Efficient synthesis of 3-aminocyclobut-2-en-1-ones: Squaramide surrogates as potent VLA-4 antagonists

Article abstract of DOI:10.1021/ol034701n

(Matrix presented) A novel series of uniquely functionalized 3-aminocyclobut-2-en-1-ones has been prepared by facile condensation of a variety of cyclobuta-1,3-diones with a phenylalanine-derived primary amine. These systems subsequently lend themselves t

Full text of DOI:10.1021/ol034701n

ORGANIC
LETTERS
2003
Vol. 5, No. 13
2343-2346
Efficient Synthesis of
3-Aminocyclobut-2-en-1-ones:
Squaramide Surrogates as Potent VLA-4
Antagonists
Stephen Brand,* Benjamin C. de Candole, and Julien A. Brown
Medicinal Chemistry, Celltech Group plc, 216 Bath Road, Slough SL1 4EN, UK
sbrand@celltech.co.uk
Received April 28, 2003
ABSTRACT
A novel series of uniquely functionalized 3-aminocyclobut-2-en-1-ones has been prepared by facile condensation of a variety of cyclobuta-
1,3-diones with a phenylalanine-derived primary amine. These systems subsequently lend themselves to substitution at C-2 by reaction with
a variety of electrophilic reagents including N-halosuccinimides, sulfenyl chlorides, and Eschenmoser’s salt. Compounds from this novel
series are potent antagonists of VLA-4.
The integrins are a large family of heterodimeric, cell surface
receptor proteins which consist of an R and a â subunit.
Restricted association of these subunits can give rise to more
than 24 characterized integrins, which bind specific peptide
ligands of the extracellular matrix.1 The integrin VLA-4 (very
late activating antigen-4, R4â1, CD49d/CD29) is expressed
on a variety of leukocytes and through interaction with its
endothelial ligand VCAM-1 (vascular cell adhesion mol-
ecule) plays an important role in recruitment, trafficking, and
infiltration of circulating lymphocytes to sites of inflamma-
tion.2 Accumulating clinical evidence from the use of
monoclonal antibodies (i.e., Antegren/Natalizumab) strongly
suggests that pharmacological antagonism of VLA-4 is a
viable method for alleviating the debilitating tissue damage
which arises in autoimmune disorders including rheuma-
toid arthritis, asthma, multiple sclerosis, and Crohn’s
disease.3
VCAM-1 has been shown to bind VLA-4 through the
sequences Ile-Asp-Ser.4 This binding motif was used as a
starting point for early proteomimetic inhibitor design efforts,
which initially gave rise to cyclic peptides such as 1 (Figure
1), being derived from this sequence.5 However, peptides
typically make poor drug candidates as they frequently have
sub-optimal absorption properties and low metabolic stability,
factors which can significantly impede systemic exposure.
N-Acylated phenylalanine derivatives have subsequently
emerged from several groups as a major class of potent
VLA-4 antagonist.6 Notable members of this useful class of
(3) Seiffge, J. J. Rheumatol. 1996, 23, 2086. Gordon, F. H.; Lai, C. W.
Y.; Hamilton, M. I.; Pounder, R. E. Gastroenterology 2001, 121, 268.
Borchers, M. T.; Crosby, J.; Farmer, S. Am. J. Physiol. 2001, 280, 813.
(4) Renz, M. E.; Chiu, H. H.; Jones, S.; Fox, J.; Kim, K. J.; Presta, L.
G.; Fong, S. J. Cell Biol. 1994, 125, 1395.
(5) Fotouhi, N.; Joshi, P.; Fry, D.; Cook, C.; Tilley, J. W.; Kaplan, G.;
Hanglow, A.; Rowan, K.; Schwinge, V.; Wolitsky, B. Bioorg. Med. Chem.
Lett. 2000, 10, 1171.
(1) Elices, M. J. In Cell Adhes. Mol. Matrix Proteins; Mouse, S. A.,
Ed.; Springer: Berlin, 1999; p 133.
(2) Hemler, M. E. Annu. ReV. Immunol. 1990, 8, 365.
(6) Porter, J. R. IDrugs 2000, 3, 788. Tilley, J. W. Expert Opin. Ther.
Pat. 2002, 12, 991.
10.1021/ol034701n CCC: $25.00 © 2003 American Chemical Society
Published on Web 06/06/2003

