CHEM 203
Topics Discussed on Nov. 30
Monosaccharides as generally chiral molecules, due to the presence of multiple stereogenic C's
Principle: virtually all of the naturally occurring monosaccharides are enantiomerically pure
The simplest aldotriose: glyceraldehyde
HO-CH2–(CHOH)–CHO
D-
and L-forms of glyceraldehyde:
CHO
CHO
enantiomers
H
H
HO
OH
HO
OH
L-(S)-(–)-glyceraldehyde
D-(R)-(+)-glyceraldehyde
(OH on the left)
(OH on the right)
Reaction of an aldehyde (or ketone) with hydrocyanic acid (HCN): formation of cyanohydrins
H C N
O
R1
R2
R1
N
C N
OH
HO
R2
R1 R2
a cyanohydrin
aldehyde or ketone
D-Glyceraldehyde
C
as the formal progenitor of all common monosaccharides
The Fischer-Kiliani (F-K) method for the synthesis of monosaccharides from – e.g. – Dglyceraldehyde
O
H
C
H
OH
OH
D-(R)-(+)-glyceraldehyde:
a triose
addition of H–CN:
formation of two
N C–H
O
C
H
H
OH
OH
diastereomeric
products:
N
C
separate the
diastereomers
H C OH
H
OH
OH
N
C
+
HO C H
H
OH
OH
N C
HO
H
C
H
OH
OH
Lecture of Nov 30
p. 2
pure
H C OH
repeat
H
OH
learn next
OH
semester how)
erythrose: a tetrose
H C OH
H
OH
OH
pentoses
repeat
hexoses
CHO
CN
pure
CHO
convert CN
to CHO (will
CN
HO C H
H
OH
OH
convert CN
to CHO (will
HO C H
H
OH
OH
learn next
semester how)
repeat
pentoses
repeat
hexoses
threose: a tetrose
Fischer structures of naturally occurring aldoses and stereochemical relationship thereof to
glyceraldehyde of – e.g. – (R)-configuration (= D-(R)-glyceraldehyde):
key stereogenic center:
defines D- or L- series
CHO
H
OH
H
OH
OH
erythrose
CHO
H
OH
H
OH
H
OH
OH
ribose
F-K 3
F-K 2
CHO
H
OH
OH
glyceraldehyde
(aldotriose)
F-K 1
(aldotetroses)
CHO
H
HO
H
OH
H
OH
OH
CHO
H
OH
OH
threose
HO
H
CHO
H
OH
H
HO
H
OH
OH
xylose
arabinose
(aldopentoses)
F-K 3
F-K 3
F-K 2
CHO
H
H
OH
OH
lyxose
HO
HO
H
F-K 3
CHO
CHO
CHO
CHO
CHO
CHO
CHO
CHO
H
H
H
H
H
H
H
OH HO
OH HO
OH HO
OH HO
H
H
H
H
H HO
H
OH
OH HO
HO
OH H
OH HO
H
H
H
H HO
H HO
H HO
H
OH
OH
OH
OH HO
H
H
H
H
OH
OH
OH
OH H
OH H
OH H
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
allose
altrose
glucose
mannose
gulose
idose
galactose
talose
H
H
H
H
(aldohexoses)
Stereochemical convention for monosaccharides: D and L sugars
Lecture of Nov 30
p. 3
enantiomers!
HO
H
HO
HO
HO
CHO
H
HO
HO
L-(S)-(–)-glyceraldehyde
(OH on the left)
CHO
H
OH
H
H
L-glucose
CHO
H
HO
H
HO
H
HO
HO
L-ribose
CHO
H
HO
H
HO
HO
L-erythrose
CHO
H
OH
OH
D-(R)-(+)-glyceraldehyde
(OH on the right)
H
HO
H
H
CHO
OH
OH
OH
OH
D-ribose
CHO
H
OH
H
OH
OH
D-erythrose
H
H
H
CHO
OH
H
OH
OH
OH
D-glucose
Principle: most natural monosaccharides belong to the D-stereochemical series. The much rarer
L-sugars are produced primarily by certain fungal and microbial organisms for specialized uses.
Facile formation of cyclic hemiacetal / hemiketal structures (5- or 6-membered rings) in
molecules incorporating both an alcohol and an aldehyde / ketone functionality:
R
HO
O R
O
aldehyde (R = H) or
ketone (R = alkyl)
R
HO
OH
cyclic hemiacetal
or hemiketal
O
O
aldehyde (R = H) or
ketone (R = alkyl)
R
OH
cyclic hemiacetal
or hemiketal
Chemical structure of carbohydrates: hemiacetal formation
hemiacetal forms of ribose (building block of RNA) and glucose:
CHO
H
OH
H
OH
H
OH
OH
Fischer projection
of D-ribose
H
HO
H
H
CHO
OH
H
OH
OH
OH
Fischer projection
of D-glucose
H
HO
H
OH
CHO
HO
HO
OH
HO
HO
HO
OH
common hemiacetal form
of D-ribose (preferred)
aldehyde (carbonyl)
form of D-ribose
H OH
OH wavy line means that
either configuration is
possible
O
H
CHO
OH
OH
aldehyde (carbonyl)
form of D-glucose
HO
O
HO
wavy line means that
OH either configuration is
possible
OH
OH
common hemiacetal form
of D-glucose (preferred)
Lecture of Nov 30
p. 4
Furanose and pyranose forms of monosaccharides
O
O
O
O
furan
tetrahydrofuran
pyran
tetrahydropyran
H
furanose form HO
of D-ribose
("D-ribofuranose")
H
OH
O
HO
O
OH
pyranose form
of D-glucose
OH ("D-glucopyranose")
HO
HO
OH
OH
Anomeric position (= anomeric carbon; anomeric center) of the cyclic form of a simple sugar
(i.e., of a monosaccharide): the former carbonyl carbon that now sustains the hemiacetal system
Existence of two diastereomers of the hemiacetal form of a monosaccharide: α and β "anomers":
α-anomer: the OH group of the hemiacetal moiety is trans relative to the CH2OH fragment
connected to the carbon atom that defines the D/L stereochemical series
β-anomer: the OH group of the hemiacetal moiety is cis relative to the CH2OH fragment
connected to the carbon atom that defines the D/L stereochemical series
anomeric carbon
H
HO
O
OH
H
HO
H
O
OH
HO
O
HO
HO
OH
α-anomer of
D-ribofuranose
(α-D-ribofuranose)
HO
OH
β-anomer of
D-ribofuranose
(β-D-ribofuranose)
OH
OH
OH
α-anomer of
D-glucopyranose
(α-D-glucopyranose)
H
HO
O
HO
OH
OH
OH
β-anomer of
D-glucopyranose
(β-D-glucopyranose)