Efficient synthesis of a novel 4-hydroxy-2,3-dioxocyclobut-1-enyl group containing amino acids

Article abstract of DOI:10.1021/ol990286g

matrix presented Syntheses of novel amino acids possessing a squaryl group, 1-3, in an optically active form are described. The syntheses of 1-3 were conducted by a concise route involving (1) an aldol addition of an enolate derived from Gly, L-Asp, or L-Glu to diisopropyl squarate (4) and (2) an efficient decarboxylation of the aldol adducts based on the electron-withdrawing property of the squaryl group. Introducing N-protected group to 2 and 3 and peptide formation reactions are also described.

Full text of DOI:10.1021/ol990286g

ORGANIC
LETTERS
1999
Vol. 1, No. 10
1663-1666
Efficient Synthesis of a Novel
4-Hydroxy-2,3-dioxocyclobut-1-enyl
Group Containing Amino Acids
,†
Tetsuro Shinada,* Ken-ichi Hayashi,† Toshihiro Hayashi,† Yasutaka Yoshida,†
Manabu Horikawa,‡ Keiko Shimamoto,‡ Yasushi Shigeri,§ Noboru Yumoto,§ and
,†
Yasufumi Ohfune*
Department of Material Science, Graduate School of Science, Osaka City UniVersity,
Osaka 558-8585, Japan, Suntory Institute for Bioorganic Research, Mishima-gun,
Osaka 618-8503, Japan, and Osaka National Research Institute (AIST, MITI),
Midorigaoka, Ikeda 563-8577, Japan
ohfune@sci.osaka-cu.ac.jp
Received September 14, 1999
ABSTRACT
Syntheses of novel amino acids possessing a squaryl group, 1−3, in an optically active form are described. The syntheses of 1−3 were
conducted by a concise route involving (1) an aldol addition of an enolate derived from Gly, L-Asp, or L-Glu to diisopropyl squarate (4) and
(2) an efficient decarboxylation of the aldol adducts based on the electron-withdrawing property of the squaryl group. Introducing N-protected
group to 2 and 3 and peptide formation reactions are also described.
Squaric acid is known as an oxocarbon possessing aroma-
ticity and strong acidity.1 These unique properties have been
extended to the development of advanced materials2 and
employed as a useful diene synthon in organic synthesis.3
Recently, in medicinal chemistry, the concept of the use of
the 4-hydroxy-2,3-dioxocyclobut-1-enyl (squaryl) group as
an isostere of carboxylic acid has arisen, in which this moiety
was incorporated into the substructure of lactivicin,4 phospho-
noacetic acid,5 glycine,6 γ-aminobutyric acid,7 N-methyl-D-
aspartic acid,7 and L-glutamate analogues.8,9 The squaryl
group of these compounds was mainly connected by a
nitrogen or a phosphine atom with other functional groups
† Osaka City University.
(4) (a) Ueda, Y.; Mikkilineni, A.; Partyka, R. A. Bioorg. Med. Chem.
Lett. 1992, 2, 1541-1546. (b) Ueda, Y.; Crast, L. B., Jr.; Mikkilineni A.
B.; Partyka, R. A. Tetrahedron Lett. 1991, 32, 3767-3770. (c) Ueda, Y.;
Mikkilineni, A. B.; Partyka, R. A. Bioorg. Med. Chem. Lett. 1991, 1, 737-
740.
(5) Kim, C. U.; Misco, P. F. Tetrahedron Lett. 1992, 33, 3961-3962.
(6) Campbell, E. F.; Park, A. K.; Kinney, W. A.; Fengl, R. W.;
Liebeskind, L. S. J. Org. Chem. 1995, 60, 1470-1472.
(7) (a) Kinney, W. A.; Abou-Gharbia, M.; Garrison, D. T.; Schmid, J.;
Kowal, D. M.; Bramlett, D. R.; Miller, T. L.; Tasse, R. P.; Zaleska, M. M.;
Moyer, J. A. J. Med. Chem. 1998, 41, 236-246. (b) Kinney, W. A.
Tetrahedron Lett. 1993, 34, 2715-2718. (c) Kinney, W. A.; Lee, N. E.;
Garrison, D. T.; Podlesny, E. J., Jr.; Simmonds, J. T.; Bramlett, D.; Notvest,
R. R.; Kowal, D. M.; Tasse, R. P. J. Med. Chem. 1992, 35, 4720-4726.
(8) Chan, P. C. M.; Roon, R. J.; Koerner, J. F.; Taylor, N. J.; Honek, J.
F. J. Med. Chem. 1995, 38, 4433-4438.
‡ Suntory Institute for Bioorganic Research.
§ Osaka National Research Institute (AIST, MITI).
(1) (a) Seitz, G.; Imming, P. Chem. ReV. 1992, 92, 1227-1260. (b) West,
R.; Niu, J. Nonbenzenoid Aromatics; Snyder, J. P., Ed.; Academic Press:
New York, 1969; pp 311-346. (c) Schmidt, A. H. Synthesis 1980, 961-
994.
(2) (a) DeSilva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley,
A. J.; Mccoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. ReV. 1997, 97,
1515. (b) Ashwell, G. J.; Jefferies, G.; Hamilton, D. G.; Lynch, D. E.;
Roberts, M. P. S.; Bahra, G. S.; Brown, C. R. Nature 1995, 375, 385-388.
(c) Law, K. Y. Chem. ReV. 1993, 93, 449-486.
(3) (a) Paquette, L. A.; Tae, J. J. Org. Chem. 1998, 63, 2022-2030. (b)
Ohno, M.; Yamamoto, Y.; Eguchi, S. Synlett 1998, 1167-1174. (c) Moore,
H. W.; Yerxa, B. R. Chemtracts: Org. Chem. 1992, 5, 273-313. (d)
Liebeskind, L. S. Tetrahedron 1989, 45, 3053-3060.
10.1021/ol990286g CCC: $18.00 © 1999 American Chemical Society
Published on Web 10/15/1999

