CHEM331 Carbohydrate Workshop Wednesday 2-6 pm, Week 4, 1999

1.

For each of the structures shown below, provide i) their general name, i.e. aldotetrose, ketopentose, etc; ii) specific name (e.g. D-ribose); iii) label the chiral centres; and iv) state whether they are optically active or not. What is the relationship between compounds a, c and d and between a and g?

CH2OH

CHO H OH HO H OH H H OH

O H OH OH

CHO HO H HO H OH H H OH

CH2OH

CH2OH

HO H H

CH2OH

b

a

H HO HO H

COOH H HO H H

OH H OH OH

d

CHO HO H H OH H HO HO H

COOH

CH2OH

OH H H OH CH2OH

c

CH2OH H OH HO H OH H H OH

CH2OH

f

e

2.

CH2OH

g α-D-Mannose, a monosaccharide found in some fruits and vegetables, has the pyranose structure shown. Pure α-D-mannose is sweet. The β-anomer is present is orange peel and is partly responsible for its characteristic bitter taste. Is it surprising that the α- and β-anomers have such different levels of sweetness? Why? A solution of α-D-mannose upon standing becomes bitter. With the aid of structures provide an explanation for this. CH2OH O H OH OH

OH OH

α-D-mannose

3.

Fructose (shown below), the principal sugar in honey, is commonly used as a sweetener.

This sugar in the β-D-pyranose form is one of the sweetest carbohydrate sugars known. The β-Dfuranose form, however, is much less sweet. Using the models provided make the Fischer projection of D-fructose and use this to make the structures of β-D-fructofuranoside and β-Dfructopyranoside. Draw the Haworth projections of the furanose and pyranose forms. The sweetness of honey decreases upon standing with increasing temperature. Explain.

CH2OH O H OH OH

HO H H

CH2OH D-Fructose

3.

What is the absolute configuration (R or S) of the chiral centres of D-erythrose? CHO H OH H OH CH2OH D-erythrose

4.

Show, using structural diagrams, the mechanism of methyl acetal formation from a hemiacetal. What reagents are required? Why are only the anomeric hydroxyls of sugars methylated under these conditions?

5.

Write the open-chain structure for a) D-galactitol b) D-galactonic acid c) D-galactaric acid Starting with D-glucose what reagents would be required to produce the three derivatives a, b and c?

6.

Explain why the Benedict's, Fehling's or Tollen's reagents cannot be used to differentiate between aldoses and ketoses, and give a reagent that can be used.

7.

Using complete structures, write out the reaction of D-mannose with: a) bromine water b) nitric acid c) sodium borohydride d) acetic anhydride If the reaction with acetic anhydride was under thermodynamic control what would be the major product of this reaction? CHO HO H HO H OH H H OH CH2OH D-mannose

8.

Reduction of D-fructose with H2 and catalyst gives D-glucitol and D-mannitol. Explain. CH2OH HO H H

O H OH OH CH2OH

D-Fructose

9.

When D-glucose is treated with dry HCl in MeOH, followed by NaOH and (CH3)2SO4, a mixture of two pentamethylated anomers is obtained. Draw the products formed after the MeOH/HCl treatment and after the final NaOH/(CH3)2SO4 step and explain why the pentamethylated anomers could not be cleanly formed by treating D-glucose with only NaOH and (CH3)2SO4.

10.

The manufacture of liquid filled chocolates containing a liquid centre is an interesting application of enzyme engineering. The flavoured liquid centre consists largely of an aqueous solution of sugars rich in fructose to provide sweetness. The technical dilemna is the following: the chocolate coating must be prepared by pouring hot melted chocolate over a solid core, yet the final product must have a liquid, fructose-rich centre. Suggest a way to solve the problem. (Hint: the solubility of sucrose is much lower than the combined solubility of glucose and fructose.)

11.

Two pairs of enantiomeric aldotetroses are known. One pair form a single optically inactive substance upon treatment with sodium borohydride. The other pair yields a pair of enantiomers. Draw the Fischer projection of the aldotetroses and their products.

12.

Draw the structures of all products that result from treatment of the following molecules with periodic acid under mild and exhaustive conditions. How many molar equivalents of periodic acid would be consumed in each case? a) D-fructose b) D-galactose c) trehalose CH2OH

OH O

H

H

OH OH

O

OH HOH2C O

OH

OH alpha-linkages trehalose

13.

