohydrate It is an aldotriose and because it contains one chirality center exists in two stereoisomeric forms the D and l enantiomers Moving up the scale m complexity next come the aldotetroses Examining their structures illustrates the application of the Fischer system to compounds that contain more than one chirality center... 

Aldoses exist almost exclusively as their cyclic hemiacetals very little of the open chain form is present at equilibrium To understand their structures and chemical reac tions we need to be able to translate Fischer projections of carbohydrates into their cyclic hemiacetal forms Consider first cyclic hemiacetal formation m d erythrose To visualize furanose nng formation more clearly redraw the Fischer projection m a form more suited to cyclization being careful to maintain the stereochemistry at each chirality center... 

Oligomer (Section 14 15) A molecule composed of too few monomer units for it to be classified as a polymer but more than in a dimer trimer tetramer etc Oligonucleotide (Section 28 6) A polynucleotide containing a relatively small number of bases Oligosaccharide (Section 25 1) A carbohydrate that gives three to ten monosacchandes on hydrolysis Optical activity (Section 7 4) Ability of a substance to rotate the plane of polanzed light To be optically active a sub stance must be chiral and one enantiomer must be present in excess of the other... 

As mentioned previously, cellulosic phases as well as amylosic phases have also been used extensively for enantiomeric separations more recently (89,90). Most of the work ia this area has been with various derivatives of the native carbohydrate. The enantioresolving abiUties of the derivatized cellulosic and amylosic phases are reported to be very dependent on the types of substituents on the aromatic moieties that are appended onto the native carbohydrate (91). Table 3 fists some of the cellulosic and amylosic derivatives that have been used. These columns are available through Chiral Technologies, Inc. and J. T. Baker, Inc. 

Table 3. Carbohydrate Derivatives Used as Hplc Chiral Stationary Phases...

Antineoplastic Drugs. Cyclophosphamide (193) produces antineoplastic effects (see Chemotherapeutics, anticancer) via biochemical conversion to a highly reactive phosphoramide mustard (194) it is chiral owing to the tetrahedral phosphoms atom. The therapeutic index of the (3)-(-)-cyclophosphamide [50-18-0] (193) is twice that of the (+)-enantiomer due to increased antitumor activity the enantiomers are equally toxic (139). The effectiveness of the DNA intercalator dmgs adriamycin [57-22-7] (195) and daunomycin [20830-81-3] (196) is affected by changes in stereochemistry within the aglycon portions of these compounds. Inversion of the carbohydrate C-1 stereocenter provides compounds without activity. The carbohydrate C-4 epimer of adriamycin, epimbicin [56420-45-2] is as potent as its parent molecule, but is significandy less toxic (139). 

Most parent structures consist essentially of an assembly of rings and/or chains, the degree of hydrogenation of which is defined (usually completely saturated or containing the maximum number of non-cumulative double bonds in cyclic portions), and having no attached functional substituents (the carbohydrates are a notable exception to this). The stereochemistry at all (or most) chiral centres is defined thus such parent structures are sometimes referred to as stereoparents . Some examples are shown (77)-(83). 

The use of carbohydrates as SM s has greatly expanded in recent years, and many cases have been summarized in a text by Hanessian.33 Several examples of such syntheses are indicated in Chart 15. Other commercially available chiral molecules such as a-amino acids or a-hydroxy acids have also been applied widely to the synthesis of chiral targets as illustrated by the last two cases in Chart 15. 

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