Configurations of aldoses

Abb. 7.15. Kiliani-Fischer method for the homologation of aldoses to give the dia-stereomeric mixture of the next higher aldose. Three-step syntheses of this kind (here D-arabinose —> D-glucose + D-mannose) played a central role when about a hundred years ago Emil Fischer managed to determine the configuration of aldoses by chemical ( ) methods. 

The method by which the absolute configuration of aldoses may be determined by conversion to glycosides of (—)-butan-2-ol followed by g.c. (see Vol 12, p. 207) has been successfully applied to the hydrolysates of several polysaccharides and glycoproteins. ... 

As shown for the aldotetroses an aldose belongs to the d or the l series accord mg to the configuration of the chirality center farthest removed from the aldehyde func tion Individual names such as erythrose and threose specify the particular arrangement of chirality centers within the molecule relative to each other Optical activities cannot be determined directly from the d and l prefixes As if furns ouf bofh d eryfhrose and D fhreose are levorofafory buf d glyceraldehyde is dexfrorofafory... 

The reaction is used for the chain extension of aldoses in the synthesis of new or unusual sugars In this case the starting material l arabinose is an abundant natural product and possesses the correct configurations at its three chirality centers for elaboration to the relatively rare l enantiomers of glucose and mannose After cyanohydrin formation the cyano groups are converted to aldehyde functions by hydrogenation m aqueous solution Under these conditions —C=N is reduced to —CH=NH and hydrolyzes rapidly to —CH=0 Use of a poisoned palladium on barium sulfate catalyst prevents further reduction to the alditols... 

Configurations of the D series of aldoses containing three through six carbon atoms. 

Fischer projections and, 977-978 glycosides and, 989-990 l- 4 links in, 997-998 origin of name. 973 photosynthesis of, 973-974 see also Aldose, Monosaccharide vaccines from, 1004-1005 Carbon, ground-state electron configuration of, 6... 

Figure 4.17 The trioses D-glyceraldehyde (aldose) and dihydroxyacetone (ketose), the pentose D-ribose, the hexoses D-galactose and D-glucose (aldoses) and the ketohexose D-fructose in their open chain forms. The configuration of the asymmetrical hydroxyl group on the carbon, the furthest away from the aldehyde or ketone group, determines the assignment of D- or L-configuration.

Figure 9.3 Stereoisomers of the D-aldoses. D-Ribose and D-arabinose differ only in their configuration about a single carbon atom (carbon 2) and are examples of epimers. Diastereoisomers are stereoisomers which are not enantiomers of each other but are chemically distinct forms, the eight D-hexoses being examples. Some, however, are also epimers of each other, for example D-allose and D-altrose. The number of aldoses in the L series is equal to that of the d series and each compound is an enantiomer of one in the other series.

Names of cyclised (hemi-acetalised) aldoses and ketoses contain the infixes pyran or furan to indicate the six- or five-membered heterocyclic structure and a stereodescriptor, a or p, to indicate the configuration of the anomeric or hemi-acetal carbon atom. 

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