Saccharides

Question 1

Biological functions of saccharides

Answer 1

Metabolic energy source
Structural components of cell walls/ECM
Recognition sites on cell surfaces for adhesion/signalling

Question 2

Examples of a polysaccharide

Answer 2

Cellulose

Question 3

Aldose

Answer 3

Reducing sugar

Question 4

Ketose

Answer 4

Non-reducing sugar

Question 5

How many monosaccharide units are in an oligosaccharide?

Answer 5

2-5

Question 6

Examples of glycoproteins

Answer 6

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

Question 7

Mono-protection of the primary alcohol at glucose C6.

Answer 7

Use trityl bromide + base

Question 8

Conditions for selection acyl/benzoyl deprotection at anomeric position.

Answer 8

Hydrazine (N2H4), AcOH

Question 9

Anomeric OH is always acetylated/benzoylated in the…

Answer 9

…beta position.

Question 10

Acetone as a protecting group

Answer 10

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 **

Question 11

Acetone =

Answer 11

Isopropylidene

Question 12

Benzaldehyde as a protecting group

Answer 12

Protects glucose in its pyranose form
Protects 1,3-diols
Reaction is catalysed by ZnCl2 (Lewis acid)

Question 13

Benzaldehyde =

Answer 13

Benzylidene

Question 14

Examples of ether protecting groups for sugars

Answer 14

Trityl (selective for primary alcohols)

Benzyl (non-selective) - but anomeric Bn group can be selectively removed using mild H2 conditions

Question 15

“Non-participating group”

Answer 15

= no carbonyl

Question 16

What is the electrophile in a glycosylation reaction?

Answer 16

The glycosyl donor

Question 17

What is the nucleophile in a glycosylation reaction?

Answer 17

The glycosyl acceptor (i.e. the polymer)

Question 18

What determines the preference for whether the nucleophile adds axial or equatorial in a glycosylation reaction?

Answer 18

The presence of a (non-)participating group on C2

Question 19

Fisher glycosylation

Answer 19

= “acid-catalysed addition of an aglycone”

Generally uses the per-acetylated sugar so the product is beta

Question 20

Koenigs-Knorr reagents

Answer 20

HBr, AcOH to give ALPHA glycosyl halide

Ag2CO3, ROH, DCM for glycosylation (driving force is formation of AgBr salt)

Question 21

Why is AgOTf a preferred Ag salt for the Koenigs-Knorr reaction?

Answer 21

AgOTf is organic soluble

Question 22

Preparation of thioglycosides

Answer 22

Prepared from the per-acetylated sugar using BF3.OEt2 and R-SH

Question 23

What are the two methods for activating thioglycosides?

Answer 23

1. Using electrophilic thiophilic promoters e.g. MeSSMe2(+)

2. Oxidation of S to the sulfoxide using mCPBA, followed by addition of a triflate

Question 24

Reagent for per-methylation of all free OH groups in a sugar.

Answer 24

Dimethyl sulfate

Me2SO4

Question 25

Conditions for glycosidic bond hydrolysis.

Answer 25

100 oC, HCl/H2O (this will also hydrolyse anomeric OME - because this group is an acetal)

Question 26

What are the 3 classes of natural products?

Answer 26

1. Alkaloids
2. Terpenoids/steroids
3. Polyketides and polyethers

Question 27

Examples of alkaloids

Answer 27

Cocaine
Curare
Opoim

Question 28

What are terpenoids derived from?

Answer 28

Isoprene

Question 29

What are steroids?

Answer 29

Cyclised squalene

Question 30

Functions of steroids.

Answer 30

Cell membrane stabilisers (decrease fluidity)
Signalling molecules e.g. hormones
Vitamin precursors