Organic Chemistry

938 CHAPTER 22 Carbohydrates

Why is there more -D-glucose than -D-glucose in an aqueous solution at equilibri-

um? The OH group bonded to the anomeric carbon is in the equatorial position in -D-

glucose, whereas it is in the axial position in -D-glucose. Therefore, -D-glucose is

more stable than -D-glucose, so -D-glucose predominates at equilibrium in an

aqueous solution.

If you remember that all the OH groups in -D-glucose are in equatorial positions,

it is easy to draw the chair conformation of any other pyranose. For example, if you

want to draw -D-galactose, you would put all the OH groups in equatorial positions,

except the OH groups at C-4 (because galactose is a C-4 epimer of glucose) and at C-1

(because it is the ). You would put these two OH groups in axial positions.

To draw an L-pyranose, draw the D-pyranose first, and then draw its mirror image.

For example, to draw -L-gulose, first draw -D-gulose. (Gulose differs from glucose

at C-3 and C-4, so the OH groups at these positions are in axial positions.) Then draw

the mirror image of -D-gulose to get -L-gulose.

PROBLEM 23

Which OH groups are in the axial position in

a. -D-mannopyranose? c. -D-allopyranose?

b. b-D-idopyranose?

b a

the OH at

C-4 is axial

OH

CH 2 OH HOCH 2

HO OH

HO

HO

OH HO

OH

O O

-D-gulose -L-gulose

the OH at

C-3 is axial

the OH at C-1 is

equatorial ( )

b b

b b

OH

CH 2 OH

HO

HO

OH

O

-D-galactose

the OH at

C-4 is axial

the OH at

C-1 is axial ( )

a-anomer

a

b

a b

a b

b

b a

3-D Molecules:

• D-Galactose;


• D-Gulose;
  b-L-Gulose

b

a

OH

CH 2 OH

HO

HO

HO

O

-D-glucose

36%

axial

OH

CH 2 OH

HO

HO

HO

O

CH O

CH 2 OH

HO

HO

HO

OH

-D-glucose

64%

equatorial