although the strong acidity of the squaryl group would be
much reduced. Only a few examples have been reported
regarding the analogues which are connected by a carbon-
carbon bond. These facts prompted us to examine the
syntheses of amino acids containing a squaryl group, nearly
equivalent to naturally occurring acidic amino acids such as
aspartic acid, glutamic acid, and tyrosine, which would
provide numerous applications to life science. In this report,
we wish to describe the syntheses of 2-squarylmethylamine
(1), (2S)-2-amino-3-squarylpropionic acid (2), and (2S)-2-
amino-4-squarylbutyric acid (3) via a facile addition of
enolates (Figure 1).
Scheme 1a
Figure 1. Squaryl group containing amino acids and diisopropyl
squarate.
7 would be removed under the decarboxylative conditions.
In fact, successive treatments of the esters 7a and 7b with
trifluoroacetic acid followed by refluxing in toluene gave in
moderate yield the decarboxylated products, 8a12 and 8b,13
respectively. Instead of heating, treatment with triethylamine
(2-3 equiv) in CH2Cl2 at room temperature for 15 min
afforded the same products.14 The isopropyl group was
removed using 12 M HCl in acetone (1:1) to give 3-hydroxy-
4-methylcyclobutene-1,2-dione13 (98%) and 3-ethyl-4-hy-
droxycyclobutene-1,2-dione15 (98%), respectively. Based on
these results, it would be reasonable to plan the synthesis of
1-3 by the following strategy: (1) addition of an ester
enolate derived from glycine, L-aspartic acid, or L-glutamic
acid as the aldol donor to diisopropyl squarate (4), (2)
decarboxylation of the resulting aldol adduct, and (3)
deprotection leading to the amino acids 1-3 (Scheme 2).
We began the studies with (1) an inspection of an
appropriate protecting group for the dihydroxy groups of
squaric acid suitable throughout the synthetic transformations
and (2) finding an efficient method for introducing an
optically active amino acid moiety where an addition of an
ester enolate derived from the amino acid to a carbonyl group
of the squarate was our choice (vide infra). Previous studies
regarding the addition of enolates to dimethyl or diethyl
squarate have shown that the yields of the corresponding
1,2-adducts are moderate due probably to simultaneous 1,4-
addition of the reagent or the amine employed as the base
to the squarate.10 To avoid the presumed side reactions, we
employed the squarate 4 protected with a sterically bulky
isopropyl group. Then, the yield of the enolate addition using
tert-butyl acetate (5a) was much improved to give 6a in 98%
yield. In the case of the enolate prepared from tert-butyl
propionate (5b), the reaction required an addition of ceric
chloride for improved yield (6b, 71%). Treatment of the
adducts with a few drops of 12 M HCl in CH2Cl2 afforded
tert-butyl 2-squaryl acetates, 7a11 and 7b, respectively
(Scheme 1).
Scheme 2
Since the squaryl group can be viewed as a potent electron-
withdrawing group, we assumed that the carboxy group in
First, we attempted an aldol addition of various types of
glycine enolate to 4. The dianionic enolate 5d underwent
smooth addition at -78 °C to give aldol adduct 6c in 72%
yield as a mixture of diastereomers, while the reaction with
the imino enolate 5c did not proceed even at ambient
(9) For related examples, see: (a) Oguz, U.; Akkaya, E. U. J. Org. Chem.
1998, 63, 6059-6060. (b) Pirrung, M. C.; Han, H.; Chen, J. J. Org. Chem.
1996, 61, 4527-4531. (c) Kamath, V. P.; Diedrich, P.; Hindsgaul, O.
Glycoconjugate J. 1996, 13, 315-319. (d) Soll, R. M.; Kenney, W. A.;
Primeau, J.; Garrick, L.; McCaully, R. J.; Colatsky, T.; Oshiro, G.; Park,
C. H.; Hartupee, D.; White, V.; McCallum. J.; Russo, A.; Dinish, J.; Wojdan,
A. Bioorg. Med. Chem. Lett. 1993, 3, 757-760. (e) Young, R. C.; Durant,
G. J.; Emmett, J. C.; Ganellin, R.; Graham, M. J.; Mitchell, R. C.; Prain,
H. D.; Roantree, M. L. J. Med. Chem. 1986, 29, 44-49.
(10) (a) Yamamoto, Y.; Nunokawa, K.; Okamoto, K.; Ohno, M.; Eguchi,
S. Synthesis 1995, 571-576. (b) Yamamoto, Y.; Ohno, M.; Eguchi, S. J.
Am. Chem. Soc. 1995, 117, 9653-9661. (c) Kraus, J. L. Tetrahedron Lett.
1985, 26, 1867-1870.
(11) Hayashi, K.; Shinada, T.; Sakaguchi, K.; Horikawa, M.; Ohfune,
Y. Tetrahedron Lett. 1997, 38, 7091-7094.
(12) Liebeskind, L. S.; Fengl, R. W.; Wirtz, K. R.; Shawe, T. T. J. Org.
Chem. 1988, 53, 2482-2488.
(13) Dehmlow, E. V.; Schell, H. G. Chem. Ber. 1980, 113, 1-8.
(14) Removal of the alkoxycarbonyl group was also effected by treatment
with LiI in DMF at 120 °C (47%).
(15) West, R.; Niu, J. Oxocarbons; Academic Press: New York, 1980;
pp 169-184.
1664
Org. Lett., Vol. 1, No. 10, 1999

