Glycosylation Reactions

The widespread occurrence of glycosides and polysaccharides requires processes for attaching sugar units to a suitable atom of an aglycone to give a glycoside, or to another sugar giving a polysaccharide. Linkages tend to be through oxygen, although they are not restricted to oxygen, since S-, N-, and C-glycosides are well known. The agent for glycosylation is a uridine diphosphosugar, e.g. UDPglucose. This is synthesized from glucose 1-phosphate and UTP, and then the glucosylation process can be envisaged as a simple SN2 nucleophilic displacement reaction [Figure 2.27(a)]. Since UDPglucose has its leaving group in the a-configuration, the product has the ^-configuration, as is most commonly found in natural glucosides. Note, however, that many important carbohydrates, e.g. sucrose and starch, possess a-linkages, and these appear to originate via double SN2 processes (see page 470). Other UDPsugars, e.g. UDPgalactose or UDPxylose, are utilized in the synthesis of glycosides containing different sugar units.

The hydrolysis of glycosides is achieved by specific hydrolytic enzymes, e.g. P-glucosidase for P-glucosides and P-galactosidase for P-galactosides. These enzymes mimic the readily achieved acid-catalysed processes [Figure 2.27(b)]. Under acidic conditions, the a- and P-anomeric hemiacetal forms can also equilibrate via the open chain

Glycosylation reactions (a) O-glucosylation

HO T OP

glucose 1-P

^OH ROH

sn2 reaction

UDPglucose

O-P-D-glucoside

(b) hydrolysis of O-glucosides

(c) C-glucosylation

HO HO

hon 'Ci

HO HO

HO{1

OPPU

UDPglucose

HO{1

OPPU

UDPglucose

HO HO

HO HO

HO HO

HOf H

HO HO

HOf H

P-D-glucose

HO HO

P-D-glucose

OH HO

OH HO

HO HO

H OH C-P-D-glucoside

Figure 2.27

H OH C-P-D-glucoside

HO I OH

a-D-glucose

Figure 2.27

b sugar. Of particular importance is that although O-, N-, and S -glycosides may be hydrolysed by acid, C-glycosides are stable to acid. C -Glycosides are produced in a similar manner to the C-alkylation process described above, where a suitable nucleophilic carbon is available, e.g. aromatic systems activated by phenol groups [Figure 2.27(c)]. The resultant C-glycoside thus contains a new carbon-carbon linkage, and cleavage would require oxidation, not hydrolysis.

Was this article helpful?

0 0
Good Carb Diet

Good Carb Diet

WHAT IT IS A three-phase plan that has been likened to the low-carbohydrate Atkins program because during the first two weeks, South Beach eliminates most carbs, including bread, pasta, potatoes, fruit and most dairy products. In PHASE 2, healthy carbs, including most fruits, whole grains and dairy products are gradually reintroduced, but processed carbs such as bagels, cookies, cornflakes, regular pasta and rice cakes remain on the list of foods to avoid or eat rarely.

Get My Free Ebook


Post a comment