An L-aldopentose A is oxidised by nitric acid to give an aldaric acid B, which is optically active. Also, compound A is chain shortened by a Ruff or Wohl degradation to an aldotetrose C. C undergoes reduction with sodium borohydride to an optically inactive aldonic acid D. Write the structures of A through to D. Provide reasonings for your choices.

14.

Trehalose is a disaccharide that is the main carbohydrate component in the blood of insects. Write down the systematic name for trehalose (hint: make a model). Will trehalose give a positive or negative test with Fehling's reagent? Draw the structure(s) obtained following periodic acid cleavage of trehalose. CH2OH

OH O

H

H

OH O

OH

OH HOH2C O

OH

OH alpha-linkages trehalose

15.

Explain, with the aid of diagrams, why lactose [O-β-D-galactopyranosyl-(1,4)-Dglucopyranose] forms a phenylosazone derivative, but sucrose [O-α-D-glucopyranosyl(1,2)-β-D-fructofuranoside] does not. Draw the phenylosazone derivative of lactose and outline a method for its preparation.

16.

Show how the following experimental evidence can be used to deduce the structure of the disaccharide AB. i.

Acid hydrolysis of AB, C12H22O11, gives equimolar quantities of D-glucose and Dgalactose. AB undergoes a similar hydrolysis in the presence of β-galactosidase.

ii.

AB is a reducing sugar and forms a phenylosazone; it also undergoes mutarotation.

iii.

Oxidation of AB with bromine water followed by hydrolysis with dilute acid gives D-galactose and D-gluconic acid.

iv.

Bromine water of AB followed by methylation and hydrolysis gives 2,3,6-tri-Omethylgluconolactone and 2,3,4,6-tetra-O-methyl-D-galactose.

v.

Methylation and hydrolysis of AB gives 2,3,6-tri-O-methyl-D-glucose and 2,3,4,6tetra-O-methyl-D-galactose.

17.

A naturally occurring compound, J, has the formula C7H14O6. It is a non-reducing sugar and does not mutarotate. Compound J is hydrolysed by aqueous HCl to compound K, C6H12O6, a reducing sugar. Oxidation of K with dilute HNO3 gives an optically inactive diacid, L, C6H10O8. Ruff degradation of K gives a new reducing sugar, M, C5H8O7. Compound J is treated successively with NaOH and dimethyl sulfate, aqueous HCl, and hot nitric acid. From the product mixture, one may isolate

α,β-dimethoxysuccinic acid and α-methoxymalonic acid. Give the structures for compounds J through to N. What structural ambiguity exists, if any? COOH H

OMe

H

OMe

COOH H

COOH

COOH

α,β-methoxymalonic acid

α,β-dimethoxysuccinic acid

18.

OMe

2-Amino-3-(β-D-glucopyranosyloxy)-γ-oxobenzenebutanoic acid (A) is a naturally occurring glucoside found in the human lens that protects the lens from UV damage. It can be synthesised from B (Scheme 1). Suggest possible reagents to get to C and A. Why couldn’t D be used to generate the glucoside instead of B?

O

CO2Me NO2

CO2Me

O

NO2

??

OH

O

CO2H NH2

??

O CH2OAc

B

O CH2OH

O

O

OAc

OH H

OAc

H

OH OAc

C

OH

A

Scheme 1 CO2H

O

NH2 OH D

19.

Most paper is made by removing the lignin from wood pulp and forming the resulting largely unoriented cellulose fibres into a sheet. Untreated paper loses most of its strength when wet with water but maintains its strength when wet with oil. Explain.

20.

A recent ‘eat-all-you-want’ drug, which was touted as a ‘starch blocker’ (and which was banned by the Food and Drug Administration) contains an α-amylase-inhibiting protein extracted from beans. (α-Amylase breaks starch down into smaller saccharide units for digestion.) If this substance really worked, as advertised, which it does not, what unpleasant side effects would result from its ingestion with a starch-containing meal?

21.

E. coli contain the protein PapG on their outer surface, which allows them to attach to α-Dgalactopyranosyl-(1-4)-β-D-galactopyranose groups that are present on the surface of urinary tract epithelial cells. This in turn allows urinary tract infection to take place. Instilling methyl-α-D-mannoside into the bladder of a mouse prevents the colonisation of its urinary tract by E. coli. What is the reason for this effect?