Figure 2. Proposed route to 3-aminocyclobut-2-en-1-ones.
investigation and embarked upon development of a robust
method which would grant access to 5 and its analogues.
Accordingly, gem-disubstituted 3-ethoxy-2-cyclobutene-
1-ones were readily accessed by the cycloaddition of a
disubstituted ketene, generated in situ by dehydrohalogena-
tion of the corresponding acid chloride, and ethoxyacetylene,
according to a previously recognized phenomenon11 (Scheme
1). For example, the reaction of cyclohexanecarbonyl
Figure 1. Potent small molecule VLA-4 antagonists.
Scheme 1. Synthesis of Cyclobuta-1,3-diones
small molecule are 27 and 3,8 which are reportedly under-
going clinical evaluation for the treatment of asthma and are
currently in phase 1 and phase 2, respectively.
Of the numerous inhibitors reported in this class, it is clear
that a high degree of diversity in the N-acyl substituent is
tolerated, although these are generally hydrophobic, and that
the amino acid R-NH is important for activity. We have
recently described the discovery of squaric acid derivatives
including CT7015 (4) as VLA-4 antagonists9 that illustrate
these observations.
As part of a search for a unique class of VLA-4 antagonists
distinct from 4, we hypothesized that the combination of a
hydrophobic spirocyclic substituent, such as that in 1, in the
context of a rigid planar scaffold such as that provided by
the 3,4-diamino-3-cyclobuten-1,2-dione (squaramide) group
of 4, would provide potent compounds with an advantageous
pharmacokinetic profile. We believed that the 3-amino-2-
cyclobuten-1-one analogues typified by 5 would be the most
appropriate compounds to test this “hybridization” hypothesis
(Figure 2) and investigated the synthesis of these by
condensation of an R-amino ester with a cyclobuta-1,3-dione
or its enol ether.10
chloride and NEt3 (1.5 equiv) in the presence of ethoxy-
acetylene12 (2.0 equiv) in Et2O afforded 3-ethoxyspiro[3.5]-
non-2-en-1-one (7e) in 81% yield.
The reaction time can be reduced from a reported 21 days
at 5 °C10a to less than 48 h by heating the reaction to reflux
temperature. Although feasible, no apparent dimerization of
the intermediate ketenes was observed at this elevated
temperature. The novel spirocyclic examples 7g-l were
prepared using an analogous transformation with comparable
yields being obtained. In all cases examined, the cycloadducts
were found to be stable and were conveniently purified by
column chromatography on silica gel.
We felt that this intriguing structural element, thus far
unutilized in the pharmaceutical arena, warranted further
(7) Sircar, I.; Gudmundsson, K. S.; Martin, R.; Liang, J.; Nomura, S.;
Jayakumar, H.; Teegarden, B. R.; Nowlin, D. M.; Cardarelli, P. M.; Mah,
J. R.; Connell, S.; Griffith, R. C.; Lazarides, E. Bioorg. Med. Chem. 2002,
10, 2051.
(8) Chen, L.; Tilley, J.; Trilles, R. V.; Yun, Weiya, Y.; Fry, D.; Cook,
C.; Rowan, K.; Schwinge, V.; Campbell, R. Bioorg. Med. Chem. Lett. 2002,
12, 137.
(9) Porter, J. R.; Archibald, S. C.; Childs, K.; Critchley, D.; Head, J. C.;
Linsley, J. M.; Parton, T. A. H.; Robinson, M. K.; Shock, A.; Taylor, R. J.;
Warrellow, G. J.; Alexander, R. P.; Langham, B. Bioorg. Med. Chem. Lett.
2002, 12, 1051.
Various attempts to convert enolether 7e directly to the
desired 3-amino derivative (i.e., 5) by substitution with
L-phenylalanine ethyl ester as a test substrate were frustrat-
(10) Wasserman, Martin, and Moore have each reported the synthesis
of a single example of the title compound using this approach, the latter
within the context of the preparation of an amino-substituted naphthol,
see: (a) Wasserman, H. H.; Piper, J. U.; Dehmlow, E. V. J. Org. Chem.
1973, 38, 1451. (b) Hasek, R. H.; Gott, P. G.; Martin, J. C. J. Org. Chem.
1964, 29, 2510. (c) Turnbull, P.; Moore, H. W. J. Org. Chem. 1995, 60,
644.
(11) Arens, J. F. AdVances in Organic Chemistry Methods and Results;
Raphael, R. A., Taylor, E. C., Ed.; Interscience Publishers: New York,
1960; Vol. 2, pp 117-212. Martin, J. C. U.S. Patent 3 288 854, 1966.
(12) Ethoxyacetylene was purchased from Lancaster Chemical Co. as
an approximately 50%w/w solution in hexanes. Yields 7a-l were calculated
based upon the amount of acid chloride used.
2344
Org. Lett., Vol. 5, No. 13, 2003

