L 434

physical descriptors has been presented above. The reason the D/L system continues to be used is essentially biochemical. For example D-(+)-glucose (4.25), the enantiomer of glucose found in biological systems, could be known as (2R, 3S, 4R, 5R)-2,3,4,5,6-pentahydroxyhexanal or (2R, 3S, 4R, 5R)-aldohexose, which does not take into account the ring structure of the molecule and the two anomeric forms. It is obviously simpler to refer to D-(+)-glucose. Also the naturally occurring amino acids, are of the L-configuration and the application of the R/S system results in a lack of consistency within the series. For example L-Serine (4.28) has the S-absolute configuration while L-cysteine (4.34) has the R-configuration as a result of the presence of the sulphur atom.

Additional complexities may also arise in the nomenclature of semisynthetic products, as in some cases both systems are used to define the structure of the molecule. For example the absolute stereochemistry of the 6-aminopenicillanic acid and 7-aminocephalosporanic acid nucleii have been determined and defined in terms of the R/S system but the addition of a side chain, e.g. ampicillin (4.35) and cephalexin (4.36), may result in the introduction of an additional chiral centre, which in the case of these two compounds is frequently

(4.35}; ampicillin

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