Synthetic studies towards anti-SARS agents: Application of an indium-mediated allylation of α-aminoaldehydes as the key step towards an intermediate

Article abstract of DOI:10.1016/j.tetlet.2004.10.146

AG7088 was identified as a good starting point for modification, leading to an efficient and bio-available inhibitor for the SARS coronavirus main proteinase (SARS-CoV M^pro). Synthesis of intermediate 1 and analogues proceeded via a highly diastereoselective indium-mediated allylation of α-aminoaldehydes.

Full text of DOI:10.1016/j.tetlet.2004.10.146

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 9501–9504
Synthetic studies towards anti-SARS agents: application of
an indium-mediated allylation of a-aminoaldehydes as the key
step towards an intermediate
Shu-Sin Chng, Truong-Giang Hoang, Wei-Woon Wayne Lee, Mun-Pun Tham,
Hui Yvonne Ling and Teck-Peng Loh*
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore
Received 22 October 2004; accepted 26 October 2004
Available online 11 November 2004
Abstract—AG7088 was identified as a good starting point for modification, leading to an efficient and bio-available inhibitor for the
SARS coronavirus main proteinase (SARS-CoV Mpro). Synthesis of intermediate 1 and analogues proceeded via a highly diaste-
reoselective indium-mediated allylation of a-aminoaldehydes.
Ó 2004 Elsevier Ltd. All rights reserved.
Severe acute respiratory syndrome (SARS) was first re-
ported in Asia in February 2003. Over a few months,
the disease spread from its likely origin in Southern
China to more than two dozen countries in North
America, South America, Europe and Asia with more
than 8400 cases of infection.1 SARS is characterized
by high fever, malaise, rigor, headache and nonproduc-
tive cough or dyspnoea and may progress to generalized
interstitial infiltrates in the lung. The fatality rate among
people with the illness is around 15% (calculated as
deaths/(deaths + surviving patients)).1a
inase (SARS-CoV Mpro) and disrupt the replication
cycle of the virus. In May 2003, Hilgenfeld and co-work-
ers showed that the substrate-binding site in SARS-CoV
Mpro is well conserved compared to those in two
other coronavirus main proteinases (HCoV 229E and
TGEV Mpro).4 In addition, they reported similarities
between substrate/inhibitor-binding modes of SARS-
CoV Mpro and the distantly related human rhinovirus
3C proteinase (HRV 3Cpro). Based on these findings, it
was suggested that the HRV 3Cpro inhibitor, AG7088,
could serve as a good starting point for modifications
leading to an efficient and bio-available inhibitor for
the SARS-CoV Mpro and other coronavirus main
proteinases. AG7088 is currently under clinical trial
for treatment of the Ôcommon coldÕ caused by HRV
Scheme 1.
A novel coronavirus has been identified as the causative
2,3
agentof SARS.
In order to find an efficient therapy
for SARS, many scientists are now focusing on the
development of drugs that inhibit the viral main prote-
O
NH
F
O
NH
O
O
O
N
+
COOEt
N
H
N
H
N
O
N
H
O
O
O
COOEt
BocHN
O
F
1
2
AG7088
Scheme 1.
Keywords: Anti-SARS agents; AG7088; Indium-mediated allylation; Diastereoselective.
*
Corresponding author. Tel.: +65 6874 7851; fax: +65 6779 1691; e-mail: chmlohtp@nus.edu.sg
0040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.10.146

