Biomolecules · Question 2

A known artificial sweetener X is composed of 4-chloro-4-deoxy-α-D-galactose and 1,6-dichloro-1,6-dideoxy-β-D-fructose joined by a glycosidic linkage. Structure of D-galactose is given below: [figure] The correct structure of X is

1. [figure]
2. [figure]
3. [figure]
4. [figure]

Answer

(1) [figure]

Solution

X is sucralose: C-1 (anomeric C) of the galactose unit is joined through O to C-2 (anomeric C) of the fructose unit, as in sucrose.

Galactose unit: D-galactose differs from D-glucose only at C-4, its C-4 substituent being axial (up in the chair). In 4-chloro-4-deoxy-galactose the Cl therefore occupies this axial (up) position at C-4, with H equatorial; the glycosidic O at C-1 is axial (down) = α.

Fructose unit: furanose with CH2Cl at C-1 (on C-2) and C-6 (on C-5), linked to the galactose by O on C-2 directly ( ).

(B) has Cl at C-4 in the equatorial (glucose-type) position; in (C) and (D) the O-bridge is attached through a CH2 group instead of the anomeric carbon of fructose, so they are wrong.

Answer: (A)