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The conformations adopted by a pyranose or furanose ring are denoted by a system in which a capital letter indicates the overall shape, C = chair, B = boat, H = half chair, S = skew boat, E = envelope [1, 2]. The first four of these conformations has four atoms in a plane; the envelope conformation has five.
The particular conformation is then denoted by assigning the letter corresponding to the shape (C, B, H, S, E); determining the four (or five) atoms that define the plane; assigning a 'top' and 'bottom' face through the use of a left-hand rule counting in the order of increasing ring carbon; and then indicating the identities and relative positions (top face = superscript and prefix; bottom face = subscript and suffix) of the remaining two atoms on that capital letter. In the case of the envelope conformation, only a single atom is located outside of the plane. For a more detailed discussion see the excellent book by J. Fraser Stoddard [3].
It should be noted that the conformational symbols for enantiomers are different. This is because the reference plane is the same, yet application of the left-hand rule results in a different 'top' and 'bottom' face. Thus the mirror image of α-D-glucose-4C1 is α-L-glucose-1C4. It is therefore important to state whether the D or L form is under consideration.
References
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Conformational nomenclature for five- and six-membered ring forms of monosaccharides and their derivatives, Pure and Appl. Chem., 1981, 53, 1901—1905. DOI: 10.1351/pac198153101901[IUPACNomenclature1981]
