Organic Chemistry

Section 22.17 Disaccharides 943

The products obtained from periodate cleavage of a six-membered-ring acetal are
different from those obtained from cleavage of a five-membered-ring acetal.

PROBLEM 26

What kind of aldohexose would form L-glyceraldehyde when its acetal is oxidized with

periodic acid?

22.17 Disaccharides

If the hemiacetal group of a monosaccharide forms an acetal by reacting with an
alcohol group of another monosaccharide, the glycoside that is formed is a disac-
charide. Disaccharidesare compounds consisting of two monosaccharide subunits
hooked together by an acetal linkage. For example, maltose is a disaccharide
obtained from the hydrolysis of starch. It contains two D-glucose subunits hooked
together by an acetal linkage. This particular acetal linkage is called an
The linkage is between C-1 of one sugar subunit and
C-4 of the other. The “prime”superscript indicates that C-4 is not in the same ring
as C-1. The linkage is an linkage because the oxygen atom in-
volved in the glycosidic linkage is in the Remember that the
is axial when a sugar is shown in a chair conformation and is down when the sugar
is shown in a Haworth projection; the is equatorial when a sugar is
shown in a chair conformation and is up when the sugar is shown in a Haworth
projection.

HO

HO

HO

CH 2 OH O

HO

HO

OH

CH 2 OH O

O

maltose

an -1,4′-glycosidic linkage

the configuration of this

carbon is not specified

(^1) 4'
b-position
a-position. a-position
a-1,4¿-glycosidic
A-1,4œ-glycosidic linkage.
2 HIO 4
H 2 O
HCl
O
O
O
OH
O
OH
HC HC O
CHHC HC O
H 2 CO
4 1 4
HC O

• HC O
  1
  (^12)
  5 5
  6
  OCH 3
  CHOH CHOH
  CH 2 OH
  O
  32
  4
  5
  6
  3 2 3
  five-membered ring
  OCH 3
  OCH 3
  2 HIO O
  4
  H 2 O
  HCl
  OH
  CH 2 OH
  H
  HC O
  4
  HC O
  HC
  H
  C
  O
  CH
  O
  HO
  H
  C
  O


• O
  1
  2
  5
  6
  CH 2 OH
  OCH 3
  5
  3
  (^21)
  6
  4
  CH 2 OH
  OH
  O
  HO
  HO
  3
  (^45)
  6
  (^21)
  six-membered ring formic acid D-glyceraldehyde