5.9. Logic Puzzles: Using Chemical Reactions to Determine Sugar Structures
It is possible to use these chemical reactions and critical thinking to determine the structures of more complex unknown (di-/tri-)saccharides. Often reaction information is accompanied by additional data to help simplify the process. Depending on the reaction(s) and information given, the difficulty can range from trivial to challenging. Typically, problems are generated to be straightforward provided all of the information is interpreted.
All structural determination questions are logic puzzles. In long answer questions part marks are awarded for showing the structural fragments and logic/rationale behind your interpretation; even if you struggle to generate a complete answer you can still achieve marks by showing your work/thought process and getting some of the pieces.
The specific details for each kind of question vary; not all considerations may be relevant for any given question. As a general guideline for the order of steps:
1. Determine D/L for each monomer (monosaccharide). Use the structure(s), convert to Fischer projection(s) if needed. For the common monosaccharides the simplest approach is to determine the absolute configuration of the last stereocentre. If the last stereocentre has an R absolute configuration, then it is a D-saccharide. If the last stereocentre has an S absolute configuration, then it is an L-saccharide. This only works for the common monosaccharides.
2. Determine the monomer(s) (monosaccharides). Determine aldose vs. ketose, then chain length, and finally stereochemistry. Use the structure(s), convert to Fischer projection(s) if needed. Checking the relative configurations typically works well. Remember that the absolute configurations can change when the saccharide is cyclized. Drawing/visualizing the open forms is often helpful.
3. Determine the type of connectivity (the type of glycosidic bond(s)). Remember that glycosidic bonds may be α or β and that the positions that are connected can vary. Often at least part of this is given as additional information. Often logic is required to determine which positions are/were involved. The reaction information is often useful.
4. Draw and/or Name the Unknown Compound. Some questions may ask for a drawing, the full name, and/or part of the name. Follow the standard rules for naming (see Sections 5.3 and 5.6.1) and drawing (see Section 5.2.1 and subsections therein).
Example:
A standard disaccharide was determined to have an α-glycosidic bond using an enzyme.
Separately, the disaccharide was treated with Ag2O and CH3I, followed by hydrolysis with aqueous HCl,
to yield the following products:
Draw and name the original disaccharide.
.
Determine D/L for each monomer.
Determine the monomers’ names.
Determine the type of connectivity (the type of glycosidic bond(s)).
Draw and/or Name the Unknown Compound. Some questions may ask for a drawing, the full name, and/or part of the name. Follow the standard rules for naming (see Sections 5.3 and 5.6.1) and drawing (see Section 5.2.1 and subsections therein).
