Structure of Sucralose Artificial Sweetener X from Galactose and Fructose

To determine the correct structure of the artificial sweetener $\mathbf{X}$ (Sucralose), we analyze its two constituent monosaccharide units and their stereochemistry:

1. Analysis of the 4-chloro-4-deoxy-$\alpha$-D-galactose Unit:

• Fischer Projection of $\text{D}$-galactose: \text{CHO} \\ | \\ \text{H}-\text{OH} \quad (\text{C2: Right}) \\ | \\ \text{HO}-\text{H} \quad (\text{C3: Left}) \\ | \\ \text{HO}-\text{H} \quad (\text{C4: Left}) \\ | \\ \text{H}-\text{OH} \quad (\text{C5: Right}) \\ | \\ \text{CH}_2\text{OH} \end{array}$$
• Stereochemistry in Pyranose Form:
  • C2: $-\text{OH}$ is on the right $\implies$ points DOWN.
  • C3: $-\text{OH}$ is on the left $\implies$ points UP.
  • C4: In $\text{D}$-galactose, $-\text{OH}$ is on the left $\implies$ points UP. Thus, in 4-chloro-4-deoxy-$\alpha$-D-galactose, the chlorine atom ($-\text{Cl}$) at $\text{C4}$ must point UP.
  • C1 ($\alpha$-anomer): The anomeric $-\text{OH}$ (or glycosidic oxygen) points DOWN (trans to the C5 group).

---

2. Analysis of the 1,6-dichloro-1,6-dideoxy-$\beta$-D-fructose Unit:

• Derived from $\beta$-$\text{D}$-fructofuranose where hydroxyl groups at $\text{C1}$ and $\text{C6}$ are replaced by chlorine atoms ($-\text{CH}_2\text{Cl}$).
• Stereochemistry in Furanose Form:
  • C2 (Anomeric Carbon): For the $\beta$-anomer in the standard fructofuranose ring layout, the $-\text{CH}_2\text{Cl}$ group at $\text{C1}$ points UP, and the glycosidic oxygen at $\text{C2}$ points DOWN.
  • C3: $-\text{OH}$ group points UP ($\text{HO}$ on top).
  • C4: $-\text{OH}$ group points DOWN.
  • C5: $-\text{CH}_2\text{Cl}$ group at $\text{C6}$ points UP.

---

3. Comparison of Options:

• Option (A):
  • At $\text{C4}$ of the galactose unit, $-\text{Cl}$ points UP.
  • At $\text{C1}$ of galactose, the glycosidic oxygen points DOWN ($\alpha$-linkage).
  • At $\text{C2}$ of the fructose unit, $-\text{CH}_2\text{Cl}$ points UP ($\beta$-linkage).
• Option (B): Incorrect because $-\text{Cl}$ at $\text{C4}$ of galactose points DOWN.
• Options (C) & (D): Incorrect because the configuration at $\text{C2}$ of the fructose ring is inverted ($-\text{CH}_2\text{Cl}$ points DOWN).

Therefore, the correct structure of $\mathbf{X}$ is given in Option (A).