An efficient approach to D-threo-3-hydroxyaspartic acid for the synthesis of novel L-threo-oxazolines as selective blockers of glutamate reversed uptake

Article abstract of DOI:10.1016/j.tetlet.2004.01.081

An efficient, stereoselective synthetic strategy to D-threo-3- hydroxyaspartic acid was developed. Starting from L-(2S,3S)-N-benzoyl-3- hydroxyaspartic acid dimethyl ester by a Deoxo-fluor-catalyzed cyclization reaction, an inversion of configuration at the β-center (erythro isomer), was observed. A base-induced epimerization reaction led to the D-trans-isomer, which was hydrolyzed to give D-threo-3-hydroxyaspartic acid with excellent stereoselectivity and overall yield. Starting from D-threo-3-hydroxyaspartic acid, L-threo-oxazolines can be stereoselectively synthesized.

Full text of DOI:10.1016/j.tetlet.2004.01.081

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 2355–2357
An efficient approach to
synthesis of novel -threo-oxazolines as selective blockers of
glutamate reversed uptake
D
-threo-3-hydroxyaspartic acid for the
L
Meri De Angelis and Giuseppe Campiani*
Dipartimento Farmaco Chimico Tecnologico, via Aldo Moro and European Research Centre for Drug Discovery and Development,
Siena University, 53100 Siena, Italy
Received 22 December 2003; revised 16 January 2004; accepted 19 January 2004
Abstract—An efficient, stereoselective synthetic strategy to D-threo-3-hydroxyaspartic acid was developed. Starting from L-(2S,3S)-
N-benzoyl-3-hydroxyaspartic acid dimethyl ester by a Deoxo-fluor-catalyzed cyclization reaction, an inversion of configuration at
the b-center (erythro isomer), was observed. A base-induced epimerization reaction led to the
to give
acid, -threo-oxazolines can be stereoselectively synthesized.
Ó 2004Elsevier Ltd. All rights reserved.
D
-trans-isomer, which was hydrolyzed
-threo-3-hydroxyaspartic
D
-threo-3-hydroxyaspartic acid with excellent stereoselectivity and overall yield. Starting from
D
L
L
-Glutamate (Glu) is an important nutrient involved in
J = 6.2 Hz
several biochemical pathways such as gluconeogenesis
and ammonia detoxification. In the mammalian central
nervous system (CNS) Glu plays an even more impor-
tant role as an excitatory neurotransmitter modulating
the function of most neuronal circuits. Clearance of Glu
from the synaptic cleft and from the extracellular space
is mediated by high affinity sodium-dependent excit-
atory amino acid transporters (EAAT) subdivided into
five different subtypes. Their dysfunction may contribute
to neurological disorders and neurodegenerative dis-
eases. EAATs may be the major source of extracellular
Glu by acting as a site of efflux from the intracellular
compartment. This latter mechanism has been suppos-
edly implicated during acute brain ischemia. We recently
developed a 3D-model for EAAT2/3 for the successful
design of the first, potent and highly selective EAAT
blockers based on an oxazoline/oxazole structure (1,2)
as novel neuroprotective agents to reduce the acute
damage of specific regions of the brain during the
ischemic insult.1
O
O
O
O
H
H
HO
OH HO
OH
N
O
N
O
(4S,5S)-1
2
The stereoselective synthesis of -b-hydroxyamino acids
L
is a topic of interest since they represent key structural
component of a variety of natural products, including
antibiotics, and other biologically active compounds.2
Among them, L-threo-3-hydroxyaspartic acid (L-t-
3OHAsp) is a potent transported substrate of EAAT2
while its erythro counterpart is less potent. On the
contrary, much less is available on the synthesis of
hydroxyamino acids such as
-t-3OHAsp.3
hydroxyamino acids can be synthesized from chiral
oxazolines, and enantioselective synthesis of these
heterocycles and their ring opening reactions are well
documented. Oxazolines are important intermediates in
organic chemistry,4–7 and in our case, potent pharma-
cological tools. Recently several authors8;9 reported that
cis oxazolines, bearing a phenyl ring at position 5 and a
methoxycarbonyl function at C-4, could be converted
into their trans epimers using catalytic amounts of bases.
D
-b-
-b-
D
L
Keywords: D-threo-3-Hydroxyaspartic acid; Glutamate; L-threo-Oxaz-
olines; L-t-3OHAsp.
* Corresponding author. Tel.: +39-0577-234172; fax: +39-0577-2343-
33; e-mail: campiani@unisi.it
0040-4039/$ - see front matter Ó 2004Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.01.081

