Design, synthesis, and evaluation of nonpeptidic inhibitors of human rhinovirus 3C protease

Article abstract of DOI:10.1021/jm960603e

The design, synthesis, and biological evaluation of reversible, nonpeptidic inhibitors of human rhinovirus (HRV) 3C protease (3CP) are reported. A novel series of 2,3-dioxindoles (isatins) were designed that utilized a combination of protein structure-bas

Full text of DOI:10.1021/jm960603e

5072
J . Med. Chem. 1996, 39, 5072-5082
Design , Syn th esis, a n d Eva lu a tion of Non p ep tid ic In h ibitor s of Hu m a n
Rh in ovir u s 3C P r otea se
Stephen E. Webber,* J ayashree Tikhe, Stephen T. Worland, Shella A. Fuhrman, Thomas F. Hendrickson,
David A. Matthews, Robert A. Love, Amy K. Patick, J ames W. Meador, Rose Ann Ferre, Edward L. Brown,
Dorothy M. DeLisle, Clifford E. Ford, and Susan L. Binford
Agouron Pharmaceuticals, Inc., 3565 General Atomics Court, San Diego, California 92121
Received August 21, 1996X
The design, synthesis, and biological evaluation of reversible, nonpeptidic inhibitors of human
rhinovirus (HRV) 3C protease (3CP) are reported. A novel series of 2,3-dioxindoles (isatins)
were designed that utilized a combination of protein structure-based drug design, molecular
modeling, and structure-activity relationship (SAR). The C-2 carbonyl of isatin was envisioned
to react in the active site of HRV 3CP with the cysteine responsible for catalytic proteolysis,
thus forming a stabilized transition state mimic. Molecular-modeling experiments using the
apo crystal structure of human rhinovirus-serotype 14 (HRV-14) 3CP and a peptide substrate
model allowed us to design recognition features into the P1 and P2 subsites, respectively, from
the 5- and 1-positions of isatin. Attempts to optimize recognition properties in the P1 subsite
using SAR at the 5-position were performed. In addition, a series of ab initio calculations
were carried out on several 5-substituted isatins to investigate the stability of sulfide adducts
at C-3. The inhibitors were prepared by general synthetic methods, starting with commercially
available 5-substituted isatins in nearly every case. All compounds were tested for inhibition
of purified HRV-14 3CP. Compounds 8, 14, and 19 were found to have excellent selectivity for
HRV-14 3CP compared to other proteolytic enzymes, including chymotrypsin and cathepsin
B. Selected compounds were assayed for antiviral activity against HRV-14-infected HI-HeLa
cells. A 2.8 Å cocrystal structure of derivative 19 covalently bound to human rhinovirus-serotype
2 (HRV-2) 3CP was solved and revealed that the isatin was situated in essentially the same
conformation as modeled.
In tr od u ction
been studied.1a,1c,1d Other approaches being investi-
gated include the disruption of attachment of the
intracellular adhesion molecule (ICAM-1) to human cells
using antibodies or a soluble ICAM-1 extracellular
domain.1b Most of the research has focused on the
discovery and development of capsid-binding antipi-
cornaviral agents.1,7 Alternatively, the selective inhibi-
tion of the HRV 3CP is also considered a valid strategy
for an antirhinoviral drug discovery program. Examples
of HRV 3CP inhibitors have been reported in the
literature.8 Having the coordinates for the apo X-ray
crystal structure of HRV-14 3CP, our research group
initiated a structure-based drug design program with
this as our target.
HRVs are a major cause of the common cold in the
United States and Western Europe in the late spring
and early fall seasons.1 They are small icosahedral
RNA viruses that belong to the picornaviridae family.2
More than 100 different serotypes of HRVs have been
indentified.3 In humans, the primary site of virus
infection and replication is the nasal epithelium. After
the virus binds to cell surface receptors, the plus-sense
genome is uncoated and translated by the host cell
machinery. The single open reading frame codes for a
230 kD polyprotein which is co- and posttranslationally
processed by virally encoded 2A and 3C proteases to
yield structural and enzymatic proteins essential for
viral replication.1a,2,4 The enzymatic activity of the 3CP
has been thoroughly studied.2,5,6a It is a very specific
cysteine protease responsible primarily for the catalytic
cleavage of glutamine-glycine peptide bonds. Only
minimal homology exists between HRV 3CPs and
prevalent mammalian enzymes. The X-ray crystal
structure of HRV-14 3CP revealed that, unlike thiol
proteases such as papain, this enzyme has an overall
backbone fold similar to trypsin-like serine proteases.6
Currently, there are no adequate drugs to circumvent
rhinoviral infections. Remedies which, at best, relieve
the symptoms of a cold are the only treatments avail-
able. Since there are numerous rhinoviral serotypes, a
vaccine seems impractical.1a,1d However, the quest to
discover efficacious therapeutic agents continues. The
use of interferon to prevent rhinoviral infections has
Molecu la r Design
A major goal of our research was to develop selective,
low molecular weight, nonpeptidic inhibitors of several
serotypes of the HRV 3CPs. Our investigation began
with the examination of the 2.3 Å X-ray crystal struc-
ture of human HRV-14 3CP.6 Several three-dimen-
sional features of this cysteine protease closely resemble
those of serine proteases in the trypsin family.6 The
HRV-14 3CP catalytic triad is comprised of a cysteine,
histidine, and glutamic acid, whereas in bovine trypsin,
the catalytic region is made up of serine, histidine, and
aspartic acid. Not having an X-ray structure with a
bound inhibitor, peptide substrates, including the oc-
tamer Glu-Thr-Leu-Phe-Gln-Gly-Pro-Val (representing
the 2C-3A viral cleavage site), were modeled into the
P5-P3′ region in order to establish possible atomic
interactions which may be critical for recognition.6a,9
X Abstract published in Advance ACS Abstracts, November 15, 1996.
S0022-2623(96)00603-6 CCC: $12.00 © 1996 American Chemical Society

Inhibitors of Human Rhinovirus 3C Protease
J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 26 5073
Ta ble 1. 1,5-Disubstituted Isatins
no.
R1
R2
mp (°C)
formulaa
C9H7NO2
HRV-14 Ki (µM)b
1c
2d
3e
4f
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
H
Cl
I
130-133
171-173
160-162
197-199
290-292 dec
196-198
213-215
258-260
268-270
190-192
229-230
153-155
193-195
178-180
176-178
186-187
114-116
289-290
261-263
220-222
230-231
230-232
269-271
235-237
136-138
205-207
190-192
>100
C9H6ClNO2
C9H6INO2
C9H6N2O4
59.0 ( 8.0
30.0 ( 0.6
NO2
9.7 ( 0.7
a
CO2H
CO2CH3
CN
C10H7NO4
10.0 ( 3.0
C11H9NO4
C10H6N2O2
C10H8N2O3·0.5H2O
C11H10N2O3·0.75H2O
C12H12N2O3
C10H8N2O2S
C11H9NO3·0.1H2O
C10H9NO3S
30.0 ( 2.0
51.0 ( 3.0
CONH2
CONHCH3
CON(CH3)2
CSNH2
COCH3
SOCH3
CONH2
CONH2
CONH2
CONH2
CONH2
CONH2
CONH2
CONH2
CONH2
CONH2
CONH2
CONH2
CONH2
CONH2
0.051 ( 0.006
12.0 ( 2.0
33.0 ( 7.0
11.0 ( 2.0
1.2 ( 0.07
8.2 ( 1.2
(E)-CH2CHdCHPh
(CH2)3Ph
C18H14N2O3
0.011 ( 0.002
0.027 ( 0.005
0.019 ( 0.002
0.048 ( 0.008
0.004 ( 0.003
0.002 ( 0.002
0.010 ( 0.002
0.006 ( 0.002
0.029 ( 0.009
0.025 ( 0.004
0.003 ( 0.001
0.011 ( 0.003
0.004 ( 0.002
0.004 ( 0.002
C18H16N2O3·0.25H2O
C16H12N2O3·0.55H2O
C22H16N2O3·1.4H2O
C20H14N2O3·0.25H2O
C18H12N2O3S
C17H24N2O3·0.5H2O
C18H16N2O3·0.75H2O
C17H14N2O4·0.6H2O
C18H16N2O5
C21H16N2O4·0.5H2O
C16H12N2O4·0.7H2Oa
C16H12N2O5·1.7H2Oa
C20H14N2O4·1.5H2O
CH2Ph
CH2(4-Ph-C6H4)
CH2-â-naphthyl
CH2-2-benzo[b]thiophene
CH2(4-Me-C6H4)
CH2(3,4-di-MeC6H3)
CH2(3-OMeC6H4)
CH2(3,5-di-OMeC6H3)
CH2(6-OMe-â-naphthyl)
CH2(3-OHC6H4)
CH2(3,5-di-OHC6H3)
CH2(6-OH-â-naphthyl)
a
All compounds were analyzed for C, H, N or C, H, N, S; the results agreed to within (0.4% of the theoretical values except for 5;
b
analysis passed only with 1.5HCl. 25: N, calcd 9.07, found 8.42. 26: N, calcd 8.17; found 7.59. See Experimental Section for method.
c Commercially available. See ref 29. e See ref 28. f See ref 30.
d
Since a primary goal was to keep the molecular
weight of the inhibitors as low as possible, we limited
our attention to the scissile cleavage, P1 recognition, and
S2 sites. The HRV-14 3CP specifically recognizes the
primary carboxamide of glutamine in the P1 subsite and
cleaves the Gln (P1)-Gly (P1′) amide bond by nucleo-
philic attack of the Cys-146 (serotype 14 numbering).
This protease most often distinguishes the aromatic
amino acids Phe and Tyr at P2. With these points in
mind, we envisioned the cyclic R-keto amide isatin
structure as a good core for molecular design.10 Peptidyl
R-keto amides are known reversible inhibitors of cys-
teine and serine proteases.11 In a similar fashion, the
isatin heterocycle possesses an electrophilic carbonyl,
but in a conformationally restricted form. Thus, the
2-keto group of isatin was superimposed upon the
scissile amide carbonyl of our octapeptide substrate
model.12 From this orientation, a carboxamide group
could be positioned nicely into the P1 recognition site
from C-5, whereas the S2 site could be accessed via
substitution at N-1. Our first target, 1-methylisatin-
5-carboxamide, 8, was synthesized and found to be a
fairly potent inhibitor of HRV-14 3CP having a Ki of 51
nM.
of HRV-14 3CP having a Ki of >100 µM. Not entirely
convinced that the activity of 8 versus 1 was due only
to P1 recognition of the primary carboxamide, we
decided to examine whether different substituents at
C-5 would have any significant effect on reactivity and
protease inhibition. To evaluate any SAR, various
1-methyl-5-substituted-isatins (Table 1) were prepared.
