Sequential aldol condensation catalyzed by DERA mutant Ser238Asp and a formal total synthesis of atorvastatin

Article abstract of DOI:10.1016/j.tetlet.2004.01.110

A mutant D-2-deoxyribose-5-phosphate aldolase (DERA), Ser238Asp, was used to prepare β-hydroxy-δ-lactol synthons and a key intermediate for atorvastatin synthesis.

Full text of DOI:10.1016/j.tetlet.2004.01.110

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 2439–2441
Sequential aldol condensation catalyzed by DERA mutant
Ser238Asp and a formal total synthesis of atorvastatin
Junjie Liu, Che-Chang Hsu and Chi-Huey Wong*
Department of Chemistry and the Skaggs Institute for Chemical Biology, The Scripps Research Institute,
10550 North Torrey Pines Road, BCC 483A, La Jolla, CA 92037, USA
Received 10 December 2003;accepted 13 January 2004
Abstract—A mutant -2-deoxyribose-5-phosphate aldolase (DERA), Ser238Asp, was used to prepare b-hydroxy-d-lactol synthons
D
and a key intermediate for atorvastatin synthesis.
Ó 2004 Elsevier Ltd. All rights reserved.
2-Deoxyribose-5-phosphate aldolase (DERA, EC
4.1.2.4), a Schiff base forming type I class aldolase,
catalyzes the reversible aldol condensation between
O
OH
R
O
O
O
OH OH O
DERA
H
R
R
OH
acetaldehyde and D-glyceraldehyde-3-phosphate (G3P)
-2-deoxyribose-5-phosphate (Scheme 1).1
Scheme 2. Sequential aldol reactions catalyzed by DERA.
to form
D
DERA has been over-expressed in Escherichia coli and
its structure and catalytic mechanism have been deter-
mined at the atomic level.2 In addition to its natural
substrates, DERA accepts a broad range of unnatural
substrates, especially different acceptor aldehydes and
thus potentially has enormous application in organic
synthesis. Recently, a chemo-enzymatic total synthesis
of epothilones A and C with DERA catalysis has been
reported.3 The C-2 stereochemistry in acceptor alde-
hydes was found to play a key role in the DERA
catalysis.3;4
condensation. Several b-hydroxy-d-lactols were pre-
pared by catalysis of the wild-type DERA.5 However,
the choice of R groups is very limited and only very
small or negatively charged groups are accepted. In
order to improve the enzymeÕs tolerance of the R group,
the Ser238 residue was mutated to Asp. Such a mutation
should still retain the hydrophilic nature of the binding
pocket,4;6 but neutral and positively charged groups
should become preferred over negatively charged
groups. Indeed, the Ser238Asp mutant shows a 2.5-fold
improvement in Kcat=KM compared to that of the wild-
type enzyme. Molecular modeling indicates that the C-3
hydroxyl hydrogen forms a hydrogen bond with carb-
oxylate of Asp238, accounting for this increase in reac-
tivity.6
One type of interesting reaction catalyzed by DERA is
the sequential synthesis of b-hydroxy-d-lactol synthons
(Scheme 2).5 Under optimal conditions, three achiral
aldehydes are catalyzed by DERA to afford enantio-
merically pure (3R,5R) dihydroxyl aldehydes. The for-
mation of lactol is expected to shift the reaction to the
Compared with the wild-type DERA, the mutant
Ser238Asp showed a great improvement in catalytic
activity toward sequential aldol reactions (Table 1):
Incubation of 3-azidopropinaldehyde 1a and acetalde-
hyde with wild-type DERA did not afford any product
2a, while the mutant Ser238Asp gave 35% yield of the
sequential aldol condensation product. The mutant also
improves the yield of the condensation between 3-chlo-
ropropinaldehyde 1b and acetaldehyde from 25% for the
wild-type enzyme to 43% for the mutant. Interestingly,
3-nitropropinaldehyde was not catalyzed by either the
2-O3PO
O
O
O
DERA
OH
2-
OPO3
OH
Scheme 1. Aldol condensation catalyzed by DERA.
OH
* Corresponding author. Tel.: +1-619-784-2487;fax: +1-619-784-2409;
e-mail: wong@scripps.edu
0040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.01.110