temperature.16 Treatment of 6c with a few drops of 12 M
HCl gave protected 2-squarylglycine 7c (84%), which, upon
exposure to excess 12 M HCl in acetone at room temperature,
underwent initial deprotection and subsequent decarboxyla-
tion to give 16 (72%). This concomitant decarboxylation in
contrast to the model study would be due to the presence of
an internal amino group which enhances the rate of decar-
boxylation at room temperature.
1.0, 6 M HCl)). The synthesis of 3 was performed in
essentially the same manner as that of the aspartate using
di-tert-butyl Cbz-L-glutamate. The aldol adduct 10 was
readily transformed into 13b, which, upon heating with 6
M HCl, gave 3: mp 160-163 °C (dec); [R]D +38.9° (c 1.0,
6 M HCl). The optical purity of these amino acids was
1
determined to be >90% ee by H NMR analyses of the
corresponding (R)-MTPA amides 14a and 14b. Thus, squaryl
amino acids 2 and 3 were synthesized from 4 in a few steps
in optically active form.
Next, we attempted to introduce an optically active alanyl
moiety into 4 (Scheme 3). Di-tert-butyl Cbz-L-aspartate was
Both 2 and 3 are potent acidic compounds, and their pKa
values were ∼0, 1.5, 9.4 for 2 and ∼0, 1.8, 9.4 for 3,
respectively.19 The smallest values are compatible with that
of 3-hydroxy-4-methylcyclobutene-1,2-dione (0.24) and 3-hy-
droxy-4-phenylcyclobutene-1,2-dione (-0.22).1b Indeed, the
inorganic salt-free 3-hydroxycyclobutene-1,2-diones 14-17
were not extracted with ethyl acetate from the 2 M HCl
solution, but from 6 M HCl.20
Scheme 3a
Peptide couplings of the N-terminal of 2 or 3 with Cbz-
D-Phe-OSu were performed under the biphasic conditions
to give 17 (Scheme 4). On the other hand, coupling of
Scheme 4a
chosen as the enolate precursor. After numerous unsuccessful
trials regarding the choice of base, solvent, and additives,17
we finally found that an addition of LiCl and 1,3-dimethyl-
3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMPU), which dra-
matically increased the solubility of the resulting dianionic
enolate in THF as reported by Seebach,18 was quite effective,
giving in 92% yield the desired aldol adduct 9 as a mixture
of diastereomers. The mixture was converted into dicar-
boxylate 11a with a few drops of 12 M HCl in CH2Cl2.
Decarboxylation at C3 of 11a was effected by triethylamine
to give 11b in the same manner as the conversion of 7 to 8.
Finally, all the protecting groups were removed, simulta-
neously, with 6 M HCl to afford the desired amino acid 2 in
87% yield. This process could be shortened substantially by
subjecting the aldol adduct 9 to 12 M HCl in acetone at room
temperature followed by refluxing in 6 M HCl to give 2 via
13a (68%, 2: mp 170-171 °C (dec); [R]D +15.6° (c
N-protected 16a with L-Val-OMe afforded unexpected 18
having two valine moieties. To avoid the side reactions, the
protected 11b was used for the coupling to give the desired
dipeptide 19. This was converted to the desired 20 by the
use of 1 M HCl in acetone. Thus, the modification of 2 and
3 at their N-terminal was achieved by the use of a Schotten-
(16) For dianionic glycine enolates, see: Evans, D. A.; Sidebottom, P.
J. J. Chem. Soc., Chem. Commun. 1978, 753-754.
(17) The reaction of 4 with a dienolate prepared from L-Asp using LDA
or KHMDS in THF, toluene, or hexane afforded the aldol adducts in poor
yield (∼10%). This was slightly improved (∼20%) when CeCl3 was added.
(18) Seebach, D.; Bossler, H.; Grundler, H.; Shoda, S. HelV. Chim. Acta
1991, 74, 197-224.
(19) Supporting Information.
(20) When the N-Boc-protected compound was employed, the organic
phase should immediately be washed with brine.
Org. Lett., Vol. 1, No. 10, 1999
1665

