Carbohydrate acetals and ketals
• Two types of structurally different derivatives of carbohydrates. The
acetal (or ketal) group of the first type of carbohydrate acetals
(ketals) originates from a carbonyl group of any carbonyl compound
and two hydroxyl groups of carbohydrate (e. g., 2,3-Oisopropylidene-D-glyceraldehyde (I) from acetone and Dglyceraldehyde). The second type originates from a carbonyl group
of a sugar and two hydroxyl groups of any alcohol (e. g., Dglyceraldehyde dimethyl acetal (II) from D-glyceraldehyde and
methanol).
.
Me
C H=O
O H O
C
O
Me
OH
O
Me
C H2 O H
I
II
C H2 O H
Me
O
OH
O
OH
HO
III
Me
Carbohydrate acetals and ketals
O
C H2O H
O
OH O
OH
O
OH
HO
III
Me
OH
OH
IV
Also glycosides are sugar acetals (derived from aldoses) or ketals
(derived from ketoses), e. g., methyl α-D-glucopyranoside (III), but
also internal glycosides, e. g., 1,6-anhydro--D-glucopyranose (IV)
(red colour shows the acetal functional group).
Carbohydrate acetals and ketals
• Acetone, benzaldehyde, acetaldehyde and formaldehyde
are most often employed carbonyl compounds for
preparation of ketals and acetals of the first type.
• According to these starting carbonyl compounds, they are
called as isopropylidene ketals (1,2;5,6-di-Oisopropylidene--D-glucofuranose, 1,2-O-isopropylidene-D-glucofuranose, 1,2;3,4-di-O-isopropylidene--Dgalactopyranose), and benzylidene (4,6-O-benzylidene-Dglucopyranose), ethylidene and methylene acetals of
carbohydrates.
1,2:5,6-di-O-isopropylidene--D-glucofuranose (I)
(obsolete name, diacetone glucose) – crystalline compound, m. p.
110 °C, []D -180° (water), soluble in water and many organic
solvents. The acid hydrolysis rate of its 5,6-O-isopropylidene group is
40-times higher than that in position 1,2. This is employed for
preparation of another important derivative, 1,2-O-isopropylidene-D-glucofuranose (II). Ketal I is employed as starting compound in
many syntheses. Thus, e. g., the intermediates, obtained either after
oxidation of its free hydroxyl group to 3-oxo derivative, or after its Osubstitution, are employed for preparation of aminosaccharides,
deoxysaccharides or branched-chain saccharides.
C H2 O H
(C H 3 ) 2 C
O
OH
OH
HO
OH
C H3C OC H3
O C H2
O
C H2 O H
O
+
H
OH
Z nC l 2
a le b o
H2S O4
O
O
I
HO
O
OH
H2 O
O
O
C (C H 3 ) 2
II
C (C H 3 ) 2
1,2;3,4-di-O-isopropylidene--D-galactopyranose
•
Is employed in synthesis of saccharides and their derivatives, e. g.,
D-fucose (6-deoxy-D-galactose) or D-galacturonic acid.
C H2 O H
HO
C H2 O H
O
OH
OH
OH
C H3C OC H3
Z nC l 2
a le b o
H2S O4
O
COOH
O
NOx
O
O
O
O
O
O
a le b o
TEMPO
O
O
TsC l, P y
C H 2 O Ts
O
O
C H3
LiA lH 4
O
O
O
O
O
O
O
O
• Carbohydrate acetals and ketals are stable in basic
and neutral solutions. In acid solutions they decompose
to the starting sugar and carbonyl compound. Their
hydrolysis rate is highest for benzylidene acetals and
decreases in the order isopropylidene ketals, ethylidene
acetals and methylene acetals. From carbohydrate
benzylidene acetals, the saccharide can be regenerated
also by hydrogenolysis on paladium, similarly as from
benzyl ethers.
C H3
O
C H3
O
H 3O +
O
HO
O
OH
OH
O
O
OH
D-fucose (6-deoxy-D-galactose)
4,6-O-benzylidene-D-glucopyranose (I)
C H2 O H
O
O
OH
OH
HO
O
Ph C H O
O
Ph
OH
OH
Zn Cl 2
O
O
Ph
OH
O
O
OH
OH
I
O
II
40 % of 4,6-O-benzylidene-D-glucopyranose (I) can be isolated by reacting
D-glucose with 1 mol of benzaldehyde. An excess of benzaldehyde gives rise
to 1,2:4,6-di-O-benzylidene-α-D-glucopyranose(II).
Ph
C H2 O H
(C H 3 ) 2 C
O
OH
OH
(C H 3 ) 2 C ( O C H 3 ) 2
O C H2
O
O
OH
HOTs
HO
O
D M F a le b o D M E
OH
O
C H2 O H
O
P hC H ( O M e ) 2
O
OH
HO
C (C H 3 ) 2
OMe
OH
O
Ph
OH
HOTs
D M F a le b o D M E
O
OMe
OH
Nowadays, more modern, transacetalization (transketalization) reagents
are employed for preparation of carbohydrate acetals and ketals;
acetone dimethyl ketal instead of acetone and benzaldehyde dimethyl
acetal instead of benzaldehyde.