9502
S.-S. Chng et al. / Tetrahedron Letters 45 (2004) 9501–9504
Retrosynthetic analysis of AG7088 led us to two key
intermediates, lactone 1 and lactam 2. The synthesis of
1 in our group has proceeded via the highly stereoselec-
tive indium-mediated allylation reaction of N-protected
valinal as illustrated in Scheme 2. Although syntheses of
fragments similar to 1 have previously been reported,
the synthetic strategy presented herein has two major
advantages.5 Firstly, the incorporation of the 5-methyl-
isoxazole-3-carbonyl group in the first step of the
synthesis increases the efficiency by doing away with
the protection and de-protection of the amino group.
In addition, our synthetic strategy easily allows us to
make modifications to AG7088. Hilgenfeld and co-
stead, the crude product from the Swern oxidation was
used directly in the subsequent indium-mediated allyl-
ation reaction. Thus, crude aldehyde 6 was reacted with
methyl 2-(bromomethyl)acrylate in the presence of in-
dium metal to furnish homoallylic alcohol 7 in overall
66% yield (from 5) with almost complete syn diastereo-
selectivity (syn/anti = 98:2).
The unusually high diastereoselectivity for the indium-
mediated allylation might be explained based on the Fel-
kin-Anh chelation model.7 As shown in Scheme 3, t hea-
nitrogen atom can coordinate to the indium metal to
form a five-membered chelation ring. We can see in
the transition state leading to the anti-isomer that there
is more pronounced steric repulsion between the i-prop-
yl group and the R group in the axial position
(R = COOMe). As a result, the syn-isomer is favoured.
workers suggested that most parts of AG7088 could be
4
accommodated by the active site of SARS-CoV Mpro
.
Based on molecular modeling, only the p-fluorobenzyl
group might be too long to fit into the pocket. In our
synthesis of 1, t hep-fluorobenzyl group is introduced
in the final step, allowing us to generate analogues of
AG7088 by simply changing the organocuprate reagent
employed.
However, ithas been reporetd by Seturer and Podlech
that when the a-amino group was protected using benz-
yloxycarbonyl (Cbz), the indium-mediated allylation of
the corresponding L-valinal gave a syn/anti diastereoiso-
meric ratio of only 82:18.8 In addition, when we per-
formed the same indium-mediated allylation reaction
on N-t-butyloxycarbonyl-L-valinal, we obtained the
homoallylic alcohol as an 87:13 mixture of syn/anti iso-
mers (Scheme 4). Clearly, chelation of the a-nitrogen
atom and the steric bulkiness of the i-propyl group can-
not totally account for the observed selectivity in our
reaction. This suggests that the isoxazole group could
have been involved.
Lactone 1 was synthesized in six steps from L-valine
methyl ester hydrochloride. The hydrochloride salt was
first coupled with 5-methylisoxazole-3-carboxylic acid
3 to give methyl ester 4 in almost quantitative yield. Sub-
sequently, NaBH4 reduction of 4 in THF methanol af-
forded b-aminoalcohol 5 in 99% yield. Alcohol 5 was
then oxidized to a-aminoaldehyde 6 under Swern condi-
tions. As a-aminoaldehydes are particularly prone to
epimerization, no purification was performed on 6.6 In-
EDC.HCl (1.5 eq)
O
N
HOBT (1.1 eq)/ DIEA (3.5 eq)
N
O
+
N
H
COOMe
CHO
COOH
O
0
HCl. H N
2
COOMe
CH Cl , 0 C to r.t
2
2
3
4
99%
NaBH (9 eq)
(COCl) (3.0 eq)
2
4
O
O
THF/MeOH (3:1)
DMSO (6.0 eq)
OH
N
O
N
N
H
N
O
H
0
Et N (9.0 eq)
3
0 C to rt
0
99%
CH Cl , -78 C
5
6
2
2
O
(2.5 eq)
COOMe
In (2.0 eq)
N
Br
O
O
NH
N
N
H
O
EtOH:H O (4:1), rt.
2
OH COOMe
OH COOMe
66% (2 steps)
7
syn:anti = 98:2
Scheme 2.
H
R'
N
R'
H
R
H
H
H
N
NHR'
In
R
Br
H
+
+
In
L
L
In
H
O
O
R
O
favoured
disfavoured
NHR'
OH
NHR'
OH
R
R
Scheme 3.

