700 / 912
692 Chapter 20
Copyright © 2017 Pearson Education, Inc.
3.
To determine the structure of
1
+
2
-galactose, we can go back to
1
-
2
-lyxose, the sugar that forms sug-
ars 7 and 8 by a Kiliani–Fischer synthesis, and oxidize it with HNO
3
. Finding that the aldaric acid is
optically active allows us to conclude that
1
+
2
-galactose is sugar 7, because the aldopentose that leads
to sugars 1 and 2 would give an optically inactive aldaric acid.
OH H
H HO
HO H
H OH
CH
2
OH
H HO
H HO
HO H
H OH
CH
2
OH
8
7
D
-galactose
D
-talose
C
H O
C
H O
53.
d-Arabinose
. The only d-aldopentoses that are oxidized to optically active aldaric acids are d-arabinose
and d-lyxose. A Wohl degradation of d-arabinose forms d-erythrose, whereas a Wohl degradation of
d-lyxose forms d-threose. Because d-erythrose forms an optically inactive aldaric acid but d-threose does
not, we can conclude that the d-aldopentose is d-arabinose.
54.
O
HO
HO
HO
H
HO
CH
2
OH
+
Cl
−
OH
HO
O
HO
HO
H
HO
CH
2
OH
OH
H
+
HO
O
HO
OH
CH
2
OH
+
O
HO
OH
CH
2
OH
OH
+
O
HO
HO
CH
2
OH
+
H
2
O
H Cl
H
H
2
O
lactose
O
HO
HO
H
CH
2
OH
OH
+
H
3
O
+
HO
O
HO
OH
HO
CH
2
OH
O
OH
CH
2
OH
OH
O
O