Saccharides Flashcards
Biological functions of saccharides
Metabolic energy source
Structural components of cell walls/ECM
Recognition sites on cell surfaces for adhesion/signalling
Examples of a polysaccharide
Cellulose
Aldose
Reducing sugar
Ketose
Non-reducing sugar
How many monosaccharide units are in an oligosaccharide?
2-5
Examples of glycoproteins
EPO (erythropoietin hormone) - secreted by the kidneys, increases production of RBCs - glycosylation increases its stability/activity
Bacterial cell wall
Cartilage - acts as a ‘shock absorber’ due to the movement of water under pressure
Heparin - polysaccharide, helps prevent blood clotting
Mono-protection of the primary alcohol at glucose C6.
Use trityl bromide + base
Conditions for selection acyl/benzoyl deprotection at anomeric position.
Hydrazine (N2H4), AcOH
Anomeric OH is always acetylated/benzoylated in the…
…beta position.
Acetone as a protecting group
Protects glucose in its furanose form (1,3-diaxial clashing in pyranose form)
Protects 1,2-diols - C1&2, C5&6
** mild H+ will only deprotect C5&6 **
Acetone =
Isopropylidene
Benzaldehyde as a protecting group
Protects glucose in its pyranose form
Protects 1,3-diols
Reaction is catalysed by ZnCl2 (Lewis acid)
Benzaldehyde =
Benzylidene
Examples of ether protecting groups for sugars
Trityl (selective for primary alcohols)
Benzyl (non-selective) - but anomeric Bn group can be selectively removed using mild H2 conditions
“Non-participating group”
= no carbonyl
What is the electrophile in a glycosylation reaction?
The glycosyl donor
What is the nucleophile in a glycosylation reaction?
The glycosyl acceptor (i.e. the polymer)
What determines the preference for whether the nucleophile adds axial or equatorial in a glycosylation reaction?
The presence of a (non-)participating group on C2
Fisher glycosylation
= “acid-catalysed addition of an aglycone”
Generally uses the per-acetylated sugar so the product is beta
Koenigs-Knorr reagents
HBr, AcOH to give ALPHA glycosyl halide
Ag2CO3, ROH, DCM for glycosylation (driving force is formation of AgBr salt)
Why is AgOTf a preferred Ag salt for the Koenigs-Knorr reaction?
AgOTf is organic soluble
Preparation of thioglycosides
Prepared from the per-acetylated sugar using BF3.OEt2 and R-SH
What are the two methods for activating thioglycosides?
- Using electrophilic thiophilic promoters e.g. MeSSMe2(+)
2. Oxidation of S to the sulfoxide using mCPBA, followed by addition of a triflate