2356
M. De Angelis, G. Campiani / Tetrahedron Letters 45 (2004) 2355–2357
According to our pharmacophoric model we designed
several blockers based on an oxazole/oxazoline skeleton.
The acidic nature and the relative position of the two
carboxylic functions is crucial for activity, and to deeply
investigate their role, it was necessary to modify the pKa
of these acidic functions by their replacement with two
hydroxamic acid groups. Furthermore, we decided to
investigate also the role of alkyl functions in place of the
aromatic ring at C-2. Structural manipulations of
oxazolines such as transformation of the carboxylic
groups of 1 to hydroxamic acid functions may be diffi-
cult due to the high instability of the oxazoline ring
system. Consequently, to synthesize analogues of com-
This reaction did not provide the desired (4S,5S)-oxaz-
olines, but the amides underwent epimerization, leading
to the epimers (4S,5R)-4, with high diastereomeric
purity, by a clean inversion of configuration at the
b-center. The configuration of compounds 4a and 4b
was determined by NMR and by comparing the ana-
lytical data of these compounds with those of L-threo-
analogues obtained using N-benzoylaspartic acid
derivatives as starting materials. The NMR spectra of
the cis-isomer 4a and its trans-epimer showed a distinct
difference in the coupling constants. To bypass the
inversion of configuration induced by the SN2 cycliza-
tion reaction, and to obtain
investigated a new synthetic strategy based upon the use
of -t-3OHAsp as a key intermediate. Low cost and
commercially available -aspartic acid was the starting
L-threo-oxazolines, we
pound 1, we decided to use L-t-3OHAsp, an easy syn-
thesis of which was recently developed,10 as starting core
system to generate a versatile synthetic approach to
these heterocyclic compounds. The chiral 3-hydroxyas-
partic acid was converted into the N-benzoyl derivative
leading to 3a,b, after protection and transformation of
the carboxylic functions. Cyclization to oxazolines was
attempted by using the Deoxo-fluor reagent (Scheme 1).7
D
L
point of our synthesis. Consequently, the first step of
our strategy was the development of an efficient method
to synthesize
analysis, -(2S,3S)-N-benzoyl-3-hydroxyaspartic acid
dimethyl ester, in turn obtained from -aspartic acid,
D-t-3OHAsp. Following our retrosynthetic
L
L
could be initially transformed into the corresponding
cis-oxazoline by a SN2 ring closure (epimerization at the
b-center) which, after treatment with a catalytic amount
J = 9.9 Hz
of DBU, could be epimerized to the
D
-trans-oxazoline.
O
O
O
O
Finally, deprotection might provide
(Fig. 1).
D
-t-3OHAsp
H
H
BnO
OBn
Deoxo-fluor,
-20 °C
BnO
O
OBn
N
OH
N
O
H
Successively by the same set of stereoselective reactions,
-t-3OHAsp may provide -threo-oxazolines. As shown
D
L
in Scheme 2, diester 6 was cyclized to the oxazoline by
using the Deoxo-fluor reagent. In this step the epimer-
ization of the b-center occurred and the cis oxazoline
was obtained in 75% yield. Subsequently, by the addi-
tion of a catalytic amount of DBU the erythro (cis)
(2S,3S)-3a
(4S,5R)-4a
O
O
O
O
BnOHN
NHOBn
Deoxo-fluor,
-20 °C
BnOHN
O
NHOBn
intermediate was converted into its
D-threo (trans)
N
OH
N
O
counterpart 7, by epimerization of the a-center, with a
trans cis ratio of 95:5dr. After separation of the two
diasteroisomers by flash chromatography, the optical
H
purity of the
rotation analysis, compared to the value reported in lit.
D-enantiomer 7 was verified by an optical
(2S,3S)-3b
Scheme 1.
(4S,5R)-4b
for the
L
-enantiomer (½aŠ )42.6° (c 1, CHCl3); lit.10 ½aŠ
D
D
+42.2° (c 1.2, CHCl3)). Oxazoline 7 was obtained with a
O
O
O
O
O
O
MeO
OMe
MeO
OMe
HO
OH
N
O
N
O
NH2 OH
R
D-threo-3-hydroxyaspartic acid
L-threo-oxazolines
O
O
O
O
O
O
MeO
OMe
MeO
O
OMe
N
O
HO
OH
N
OH
NH2 OH
H
L-threo-3-hydroxyaspartic acid
Figure 1. Retrosynthetic analysis to
D
-threo-3-hydroxyaspartic acid and
L
-threo-oxazolines.