From this set of analogs, it was concluded that a
carbonyl or a carbonyl isostere was required at C-5. The
primary amide was found to be the best group, having
a positive effect of >200× on activity when compared
to the secondary and tertiary amides and the thioamide,
>20× when compared to the methyl ketone, and >150×
when compared to either the nitro or sulfoxide deriva-
tive.
In addition to SAR, a series of ab initio quantum
mechanical calculations were performed to help probe
whether substituents at C-5 were contributing to Ki
enhancement by influencing thiol reactivity at C-3, as
opposed to molecular recognition factors in the P1
subsite. According to this hypothesis, if the thermody-
namic stability of the thiol adduct was the predominate
factor, the enthalpy change for the most active C-5
derivative should be the lowest. In Table 2 we have
tabulated the calculated absolute and relative enthalpy
changes for the reaction of methyl mercaptan to six
isatins and acetaldehyde. The electronic energies were
computed at both the Hartee-Fock 6-31G(d) and MP2
6-31(d) levels.12,13,14 Since electronic correlations are
included, we believe the MP2 calculations are more
accurate. Listed in Table 2 are the absolute gas phase
We became concerned that the activity of 8 may be
attributed solely to the inherent reactive nature of isatin
toward thiols after an important observation was made.
The excess addition of the antioxidant dithiothreitol
(DTT) to the assay inactivated 8, but not the protease.
Therefore, 1-methylisatin, 1, in the absence of exogenous
thiol, was tested next and shown to be a poor inhibitor

5074 J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 26
Webber et al.
F igu r e 1. Stereoview of compound 14 modeled into the active site of HRV-14 3CP. The 2.3 Å apo crystal structure of HRV-14
3CP is colored orange, with the catalytic triad (Cys-146, His-40, and Glu-71) and the S1 His-160 functionalites highlighted. A
solvent accessible surface on the protease is colored white. The modeled octapeptide substrate is colored blue. The C-3 carbonyl
and C-5 carboxamide of isatin 14 were superimposed upon the octapeptide Gln R-carbonyl and γ-carboxamide, respectively. The
S2 site is occupied by the N-1 cinnamyl group. Individual atoms are displayed in green (carbon), red (oxygen), blue (nitrogen), and
yellow (sulfur).
Ta ble 2. Ab Initio Quantum Mechanics Calculationsa
that molecular recognition in the P1 subsite is the
predominant factor for inhibition of HRV-14 3CP.
We now turned our attention to the S2 site. Using
our original model as described previously, a propyl or
allyl group placed at N-1 could reach into this confined
hydrophobic pocket. A phenyl ring substituted at the
γ-position of either of these two groups lies almost
exactly upon the phenyl ring of the octapeptide Phe at
P2.12 The trans-cinnamyl derivative 14 was prepared
and was found to be ∼5× (11 nM) more active against
R2
∆Hb,c (HF)d ∆∆H (HF) ∆Hb,c (MP2)e ∆∆H (MP2)
HRV-14 3CP than 8. Compound 14, superimposed upon
the peptide substrate model, is illustrated in Figure 1.
The n-propylphenyl analog 15, a 27 nM inhibitor of
HRV-14 3CP, was easily derived from compound 14.
Modeling the less extended N-1 benzyl group positions
the phenyl ring snugger into the S2 pocket and more
adjacent to the imidazole of His-40 as compared to
compound 14. Several N-1 arylmethylene derivatives
were therefore proposed and synthesized. Of these, the
benzo[b]thiophene derivative, 19, was found to be the
most potent inhibitor of HRV-14 3CP.
H
8.39
7.12
10.39
7.42
0.71
6.66
7.68
6.42
9.68
6.71
0
-7.63
-9.13
7.29
5.78
3.73
5.89
0
CONH2
CO2CH3
CO2H
CN
-11.18
-9.02
-14.92
-9.67
NO2
5.96
5.25
CH3CHO
3.17
2.46
-5.95
8.96
a
b
See the Experimental Section for details. All energies are
in kcal/mol. c ∆H ) ∆Eelectronic + ∆Evibrational + ∆Etranslation,
rotation
+ RT. Hartree-Fock electronic energy. e Second-order Møller-
d
Plesset perturbation energy.
A 2.8 Å cocrystal structure of 19 and HRV-2 3CP was
solved.15 Displayed in Figure 2 is an electron density
map of this complex. Close examination of this X-ray
structure revealed several important atomic interactions
between the protein and small molecule. A covalent
bond between Cys-147 and the electrophilic C-3 of isatin
was evident, positioning the oxygen in a classical “oxy-
anion hole” arrangement. The carboxamide makes
H-bonds to the imidazole of His-161 and the hydroxyl
and R-carbonyl of Thr-142. The benzo[b]thiophene is
tightly situated in the S2 pocket, although the orienta-
tion of this bicyclic ring is ambiguous. The electron
density maps reveal an equal population of two confor-
mations differing by 180°. One conformation exposes
the sulfur atom to solvent, while the other directs the
sulfur toward the â-sheet of the protease.
enthalpy changes and the enthalpy changes relative to
the lowest energy reaction. The data indicates that the
large enthalpic differences are dependent on the level
of ab initio theory used, possibly due in part to the
relatively large size of the system, the high degree of
polarizability of the aromatic ring and its’ substituents,
and the fact that a polarizable sulfur atom is involved.
In both sets of calculations the lowest enthalpic change
is for the cyano derivative. If one were to predict the
most active compound based on calculated thermody-
namic stability, the cyano derivative would be the best.
The calculated data at the MP2 level shows that the
nitrile analog is 5.79 kcal/mol more stable than the
carboxamide analog even though the Ki for nitrile is 1
order of magnitude greater. From this result we infer

Inhibitors of Human Rhinovirus 3C Protease
J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 26 5075
F igu r e 2. Stereoview of the electron density map of the crystal structure of compound 19 complexed with HRV-2 3CP at 2.8 Å
resolution computed with coefficients 2Fobs - Fcalc and phases from the protein structure alone (without 19). The electron density
is consistent for a covalent bond formed between Cys-147 and C-3 of 19.
Sch em e 1. Original Synthesis: Preparation of 3, 5, 6, 8, 14, and 15a
a
(a) NaH, DMF, MeI, or (E)-cinnamyl bromide; (b) CO, MeOH, DMF, Et3N, (Ph3P)2PdCl2; (c) K2CO3, MeOH, H2O; (d) N-
hydroxysuccinimide, EDC, CH2Cl2; (e) Et3N, H2O, CH2(NH2)2‚2HCl; (f) (1) H2, Pd/C, (2) MnO2, CH2Cl2.
Ch em istr y
Attempts to quickly generate the acid chloride and
immediately expose it to ammonia were also unsuccess-
ful. We finally resorted to preparing the activated
N-hydroxysuccinimide ester, which was subsequently
treated with diaminomethane.16 The yield for this two-
step process was a poor 20%.
Isatin analogs 1-27, listed in Table 1, were prepared
as illustrated in Schemes 1-3. We originally devised
a five-step synthetic route to 8 starting with com-
mercially available 5-iodoisatin (Scheme 1). This pro-
cedure was also used to prepare 14, but was not very
practical due to the following reasons. The cyclic keto
amide of 6 was sensitive to ring cleavage under standard
saponification conditions with hydroxide. The hydroly-
sis of the methyl ester 6 to carboxylic acid 5 was
eventually achieved by using potassium carbonate.
Conversion of 5 to amide 8 was problematic. The
synthesis and isolation of the acid chloride of 5 failed.
A second, more general synthetic route was devised.
It involved the same number of steps as our original
plan, but turned out to be much more effective. As
illustrated in Scheme 2, ketal protection of the C-3 isatin
carbonyl alleviated some of the synthetic difficulties.17
The primary carboxamide was efficiently introduced via
a palladium-catalyzed aryl cyanation18 of the iodide at

5076 J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 26
Sch em e 2. General Synthesis of 1,5-Disubstituted Isatinsa
Webber et al.
a
(a) NaH, DMF, R1X; (b) CO, MeOH, DMF, (Ph3P)2PdCl2, NEt3; (c) H2CdC(OEt)Sn(n-Bu)3, (Ph3P)4Pd, BHT, 1,4-dioxane; (d) MeOH,
(MeO)3CH, p-TsOH; (e) KCN, THF, (Ph3P)4Pd; (f) H2SO4, H2O, MeOH; (g) H2O2, Na2CO3, EtOH; (h) H2S, NEt3, pyridine; (i) LiOH, MeOH,
H2O; (j) ClC(O)C(O)Cl, CH2Cl2, DMF; (k) MeNH2, THF; (l) Me2NH, THF; (m) BBr3, CH2Cl2.
Sch em e 3. Synthesis of 13a
a
(a) NaH, DMF, CH3I; (b) m-CPBA, CH2Cl2.
Ta ble 3. Isatins 8 and 19 versus Different Serotypesa
C-5, followed by basic peroxide hydrolysis. Having the
carbonyl protected also allowed us to prepare the acid
Ki (nM)
chloride of 33, which was a common intermediate for
amides 9 and 10.
no.
HRV-14
HRV-89
HRV-2
HRV-16
8
19
51.0 ( 6.0
1.7 ( 1.8
35.0 ( 3.0
1.1 ( 0.02
77.0 ( 6.0
5.2 ( 0.07
40.0 ( 2.0
12.0 ( 11.0
The methyl ketone, 12, was prepared directly from 3
by a Stille-type palladium-catalyzed coupling of tributyl-
(1-ethoxyvinyl)tin,19 whereas sulfoxide 13 was synthe-
sized from 5-(methylthio)isatin20 as shown in Scheme
3.
a
See the Experimental Section for details.
in Tables 1 and 3. Although inconclusive, a comparison
of the Ki values for isatins 8 and 19 for HRV-2, -14, -16,
and -89 3CPs shown in Table 3 suggests that this class
of inhibitors may have a broad range of activity against
many serotypes.