ingly unproductive. We were consequently forced to examine
formation of the desired vinylogous amides by combination
of an amine with a cyclobuta-1,3-dione. To this end,
cyclobutenones 7a-l could be hydrolyzed to cyclobutadiones
8a-l in good yield by treatment with 2 M HCl at 20 °C for
24 h. A notable aspect of these conversions is that hydrolysis
proceeds without detriment to the four-membered carbocycle,
in contrast to analogues lacking geminal substitution at C4
which undergo a facile ring-opening under these conditions
to give â-ethoxycrotonic acids.10a
Scheme 3. Condensation of a Phenylalanine-Derived
R-Amino Ester with Cyclobuta-1,3-diones
In an effort to expand the scope of the ring-forming
process, we attempted substitution at the 2-position of the
cyclobutenone by employing a variety of substituted alkoxy-
acetylenes. Consequently, cycloaddition precursors 9a,b were
prepared directly by alkylation of the acetylide anion of
n
ethoxyacetylene (1.1 equiv of Buli, 2.2 equiv of HMPA)
with benzyl bromide and n-hexyl iodide, respectively,
according to literature procedure.13 Alkynes 9c-e were
prepared by reaction of chloroacetaldehyde dibutylacetal with
2.2 equiv LDA/HMPA followed by alkylation with the
appropriate alkyl bromide, RBr.14 Alkynes 9a-e were also
found to undergo cycloaddition with dimethylketene to give
enol ethers 10a-e (Scheme 2), and these were hydrolyzed
without incident under similar conditions to give diones
11a-e.
Scheme 2. Synthesis of Trisubstituted Cyclobuta-1,3-diones
We speculate that this may either be a steric phenomenon
or alternatively that the C2 substituent stabilizes the relatively
unreactive enolic form of the dione, reducing the relative
concentration of the reactive dicarbonyl tautomer. Interest-
ingly, the condensation reaction of meso-diones 8b, c and
k, which contain prochiral centers, gave inseparable diaster-
eomeric products with the reaction showing only weak
diasteroselectivity (<30%). Contrastingly, dione 8l reacted
to give compound 13l as a single isomer, the stereochemistry
of which we were unable to determine conclusively.
With the intention of providing access to novel C-2
substituted versions of 5, we surveyed the reactivity of 13e
to a variety of electrophiles and the results of this study are
summarized in Scheme 4. In keeping with the observation
that 7a undergoes bromination,10a vinylogous amide 13e was
found to be rapidly convertible to the 2-bromo analogue 14c
in 95% yield, by reaction with bromine or NBS in THF at
room temperature (Scheme 4). Chlorination and iodination
are achieved using NCS and NIS respectively and fluorina-
tion was performed using Selectfluor16 to give 14b, 14d, and
14a, respectively. The thioalkyl substituents of 14e,f were
introduced by reaction with the appropriate sulfenyl chlo-
rides, which were produced in situ by reaction of the
corresponding disulfide and SO2Cl217 prior to addition of 13f.
Compound 14b can arise as a minor byproduct of these
reactions, presumably through chlorination of 13e by un-
In all cases described, the expected regiochemistry of the
cycloaddition was complete and no regioisomeric material
was detected.
Subsequent reaction of cyclobuta-1,3-diones 8a-i with the
phenylalanine derivative 12 was found to proceed with
suprising speed when mixed in THF at room temperature,
to give the desired vinylogous amide derivatives 13a-l in
<24 h15 (Scheme 3, conditions a). Contrastingly, those
reactions involving C-2 susbstituted cyclobuta-1,3-diones
11a-e were wholly unreactive with 12 under these condi-
tions and required heating at 100 °C in nitromethane for 48
h to achieve complete conversion to the corresponding
analogues 13m-q (Scheme 3, conditions b).
(13) Pons, J.-M.; Kocienski, P. Tetrahedron Lett. 1989, 30, 1833. 9a
and 9b were prepared in 99% and 92% yields, respectively.
(14) A modification of the method reported in: Newman, M. S.; Geib,
J. R.; Stalick, W. M. Org. Prep. Proced. Int. 1972, 4, 89.
(16) Banks, R. E.; Murtagh, V.; Tsiliopoulous, E. J. Fluorine Chem. 1991,
52, 389.
(15) Contrastingly, the reaction of 4 with cyclopenta-1,3-dione required
several days for completion under these conditions.
(17) Still, I. W. J.; Kutney, G. W.; McLean, D. J. Org. Chem. 1982, 47,
560.
Org. Lett., Vol. 5, No. 13, 2003
2345

