Arabinose, configuration

Arabinose, configuration of. 982 Kiliani-Fischer synthesis on. 995 Arachidic acid, structure of, 1062 Arachidonic acid, eicosanoids from, 1069-1070... 

The small capital letter prefix refers to configuration, related to n glyceraldehyde, and not to the direction of optical rotation. The sign of optical rotation is expressed as (+) and (—) or as d and I or by the words dextro and Uuvo. Th is we have n.( —)-fructose and ,-(+).arabinose. 

An analogous pattern of configurations can be seen m the aldopentoses when they are arranged m the order ribose arabinose xylose lyxose (RAXL is an easily remem bered nonsense word that gives the correct sequence) This pattern is discernible even m the aldotetroses erythrose and threose... 

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... 

Deoxygalactose (fucose) and arabinose both exist in Nature in d and l forms, and both are related in configuration to the corresponding galactoses. 

The asymmetric synthesis of / -branched carboxylic acid derivatives was accomplished by conjugate addition of mixed organoaluminum reagents to optically active Arabinose-derived c -unsaturated A-acyloxazolidinones (Scheme 47). Efficient stereocontrol was achieved using different optically active bicyclic oxazolidinones, yielding (.R)- or ( -configured / -branched carboxylic acid derivatives.136a... 

Digressing from reductive desulfurization into stereochemistry, we may use this experimental proof of the equivalent symmetry of D-mannitol as a basis for an independent proof of the configurations of D-mannitol and D-arabitol. The reduction of D-arabinose yields the optically active pentitol, D-arabitol application of the Sowden-Fischer synthesis to D-arabinose yields D-mannose86 which upon reduction gives D-mannitol. 

A more direct confirmation that the dimethyl-L-arabinose possesses a pyranose configuration was obtained from the study of the rate with which its lactone derivative is hydrolyzed to the corresponding open chain acid.8... 

Charon and Szabo17 demonstrated that chromophore 4 may be produced to a significant extent from a 5-O-substituted derivative of KDO (which does not contain a free diol grouping at C-4-C-5). These authors synthesized 3-deoxy-5-0-methyl-2-octulosonic acid (7 configuration at C-5-C-7, arabino at C-4, unknown) by the Comforth reaction18,19 from 4-0-formyl-2-0-methyl-D-arabinose (see Scheme 4 and Section IV,1). Compound 7 gave a millimolar extinction coefficient of 13 in the TBA assay (as compared to 92 5 for KDO). Based on this result, Charon and Szabo17 formulated for the TBA reaction of 5-0-... 

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.

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