En zym e In h ibition a n d An tivir a l Assessm en t
The HRV 3CP inhibition constants for compounds
1-27 were measured using a continuous fluorescence
resonance energy transfer assay.21 The data is reported
Isatins 8, 14, and 19 were found to have excellent
selectivity for HRV 3CPs over other readily available

Inhibitors of Human Rhinovirus 3C Protease
J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 26 5077
Ta ble 4. Protease Selectivity
Exp er im en ta l Section
% inhibition
Proton magnetic resonance spectra (NMR) were determined
using a Tech-Mag spectrometer operating at a field strength
of 300 MHz. Chemical shifts are reported in parts per million
(δ) with references set such that in CDCl3 the CHCl3 is at 7.26
ppm, in acetone-d6 the acetone is at 2.02 ppm, and in DMSO-
d6 the DMSO is at 2.49 ppm. Standard and peak multiplicities
are designated as follows: s, singlet; d, doublet; dd, doublet of
doublets; t, triplet; q, quartet; br s, broad singlet; m, multiplet.
Mass spectra were determined at the University of California,
Riverside, or Scripps Research Institute, San Diego, CA, Mass
Spectrometry Facilities. Infrared absorption (IR) spectra were
taken on a MIDAC Corp. FTIR or a Perkin-Elmer 1600 series
FTIR spectrometer. Elemental microanalyses were performed
by Atlantic Microlab Inc., Norcross, GA and gave results for
the elements stated with (0.4% of the theoretical values.
Flash column chromatography was performed using silica gel
60 (Merck Art 9385). Thin layer chromatographs were per-
formed on precoated sheets of silica 60 F254 (Merck Art 5719).
Melting points were determined on a Mel-Temp apparatus and
are uncorrected. Tributyl(1-ethoxyvinyl)tin was purchased
from Aldrich Chemical Co. Anhydrous N,N-dimethylform-
amide (DMF), dichloromethane (CH2Cl2), and dimethyl sul-
foxide (DMSO) were used as is. Tetrahydrofuran (THF) was
distilled from sodium benzophenone ketyl under nitrogen. Et2O
refers to diethyl ether. Pet ether refers to petroleum ether of
bp 36-53 °C.
(compound concentration)a
enzyme
8
14
19
trypsin (bovine)
0 (100 µM)
0 (100 µM)
0 (100 µM)
chymotrypsin (bovine) 0 (100 µM) 67 (100 µM) 25 (50 µM)
thrombin (human)
elastase
(human neutrophil)
cathepsin B
0 (100 µM) 34 (100 µM)
0 (100 µM) 40 (100 µM)
0 (100 µM)
0 (100 µM)
0 (100 µM) 27 (100 µM)
0 (10 µM)
(human liver)
a
See the Experimental Section and ref 22 for details.
Ta ble 5. Antiviral Activity (HRV-14)a
no.
EC50 (µM)b
TC50 (µM)c
TId
Ki (µM)
3
5
6
71
>100
56
>100
>100
>100
270
9.6
>1
30.0
10.0
30.0
>2
4
1.5
8
66
6.3
0.051
0.011
0.027
0.019
0.004
0.002
0.029
0.011
0.004
14
15
16
18
19
22
25
26
>7.1
14.1
>10
>5.6
>20
>20
>100
7.1
50.1
10
3.5
5.6
20
20
>100
In h ibition Assa ys. All strains of human rhinovirus (HRV)
were purchased from American Type Culture Collection
(ATCC) except for HRV serotype-14 which was obtained from
the supernatant of HeLa cells transfected with an RNA
transcript derived from a cDNA clone constructed and supplied
to us by Drs. R. Rueckert and W.-M. Lee at the Institute for
Molecular Virology, University of Wisconsin, Madison, WI.25
HRV-14 3CP was expressed and purified as described in the
literature.6a To obtain 3CPs from other serotypes, cytoplasmic
RNA was prepared from HeLa cells infected with HRV-2, -16,
or -89.4 cDNAs were synthesized using a primer coincident
with the 3′ terminus of the 3C coding regions. A region of the
genome corresponding to the carboxy terminus of 3A, all of
3B, and all of 3CP was amplified from the cDNA by PCR and
cloned into an Escherichia coli expression vector. Protein was
purified from the insoluble fraction of E. coli as described6a or
purified from the lysate supernatant by chromatography on
Blue Sepharose and Sephacryl 100, both supplied by Phar-
macia. Human liver cathepsin B and human neutrophil
elastase were purchased from Calbiochem. Bovine chymotryp-
sin and human thrombin were purchased from Boehringer
Mannheim. Bovine trypsin was purchased from Sigma.
The HRV 3CP activity was measured by a continuous
fluorescence resonance energy transfer assay using a dimodi-
fied decapeptide substrate (s), DABCYL-Gly-Arg-Ala-Val-Phe-
Gln-Gly-Pro-Val-Gly-EDANS (kcat/Km ) 3.4 × 104 M-1 s-1) or
a dimodified nonapeptide substrate (s), DABCYL-Gly-Gln-Val-
Leu-Phe-Gln-Gly-Pro-Val-EDANS (kcat/Km ) 6.7 × 104 M-1
s-1).21 Inhibition was measured as a change in initial velocity
(V0) as a function of inhibitor (I) concentration in substrate
start assays after a 10 min preincubation of enzyme (E) and
inhibitor. Assays (3 mL at 30 °C) contained 50 mM Tris, pH
7.6, 1 mM EDTA, 1 µM substrate, 50 nM HRV-14 3CP, 2%
DMSO, and inhibitor as needed. Fluorescence was monitored
at 336 nm (excitation) and 490 nm (emission). Other serotypes
were assayed at 100-200 nM enzyme concentrations. Data
was analyzed with the nonlinear regression analysis program
ENZFITTER26 using the equation
pirodavire
0.03
>10
>300
a
b
See the Experimental Section for assay conditions. 50%
effective concentration. c 50% toxic concentration. Therapeutic
d
index. e Control compound; J anssen Pharmaceuticals.
cysteine and serine proteases as depicted in Table 4.22
Against the enzymes tested, compound 8 was inactive,
whereas 14 and 19 displayed meager inhibition of
chymotrysin, possibly due to aromatic S1 recognition.
The antirhinoviral activity was measured for twelve
isatin analogs using a cell protection assay. The data
is recorded in Table 5. These compounds were checked
for their ability to protect HI-Hela cells against HRV
infection using the known technique of measuring XTT
dye reduction.23 Unfortunately these compounds were
either relatively inactive or did not display efficacy
significantly below their toxic concentrations (TC50). In
certain cases the poor antiviral activity may arise from
inadequate cell permeability; however, it nevertheless
remains unclear whether the reactive nature of these
molecules is responsible for cell toxicity.24
Con clu d in g Rem a r k s
In summary, we have designed and prepared a novel
class of very active, reversible, nonpeptidic HRV 3CP
inhibitors joining protein structure-based drug design
with molecular modeling, SAR, and ab initio quantum
mechanical calculations. Our research demonstrates
that molecular recognition, particularly in the P1 sub-
site, is a crucial aspect when considering the design of
nonpeptidic HRV 3CP inhibitors. The cocrystal struc-
ture of 19 covalently bound to HRV-2 3CP has validated
much of our modeling and design process. Disappoint-
edly, the lack of in vitro antiviral potency has raised
several puzzling and unanswered questions concerning
the toxicity and physiochemical properties of these
molecules. The solutions to these problems, together
with the further utilization of protein structure-based
drug design may lead to the discovery of an improved
class of nonpeptidic HRV 3CP inhibitors.
V(I) ) V0(EI/Etotal) where Ki ) (Etotal - EI)(Itotal - EI)/EI
Because the Km values for the fluorescent substrates are 12
µM for the 9-mer and 18 µM for the 10-mer (s , Km), no
corrections for an s/Km term were used.
An tivir a l Assa ys. HRV stocks were propagated, and
antiviral assays were performed in HI-HeLa cells, purchased
from ATCC. Cells were grown in minimal essential medium
with 10% fetal bovine serum.

5078 J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 26
Webber et al.
The ability of compounds to protect cells against HRV
infection was measured by the XTT dye reduction method.23
HI-Hela cells were infected with HRV-14 at a multiplicity of
infection (moi) of 0.13 or mock-infected with medium only.
Infected or uninfected cells were resuspended at 8 × 105 cells/
mL and incubated with appropriate concentrations of com-
pounds. Two days later, XTT/PMS was added to test plates,
and the amount of formazan produced was quantified spec-
trophotometrically at 450/650 nm. The EC50 was calculated
as the concentration of compound that increased the percent-
age of formazan production in compound-treated, virus-
infected cells to 50% of that produced by compound-free,
uninfected cells. The 50% cytotoxic dose (CC50) was calculated
as the concentration of compound that decreased the percent-
age of formazan produced in compound-treated, uninfected
cells to 50% of that produced in compound-free, uninfected
cells. The therapeutic index (TI) was calculated by dividing
(1.67 g, 12 mmol). The reaction mixture was stirred at 55-
60 °C for 24 h. Upon evaporation of the reaction solvents, the
residue was acidified with 1 N HCl and extracted with EtOAc.
The organic layers were dried over anhydrous MgSO4, filtered,
and concentrated. (Further purification of the product could
be done by flash chromatography on silica using EtOAc/MeOH,
70:30.) An orange solid (0.171 g) was isolated in 91% yield:
1
mp 290-292 °C dec; H NMR (DMSO-d6) δ 3.24 (s, 3H), 7.06
(d, J ) 6.0 Hz, 1H), 7.98 (s, 1H), 8.19 (d, J ) 6.0 Hz, 1H); IR
(KBr) 3400, 1739, 1733, 1618, 1408 cm-1
. Anal. (C10H7-
NO4‚1.5HCl) C, H, N.
