Exercício de Química Sintética e Organica Mista (710)

Prévia do material em texto

Mannose differs from glucose in configuration at C-2. All hydroxyl groups are equatorial in
�-D-glucopyranose; the hydroxyl at C-2 is axial in �-D-mannopyranose.
(c) The conformational depiction of �-L-mannopyranose begins in the same way as that of
�-D-mannopyranose. L-Mannose is the mirror image of D-mannose.
To rewrite the eclipsed conformation of L-mannose in a way that permits hemiacetal forma-
tion between the carbonyl group and the C-5 hydroxyl, C-5 is rotated 120° in the clockwise
sense.
Translating the Haworth formula into a proper conformational depiction requires that a choice
be made between the two chair conformations shown.
(d ) The Fischer projection formula for L-ribose is the mirror image of that for D-ribose.
CHO
CH2OH
H
H
H OH
OH
OH
d-Ribose
CHO
CH2OH
HO
HO
HO
H
H
H
l-Ribose Eclipsed conformation of l-ribose is
oriented properly for ring closure.
HO
H
HO
H
HO
H
CH2OH
Haworth formula of
�-l-ribopyranose
HO
H
HO
H
HO
H
H
O
OH
C
O
H
HO
H
H
CH2OH
HO
H
OH
H
H
O
OH
Haworth formula of
�-l-mannopyranose
Less stable chair conformation;
CH2OH is axial
OH
H H
CH2OH
H
H
HO
H
OH
OH
O
More stable chair conformation;
CH2OH is equatorial
OH
OH
OH
H
H
H
H
HO
OHOCH2
H
HO
H
HO
H
HO
HH
OH
CH2OH
C
H
O O1
23
4
5 5
6
HO
H
H
CH2OH
HO
HH
OH
OH
C
H
1
23
4
�-l-Mannopyranose
(remember, the anomeric
hydroxyl is down in the l series)
HO
H
H
CH2OH
HO
HH
OH
H
OH
O
rotate about
C-4 C-5 bond
CHO
CH2OH
H
HHO
HHO
OH
H OH
l-Mannose
CHO
CH2OH
HO
OHH
OHH
H
HO H
d-Mannose Eclipsed conformation
of l-mannose
HO
H
H
H
HO
HO
H
OH
CH2OH
C
O
H
CARBOHYDRATES 705