Chemical-enzymatic synthesis of azasugar phosphonic acids as glycosyl phosphate surrogates

Article abstract of DOI:10.1016/S0040-4039(01)00137-X

Starting from achiral materials two stereoisomeric phosphonylated dihydroxypyrrolidines containing four stereogenic centers were synthesized enantioselectively employing a combination of enzymatic and transition-metal-mediated methods. Both compounds contain features of the transition state of the enzyme-catalyzed fucosyl transfer reaction and represent building blocks of potential inhibitors against this class of enzymes.

Full text of DOI:10.1016/S0040-4039(01)00137-X

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 2289–2291
Chemical-enzymatic synthesis of azasugar phosphonic acids as
glycosyl phosphate surrogates
Matthias Schuster,* Wei-Fang He and Siegfried Blechert*
Institut fu¨r Organische Chemie, Technische Universita¨t Berlin, Strasse des 17 Juni 135, D-10623 Berlin, Germany
Received 23 December 2000; accepted 16 January 2001
Abstract—Starting from achiral materials two stereoisomeric phosphonylated dihydroxypyrrolidines containing four stereogenic
centers were synthesized enantioselectively employing a combination of enzymatic and transition-metal-mediated methods. Both
compounds contain features of the transition state of the enzyme-catalyzed fucosyl transfer reaction and represent building blocks
of potential inhibitors against this class of enzymes. © 2001 Elsevier Science Ltd. All rights reserved.
Glycosyl phosphates are central intermediates of the
primary metabolism and serve as glycosyl donors in the
biosynthesis of complex carbohydrate structures. More-
over, they are involved in the regulation of biochemical
pathways. Accordingly, much effort has been devoted
to the development of glycosyl phosphate mimics1 as
effectors of cellular processes. Substitution of the phos-
phoester oxygen with a carbon atom leads to struc-
turally very similar, yet hydrolytically stable phosphono
analogues such as 1 (Scheme 1). Most glycosyltrans-
ferases require glycosyl phosphates or derivatives
thereof as glycosyl donors. Like glycoside hydrolysis,2
glycosyl transfer has been postulated to proceed
through transition states exhibiting significant oxocar-
benium character (e.g. 2).3 Whereas polyhydroxylated
pyrrolidines like 3 are potent transition state analogues
of glycoside hydrolysis, they are very poor inhibitors of
glycosyltransferases.4 This may be due to the lack of
structural elements representing the phosphoester leav-
ing group.
The objective of this study was to develop a variable
enantioselective approach towards azasugar phospho-
nic acids of type 4 combining the functional properties
of glycosyl phosphonates like 1 and azasugars like 3.
These compounds represent a minimal structural motif
of glycosyltransferase transition state analogues and
will serve as stable building blocks for inhibitors against
this pharmaceutically relevant5 class of enzymes.
A retrosynthetic outline is depicted in Scheme 2. The
pyrrolidine ring can be formed by diastereoselective
reductive amination of a suitable phosphonoazidoke-
tone already containing three stereogenic centers. The
latter should be accessible both enantio- and
diastereoselectively from an enantiomerically pure a-
azidoaldehyde and 3-keto-4-hydroxy butanoic acid 5
using an enzyme-mediated aldol reaction.6 Of the four
dihydroxyacetone phosphate-depending aldolases with
complementing stereoselectivity at least two—fructose-
1,6-diphosphate aldolase and rhamnulose-1-phosphate
B
H
+
H2
OH
H
N
O
OH
O-
O-Acceptor
OH
O
N
2
P
O
δ+
HO
HO
3
OH
O-
OH
4
HO
OH
1
HO
OH
O
O-GMP
HO
OH
δ-
P
P
O-
O
O-
O
Scheme 1. Methylphosphonate analogue of glucose-1-phosphate (1), fucosyltransferase transition state (2, GMP=guanosin
monophosphate), fucosidase transition state analogue (3), azasugar phosphonic acid (4).
* Corresponding authors. Tel.: 30/31423619; e-mail: mschuster@chem.tu-berlin.de
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00137-X