1-Meth ylisa tin -5-ca r boxylic Acid N-h yd r oxysu ccin im -
id e Ester . To a solution of the carboxylic acid 5 (51 mg, 0.249
mmol) in 8 mL of anhydrous DMF at 0 °C were added
N-hydroxysuccinimide (35 mg, 0.3 mmol) and EDC (58 mg,
0.3 mmol). The solution was stirred for 2 h at room temper-
ature. The reaction mixture was evaporated to dryness, and
the resulting residue was taken up in EtOAc and washed with
water. The organic layers were dried over anhydrous MgSO4
and concentrated. The compound was purified by flash column
chromatography on silica with EtOAc to give a yellow solid
(42 mg, 56%): mp 230-232 °C; 1H NMR (DMSO-d6) δ 2.89 (s,
4H), 3.21 (s, 3H), 7.38 (d, J ) 9.0 Hz, 1H), 8.04 (s, 1H), 8.36
(d, J ) 9.0 Hz, 1H); IR (KBr) 1761, 1751, 1734, 1622, 1207
the CC50 by the EC50
.
Ab-In itio Qu a n tu m Mech a n ics Ca lcu la tion s. The en-
ergy changes for two types of reactions were studied; the
addition of methyl mercaptan to a series of substituted isatins
and the addition of methyl mercaptan to acetaldehyde.
A
protonated thiol was used as a model since there was evidence
to suggest that the active site cysteine in HRV-14 3CP is not
ionized in the resting state of the enzyme.27 Therefore, the
calculations should reflect the inherent stability of the adducts
with respect to the isolated components without any compli-
cating factors of thiol ionization, and the ion’s relative stability
in the environment of the enzyme versus the gas phase. All
ab initio calculations were performed using the GAUSSI-
AN9213 program package. Input structures were created using
the MacroModel12 program and were energy minimized using
the AMBER* force field. These structures were then converted
to the Z-matrix format using the utility program NewZmat
and then geometry optimized in GAUSSIAN at the restricted
Hartree-Fock (RHF) 6-31G(d) level. Harmonic vibrational
frequencies were calculated at this level of theory (RHF/6-31G-
(d)//RHF/6-31G(d)), and from these calculations were derived
vibrational and zero point energies and entropies at 298.15
K. The effects of electron correlation on the electronic energies
were obtained by performing second-order Møller-Plesset
perturbation theory14 at the MP2/6-31G(d)//RHF/6-31G(d)
level.
cm-1
.
1-Meth ylisa tin -5-ca r boxa m id e (8). Triethylamine (0.17
mL, 1.2 mmol) and diaminomethane dihydrochloride16 (24 mg,
0.2 mmol; dissolved in minimum amount of water) were added
at room temperature to a solution of the 1-methylisatin-5-
carboxylic acid N-hydroxysuccinimide ester (0.121 g, 0.401
mmol) in 40 mL of 1,4-dioxane. The solution was stirred for
24 h at room temperature. The residue obtained upon
evaporation of all reaction solvent was chromatographed on a
silica column using EtOAc. The product was isolated as an
1
orange solid (30 mg) in 37% yield: mp 258-260 °C; H NMR
(DMSO-d6) δ 3.19 (s, 3H), 7.21 (d, J ) 9.0 Hz, 1H), 7.44 (br s,
2H), 8.04 (s, 2H), 8.18 (d, J ) 9.0 Hz, 1H); IR (KBr) 3439, 3348,
1741, 1662, 1622, 1391, 1334, 1094 cm-1; MS (EI) m/ z 204
(M+), 176, 148, 132, 104, 77, 63, 51. Anal. (C10H8N2O3‚0.5H2O)
C, H, N.
1(E)-Cin n a m ylisa tin -5-ca r boxa m id e (14): mp 178-180
°C; 1H NMR (CDCl3) δ 4.59 (d, J ) 6.0 Hz, 2H), 5.75 (br s,
2H), 6.18 (dt, J ) 15.0, 6.0 Hz, 1H), 6.70 (d, J ) 15.0 Hz, 1H),
7.08 (d, J ) 9.0 Hz, 1H), 7.32 (m, 5H), 8.01 (s, 1H), 8.18 (d, J
) 9.0 Hz, 1H); IR (KBr) 3448, 3375, 1743, 1662, 1653, 1618,
1391, 1342 cm-1; MS (EI) m/ z 306 (M+), 189, 117, 105, 91, 83.
Anal. (C18H14N2O3) C, H, N.
Or igin a l Syn th esis: 1-Meth yl-5-iod oisa tin (3).28 To a
solution of 5-iodoisatin (3.00 g, 10.99 mmol) in 30 mL of
anhydrous DMF at 0 °C was added NaH (0.528 g, 13.19 mmol;
60% dispersion in mineral oil). The heterogeneous mixture
was stirred for 1 h at room temperature. To the reaction
mixture was added iodomethane (0.89 mL, 14.29 mmol). The
reaction mixture was stirred at room temperature overnight.
Ice cold water was added to the reaction mixture whereupon
a red solid precipitates. The solid was filtered, washed with
water and petroleum ether, and dried under vacuum to yield
2.69 g (85%) of a red solid. The product was used without
1-[1-(3-P h en yl-n -p r op yl)]isa tin -5-ca r boxa m id e (15). A
mixture of 14 (0.101 g, 0.330 mmol) and 10% Pd/C (10.2 mg)
in 50 mL of EtOAc was stirred at room temperature for 15
min under an atmosphere of hydrogen (balloon). The resulting
black suspension was filtered through a pad of Celite. The
yellow filtrate was concentrated to give a 1:1 mixture (86.0
mg) of the desired product along with 1-[1-(3-phenyl-n-propyl)]-
3-hydroxy-2-oxindole-5-carboxamide. This mixture was dis-
solved in 15 mL of CH2Cl2 and oxidized with MnO2 (0.040 g,
0.461 mmol). The black suspension was stirred at room
temperature for 45 min and then filtered through Celite. The
yellow filtrate was concentrated to give a pure product as a
yellowish orange solid (0.074 g; 86%): mp 176-178 °C; (CDCl3)
δ 2.07 (dt, J ) 7.5, 7.5 Hz, 2H), 2.74 (t, J ) 7.5 Hz, 2H), 3.80
(t, J ) 7.5 Hz, 2H), 5.85 (br s, 2H), 6.83 (d, J ) 9.0 Hz, 1H),
7.25 (m, 5H), 7.93 (s, 1H), 8.17 (d, J ) 9.0 Hz, 1H); IR (KBr)
3479, 3346, 1739, 1664, 1608, 1383 cm-1; MS (EI) m/ z 308
(M+), 280, 204, 175, 149, 118, 91. Anal. (C18H16N2O3‚0.25H2O)
C, H, N.
Gen er a l Syn th esis: 1-(2-Na p h th ylm eth yl)-5-iod oisa tin
Dim eth yl Keta l (28d ). A suspension of 1-(2-naphthylmeth-
yl)-5-iodoisatin (3d ) (prepared as described for 3 from 5-iodo-
isatin and 2-(bromomethyl)naphthalene; 0.186 g, 0.451 mmol)
and p-toluene sulfonic acid (8 mg, 0.0415 mmol) in 7 mL of
anhydrous methanol and trimethyl orthoformate (1.5 mL, 13.5
mmol) was refluxed for 44 h at 80 °C. Upon evaporation of
all solvent, the residue was taken up in CH2Cl2 and washed
with saturated aqueous NaHCO3. The organic layers were
dried and concentrated. When dried under vacuum, a yellow-
1
further purification: mp 160-162 °C; H NMR (DMSO-d6) δ
3.10 (s, 3H), 7.00 (d, J ) 9.0 Hz, 1H), 7.79 (s, 1H), 7.99 (d, J
) 9.0 Hz, 1H); IR (KBr) 1747, 1731, 1615, 1441, 1325, 1102
cm-1. Anal. (C9H6INO2) C, H, N.
1-Meth yl-5-ca r bom eth oxyisa tin (6). A mixture of 3 (0.5
g, 1.74 mmol) and bis(triphenylphosphine)palladium chloride
(0.055 g, 0.0783 mmol) in 10 mL of anhydrous DMF, 20 mL of
methanol, and triethylamine (0.76 mL, 5.48 mmol) was heated
at 50-60 °C for 18 h in a carbon monoxide atmosphere. The
resulting solution was evaporated to dryness. The residue was
taken up in 1 N HCl. The mixture was extracted with EtOAc,
and the combined organic extracts were washed with brine,
dried over MgSO4, and concentrated. The crude product was
chromatographed on a silica column using CHCl3. The product
was isolated as a yellowish orange solid (0.190 g; 99%): mp
1
196-198 °C; H NMR (DMSO-d6) δ 3.18 (s, 3H), 3.86 (s, 3H),
7.27 (d, J ) 9.0 Hz, 1H), 7.94 (s, 1H), 8.24 (d, J ) 9.0 Hz, 1H);
IR (KBr) 1751, 1709, 1631, 1309, 771, 473 cm-1. Anal. (C11H9-
NO4) C, H, N.
1-Meth ylisa tin -5-ca r boxylic Acid (5). Methyl ester 6 (0.2
g, 0.913 mmol) was dissolved in 100 mL of methanol and 25
mL of water. To this solution was added potassium carbonate

Inhibitors of Human Rhinovirus 3C Protease
J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 26 5079
ish orange solid (0.189 g, 92%) was formed. The product was
used without further purification: mp 118-119 °C; H NMR
(CDCl3) δ 3.62 (s, 6H), 5.00 (s, 2H), 6.52 (d, J ) 9.0 Hz, 1H),
7.35 (d, J ) 9.0 Hz, 1H), 7.45-7.53 (m, 3H), 7.69 (s, 2H), 7.76-
7.81 (m, 3H); IR (KBr) 2932, 1732, 1606, 1481, 1423, 1176,
1138 cm-1. Anal. (C21H18INO3) C, H, N.
