Configurations, sugars Fischer formula

Sometimes the terms erythro and threo are used to specify fee relative configuration of two adjacent stereogenic centers. The terms are derived fom fee sugars erythrose and threose. The terms were originally defined such feat a Fischer projection formula in which two adjacent substituents were on the same side was fee erythro isomer and feat in whidi the substituents were on opposite sides was the threo isomer. 

By convention, one of these two forms is designated the d isomer, the other the l isomer. As for other biomolecules with chiral centers, the absolute configurations of sugars are known from x-ray crystallography. To represent three-dimensional sugar structures on paper, we often use Fischer projection formulas (Fig. 7 2). 

Arabinose—as could be shown by chain extension and degrading of glucose and mannose—forms a portion of both of these hexoses its configuration can be neither that of ribose, nor that of xylose, because, in contrast to arabinose, these pentoses form inactive dicarboxylic acids. The formula (7) (lyxose) is ruled out for arabinose, since a one-carbon addition to it leads to an inactive (galactaric) acid and an active (talaric) acid. Consequently, D-arabinose must have formula (5) and with that conclusion, (10) had been derived for D-(+)-glucose—but with one restriction. Fischer had to choose between (10) and its enantiomorph he chose (10), and wqs aware of the arbitrary nature of this decision. The same decision fixed the form of all the other series-related sugars represented in Fig. 1. That Fischer had accidentally chosen the correct absolute configuration was not realized until after his death. 

Fischer projections choose the cyclic and planar, //-eclipsed conformation of the carbon skeletons of sugars as a basis, then pull the cycle straight to form a line and orient it so that the more highly oxidized end Cl is located at the top. This projection procedure remained alive for almost a century because inherent symmetry properties in carbohydrates become apparent here in the most simple way. Always remember Fischer projection formulas are the short notations of a/l-eclipsed cyclic conformations, although they look linear (Fig. 4.2.1). The terminal chiral center, C5 in hexoses, is usually R- or, in the Fischer notation, D-configured. 

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