The ability of vinyl iodide 14d to act as a participant in a
Stille-type reaction would allow more convenient parallel
synthesis of C2-substituted compounds, thereby circumvent-
ing the need to synthesize individual acetylenes. To this end,
14d undergoes a Pd(PPh3)4-catalyzed Stille coupling with
3-tributylstannylpyridine and tributylstannyl-1,4-pyridazine
in 41% and 53% yield, respectively, as representative
examples of this strategy.
Scheme 4. Reaction of 13e with Electrophiles
As a prelude to biological testing, ethyl esters 14a-m were
required to be converted to their corresponding acids 15a-
m, and this could be achieved by reaction with 1M LiOH in
THF, in high yield (>85%). It is noteworthy that the
aminocyclobutenone core of all compounds were found to
be stable under these basic conditions (with the exception
of 14j), an indication of the “amidic” stabilization of the
enaminone moiety. The ability of carboxylic acids 15a-m
to disrupt binding of VLA4 to VCAM was evaluated in a
cell based assay using E6.1 Jurkat cells (human T-cell line)
and immobilized VCAM-1 in the presence of 1%BSA
(bovine serum albumin), and all demonstrated an IC50 < 1
µM. Notably compound 14m has an IC50 of 3 nM in this
assay. A fuller analysis of the biological properties of
compounds 15a-m and their congeners shall be reported in
due course.
In summary, we have developed the synthesis of a variety
of highly functionalized 3-amino-2-cyclobutenones by con-
densation of an amine with a cyclobuta-1,3-dione as the key
step. These uncommon functional groups have subsequently
shown potential application as squaramide isosteres in a
biological context.
reacted SO2Cl2, although this side-reaction could be avoided
by using an excess of disulfide during the sulfenyl chloride
preparation step.
Compounds 14g,h were prepared in an analogous fashion
by reaction with commercially available phenylsulfenyl and
-selenenyl chloride. A dimethylaminomethyl group can be
introduced onto 13e by reaction with Eschenmoser’s salt
Acknowledgment. We thank Dr. Richard Taylor and
Barbara Mason for providing spectral and analytical data and
Dr. Tony Shock and Paul Hale for cell assay data.
+
[H2CdN(Me)2 I-]. Hydroxylation of the unsubstituted
Supporting Information Available: Preparative methods
and spectral and analytical data for compounds 7e-l, 13a-
q, and 14a-m. This material is available free of charge via
the Internet at http://pubs.acs.org.
vinylic carbon was achieved by oxidation with Pb(OAc)4 to
give compound 14j. π-Electrophiles are also found to react
with the nucleophilic amino-cyclobutenone system as evi-
denced by reaction of 13e with 1,3-dithienium tetrafluoro-
borate to give 14k. The heterocyclic sulfenyl chlorides used
in reactions l and m were prepared in-situ directly from the
corresponding heterocyclic mercaptans by reaction with NCS
in THF18 and these were found to react predictably with 13e
to give 14l,m.
OL034701N
(18) 1-Methyltetrazole-5-sulfenyl chloride has been previously prepared
by chlorination of the mercaptan with chlorine. See: Postle, S. R. U.S.
Patent 4 387 158, 1983.
2346
Org. Lett., Vol. 5, No. 13, 2003