N,N-Dim eth yl-1-m eth ylisa tin -5-ca r boxa m id e Dim eth -
yl Keta l (35). A solution of dry 1-methylisatin-5-carboxylic
acid dimethyl ketal (33) (0.251 g, 1.0 mmol) in 15 mL of
anhydrous CH2Cl2 was cooled to 0 °C. One drop of DMF was
added to this solution preceded by the slow addition of oxalyl
chloride (0.14 g, 1.1 mmol). The stirred solution was brought
to room temperature over a period of 1 h. At this time the
solution was concentrated under vacuum to give the crude acid
chloride which was used immediately. The acid chloride was
taken up in 15 mL of anhydrous THF. The solution was cooled
to 0 °C, and an excess of dimethylamine gas was bubbled into
the reaction vessel. The mixture was warmed to room tem-
perature and concentrated to give a residue which was purified
by column chromatography on silica with 80% EtOAc in pet.
ether to give a tan solid in 72% yield: mp 70-72 °C; 1H NMR
(CDCl3) δ 3.06 (br s, 6H), 3.18 (s, 3H), 3.56 (s, 6H), 6.85 (d, J
) 9.0 Hz, 1H), 7.47-7.50 (m, 2H); IR (KBr) 2941, 1732, 1631,
1620, 1491, 1390, 1205, 1114, 1045, 829, 769 cm-1. MS (EI)
m/ z 278 (M+), 247, 206, 188, 160, 104.
1
1-(2-Na p h t h ylm et h yl)-5-cya n oisa t in Dim et h yl Ket a l
(29d ). To a solution of the 1-(2-naphthylmethyl)-5-iodoisatin
dimethyl ketal (28d ) (0.108 g, 0.235 mmol) in 2 mL of
anhydrous THF were added KCN (31 mg, 0.470 mmol) and
tetrakis(triphenylphosphine)palladium (5 mg, 4.0 µmol). The
suspension was refluxed for 21 h. The reaction mixture was
evaporated to remove all solvent and the resultant residue was
taken up in CH2Cl2 and washed with water. The organic
layers were dried over anhydrous MgSO4, filtered, and con-
centrated. The product was purified by flash chromatography
on silica using EtOAc/hexanes, 30:70. A tan solid (71 mg, 94%)
was isolated: mp 120-121 °C; 1H NMR (CDCl3) δ 3.64 (s, 6H),
5.05 (s, 2H), 6.83 (d, J ) 9.0 Hz, 1H), 7.35 (d, J ) 9.0 Hz, 1H),
7.48-7.54 (m, 3H), 7.67 (s, 1H), 7.71 (s, 1H), 7.78-7.84 (m,
3H); IR (KBr) 3055, 2989, 2951, 2226, 1738, 1616, 1595, 1493,
1346, 1263, 1165, 1033, 827, 504 cm-1. Anal. (C22H18N2O3)
C, H, N.
N,N-Dim eth yl-1-m eth ylisa tin -5-ca r boxa m id e (10): mp
1
190-192 °C; H NMR (CDCl3) δ 3.06 (br s, 6H), 3.29 (s, 3H),
6.96 (d, J ) 9.0 Hz, 1H), 7.67 (s, 1H), 7.78 (d, J ) 9.0 Hz, 1H);
IR (KBr) 1736, 1624, 1612, 1334, 1168, 1102 cm-1: MS (EI)
m/ z 232 (M+), 203, 188, 160, 77. Anal. (C12H12N2O3) C, H,
N.
1-(2-Na p h th ylm eth yl)isa tin -5-ca r boxa m id e Dim eth yl
Keta l (31d ). To a suspension of the 1-(2-naphthylmethyl)-5-
cyanoisatin dimethyl ketal (29d ) (0.630 g, 1.685 mmol) in 60
mL of EtOH at room temperature were added 6 mL of 3 N
aqueous Na2CO3 and 5 mL of 30% H2O2. The reaction mixture
was stirred at room temperature for 4 h. The white suspension
was diluted with water and extracted with EtOAc. The
combined organic layers were dried over anhydrous MgSO4,
filtered, and evaporated to dryness. A white solid (0.618 g,
97%) was isolated which was used without further purifica-
tion: mp 191-193 °C; 1H NMR (CDCl3) δ 3.65 (s, 6H), 5.06 (s,
2H), 5.20-5.60 (br s, 2H), 7.37 (d, J ) 9.0 Hz, 1H), 7.46-7.49
(m, 2H), 7.70 (d, J ) 9.0 Hz, 1H), 7.73 (s, 1H), 7.77-7.83 (m,
3H), 7.90 (s, 1H); IR (KBr) 3412, 3181, 2949, 1730, 1668, 1616,
1373, 1143, 1044, 812, 755 cm-1. Anal. (C22H20N2O4) C, H,
N.
1-Meth yl-5-iod oisa tin d im eth yl k eta l (28): mp 88-90
1
°C; H NMR (CDCl3) δ 3.14 (s, 3H), 3.56 (s, 6H), 6.63 (d, J )
9.0 Hz, 1H), 7.68-7.70 (m, 2H); IR (KBr) 1738, 1607, 1474,
1358, 1251, 1102, 1052 821 cm-1. Anal. (C11H12NO3I) C, H,
N.
1-Meth yl-5-cya n oisa tin d im eth yl k eta l (29): mp 130-
1
131 °C; H NMR (CDCl3) δ 3.20 (s, 3H), 3.58 (s, 6H), 6.92 (d,
J ) 9.0 Hz, 1H), 7.66 (s, 1H), 7.71 (d, J ) 9.0 Hz, 1H); IR
(KBr) 2954, 2217, 1738, 1615, 1507, 1358, 1110, 821 cm-1
Anal. (C12H12N2O3) C, H, N.
.
1-Meth ylisatin -5-th iocar boxam ide Dim eth yl Ketal (30).
In a glass pressure tube, equipped with a threaded Teflon cap,
a solution of 1-methyl-5-cyanoisatin dimethyl ketal (29) (0.23
g, 1.0 mmol) in 7 mL of pyridine and 3 mL of NEt3 is cooled to
0 °C. The solution is saturated with H2S gas, sealed, and
brought to ambient temperature. (CAUTION: This procedure
should be carried out in a fume hood using a safety shield! )
After 48 h the vessel is cooled to 0 °C and vented. Excess H2S
is displaced by argon, and the solvents are remove under
vacuum. The green residue is purified by column chromatog-
raphy on silica with 25% EtOAc in pet. ether to give a pale
yellow solid in 96% yield: 180-181 °C; 1H NMR (CDCl3) δ
3.21 (s, 3H), 3.59 (s, 6H), 6.85 (d, J ) 9.0 Hz, 1H), 7.12 (br s,
1H), 7.51 (br s, 1H), 7.98-8.02 (m, 2H); IR (KBr) 3379, 3171,
1722, 1615, 1350, 1109, 1059 cm-1. Anal. (C12H14N2O3S) C,
H, N, S.
1-(2-Na p h t h ylm et h yl)isa t in -5-ca r b oxa m id e (18).
A
slightly turbid solution of 1-(2-naphthylmethyl)isatin-5-car-
boxamide dimethyl ketal (31d ) (8.8 mg, 0.023 mmol) in 1.5
mL of MeOH and 0.5 mL of H2O was cooled to 0 °C. To this
cold solution was gradually added concentrated H2SO4 (4
drops). Heat (70-75 °C) was applied for 7 h. Upon completion
of reaction a yellow solid precipitated out of solution. This
yellow solid was filtered, washed with water, and dried under
vacuum to yield 4 mg (57%) of pure product: mp 289-290 °C;
1H NMR (DMSO-d6) δ 5.10 (s, 2H), 7.05 (d, J ) 9.0 Hz, 1H),
7.40 (br s, 1H), 7.47-7.50 (m, 3H), 7.56 (d, J ) 9.0 Hz, 1H)
7.82-7.91 (m, 3H), 8.01 (s, 1H), 8.03 (d, J ) 9.0 Hz, 1H), 8.09
(s, 1H); IR (KBr) 3474, 3369, 1749, 1732, 1664, 1622, 1587,
1369, 1340 cm-1. Anal. (C20H14N2O3‚0.25 H2O) C, H, N.
1-Meth ylisa tin -5-th ioca r boxa m id e (11): mp 229-230
°C; 1H NMR (DMSO-d6) δ 3.16 (s, 3H), 7.19 (d, J ) 9.0 Hz,
1H), 8.11 (s, 1H), 8.31 (d, J ) 9.0 Hz, 1H), 9.55 (br s, 1H), 9.89
(br s, 1H); IR (KBr) 3414, 3310, 3225, 1745, 1618, 1342, 1085
cm-1. MS (EI) m/ z 220 (M+), 186, 158, 130, 103, 76, 50. Anal.
(C10H8N2O2S) C, H, N, S.
1-Meth yl-5-ca r bom eth oxyisa tin Dim eth yl Keta l (32).
This compound was prepared from 6 using the procedure to
prepare 1-(2-naphthylmethyl)-5-iodoisatin dimethyl ketal (28d)
1
in 85% yield: mp 83-85 °C; H NMR (CDCl3) δ 3.17 (s, 3H),
3.56 (s, 6H), 3.88 (s, 3H), 6.85 (d, J ) 9.0 Hz, 1H), 8.04 (s,
1H), 8.09 (d, J ) 9.0 Hz, 1H); IR (KBr) 29157, 2833, 1733,
1-Meth yl-5-a cetylisa tin (12). To a solution of 3 (0.4 g, 1.39
mmol) in 20 mL of anhydrous 1,4-dioxane was added tributyl-
(1-ethoxyvinyl)tin (0.61 mL, 1.81 mmol), tetrakis(triphenylphos-
phine)palladium (80 mg, 0.07 mmol), and few crystals of 2,6-
di-tert-butyl-4-methylphenol. The reaction mixture was heated
at 90-95 °C for 21 h. The reaction mixture was evaporated
to remove all solvent. The resultant residue was acidified with
1 N HCl and extracted with EtOAc. The combined organic
layers were dried over anhydrous MgSO4 and concentrated.
The product was purified by flash chromatography on a silica
column using hexane/EtOAc, 60:40, to give an orange solid.
In order to get rid of residual tin byproducts, the orange solid
was dissolved in CH3CN and washed with hexanes and the
layers were separated. The CH3CN layer was dried and
evaporated to dryness to give a pure orange solid (60 mg): mp
153-155 °C; 1H NMR (CDCl3) δ 2.59 (s, 3H), 3.13 (s, 3H), 7.00
(d, J ) 9.0 Hz, 1H), 8.17 (s, 1H), 8.30 (d, J ) 9.0 Hz, 1H); IR
1700, 1609, 1303, 1228, 1113, 1071, 769, 740 cm-1
(C13H15NO5) C, H, N.
. Anal.
1-Meth ylisa tin -5-ca r boxylic Acid Dim eth yl Keta l (33).