M. De Angelis, G. Campiani / Tetrahedron Letters 45 (2004) 2355–2357
2357
O
O
O
O
O
O
MeO
OMe
1. Deoxo-fluor
2. DBU cat.
MeO
O
OMe
Ref. 9
N
O
HO
OH
N
OH
H
NH2
5, L-aspartic acid
6
D-7
HCl 6N
O
1. SOCl2,
MeOH
O
O
O
O
O
O
HCl 6N
MeO
OMe
2. RCOCl
HO
OH
HO
OH
NH2 OH
N
NH2 OH
3. Deoxo-fluor
4. DBU cat.
R
8, D-t-3-OHAsp
10, L-t-3-OHAsp
9a-c
L-threo-oxazolines
Scheme 2.
Sinclair, C.; Fumagalli, E.; Mennini, T. J. Med. Chem.
2001, 44, 2507.
high optical purity. Compound 7 was hydrolyzed to
provide D-t-3OHAsp 8 in good overall yield (38%) and
high optical purity (½aŠ )7.4° (c 0.9, 5 N HCl); lit.3 ½aŠ
2. (a) Zhang, A. J.; Burgess, K. Angew. Chem. 1999, 111, 666;
(b) Evans, D. A.; Britton, T. C.; Ellman, J. A.; Dorow,
R. L. J. Am. Chem. Soc. 1987, 109, 6881; (c) Davidson, B.
S. Chem. Rev. 1993, 93, 1771; (d) Gant, T. G.; Meyers, A.
I. Tetrahedron 1994, 50, 2297.
3. Dudding, T.; Hafez, A. M.; Taggi, A. E.; Wagerle, T. R.;
Lectka, T. Org. Lett. 2002, 4, 387.
4. Frump, J. A. Chem. Rev. 1971, 71, 483.
5. Wipf, P.; Miller, C. P. Tetrahedron Lett. 1992, 33, 907.
6. Wipf, P.; Xu, W. J. Org. Chem. 1996, 61, 6556.
7. Phillips, A. J.; Uto, Y.; Wipf, P.; Reno, M. J.; Williams, D.
R. Org. Lett. 2000, 2, 1165.
8. Evans, D. A.; Janey, J.; Magomedov, N.; Tedrow, J. S.
Angew. Chem., Int. Ed. 2001, 40, 1884–1888.
9. Lee, S.-H.; Yoon, J.; Nakamura, K.; Lee, Y.-S. Org. Lett.
2000, 2, 1243–1246.
D
D
D-t-3OHAsp was
)7.5° (c 1, 5 N HCl)). Subsequently,
converted to the N-acyl intermediates by using acetyl
chloride, benzoyl chloride, and 2,2-dimethylpropionyl
chloride, respectively. These compounds were then
treated with Deoxo-fluor reagent followed by catalytic
DBU to give the desired L-threo oxazolines (9a–c) after
double epimerization at b- and a-centers.11 (4S,5S)-2-
Phenyloxazoline-4,5-dicarboxylic acid dimenthyl ester
9b showed spectroscopic data comparable to the com-
pound reported in lit. (½aŠ +41.8° (c 0.8, CHCl3); lit.10
D
½aŠ +42.2° (c 1.2 CHCl3)). Furthermore, ring-opening
D
reaction under acidic conditions of compounds 9a–c
provided
L
-t-3OHAsp (10) as a pure enantiomer (½aŠ
D
+7.6° (c 1.1, 5 N HCl); lit.10 ½aŠ +7.5° (c 1, 5 N HCl)).
D
10. Cardillo, G.; Gentilucci, L.; Tolomelli, A.; Tomasini, C.
Synlett 1999, 11, 1727–1730.
11. D-threo-3-Hydroxyaspartic acid dimethyl ester hydrochlo-
In conclusion we have shown an efficient stereoselective
ride. 1H NMR (MeOH-d4) d 3.85 (s 3H), 3.90 (s, 3H), 4.45
(br s, 1H).
(4R,5R)-N-Pivaloyl-3-hydroxyaspartic acid dimethyl ester.
synthesis of
from -threo-3-hydroxyaspartic acid.
used as a key structural intermediate for a general and
versatile synthesis of -threo-oxazolines.
D
-threo-3-hydroxyaspartic acid starting
L
D
-t-3OHAsp was
73% yield as colorless prisms (mp: 94–95 °C): ½aŠ )69.2°
D
L
1
(c 1.3, CHCl3); H NMR (CDCl3) d 1.18 (s, 9H), 3.78 (s,
3H), 3.79 (s, 3H), 4.69 (d, 1H, J ¼ 1:8 Hz), 5.09 (dd, 1H,
J ¼ 1:8, 8.8 Hz), 6.35 (br s, 1H).
Acknowledgement
(4S,5S)-2-(tert-Butyl)oxazoline-4,5-dicarboxylic acid dimethyl
ester (9c). Deoxo-fluor-catalyzed cyclization: erythro-
Financial support from MIUR––Rome (PRIN 2002).
isomer (4R,5S): ½aŠ )115.8° (c 1.2, CHCl3); 1H NMR
D
(CDCl3) d 1.27 (s, 9H), 3.71 (s 3H), 3.73 (s, 3H), 4.97 (d,
1H, J ¼ 10:3 Hz), 5.06 (d, 1H, J ¼ 10:8 Hz). DBU-cata-
References and notes
lyzed epimerization of a-center (4S,5S)-9c: ½aŠ +147.1° (c
D
1. Campiani, G.; De Angelis, M.; Armaroli, S.; Fattorusso,
C.; Catalanotti, B.; Ramunno, A.; Nacci, V.; Novellino,
E.; Grewer, C.; Ionescu, D.; Rauen, T.; Griffiths, R.;
0.7, CHCl3); 1H NMR (CDCl3) d 1.25 (s, 9H), 3.78 (s,
3H), 3.80 (s, 3H), 4.72 (d, 1H, J ¼ 6:0 Hz), 5.16 (d, 1H,
J ¼ 6:0 Hz).