Structural Characterization of Disaccharide Rings and Glycosidic Linkages

To determine the correct statement about the given disaccharide, we analyze the structure of each ring:

1. Ring (a):

   • Ring Size: Ring (a) consists of 5 carbon atoms and 1 oxygen atom forming a six-membered cyclic system. Therefore, it is a pyranose ring.
   • Anomeric Configuration ($\alpha$ or $\beta$):
     • The $-\mathrm{CH_2OH}$ group at $\text{C-5}$ points upwards (above the plane of the ring), which is characteristic of the $\text{D}$-configuration.
     • The glycosidic substituent ($-\mathrm{OCH_2CH_2O-}$) at the anomeric carbon ($\text{C-1}$) is directed downwards (below the plane of the ring).
     • Because the $-\mathrm{CH_2OH}$ reference group at $\text{C-5}$ and the glycosidic oxygen at $\text{C-1}$ are located on opposite faces of the ring (trans to each other), the anomeric carbon has the $\alpha$-configuration.
   • Thus, Ring (a) is a pyranose with an $\alpha$-glycosidic linkage.

2. Ring (b):

   • Ring (b) consists of 4 carbon atoms and 1 oxygen atom forming a five-membered cyclic system, meaning it is a furanose ring.
   • At the anomeric carbon (the ketal carbon on the left), the glycosidic oxygen is directed downwards while the $-\mathrm{CH_2OH}$ is directed upwards.
   • The reference $-\mathrm{CH_2OH}$ group at the far stereocenter (rightmost carbon) is directed downwards. Since the glycosidic oxygen and the reference $-\mathrm{CH_2OH}$ group are on the same side (cis), this corresponds to a $\beta$-glycosidic linkage, making statement C incorrect.

Therefore, the only correct statement is that Ring (a) is pyranose with an $\alpha$-glycosidic link.

Correct Answer: A