To a solution of 1-methyl-5-carbomethoxyisatin dimethyl ketal
(32) (0.53 g, 2.0 mmol) in 18 mL of MeOH was added an
aqueous solution of LiOH (1.02 g, 2.43 mmol; 8 mL of H2O).
After being stirred at room temperature for 12 h, the mixture
was washed twice with 20 mL of Et2O. The aqueous layer
was carefully neutralized at 0 °C with dilute HCl and extracted
three times with 40 mL of EtOAc. The organic layers were
combined, dried over MgSO4, filtered, and concentrated to give
0.46 g (91% yield) of the crude acid as an off-white solid: mp
181-183 °C; 1H NMR (CDCl3) δ 3.21 (s, 3H), 3.59 (s, 6H), 6.92
(d, J ) 9.0 Hz, 1H), 8.13 (s, 1H), 8.16 (d, J ) 9.0 Hz, 1H); MS
(EI) m/ z 251 (M+), 223, 208, 178, 149.

5080 J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 26
Webber et al.
(KBr) 1743, 1682, 1606, 1375, 1317, 1259, 1102 cm-1; MS (EI)
m/ z 203 (M+), 160, 132, 91 63. Anal. (C11H9NO3‚0.1H2O) C,
H, N.
1-(4-Meth ylben zyl)isa tin -5-ca r boxa m id e (20): mp 220-
1
222 °C; H NMR (DMSO-d6) δ 2.26 (s, 3H), 4.89 (s, 2H), 7.01
(d, J ) 9.0 Hz, 1H), 7.14 (d, J ) 9.0 Hz, 2H), 7.32 (d, J ) 9.0
Hz, 2H), 7.41 (s, 1H), 8.01(s, 1H), 8.03-8.08 (m, 2H); IR (KBr)
3387, 3181, 1736, 1662, 1616, 1400, 1350 cm-1; MS (EI) m/ z
294 (M+), 237, 189, 105, 77. Anal. (C17H14N2O3‚0.5H2O) C,
H, N.
1-Meth yl-5-(m eth ylth io)isa tin (36). This intermediate
was prepared from 5-(methylthio)isatin20 and iodomethane
using the N-alkylation procedure describe above: mp 156-
1
158 °C; H NMR (CDCl3) δ 2.53 (s, 3H), 3.29 (s, 3H), 6.84 (d,
J ) 9.0 Hz, 1H), 7.53 (m, 2H); IR (KBr) 3421, 1736, 1616, 1464,
1-(3,4-Dim eth ylben zyl)isa tin -5-ca r boxa m id e (21): mp
1332, 1119, 490 cm-1
.
1
230-231 °C; H NMR (DMSO-d6) δ 2.16 (s, 6H), 4.83 (s, 2H),
7.00 (d, J ) 9.0 Hz, 1H), 7.13 (m, 3H), 7.40 (br s, 1H), 8.01 (br
s, 1H), 8.07 (m, 2H); IR (KBr) 3468, 3377, 3170, 1741, 1668,
1616, 1406, 1338 cm-1; MS (EI) m/ z 308 (M+), 189, 133, 119,
89, 59. Anal. (C18H16N2O3‚0.75 H2O) C, H, N.
1-Meth yl-5-(m eth ylsu lfin yl)isa tin (13). To a stirred
solution of the 1-methyl-5-(methylthio)isatin (36) (30 mg, 0.145
mmol), in 10 mL of CH2Cl2 at room temperature was added
m-CPBA (39 mg, 0.145 mmol, 65%). The reaction mixture was
stirred at room temperature for 1 h. Upon completion, the
reaction mixture was diluted with CH2Cl2 and washed with
saturated aqueous NaHCO3. The organic phase was dried over
anhydrous MgSO4 and evaporated to dryness. Purification of
the product by flash chromatography on a silica column using
CHCl3/MeOH, 99:1, gave 20 mg of an orange solid (63%): mp
193-195 °C; 1H NMR (CDCl3) δ 2.75 (s, 3H), 3.32 (s, 3H), 7.10
(d, J ) 9.0 Hz, 1H), 7.81 (s, 1H), 8.02 (d, J ) 9.0 Hz, 1H); IR
(KBr) 3429, 1734, 1604, 1476, 1062, 475 cm-1; MS (EI) m/ z
223 (M+), 208, 180, 152, 97, 63. Anal. (C10H9NO3S) C, H, N,
S.
1-(3-Meth oxyben zyl)isatin -5-car boxam ide (22): mp 230-
1
232 °C; H NMR (DMSO-d6) δ 3.71 (s, 3H), 4.89 (s, 2H), 6.84
(d, J ) 9.0 Hz, 1H), 6.97-7.02 (m, 3H), 7.01 (t, J ) 7.5 Hz,
1H), 7.41 (br s, 1H), 8.02 (br s, 1H), 8.08 (m, 2H); IR (KBr)
3475, 3213, 1739, 1662, 1616, 1585, 1491, 1346 cm-1; MS (EI)
m/ z 310 (M+), 253, 210, 189, 121, 91, 78, 65. Anal. (C17H14
N2O4‚0.6 H2O) C, H, N.
-
1-(3,5-Dim eth oxyben zyl)isatin -5-car boxam ide (23): mp
1
269-271 °C; H NMR (DMSO-d6) δ 3.81 (s, 6H), 4.83 (s, 2H),
6.39 (s, 1H), 6.60 (s, 2H), 6.99 (d, J ) 9.0 Hz, 1H), 7.40 (br s,
1H), 8.01 (br s, 1H), 8.08 (m, 2H); IR (KBr) 3468, 3344, 1747,
1676, 1616, 1602, 1356, 1209, 1151, 1068 cm-1; MS (EI) m/ z
340 (M+), 189, 151, 121, 105, 77. Anal. (C18H16N2O5) C, H,
N.
1
1-Meth yl-5-ch lor oisa tin (2):29 mp 171-173 °C; H NMR
(DMSO-d6) δ 3.13 (s, 3H), 7.18 (d, J ) 9.0 Hz, 1H), 7.60 (s,
1H), 7.73 (d, J ) 9.0 Hz, 1H); IR (KBr) 1749, 1726, 1608, 1471,
1327 cm-1. Anal. (C9H6NO2Cl) C, H, N.
1-Meth yl-5-n itr oisa tin (4):30 mp 197-199 °C; 1H NMR
(DMSO-d6) δ 3.21 (s, 3H), 7.36 (d, J ) 9.0 Hz, 1H), 8.23 (s,
1H), 8.55 (d, J ) 9.0 Hz, 1H); IR (KBr) 3057, 2941, 1757, 1743,
1610, 1516, 1467, 1338, 1292, 1111 cm-1. Anal. (C9H6N2O4)
C, H, N.
1-Meth yl-5-cya n oisa tin (7): mp 213-215 °C; 1H NMR
(CDCl3) δ 3.32 (s, 3H), 7.04 (d, J ) 9.0 Hz, 1H), 7.89 (d, J )
9.0 Hz, 1H), 7.92 (s, 1H); IR (KBr) 2238, 1757, 1743, 1734,
1620, 1589, 1489, 1331, 1109, 837 cm-1. Anal. (C10H6N2O2)
C, H, N.
1-[[2-(6-Meth oxyn a p h th yl)]m eth yl]isa tin -5-ca r boxa m -
id e (24). This compound was prepared by the five-step
general synthesis outlined above. 2-(Chloromethyl)-6-meth-
oxynaphthalene, used in the N-alkylation step, was prepared
as described in the literature:32 mp 235-237 °C; 1H NMR
(DMSO-d6) δ 3.84 (s, 3H), 5.05 (s, 2H), 7.06 (d, J ) 9.0 Hz,
1H), 7.15 (d, J ) 9.0 Hz, 1H), 7.29 (s, 1H), 7.40 (br s, 1H), 7.50
(d, J ) 9.0 Hz, 1H), 7.75 (d, J ) 9.0 Hz, 1H), 7.80 (d, J ) 9.0
Hz, 1H), 7.90 (br s, 1H), 8.05 (m, 3H); IR (KBr) 3412, 1730,
1676, 1635, 1616, 1408 cm-1; MS (EI) m/ z 360 (M+), 189, 171,
128. Anal. (C21H16N2O4‚0.5 H2O) C, H, N.
1-Meth ylisatin -5-(N-m eth yl)-car boxam ide (9): mp 268-
1-(3-Hyd r oxyben zyl)isa tin -5-ca r boxa m id e (25). (The
following procedure was used to prepare compounds 25-27.)
To a solution of 22 (0.050 g, 0.162 mmol) in 16 mL of
anhydrous CH2Cl2 was added BBr3 (1 M in CH2Cl2; 0.4 mL,
0.4 mmol) at 0 °C. The reaction mixture was stirred at room
temperature for 18 h whereupon the resulting white suspen-
sion was diluted with CH2Cl2 and washed with saturated
aqueous NaHCO3. The organic layers were dried and concen-
trated to give the product as an orange solid (46 mg; 96%):
mp 136-138 °C; 1H NMR (DMSO-d6) δ 4.83 (s, 2H), 6.65 (d, J
) 9.0 Hz, 1H), 6.77 (s, 1H), 6.83 (d, J ) 6.0 Hz, 1H), 7.04 (d,
J ) 9.0 Hz, 1H), 7.11 (t, J ) 7.5 Hz, 1H), 7.41 (br s, 1H), 8.02
(br s, 1H), 8.09 (m, 2H), 9.42 (s, 1H); IR (KBr) 3396, 3256,
1739, 1662, 1618, 1591, 1449 cm-1; MS (EI) m/ z 296 (M+),
239, 189, 121, 107, 77, 61. Anal. (C16H12N2O4‚0.7H2O) C, H;
N: calcd, 9.07; found, 8.42.
1
270 °C; H NMR (DMSO-d6) δ 2.76 (d, J ) 6.0 Hz, 3H), 3.13
(s, 3H), 7.21 (d, J ) 9.0 Hz, 1H), 7.99 (s, 1H), 8.15 (d, J ) 9.0
Hz, 1H), 8.52 (br s, 1H); IR (KBr) 3536, 3437, 3325, 1734, 1633,
1616, 1558, 1482, 1317, 1110 cm-1; MS (EI) m/ z 218 (M+),
160, 132, 104, 77, 63. Anal. (C11H10N2O3‚0.75H2O) C, H, N.