2440
J. Liu et al. / Tetrahedron Letters 45 (2004) 2439–2441
Table 1. Sequential aldol reactions catalyzed by mutant Ser238Asp
N
O
O
3
OH OH
O
a
O
O
N
3
OR
6a R=Me
2a
R
O
O
6b R=t-Bu
Br2, BaCO3
DERA
6 d
OH
R-CHO
b
OH
R–CHO
Wild type (yield %)
N.R.b
Ser238Asp (yield %)
N
3
O
O
N
3
O
(35)a
(43)a
O
O
O
O
O
O
c
1a
H N
2
OMe
2a
N
3
OMe
OH
Cl
O
O
Cl
O
O
8
7
Cl
O
(25)a
Scheme 4. Reagents and conditions: (a) MeONa, MeOH, 83%;
t-BuOK, t-BuOH, 72%;(b) camphorsulfonic acid, 2,2-dimethoxypro-
pane, 76%;(c) Ph 3P, 3d, 88%.
1b
OH
2b
OH
2b
O N
2
N.R.b
N.R.b
O
1c
a For two steps.
b N.R.: no reaction.
O
O
O
O
O
O
a
N
3
OMe
N
3
OH
7
9
wild-type or the mutant enzymes, presumably because of
the negative charge on the oxygen atom of the nitro
group, which is less favored by the mutant than the wild-
type enzyme.
b
O
O
O
c
O
O
O
The sequential aldol reaction product 2a could easily be
transformed to molecules such as 5, a key chiral com-
ponent of atorvastatin 3 (Lipitorâ, Sortisâ), which is an
inhibitor of HMG-CoA reductase. Previous syntheses of
atorvastatin involve a Paal–Knorr pyrrole formation
reaction, which couples aromatic diketone 4 with chiral
ester 5 (Scheme 3). Several different approaches to the
key intermediate 5 or its analogues have been reported.7
N
O
3
H N
2
O
5
10
Scheme 5. Reagents and conditions: (a) LiOH, MeOH–H2O, 83%;
(b) Boc2O, DMAP, 86%;(c) Ph 3P, 3d, 72%.
reduction of 10 with Ph3P. Following previously
described methodology,7d atorvastatin can be synthe-
sized from 5.
In our research, the lactone ring of 2a was first opened
under basic conditions to afford methyl or tert-butyl
esters 6a and 6b (Scheme 4). Methyl ester 6a was then
protected with 2,2-dimethoxypropane to afford the
acetonide 7. Ph3P was employed to reduce the azido
group to the primary amine in high yield. However, we
found that tert-butyl ester 6b also became methyl ester 7
when 6b was treated with 2,2-dimethylpropane under
acid conditions. The tert-butyl ester amine 5 could be
prepared from methyl ester 7 in a three-step procedure
(Scheme 5).10 The methyl ester was first hydrolyzed
under basic conditions.8 Esterification of 9 with Boc2O
afforded tert-butyl ester 10,9 and 5 was synthesized after
In conclusion, we examined the mutant DERA
Ser238AspÕs activity toward different substrates and
found that the substrate specificity of the mutant
enzyme had opened up to tolerate more unnatural
substrates. A new way to prepare tert-butyl[(4R,6R)-
6-aminoethyl-2,2-dimethyl-1,3-dioxn-4-yl]acetate, a key
chiral intermediate in the synthesis of atorvastatin, was
developed from the product of enzymatic catalysis 2a.
Compared to the previous syntheses, the new chemo-
enzymatic route described here is much shorter and
more efficient.
F
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