Conformational analysis of the carbohydrate ketals
and acetals
Ketones (R1-CO-R2), reacting with
hydroxyl groups of carbohydrates,
preferentially provide the
termodynamically more favourable
five-membered cyclic ketals of the
1,3-dioxolane type. The
characteristic examples are Oisopropylidene ketals (R1 = R2 = Me).
1
carbohydrate moiety
2
The reason is that both the bulky
substituents R1, R2 are placed in
equivalent, degenerated quasiequatorial (or quasi-axial) positions.
carbohydrate moiety
Aldehydes (R-CH=O), reacting with
hydroxyl groups of carbohydrates,
preferentially provide the
termodynamically more favourable
six-membered cyclic ketals of the
1,3-dioxane type. The
characteristic examples are Obenzylidene acetals (R = Ph).
In this case, the bulky substituent R
is placed in equatorial position and
hydrogen atom in the axial position.
Isopropylidene ketals of common aldohexoses
C H2 O H
(C H 3 ) 2 C
O
OH
OH
HO
OH
D -g lu kó za
C H3C O C H3
O C H2
O
OH
Z nC l2
a le b o
H2S O4
O
Me
O
O
O
O
Me
Me
O
O
HO
O
Me
C (C H 3 ) 2
O
C H2 O H
(C H 3 ) 2 C
O
O HH O
OH
HO
D -m a n ó za
C H3C O C H3
O C H2
O
O
O
Z nC l2
a le b o
H2S O4
OH
OH
D -g a la któ za
OH
Me
C H2 O H
O
OH
O
C H3C O C H3
Z nC l2
a le b o
H2S O4
O
OH
O
C (C H 3 ) 2
C H2 O H
HO
O
Me
O
O
Me
Me
OH
O
O
O
O
O
(C H 3 ) 2 C
O
C (C H 3 ) 2
Me
Me
O
O
O
Me
Me
O
C H2 O H
(C H 3 ) 2 C
O
O HH O
HO
(C H 3 ) 2 C (O C H 3 ) 2
OH
O C H2
O
O
Me
O
HOTs
D M F a le b o D M E
O
Me
OMe
O
O
OMe
C (C H 3 ) 2
O
O
Me
Me
D -m a n ó za
In case of employing more modern transketalization reagent,
acetone dimethyl ketal instead of acetone, D-mannose does not
afford 1,2;5,6-di-O-isopropylidene--D-mannofuranose, but its
glycoside, methyl 1,2;5,6-di-O-isopropylidene--D-mannofuranoside.
Acetals and ketals of alditols
OH
OH
OH
O
PhC H O
HO
OH
+
OH
Ph
Ph
HO
H
O
O
O
Ph
Ph
O
O
Ph
O
O
O
O
OH
OH
O
OH
OH
O
Ph
OH
D-glucitol
2,4-Obenzylidene-D-glucitol
(sorbitol)
OH
HO
1,3:2,4-di-Obenzylidene-D-glucitol
1,3:2,4:5,6-tri-Obenzylidene-D-glucitol
1,2:5,6-di-O-izopropylidén-D-manitol
C H 3C O C H 3
HO
OH
OH
OH
D-manitol
k ys e lin a
(rôzne
podmienky)
alebo
1,2:3,4:5,6-tri-O-izopropylidén-Dmanitol
Acetals and ketals of alditols
OH
OH
OH
O
PhC H O
HO
OH
+
OH
Ph
Ph
HO
H
O
O
O
Ph
Ph
O
O
Ph
O
O
O
O
OH
OH
O
OH
OH
O
Ph
OH
D-glucitol
1,3:2,4:5,6-tri-Obenzylidene-D-glucitol
(sorbitol)
OH
HO
1,2:5,6-di-O-isopropylidene-D-mannitol
C H 3C O C H 3
HO
OH
OH
OH
D-mannitol
k ys e lin a
Acid
(rôzne
(different
podmienky)
condition)
or
1,2:3,4:5,6-tri-O-isopropylidene-D-mannitol
Synthetic employment of carbohydrate acetals
and ketals
OH
OH
O
O
Ph
HO
N aIO 4
O
OH
+
OH
H
Ph
HO
H2O
O
OH
O
HO
HO
H2 O
OH
OH
O
HO
O
OH
OH
2,4-O-benzylideneD-glucitol
O CH2
O
L-xylose
O CH2
O CH2
O
O
O
NaBH4
PDC
OH
O
O
1,2;5,6-di-O-isopropylidene-D-glucofuranose
O
O
+
H
O
O
O
O
HO
H 2O
D-allose
O
1,2;5,6-di-O-isopropylidene-D-allofuranose
Free sugars can be practically released from all sugar acetals or ketals by hydrolysis
with a 3 N strong acid at room temperature within 48 hours.