1-Ben zylisa tin -5-ca r boxa m id e (16): mp 186-187 °C; 1H
NMR (CDCl3) δ 4.98 (s, 2H), 5.90 (br s, 2H), 6.90 (d, J ) 9.0
Hz, 1H), 7.32 (m, 5H), 7.99 (s, 1H), 8.10 (d, J ) 9.0 Hz, 1H);
IR (KBr) 3429, 3214, 1734, 1662, 1618, 1325, 688 cm-1; MS
(EI) m/ z 280 (M+), 223, 189, 180, 105, 91, 65. Anal.
(C16H12N2O3‚0.55H2O) C, H, N.
1-(4-P h en ylben zyl)-isatin -5-car boxam ide (17): mp 114-
1
116 °C; H NMR (DMSO-d6) δ 4.85 (s, 2H), 6.58 (d, J ) 9.0
Hz, 1H), 7.28-7.51 (m, 9H), 8.03 (m, 4H); IR (KBr) 3377, 1743,
1736, 1676, 1618 cm-1; MS (EI) m/ z 356 (M+), 189, 165, 152,
141, 105. Anal. (C22H16N2O3‚0.25H2O) C, H, N.
1-(3,5-Dih yd r oxyben zyl)isa tin -5-ca r boxa m id e (26): mp
1
2-(Br om om eth yl)ben zo[b]th iop h en e.31 (This alkylating
agent was used in the first step of the five-step general
synthesis of 19.) To a solution of alcohol (0.210 g, 1.28 mmol)
in 10 mL of CH2Cl2 at 0 °C was added CBr4 (0.552 g, 1.67
mmol). The reaction mixture was stirred for 30 min at room
temperature. The reaction mixture was diluted with CH2Cl2
and washed with water. The organic layers were dried over
anhydrous MgSO4, filtered, and concentrated. Purification of
the crude by flash chromatography on silica using hexanes/
EtOAc, 95:5, yielded an oil (0.107 g; 36%). The product being
unstable at room temperature was used immediately: 1H NMR
(CDCl3) δ 4.79 (s, 2H), 7.33-7.36 (m, 3H), 7.73 (t, J ) 4.5 Hz,
1H), 7.81 (t, J ) 4.5 Hz, 1H).
205-207 °C; H NMR (DMSO-d6) δ 4.74 (s, 2H), 6.10 (s, 1H),
6.22 (s, 2H), 7.05 (d, J ) 9.0 Hz, 1H), 7.42 (br s, 1H), 8.05 (br
s, 1H), 8.08 (s, 1H), 8.11 (d, J ) 9.0 Hz, 1H), 9.25 (s, 2H); IR
(KBr) 3383, 3211, 1741, 1662, 1618, 1350, 1176 cm-1. Anal.
(C16H12N2O5‚1.7 H2O) C, H; N: calcd, 8.17; found, 7.59.
1-[[2-(6-Hyd r oxyn a p h th yl)]m eth yl]isa tin -5-ca r boxa m -
id e (27): mp 190-192 °C; 1H NMR (DMSO-d6) δ 5.02 (s, 2H),
7.06 (m, 3H), 7.42 (m, 2H), 7.67 (t, J ) 9.0 Hz, 2H), 7.84 (s,
1H), 8.04 (m, 3H), 9.73 (s, 1H); IR (KBr) 3387, 3208, 1739,
1664, 1616, 1400 cm-1; MS (EI) m/ z 346 (M+), 289, 189, 157,
128, 61. Anal. (C20H14N2O4‚1.5 H2O) C, H, N.
Ack n ow led gm en t. We are very grateful to Drs.
Wai-Ming Lee and Roland Rueckert of the Institute for
Molecular Virology, University of Wisconsin, Madison,
WI, for supplying us with the infectious cDNA clone for
HRV serotype-14. We also thank Dr. P. Dragovich of
the medicinal chemistry group at Agouron for suggest-
ing, and providing details for, the palladium-catalyzed
1-(2-Be n zo[b]t h iop h e n ylm e t h yl)isa t in -5-ca r b oxa m -
id e (19): mp 261-263 °C; 1H NMR (DMSO-d6) δ 5.23 (s, 2H),
7.34 (m, 3H), 7.44 (br s, 1H), 7.57 (s, 1H), 7.77 (d, J ) 9.0 Hz,
1H), 7.91 (d, J ) 9.0 Hz, 1H), 8.04 (br s, 1H), 8.09 (s, 1H), 8.15
(d, J ) 9.0 Hz, 1H); IR (KBr) 3470, 3371, 1736, 1668, 1624,
1591, 1371, 763 cm-1: MS (EI) m/ z 336 (M+), 308, 279, 189,
147, 134, 103. Anal. (C18H12N2O3S) C, H, N, S.

Inhibitors of Human Rhinovirus 3C Protease
J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 26 5081
1995, 1081-1082. (g) Brill, G. M.; Kati, W. M.; Montgomery,
D.; Karwowski, J . P.; Humphrey, P. E.; J ackson, M.; Clement
J . J .; Kadam, S.; Chen, R. H.; McAlpine, J . B. Novel Triterpene
Sulfates from Fusarium compactum Using a Rhinovirus 3C
Protease Inhibitor Screen. J . Antibiot. 1996, 49, 541-546.
(9) Models of peptides bound to HRV-14 3CP were created from a
combination of constrained molecular dynamics and molecular
mechanics minimizations using the CHARMM software: Brooks,
B. R.; Bruccoleri, R. E.; Olafson, B. D.; States, D. J .; Swami-
nathan, S.; Kaplus, M. CHARMM: A Program for Macromo-
lecular Energy, Minimization, and Dynamics Calculations. J .
Comput. Chem. 1983, 4, 187-217.
(10) During the course of our research, isatin derivatives have been
reported to be inhibitors of serine and cysteine proteases. (a)
This class of compounds was discovered by a screening program;
Spitzer, W. A. Lilly Research Laboratories, Indianapolis, IN
46285, personal communication. (b) Iyers, R. A.; Hanna, P. E.
N-(Carbobenzyloxy)isatin: A Slow Binding R-Keto Lactam In-
hibitor of R-Chymotrypsin. Bioorg. Med. Chem. Lett. 1995, 5,
89-92.
coupling reaction of tributyl(1-ethoxyvinyl)tin with
1-methyl-5-iodoisatin.
Refer en ces
(1) (a) Couch, R. B. Rhinoviruses. In Virology; Fields, B. N., Knipe,
D. M., Eds.; Raven Press: New York, 1990; Vol. 1, Chapter 22,
pp 607-629. (b) See: McKinlay, M. A.; Pevear, D. C.; Rossman,
M. G. Treatment of the Picornavirus Common Cold by Inhibitors
of Viral Uncoating and Attachment. Annu. Rev. Microbiol. 1992,
46, 635-654 and references cited therein. (c) Phillpotts, R. J .;
Tyrell, D. A. J . Rhinovirus Colds. Br. Med. Bull. 1985, 41, 386-
390. (d) Gwaltney, J . M. Rhinoviruses. In Viral Infections of
Humans; Evans, A. S., Ed.; Plenum Publishing Corp.: New York,
1982; Chapter 20, pp 491-517. (e) Gwaltney, J . M. The Common
Cold. In Principles and Practices of Infectious Diseases; Mandell,
G. L., Douglas, R. G., Bennett, J . E., Eds.; J ohn Wiley & Sons:
New York, 1985; Chapter 38, pp 351-355.
(2) See: Kra¨usslich, H.-G.; Wimmer, E. Viral Proteinases. Annu.
Rev. Biochem. 1988, 57, 701-754 and references cited therein.
(3) Hamparian, V. V.; Colonno, R. J .; Cooney, M. K.; Dick, E. C.;
Gwaltney, J . M.; Hughes, J . H.; J ordan, W. S.; Kapikian, A. Z.;
Mogabgab, W. J .; Monto, A.; Phillips, C. A.; Rueckarts, R. R.;
Schieble, J . H.; Stott, E. J .; Tyrrell, D. A. J .; Benade, L. E.; Riker,
D. K. A Collaborative Report: Rhinoviruses - Extension of the
Numbering System from 89 to 100. Virology 1987, 159, 191-
192.
(4) (a) Callahan, P. L.; Mizutani, S.; Colonno, R. J . Molecular cloning
and complete sequence determination of the RNA genome of
human rhinovirus type 14. Proc. Natl. Acad. Sci. U.S.A. 1985,
82, 732-736. (b) Stanway, G.; Hughes, P. J .; Mountford, R. C.;
Minor, P. D.; Almond, J . W. The complete nucleotide sequence
of the common cold virus: human rhinovirus 14. Nucleic Acids
Res. 1984, 12, 7859-7875. (c) Lee, W.-M.; Wang, W.; Rueckart,
R. R. Complete sequence of the RNA genome of human rhinovi-
rus 16, a clinically useful common cold virus belonging to the
ICAM-1 receptor group. Virus Genes 1995, 9, 177-181.
(5) (a) Orr, D. C.; Long, A. C.; Kay, J .; Dunn, B. M.; Cameron, J . M.
Hydrolysis of a Series of Synthetic Peptide Substrates by the
Human Rhinovirus 14 3C Protease, Cloned and Express in
Escherichia coli. J . Gen. Virol. 1989, 70, 2931-2942. (b) Cord-
ingly, M. G.; Register, R. B.; Callahan, P. L.; Garsky, V. M.;
Colonno, R. J . Cleavage of Small Peptides In Vitro by Human
Rhinovirus 14 3C Protease Expressed in Escherichia coli. J .
Virol. 1989, 63, 5037-5045.
(6) (a) Matthews, D. A.; Smith, W. A.; Ferre, R. A.; Condon, B.;
Budahazi, G.; Sisson, W.; Villafranca, J . E.; J anson, C. A.;
McElroy, H. E.; Gribskov, C. L.; Worland, S. Structure of Human
Rhinovirus 3C Protease Reveals a Trypsin-like Polypeptide Fold,
RNA-Binding Site, and Means for Cleaving Precursor Polypro-
tein. Cell 1994, 77, 761-771. (b) Bazan, J . F.; Fletterick, R. J .
