Continued from 2-Carb-3 & 2-Carb-4
2-Carb-5.1. Ring size
Most monosaccharides exist as cyclic hemiacetals or hemiketals. Cyclic forms with a three-membered ring are called oxiroses, those with a four-membered ring oxetoses, those with a five-membered ring furanoses, with a six-membered ring pyranoses, with a seven-membered ring septanoses, with an eight-membered ring octanoses, and so on. To avoid ambiguities, the locants of the positions of ring closure may be given; the locant of the carbonyl group is always cited first, that of the hydroxy group second (for relevant examples of this see 2-Carb-6.4). Lack of ring size specification has no particular implication.
Note. The 'o' of oxirose, oxetose, and octanose is not elided after a prefix ending in 'o'.
Nonooctanose, not nonoctanose.
If it is to be stressed that an open-chain form of an aldose is under consideration, the prefix 'aldehydo-' may be used. For ketoses, the prefix is 'keto-'
2-Carb-5.2. The Fischer projection
If a cyclic form of a sugar is to be represented in the Fischer projection, a long bond can be drawn between the oxygen involved in ring formation and the (anomeric) carbon atom to which it is linked, as shown in the following formulae for cyclic forms of α-D-glucose (see 2-Carb-6 for the meaning of α and β):
2-Carb-5.3. Modified Fischer projection
To clarify steric relationships in cyclic forms, a modified Fischer projection may be used. The carbon atom bearing the ring-forming hydroxy group, C-n (C-5 for glucopyranose) is rotated about its bond to C-(n - 1) (C-4 for glucopyranose) in order to bring all ring atoms (including the oxygen) into the same vertical line. The oxygen bridge is then represented by a long bond; it is imagined as being behind the plane of the paper. Examples are given below.
Thus the trans relationship between the hydroxymethyl group and the C-1 hydroxy group in α-D-glucopyranose, and the cis relationship between the methyl group and the C-1 hydroxy group in β-L-fucopyranose, are clearly shown. Note that representation of ketoses may require a different modification of the Fischer projection, as shown in the fructofuranose example above. Here C-2 is rotated about the bond with C-3 to accommodate the long bond to C-2 from the oxygen at C-5.
2-Carb-5.4 The Haworth representation
This is a perspective drawing of a simplified model. The ring is orientated almost perpendicular to the plane of the paper, but viewed from slightly above so that the edge closer to the viewer is drawn below the more distant edge, with the oxygen behind and C-1 at the right-hand end. To define the perspective, the ring bonds closer to the viewer are often thickened.
The following schematic representation of pyranose ring closure in D-glucose shows the reorientation at C-5 necessary to allow ring formation; this process corresponds to the change from Fischer to modified Fischer projection.
The orientation of the model described above results in a clockwise numbering of the ring atoms. Groups that appear to the right of the modified Fischer projection appear below the plane of the ring; those on the left appear above. In the common Haworth representation of the pyranose form of D-aldohexoses, C-6 is above the plane.
Generally, the configuration at the centre that yields the ring oxygen determines whether the rest of the carbon chain is below or above the plane of the ring.
Examples (for the use of α and β see 2-Carb-6):
Note. In writing Haworth formulae, the H atoms bound to the carbon atoms of the ring are often omitted to avoid crowding of the lettering in the ring. For the sake of clarity, the form with H atoms included is preferred in this document.
2-Carb-5.5. Unconventional Haworth representations
It is sometimes desirable to draw Haworth formulae with the ring in other orientations (see Chart II), when there are bulky substituents to be represented, or when linkages in oligo- or poly-saccharides are to be shown. If the ring is inverted [as in (g)-(l)], the numbering runs counterclockwise.
2-Carb-5.6. The Mills depiction
In some cases, particularly where additional rings are present, structural formulae can be clarified by use of the Mills depiction. Here the main hemiacetal ring is drawn in the plane of the paper; dashed bonds denote substituents below this plane, and thickened bonds those above.
2-Carb-5.7. Depiction of conformation
The Haworth representation implies a planar ring. However, monosaccharides assume conformations that are not planar: these may be represented by Haworth conformational formulae. The nomenclature of conformations is described in 2-Carb-7. For example, β-D-glucopyranose assumes a chair conformation:
Note. The hydrogen atoms bonded to carbon are frequently omitted, but their inclusion may be necessary to make a stereochemical point.
2-Carb-5.8. Conformations of acyclic chains
Conformational depictions of acyclic sugar chains are conveniently expressed by locating certain atoms in the plane of the paper and orientating the remaining atoms or groups appropriately above and below that plane, as shown for D-arabinitol and xylitol (it should be recognized that the favoured conformation does not necessarily have all the carbon atoms in the same plane):
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