Sialic acids ring conformation

Since the numbering of carbon atoms in JV-acetylneuraminic acid starts at the carboxylate group, to obtain the equivalent conformation of a hexopyranoside (Figure 2.6), 1 should be subtracted from the carbon numbers - but not the ring oxygen. Thus, sialic acid 2,5 is the counterpart of hexopyranoside 4, 82 the counterpart of and S the counterpart of... 

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