Question

A derivative of sucrose that is found in sugar beets and higher plants was analyzed and found to have the properties listed in the following.

(a) Mol. Formula: C₁₈H₂₂O₁₆    MW = 504.4

Assuming the compound is made up of C₆ sugars, how many sugars to you expect to find in the molecule?

(b) The compound was subjected to acid hydrolysis and gas chromatographic (GC) analysis of its sugar components by means of their per-O-trimethylsilyl ether derivatives. The sugar components appeared as three sets of double peaks (peaks running close together) that together indicated a Glc:Gal:Fru ratio of 1:1:1, using authentic standards.

Show the products of the hydrolysis and trimethylsilylation reactions. Account for the doubled peaks for the sugars.

(c) A part of the above sugar hydrolysis mixture was subjected to reduction with NaBH₄, and the reduction products were isolated and converted with Ac₂O/pyridine to their alditol acetates. The number of peaks in the gas chromatographic (GC) trace = 3. These corresponded to ratios of Glc 1.3:Gal 1.0:Man 0.7, based on analysis of their per-O-acetyl alditol derivatives.

How did per-O-acetyl-d-mannitol get into the picture? Why did the Glc ratio go to 1.3? Reconcile with the findings in (b).

(d) The original compound has an [α]_D²¹ 105° (c 1, H₂O) and it does not mutarotate. The compound does not reduce Fehling’s solution and is not oxidized by Br₂/H₂O.

What is the significance of these findings?

(e) Treatment of the compound in water with α-galactosidase, an enzyme that specifically hydrolyzes α-d-galactose linkages, gives Gal and sucrose.

What does this indicate?

(f) Exhaustive methylation of the compound with KOH and (MeO)₂SO₂ (dimethyl sulfate) gives a compound with 11 Me groups. Hydrolysis of the methylated derivative gives three identifiable compounds:

2,3,4,6-tetra-O-methyl-d-galactopyranose

2,3,4-tri-O-methyl-d-glucopyranose

1,3,4,6-tetra-O-methyl-d-fructofuranose

Draw each of these structures. Indicate what information about the total structure is revealed.

(g) Collectively using these data, derive the structure for the compound and accurately draw its structure, being careful to show proper stereochemistry. Be careful in depicting the absolute stereochemistry for each sugar component and for each glycosidic linkage. Label each sugar unit (Gal, Glc or Fru), indicate the anomeric linkage (β or α).

Answer 1