2290
M. Schuster et al. / Tetrahedron Letters 42 (2001) 2289–2291
N3
+
H2
N
O
O
OH
O-
N3
*
OH
*
*
O
OH
R
ii
*
*
i
R
P
+
O
OH
P
H
*
R
OH
P
*
*
HO
OH
OH
O
HO
OH
5
O
Scheme 2. Retrosynthesis of five-membered azasugar phosphonic acids: (i) reduction; (ii) aldolase reaction.
aldolase7—accept 5 instead of the natural substrate
DHAP. Thus, a variety of diastereomeric cyclic prod-
ucts should be accessible from a chosen aldehyde
substrate.
that under neutral conditions R-10 undergoes slow
isomerization to S-10, which is converted much faster.
After ion exchange chromatography 11 and 12 were
obtained in 85 and 67% yield, respectively.13 The abso-
lute configurations of the two new stereogenic centers
of 11 and 12 have not been determined experimentally,
but were derived from the stereoselectivity of RAMA.6
The final hydrogenation step was performed in aqueous
solution under atmospheric pressure using 10% Pd on
coal. Compounds 4 and 13 were obtained as white
solids after gel permeation chromatography on Bio-Gel
P2 in 56 and 45% yield, respectively.14 In both cases the
reductive amination proceeded diastereoselectively. The
hydrogen atoms are delivered from the face of the
hydroxyl group adjacent to the carbonyl carbon of the
cyclic imine intermediate as has been observed in the
reduction of comparable azidoketones lacking the
phosphonyl substituent.15 This assignment of the abso-
lute configuration is supported by the presence of weak
NOE signals involving the hydrogen atom introduced
at the imine carbon atom of both 4 and 13 (Scheme 4).
Two diastereomers of an azasugar phosphonic acid (4
and its 5-epimer 13) containing characteristic features
of the fucosyltransferase transition state were chosen as
a starting point to demonstrate the viability of our
approach. As shown in Scheme 3, both enantiomers of
a-azidopropionic aldehyde 10 serving as aldol acceptors
were obtained from cinnamoic aldehyde, which was
first transformed into racemic alcohol 7. R-7 was selec-
tively acetylated using Amano lipase AK to give R-8
with >95% ee.8 After separation by silica gel chro-
matography the remaining S-7 (>95% ee) was acety-
lated using acetic anhydride in pyridine. Subsequently,
Pd(0)-catalyzed allylic substitution was applied to
transform both S-8 and R-8 into the corresponding
azides 9 with complete retention of the absolute
configuration.9 Notably, no regioisomer is formed dur-
ing this reaction. a-Azidoaldehydes 10 were generated
from S-9 and R-9, respectively, by ozonolysis at −78°C
in MeOH/CH2Cl2 followed by reduction with dimethyl
sulfide in the presence of water.10 DHAP analogue 5
was prepared according to Page et al.11 Although this
compound has been demonstrated to be a viable sub-
strate analogue of dihydroxyacetonephosphate,7 we are
aware of only one preparative application.12
In summary, a flexible enantioselective synthesis of
dihydroxylated pyrrolidines bearing an ethylphospho-
nyl substituent, has been developed. It is based on an
aldolase-catalyzed enantio- and diastereoselective C,C-
bond formation utilizing an unnatural aldol donor sub-
strate. The coupling of products
4 and 13 to
guanosinmonophosphate (GMP) in order to obtain
fully functional analogues of the fucosyltransferase
transition state 2 is currently in progress.
Commercially available rabbit muscle fructose bisphos-
phate aldolase (RAMA) catalyzed the aldol reaction of
each S- and R-10 with 0.7 equiv. of 5. S-10, exhibiting
the same absolute configuration as the natural substrate
Acknowledgements
D
-glycerinaldehyde, was converted at a significantly
higher rate than R-10. Whereas 11 was formed in
diastereomerically pure form, 12 contained up to 20%
of isomer 11 depending on the reaction time, indicating,
Financial support by the Fonds der Chemischen Indus-
trie is gratefully acknowledged.
OAc
OR
OH
CHO
a
b
+
R-8
6
7
S-7
S-8
N3
c
N3
N3
d
d
e
O
S-8
R-8
S-9
S-10
H
H
N3
e
O
R-10
R-9
Scheme 3. Enantioselective synthesis of a-azido aldehydes S-10 and R-10. (a) MeLi, THF (90%); (b) Amano lipase AK, vinyl
acetate, hexane; (c) Ac2O, pyr (95%); (d) NaN3, 0.01 equiv. Pd(dppb)2, THF–H2O (2.5:1), 50°C (85%); (e) i. O3, CH2Cl2–MeOH
(1:1), −78°C, ii. Me2S, H2O (90%).

M. Schuster et al. / Tetrahedron Letters 42 (2001) 2289–2291
2291
2%
+
H2
N
N3
OH
OH
O
OH
H
O
OH
O-
H
P
a
b
P
OH
S-10
R-10
11 (85%)
OH
O
4 (56%)
HO
OH
2.5%
+
H2
N
H
O
OH
O-
N3
O
OH
P
b
a
P
OH
H
13 (45%)
12 (67%)
OH
O
HO
OH
Scheme 4. Aldolase-catalyzed synthesis of azasugar phosphonates 4 and 13 (NOE indicated by arrows). (a) RAMA, 3 equiv. 5,
H2O, pH 7.0; (b) H2, 10% Pd-C, H2O, pH 1.
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.