Organic Chemistry

936 CHAPTER 22 Carbohydrates

The Haworth projection of a D-furanose is viewed edge on, with the ring oxygen

away from the viewer. The anomeric carbon is on the right-hand side of the molecule,

and the primary alcohol group is drawn upfrom the back left-hand corner.

Ketoses also exist predominantly in cyclic forms. D-Fructose forms a five-membered-

ring-hemiketal as a consequence of the C-5 OH group reacting with the ketone carbonyl

group (Section 18.7). If the new asymmetric carbon has the OH group on the right in a

Fischer projection, the compound is -D-fructose; if the OH group is on the left, the

compound is -D-fructose. These sugars can also be called -D-fructofuranose and

• D-fructofuranose. Notice that in fructose the anomeric carbon is C-2, not C-1 as in
  aldoses. D-Fructose can also form a six-membered ring by using the C-6 OH group. The
  pyranose form predominates in the monosaccharide, whereas the furanose form
  predominates when the sugar is part of a disaccharide. (See the structure of sucrose in
  Section 22.17.)

Haworth projections are useful because they allow us to see easily whether

the OH groups on the ring are cis or trans to each other. Because five-membered

rings are close to planar, furanoses are well represented by Haworth projections.

However, Haworth projections are structurally misleading for pyranoses because a

six-membered ring is not flat but exists preferentially in a chair conformation

(Section 2.12).

b

b a

a

HO H O

OH

HOH

H

CH 2 OH

H

H OH

HO H O

OH

HOH

H

CH 2 OH

H

HO H

HOHO

OH

HOH

H

CH 2 OH

H

HOHO

H

HOH

H

CH 2 OH

HO

HOCH 2 HOCH 2

O O

OH

OH

OH OH

OH OH

HO

HOCH 2

OH

OH

OH

HO

HOCH 2

OH

OH OH

O O

1 2 3 4 5 6

1 2 3 4 5 6 5

4 1 4 1

(^32)
5
3 2
Fischer projections
Haworth projections

• D-glucopyranose -D-glucopyranose -D-ribofuranose -D-ribofuranose

3-D Molecules:

• D-Glucopyranose;


• D-Glucopyranose;


• D-Ribofuranose;
  b-D-Ribofuranose

a

b

a

HO H O

OH

HOH

H

CH 2 OH

HOCH 2

HO H O

CH 2 OH

HOH

H

CH 2 OH

HO

HO H O O

OH

HOH

HOH

CH 2

HOCH 2

HO H

CH 2 OH

HOH

HOH

CH 2

HO

CH 2 OH

HOCH (^2) CH 2 OH CH 2 OH
O O
OH
OH
HO HO
HO
HO
HO HO
OH
O
HO
HO
HOCH 2
CH 2 OH
OH
HO HO
O

• D-fructofuranose -D-fructofuranose -D-fructopyranose -D-fructopyranose

Fischer projections

Haworth projections

(^12)
3
4
5
6
(^12)
3 4 5 6 1 6
(^252)
1
5
(^43)
6
43
Sir Walter Norman Haworth
(1883–1950)was born in England.
He received a Ph.D. in Germany
from the University of Göttingen and
later was a professor of chemistry at
the Universities of Durham and
Birmingham in Great Britain. He was
the first to synthesize vitamin C and
was the one who named it ascorbic
acid. During World War II, he worked
on the atomic bomb project. He re-
ceived the Nobel Prize in Chemistry
in 1937 and was knighted in 1947.