S.-S. Chng et al. / Tetrahedron Letters 45 (2004) 9501–9504
9503
NHR'
O
NHR'
NHR'
OH
In
Br
H
H
COOMe
COOMe
+
+
+
COOMe
OH
R' = Cbz, syn:anti = 82:18
R' = Boc, syn:anti = 87:13
R' = isoxazole, syn:anti = 98:2
NHR'
O
NHR'
In
NHR'
Br
COOMe
COOMe
+
COOMe
OH
OH
R' = Cbz, syn:anti = 75:25
R' = isoxazole, syn:anti = 94:6
Scheme 4.
Therefore, we propose that the nitrogen atom of the
isoxazole ring coordinates to the indium atom, forming
a second five-membered chelated ring (Scheme 5).
The interaction of the isoxazole ring with indium metal
causes the transition state to be more rigid (compare
Schemes 3 and 5). Less puckering of the transition state
(R0 = isoxazole) is now possible (compared to R0 = Cbz
or Boc) to minimize the steric repulsion between the
i-propyl group and the COOMe group. The transition
state leading to the anti product is therefore further
destabilized, resulting in the increase in the syn/anti
selectivity from 82:18 and 87:13 to 98:2, respectively.
In our synthesis of the L-leucine analogue of lactone
1, an improvementin hte syn/anti selectivity was also
clearly demonstrated (Scheme 4). The excellentdiastereo-
selectivity in the allylation step makes our synthetic
strategy of 1 very efficient and paves the way for control-
ling the second stereogenic centre in the molecule.
dichloromethane to afford a,b-unsaturated lactone 8 in
high yield (89%), (Scheme 6). Conjugate addition of
the organocuprate reagent (formed in situ from p-fluoro-
benzylmagnesium bromide and copper(I) cyanide) to 8
finally provided key intermediate lactone 1 in 50% yield
as a mixture of isomers (cis/trans = 39:61). The desired
trans-isomer 1a9 was obtained as the major product
(confirmed by NOE experiments).
Herein, we have reported the asymmetric synthesis of a
key intermediate in our synthetic approach towards
AG7088 and its analogues. The critical step in the syn-
thesis involved a highly diastereoselective indium-medi-
ated allylation reaction. Syntheses of the second key
intermediate and analogues of AG7088 are now under-
way in our laboratory, and the compounds obtained will
be tested to examine their inhibitory abilities on the rep-
lication and spread of the SARS coronavirus.
Upon accomplishing this critical step, homoallylic alco-
hol 79 was cyclized using p-toluenesulfonic acid in
Acknowledgements
We gratefully acknowledge the A*STAR Biomedical
Research Council (BMRC) and the National University
of Singapore (N.U.S.) for funding this research.
O
O
H
H
N
O
H
N
H
N
N
R
R
O
In
In
H
H
O
O
favoured
strongly disfavoured
References and notes
NHR'
OH
NHR'
OH
1. (a) World Health Organisation, ÔCumulative Number of
Reported Probable Cases of SARSÕ, http://www.who.int/
csr/sars/country/en/; (b) Centre for Disease Control and
Prevention, ÔSevere Acute Respiratory SyndromeÕ, http://
www.cdc.gov/ncidod/sars/.
R
R
Scheme 5.
O
N
O
p-TsOH (1.0 eq)
N
N
N
H
O
H
O
CH2Cl2, rt
89%
O
OH COOMe
F
O
7
8
F
F
O
N
O
(6.0 eq)
CuCN (3.0 eq)
BrMg
N
N
+
N
H
O
O
H
O
ether, -78 o
50%
C
O
O
O
1a
1b
Scheme 6.

9504
S.-S. Chng et al. / Tetrahedron Letters 45 (2004) 9501–9504
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J = 10.03Hz, 1H), 6.39 (s, 1H), 6.18 (s, 1H), 5.66 (s, 1H),
4.01 (ddd, J = 1.60, 4.32, 8.13Hz, 1H), 3.73–3.67 (m, 1H),
3.69 (s, 3H); 2.48 (dd, J = 4.22, 13.85Hz, 1H), 2.42 (s, 3H),
2.39 (dd, J = 8.03, 14.05Hz, 1H), 2.00–1.88 (m, 1H), 0.96
(d, J = 6.83Hz, 3H), 0.92 (d, J = 6.82Hz, 3H). 13C NMR
(300MHz, CDCl3) d 171.0, 168.2, 159.4, 158.6, 136.8, 128.6,
101.4, 69.5, 58.5, 52.0, 38.6, 30.2, 19.6, 19.3, 12.1. FTIR
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¨
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`
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¨
¨
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950, 818cmꢀ1 HRMS Calcd for C15H22N2O5 [M+]:
.
310.1529. Found: 310.1516.
Data for molecule 1a: 1H NMR (300MHz, CDCl3) d 7.14–
7.08 (m, 2H), 7.00–6.94 (m, 2H), 6.74 (d, J = 10.04Hz, 1H),
6.40 (d, J = 0.80Hz, 1H), 4.47 (ddd, J = 2.01, 6.32, 8.13Hz,
1H), 3.91 (ddd, J = 2.01, 8.33, 10.14Hz, 1H), 3.04–2.80 (m,
3H); 2.47 (s, 3H), 2.19–2.01 (m, 2H), 1.93 (d (hept),
J = 6.83, 8.03Hz, 1H), 0.98 (d, J = 6.83Hz, 3H), 0.95 (d,
J = 6.82Hz, 3H). 13C NMR (300MHz, CDCl3) d 178.4,
171.6, 161.9, 160.0, 158.1, 133.3, 130.5, 115.6, 101.4, 77.6,
57.3, 41.0, 35.8, 30.6, 29.4, 19.7, 19.2, 12.3. 19F NMR
(300MHz, CDCl3) d ꢀ40.36 (s, 1F). FTIR (KBr): 3391,
3328, 2928, 2851, 2360, 1771, 1682, 1627, 1536, 1509, 1219,
1154, 1020, 805cmꢀ1. Norminal MS(EI) for C20H23FN2O4
[M+]: 374.1 (100%), 375.1 (41%).
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