5
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Looking at this structure, I would say this molecule has five chiral carbons. However, its IUPAC name (3R,4R,5S,6R)-3-amino-6-(hydroxymethyl)oxane-2,4,5-triol suggests that there are only four. Which is it?

enter image description here

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  • $\begingroup$ It is the hemiacetal carbon atoms. It is chiral but racemization can occur through the open form of the molecules. $\endgroup$ – Marko Feb 22 '16 at 14:43
  • $\begingroup$ The diagram you drew doesn't indicate the stereochemistry, so I am curious as to how you came up with that IUPAC name. $\endgroup$ – orthocresol Feb 22 '16 at 17:05
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The structure as drawn indeed contains 5 chiral carbon atoms. However, the one carbon that is bound to two oxygen atoms is called the anomeric carbon. It is part of a hemiacetal moiety in the structure you have drawn. That means that at room temperature and in aqueous solution, it will equilibrate reasonably quickly between:

  1. One enantiomer of hemiacetal
  2. An open-chain non-cyclic molecule where the anomeric carbon is an aldehyde moiety and is thus not chiral.
  3. The other enantiomer of hemiacetal.

Thus, for many applications it is probably better to think of this molecule as having only four "fixed" stereocenters.

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