Bauman-type method, and the coupling at their C-terminal
was performed using the protected intermediate 11b.
In summary, we have synthesized three kinds of the amino
acids, 1, (2S)-2, and (2S)-3. To our knowledge, (2S)-2 and
(2S)-3 are the first examples of the synthesis of amino acid
involving a squaryl group with a carbon-carbon bond.
Among them, 2 is an analogous amino acid to glutamic acid
which plays an important role as an excitatory neurotrans-
mitter in the mammalian central nervous system.21 Further
studies focusing on their biological activities and their
incorporation into biologically active peptides are in progress
in our laboratory.
Acknowledgment. This work was supported by a Grant-
in-Aid from the Ministry of Education, Science, Sports, and
Culture, Japan.
Supporting Information Available: Experimental pro-
cedures, spectral data for all compounds, and titration graphs
for determination of the pKa values of 2 and 3. This material
is available free of charge via the Internet at http://pubs.acs.org.
(21) Preliminary pharmacological assays of the synthetic compounds 2
and 3, for both ionotropic glutamate receptors (KA, AMPA, and NMDA
subtypes) in rat brain synaptic membranes and metabotropic glutamate
receptors (cloned rat mGluR1, -2, and -4 expressed on CHO cell) were
performed. Radioligand binding assays using [3H]KA for KA receptors,
[3H]AMPA for AMPA receptors, and [3H]CGP39653 for NMDA receptors
revealed that 3 was a potent agonist of KA and AMPA receptors while the
magnitude of its activity was slightly weaker than those of glutamate. 3:
OL990286G
KA (IC50 ) 2.0 µM), AMPA (0.8 µM), and NMDA (31.6 µM). L-Glutamic
acid: KA (0.12 µM), AMPA (0.12 µM), and NMDA (0.32 µM). On the
other hand, 2 and 3 did not activate mGluRs even at 1 mM concentrations.
In addition, they did not inhibit uptake of [14C]glutamate in COS-1 cells
expressing human excitatory amino acid transporters (EAAT1 and -2).
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Org. Lett., Vol. 1, No. 10, 1999