A potential limitation on the use of FDP aldolases for the synthesis of monosaccharides is that the products are always ketoses with fixed stereochemistry at the
newly generated chiral centers on C-3 and C-4. One way to overcome this limitation
and to obtain aldoses instead of ketoses makes use of a monoprotected dialdehyde as
the acceptor substrate (Scheme 2.186). After the RAMA-catalyzed aldol reaction, the
resulting ketone is reduced in a diastereoselective fashion with polyol dehydrogenase.
The remaining masked aldehyde is then deprotected to yield a new aldose.
Aldol additions catalyzed by DHAP-dependent aldolases which exhibit a complementary stereospecificity to RAMA have been used to a lesser extent (Schemes
2.180 and 2.187) [1550, 1551]. Although the selectivity with respect to the center
on carbon 3 is absolute, in some cases the corresponding C-4 diastereomer was
formed in minor amounts depending on the R substitutent on the aldehyde. In those
cases shown in Scheme 2.187, however, only a single diastereomer was obtained.
One drawback common to group I aldolases is that they require the expensive
and sensitive phosphorylated donor dihydroxyacetone phosphate. This molecule is
not very stable in solution (t 1/2 ~ 20 h at pH 7), and its synthesis is not trivial
[1552]. DHAP may be obtained from the hemiacetal dimer of dihydroxyacetone by
chemical phosphorylation with POCl 3 [1553, 1554], or by enzymatic
O
O
O
H
O
H
OH
OH
OH
H
O
O H
OH
OH
OH
O
O
O
O
O
OH
OH
OH
+
FDP aldolase
DHAP
polyol dehydrogenase
NADH-recycling
2-deoxy-L-idose
*
*
* newly formed stereocenters
4
3
H
+
H 2 O
phosphatase
Scheme 2.186 Synthesis of aldoses using FDP aldolase
O
H
R
O
O
HO
OH
OH O
O
R
OH
OH O
O
R
CHOH-CH 2 F, (EtO) 2 CH-CH 2 ,
CHOH-CH 2 -OMe, CHOH-CH 2 N 3 ,
H, CH 3 , i-Pr, CH 2 OH, (CH 2 ) 2 OH,
CHOH-CH 2 F, DL-CHOH-CH 2 OH
CHOH-CH 2 -OMe, CHOH-CH 2 N 3 ,
(CH 2 ) 2 CO 2 H,
CH 2 OH, DL-CHOH-CH 2 OH, 2-pyridyl,
H, CH 3 , i-Pr, CH 2 OH, (CH 2 ) 2 OH,
4
3
4
3
Rhamnulose1-phosphate
aldolase
Fuculose1-phosphate
aldolase
= phosphate
DHAP
R
P
P
P
P
R
Scheme 2.187 Aldol reactions catalyzed by fuculose- and rhamnulose-1-phosphate aldolase
212
2 Biocatalytic Applications
newly generated chiral centers on C-3 and C-4. One way to overcome this limitation
and to obtain aldoses instead of ketoses makes use of a monoprotected dialdehyde as
the acceptor substrate (Scheme 2.186). After the RAMA-catalyzed aldol reaction, the
resulting ketone is reduced in a diastereoselective fashion with polyol dehydrogenase.
The remaining masked aldehyde is then deprotected to yield a new aldose.
Aldol additions catalyzed by DHAP-dependent aldolases which exhibit a complementary stereospecificity to RAMA have been used to a lesser extent (Schemes
2.180 and 2.187) [1550, 1551]. Although the selectivity with respect to the center
on carbon 3 is absolute, in some cases the corresponding C-4 diastereomer was
formed in minor amounts depending on the R substitutent on the aldehyde. In those
cases shown in Scheme 2.187, however, only a single diastereomer was obtained.
One drawback common to group I aldolases is that they require the expensive
and sensitive phosphorylated donor dihydroxyacetone phosphate. This molecule is
not very stable in solution (t 1/2 ~ 20 h at pH 7), and its synthesis is not trivial
[1552]. DHAP may be obtained from the hemiacetal dimer of dihydroxyacetone by
chemical phosphorylation with POCl 3 [1553, 1554], or by enzymatic
O
O
O
H
O
H
OH
OH
OH
H
O
O H
OH
OH
OH
O
O
O
O
O
OH
OH
OH
+
FDP aldolase
DHAP
polyol dehydrogenase
NADH-recycling
2-deoxy-L-idose
*
*
* newly formed stereocenters
4
3
H
+
H 2 O
phosphatase
Scheme 2.186 Synthesis of aldoses using FDP aldolase
O
H
R
O
O
HO
OH
OH O
O
R
OH
OH O
O
R
CHOH-CH 2 F, (EtO) 2 CH-CH 2 ,
CHOH-CH 2 -OMe, CHOH-CH 2 N 3 ,
H, CH 3 , i-Pr, CH 2 OH, (CH 2 ) 2 OH,
CHOH-CH 2 F, DL-CHOH-CH 2 OH
CHOH-CH 2 -OMe, CHOH-CH 2 N 3 ,
(CH 2 ) 2 CO 2 H,
CH 2 OH, DL-CHOH-CH 2 OH, 2-pyridyl,
H, CH 3 , i-Pr, CH 2 OH, (CH 2 ) 2 OH,
4
3
4
3
Rhamnulose1-phosphate
aldolase
Fuculose1-phosphate
aldolase
= phosphate
DHAP
R
P
P
P
P
R
Scheme 2.187 Aldol reactions catalyzed by fuculose- and rhamnulose-1-phosphate aldolase
212
2 Biocatalytic Applications