I'm wondering how do you count the carbon atom as shown in
http://en.wikipedia.org/wiki/Haworth_projection for a fructose?
Looking at the structural diagram on that page, isn't the "carbon numbering" on the diagram molecule what you want?
As to
why it is numbered like that, all I can offer is the link to that old, and rather difficult to read, IUPAC document and on page 2 it suggests
to me one starts numbering from the carbonyl when the molecule is in straight chain form. Of course, if there isn't a straight chain form of the molecule then that method won't work.
I'll make it clear this info is purely from Googling and reading. I'm not an expert on this - anyone else have a better, clearer reference than what I have provided?
Clive
Edit
Is this relevant?
http://www.chem.qmul.ac.uk/iupac/misc/psac.html1.3. Atomic Numbering
The atom numbering of two monosaccharide units (a hexopyranose and a hexofuranose) is shown in Fig. 1. The notation used here conforms with that being proposed for specifying polynucleotide conformation. Atoms are thus designated C3, O2, H4, etc. The hydrogen atoms of a methylene group may be distinguished by an additional number, e.g. H61 and H62 where the lower number is selected for the pro-S atom [7]. When it is necessary to indicate the particular saccharide unit its number may be added in parenthesis, e.g. O3(i ), C4(i+1), H61(i-1).
(note to self. when in hole, stop digging)