Viral cysteine proteases are homologous to the trypsin-like
family of serine proteases: Structural and functional implica-
tions. Proc. Natl. Acad. Sci. U.S.A. 1988, 85, 7872-7876. (c)
Gorbalenya, A. E.; Blinov, V. M.; Donchenko, A. P. Poliovirus-
encoded Proteinase 3C: A Possible Evolutionary Link Between
Cellular Serine and Cysteine Proteinase Families. FEBS Lett.
1986, 194, 253-257. (d) Allaire, M.; Chernala, M. M.; Malcolm,
B. A.; J ames, M. N. G. Picornaviral 3C cysteine proteinases have
a fold similar to chymotrypsin-like serine proteinases. Nature
1994, 369, 72-76.
(11) (a) Harbeson, S. L.; Abelleira, S. M.; Akiyama, A.; Barrett, R.;
Carroll, R. M.; Straub, J . A.; Tkacz, J . N.; Wu, C.; Musso, G. F.
Stereospecific Synthesis of Peptidyl R-Keto Amides as Inhibitors
of Calpain. J . Med. Chem. 1994, 37, 2918-2929. (b) Li, Z.; Patil,
G. S.; Golubski, Z. E.; Hori, H.; Tehrani, K.; Foreman, J . E.;
Eveleth, D. D.; Bartus, R. T.; Powers, J . C. Peptide R-Keto Ester,
R-Keto Amide, and R-Keto Acid Inhibitors of Calpains and Other
Cysteine Proteases. J . Med. Chem. 1993, 36, 3472-3480. (c)
Fusetani, N.; Matsunaga, S.; Matsumoto, H.; Takebayashi, Y.
Cyclotheonamides, Potent Thrombin Inhibitors, from a Marine
Sponge Theonella sp. J . Am. Chem. Soc. 1990, 112, 7053-7054.
(12) Modeling exercises were performed with MacroModel: Moha-
madi, F.; Richards, N. G. J .; Guida, W. C.; Liskamp, R.; Lipton,
M.; Caufield, C.; Chang, G.; Hendrickson, T.; Still, W. C.
MacroModel - An Integrated Software System for Modeling
Organic and Bioorganic Molecules Using Molecular Mechanics.
J . Comput. Chem. 1990, 11, 440-467.
(13) See the Experimental Section for methods and details of the ab
initio quantum mechanical calculations.
(14) (a) Møller, C.; Plesset, M. S. Note on the Approximation
Treatment for Many-Electron Systems. Phys. Rev. 1934, 46,
618-622. (b) Pople, J . A.; Seeger, R.; Krishnan, R. Variational
Configuration Interaction Methods and Comparison with Per-
turbation Theory. Int. J . Quant. Chem. Symp. 1977, 11, 149-
163. (c) Krishnan, R.; Frisch, M. J .; Pople, J . A. Contribution of
triple substitutions to the electron correlation energy in fourth
order perturbation theory. J . Chem. Phys. 1980, 72, 4244-4245.
(15) Excess 18 was incubated with HRV-2 3CP and crystallized using
protocols to be described elsewhere. Diffraction data from a
single crystal grown in space group P212121 were collected at 4
°C on a Rigaku AFC-5 diffractometer equipped with a graphite
monochromator and a dual chamber area detector system of the
Xuong-Hamlin design. Cell parameters are a ) 34.43, b ) 66.48,
c ) 77.86; R ) â ) γ ) 90°. These data are 94% complete to 2.8
Å resolution (average redundancy is 4.5) with a scaling R factor
on intensities of 0.113.
(16) Galaverna, G.; Corradini, R.; Dossena, A.; Marchelli, R. Diami-
nomethane dihydrochloride, a novel reagent for the synthesis
of primary amides of amino acids and peptides from active esters.
Int. J . Pept. Protein Res. 1993, 42, 53-57.
(17) Wenkert, E.; Hudlicky, T. Reactions of Isatin Dimethyl Ketal
and its Ethyl Imino Ether with Methyllithium. Synth. Commun.
1977, 7, 541-547.
(7) See Oren, D. A.; Zhang, A.; Nesvadba, H.; Rosenwirth, B.;
Arnold, E. Synthesis and Activity of Piperizine-containing
Antirhinoviral Agents and Crystal Structure of SDZ 880-061
Bound to Human Rhinovirus 14. J . Mol. Biol. 1996, 259, 120-
134 and references cited therein.
(18) Heck, R. F. In Palladium Reagents in Organic Syntheses;
Katritky, A. R., Meth-Cohn, O., Rees C. W., Eds.; Academic
Press: London, 1985; Chapter 6, pp 308-311.
(8) (a) Skiles, J . W.; McNeil, D. Spiro Indolinone Beta-lactams,
Inhibitors of Poliovirus and Rhinovirus 3C-Proteinases. Tetra-
hedron Lett. 1990, 31, 7277-7280. (b) Kadam, S.; Poddig, J .;
Humphrey, P.; Karwowski, J .; J ackson, M.; Tennent, S.; Fung,
L.; Hochlowski, J .; Rasmussen, R.; McAlpine, J . Citrinin Hydrate
and Radicinin: Human Rhinovirus 3C-Protease Inhibitors Dis-
covered in a Target-directed Microbial Screen. J . Antibiot. 1994,
47, 836-839. (c) Singh, S. B.; Cordingley, M. G.; Ball, R. G.;
Smith, J . L.; Dombrowski, A. W.; Goetz, M. A. Structure and
Stereochemistry of Thysanone: A Novel Human Rhinovirus 3C-
Protease Inhibitor from Thysanophora penicilloides. Tetrahedron
Lett. 1991, 32, 5279-5282. (d) Kaldor, S. W.; Hammond, M.;
Dressman, B. A.; Labus, J . M.; Chadwell, F. W.; Kline, A. D.;
Heinz, B. A. Glutamine-derived Aldehydes for the Inhibition of
Human Rhinovirus 3C Protease. Bioorg. Med. Chem. Lett. 1995,
5, 2021-2026. (e) Malcolm, B. A.; Lowe, C.; Shechosky, S.;
McKay, R. T.; Yang, C. C.; Shah, V. J .; Simon, R. J .; Vederas, J .
C.; Santi, D. V. Peptide Aldehyde Inhibitors of Hepatitis A Virus
3C Proteinase. Biochemistry 1995, 34, 8172-8179. (f) Sham, H.
L.; Rosenbrook, W.; Kati, W.; Betebenner, D. A.; Wideburg, N.
E.; Saldivar, A.; Plattner, J . J .; Norbeck, D. W. Potent inhibitor
(19) McKean, D. R.; Parrinello, G.; Renaldo, A. F.; Stille, J . K.
Synthesis of Functionalized Styrenes via Palladium-Catalyzed
Coupling of Aryl Bromides with Vinyl Tin Reagents. J . Org.
Chem. 1987, 52, 422-424.
(20) Brand, K.; Vo¨lcker, E. U¨ ber Methylmerkaptoatophane. Arch.
Pharm. 1934, 272, 257-268.
(21) (a) Fuhrman, S. A.; Worland, S.; Meador, J . W.; DeLisle, D.;
Brown, E. L.; Okano, K. A Continuous Assay for Rhinovirus 3C
Protease and its use in Drug Discovery. Presented at the IXth
Meeting of the European Study Group on the Molecular Biology
of Picornaviruses, May 18-24, 1996, abstr. D25. (b) Wang, G.
T.; Matayoshi, E.; Huffaker, H. J .; Krafft, G. A. Design and
Synthesis of New Fluorogenic HIV Protease Substrates Based
on Resonance Energy Transfer. Tetrahedron Lett. 1990, 31,
6493-6496.
(22) Assays were performed using commercially available proteases
essentially as described by the supplier.
(23) Weislow, O. S.; Kiser, R.; Fine, D. L.; Bader, J .; Shoemaker, R.
H.; Boyd, M. R. New Soluble-Formazan Assay for HIV-1 Cyto-
pathic Effects: Application to High-Flux Screening of Synthetic
and Natural Products for Aids-Antiviral Activity. J . Natl. Cancer
Inst. 1989, 81, 577-586.
of the human rhinovirus (HRV) 3C protease containing
a
backbone modified glutamine. J . Chem. Soc., Perkin Trans. 1

5082 J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 26
Webber et al.
(24) See: Zolta´n, G.; Klaassen, C. D. Mechanisms of Toxicology. In
Toxicology; The Basic Science of Poisons; Klaassen, C. D., Amdur,
M. O., Doull, J ., Eds.; McGraw-Hill: New York, 1996; Chapter
3, pp 35-74.
(30) Borsche, W.; Weussmann, H.; Fritzsche, A. Untersuchungen u¨ber
Isatin und verwandte Verbindungen, V: U¨ ber Nitro-isatine.
Chem. Ber. 1924, 57, 1149-1152.
(31) Yamamoto, M.; Kikinuma, M.; Kohmoto, S.; Yamada, K. Som-
melet-Hauser Rearrangement of Oxygen and Sulfur Containing
Heteroaromatic Sulfonium Ylides. Bull. Chem. Soc. J pn. 1989,
62, 958-960 and references cited therein.
(32) Haydock, D. B.; Mulholland, T. P. C.; Telford, B.; Thorp, J . M.;
Wain, J . S. Analogues of clofibrate and clobuzarit containing
fluorine in the side chains. Eur. J . Med. Chem. 1984, 19,
205-214.
(25) Lee, W.-M.; Monroe, S. S.; Rueckart, R. R. Role of Maturation
Cleavage in Infectivity of Picornaviruses: Activation of an
Infectosome. J . Virol. 1993, 67, 2110-2122.
(26) Leatherbarrow, R. J . ENZFITTER, Biosoft, Ferguson, MO 63135.
(27) The details pertaining to this protonated thiol model will be
presented elsewhere.
(28) Borsche, W.; Weussmann, H.; Fritzsche, A. Untersuchungen u¨ber
Isatin und verwandte Verbindungen, VI: U¨ ber 5-J od-isatin und
5.5′-Dijod-indigo. Chem. Ber. 1924, 57, 1770-1775.
(29) Mangini, A.; Passerini, R. Ricerche sugli eterociclici: spettri di
assorbimento u.v. e proprieta cromoforiche. - Nota II. Isatine.
Gazz. Chim. Ital. 1955, 85, 840-880.
J M960603E