ABSTRACT
Figure 1.3 Fischer projection of the chiral glyceraldehyde.The unequivocal advantage of the Fischer projection is that the absolute structure can be inferred, according to the following definition: the horizontal lines of the central cross lines are pointing towards the observer, while the two vertical lines are directing into the plane. Addition of an additional H-C-OH group (aldotetroses) gives rise to a second stereogenic centre. As a consequence, 2n = 4
stereoisomers can be expected, i.e., two pairs of enantiomers, where the stereochemical relation between these two pairs is denominated with the term diastereomer. It is worth noting that enantiomers basically exhibit the same physical parameters, while diastereomers may show different ones, provided that no enantioselective interactions with other chiral molecules or polarized light is encountered. In the case of aldopentoses, three stereogenic centres are present, and for aldohexoses four stereogenic centres are found, resulting in 23 = 8 and 24 = 16 stereoisomers, respectively. For example, in the case of the aldohexoses the eight stereoisomers shown in Fig. 1.2 plus the respective mirror images (enantiomers) exist. Basically, at each stereogenic centre a d-or l-notation is conceivable. However, in order to facilitate the nomenclature, a clear definition allows an assignment to the d-and l-notation: the stereogenic centre that is most remote of the carbonyl function (carbon exhibiting the highest oxidation number) determines the assignment to the group of d-and l-stereoisomers. However, it has to be noted that the absolute structure of the molecule cannot be directly inferred from the name; it can only be indirectly obtained on the background of additional knowledge of the glucose structure. Diastereoisomers that exhibit the opposite configuration at only one of two or more tetrahedral stereogenic centres present in the respective molecular entities are called epimers. This holds, for example, for the diastereomeric pair d-glucose and d-mannose (see Fig. 1.2).Alternatively, the R-/S-notation that is based on the CahnIngold-Prelog (CIP) rules can be applied to carbohydrates with several stereogenic centres. For example, strict assignment of each stereogenic centre in d-glucose to the R-/S-notation results in the exact IUPAC name (2R,3S,4R,5R,6)-pentahydroxyhexanal. In contrast to the d/l notation, this name allows an exact assignment of each stereogenic centre to its absolute structure. However, as this name is very inconvenient in publications, the name d-glucose as well as the respective names of other sugars according to the d/l notation are still dominating. 1.2.1.2 Ring structures of carbohydratesAs early as 1883 Tollens had recognized that simple aldohexoses such as glucose do not show all characteristics of aldehydes. In
particular, no addition of sodium hydrogensulfate or ammoniak were observed, and no reaction with Schiff ’s reagent takes place. Tollens inferred from these observations that simple sugars do not prefer the aldehyde or keto form arrangement. Instead, an intramolecular hemiacetal formation may lead to cyclic structures, as shown in Fig. 1.4. These Tollens formulae suggest that basically five-or six-membered rings can be formed by reaction of the carbonyl function with the hydroxy group at C4 or C5. Furthermore, it is evident that an additional stereogenic centre is formed at the C1 position. If the hydroxy group bonded to the new asymmetric centre shows to the right the hemiacetal is called a-d-glucose, and if the hydroxy group points to the left it is b-d-glucose. a-d-Glucose and b-d-glucose are called anomers. Anomers are two sugars that differ in configuration only at the carbon that was the carbonyl carbon in the open-chain form. Ano is the Greek word for “upper”, which indicates that anomers differ in configuration at the uppermost asymmetric carbon. As a consequence, a-d-glucose and b-d-glucose are diastereomers (not enantiomers) exhibiting different physical parameters such as solubility, melting point, and specific rotation.
