Section 22.10/11. Cyclic structures: anomers.

These are the two anomers (diastereoisomers differing only at the acetal/hemiacetal site) of glucopyranose, a cyclic form of glucose with a six-membered, pyran-like ring. The anomers exist because a new asymmetric center is created when the hemiacetal forms. In general, the anomer with the hemiacetal OH group trans to the terminal -CH₂OH group is the alpha anomer, that with the two groups cis is the beta anomer. The model on the left is of the alpha anomer, that on the right of the beta. In aqueous solution the two are in equilibrium (via the open-chain form), though the beta form dominates in this case, 64% to 36%, as it has all substituent groups equatorial. At equilibrium, only 0.02% is the open-chain form.