Design and synthesis of novel HIV-1 protease inhibitors incorporating oxyindoles as the P′2-ligands

Article abstract of DOI:10.1016/j.bmcl.2006.01.011

A series of novel oxyindole-derived HIV-1 protease inhibitors were designed and synthesized based upon our X-ray crystal structure of inhibitor 2 (TMC-114) bound to HIV-1 protease. The effects of substituents, spirocyclic rings, and ring sizes have been investigated. A number of inhibitors exhibited low nanomolar inhibitory potencies against HIV protease.

Full text of DOI:10.1016/j.bmcl.2006.01.011

Bioorganic & Medicinal Chemistry Letters 16 (2006) 1869–1873
Design and synthesis of novel HIV-1 protease
inhibitors incorporating oxyindoles as the P02-ligands
Arun K. Ghosh,a,* Gary Schiltz,a,b Ramu Sridhar Perali,a Sofiya Leshchenko,a,b
Stephanie Kay,b D. Eric Walters,c Yasuhiro Koh,d Kenji Maedad and Hiroaki Mitsuyad,e
aDepartments of Chemistry and Medicinal Chemistry, Purdue University, West Lafayette, IN 47907, USA
bDepartment of Chemistry, University of Illinois at Chicago, 845 West Taylor Street, Chicago, IL 60607, USA
cDepartment of Biological Chemistry, Rosalind Franklin University of Medicine and Science, North Chicago, IL 60064, USA
dDepartment of Hematology and Infectious Diseases, Kumamoto University School of Medicine, Kumamoto 860-8556, Japan
eExperimental Retrovirology section, HIV and AIDS Malignancy Branch, National Cancer Institute, Bethesda, MD 20892, USA
Received 31 October 2005; revised 28 December 2005; accepted 4 January 2006
Abstract—A series of novel oxyindole-derived HIV-1 protease inhibitors were designed and synthesized based upon our X-ray crys-
tal structure of inhibitor 2 (TMC-114) bound to HIV-1 protease. The effects of substituents, spirocyclic rings, and ring sizes have
been investigated. A number of inhibitors exhibited low nanomolar inhibitory potencies against HIV protease.
Ó 2006 Elsevier Ltd. All rights reserved.
The AIDS epidemic has grown into one of the most press-
ing medical concerns of our time.1 The advent of highly
active antiretroviral therapy (HAART) with HIV prote-
ase inhibitors and reverse transcriptase inhibitors has
resulted in an improved quality of life, enhanced HIV
management, and halted the progression of AIDS.2 How-
ever, drug side effects and the emergence of drug-resis-
tance are making these therapies ineffective.3 In our
continuing effort to develop new inhibitors that maintain
their potencies against mutant strains of HIV, we have
recently reported the design and synthesis of a novel
inhibitor (2, now known as TMC-114 or Darunavir,
Fig. 1) which is currently undergoing phase III clinical
trials.4,5 This inhibitor is exceedingly potent against
wild-type (Ki = 15 1 pM, n = 4 and ID50 = 1.4
0.25 nM, n = 5) as well as resistant viruses.4
accepts two hydrogen bonds from the N–H of Ile 50 and
Ile 50’ amides of the HIV protease. This tight bound water
molecule is also present in saquinavir-bound HIV-1 pro-
tease as well as other protein–ligand complexes.7 Based
on this key interaction, we postulated that an oxyindole
derivative could be designed to interact with this critical
water molecule as well as to fill the S02 region of the enzyme
active site effectively. Such inhibitor with a basic amine
O
H2N
H
N
O
O
OH
H
H
N
N
N
H
O
Ph
N
H
Saquinavir (1)
NH2
OH
H
Subsequently, to gain molecular insight into the ligand-
binding site interaction, we determined a high resolution
X-ray crystal structure of this inhibitor bound to HIV-1
protease.6 An intriguing feature of this structure is the
presence of a tetracoordinated critical water molecule
that donates its hydrogen bonds to the urethane carbonyl
and one of the sulfonamide oxygens of the inhibitor and
H
H
O
H
N
N
S
O
H
O
O
O
Ph
O
O
TMC-114 (2)
OH
N
H
N
Me
O
H
O
H
O
Keywords: HIV proptease; Inhibitors; Oxyindole; TMC-114; Design;
Synthesis.
N
H
O
Ph
3
*
Corresponding author. Tel.: +1 765 494 5323; fax: +1 765 496 1612;
e-mail: akghosh@purdue.edu
Figure 1. HIV protease inhibitors.
0960-894X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2006.01.011

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A. K. Ghosh et al. / Bioorg. Med. Chem. Lett. 16 (2006) 1869–1873
functionality may improve absorption profiles. Oxyin-
doles have been previously utilized in several FDA-
approved drugs.8 Herein, we report our preliminary
results of these investigations in which an oxyindole ring
has been incorporated in the P02 position of inhibitor 2.
This has resulted in a series of inhibitors with subnanomo-
lar enzyme inhibitory potencies. We have also examined
the feasibility of spirocyclic oxyindole derivatives as
P02-ligands. However, acyclic inhibitors were more potent
than their cyclic counterparts.
derivative 8 as a mixture (1:1 ratio by 1H NMR analysis)
of diastereomers in excellent yields (81–94%). Both dia-
stereomers for inhibitor 3 were separated by silica gel
chromatography. Catalytic hydrogenation of various
azides 8 with optically active bis-tetrahydrofuranyl
Table 1. Inhibitory activity of oxyindole derivatives
Entry
1
Compound
Ki (nM)
OH
H
Me
H
O
N
N
O
6
3
7
0.6
The general synthesis of various oxyindole-derived
inhibitors is outlined in Scheme 1. As shown, commer-
cially available isatin was reacted with 2.2 equiv of the
appropriate alkyl Grignard reagent at 0 °C to provide
the corresponding tertiary alcohol in 57–72% yield.9
Chlorination of the resulting alcohol using thionyl
chloride and triethylamine in CH2Cl2 produced chloride
5 in good overall yield (57–76%).10 Reaction of optically
active azido epoxide 611 with the appropriate amine in
isopropanol at reflux gave the corresponding secondary
amine 7 in essentially quantitative yield. Reaction of the
respective amine 7 with chloride 5 (R1=Me) and trieth-
ylamine in acetonitrile smoothly provided oxyindole
H
O
H
O
Ph
N
H
3a
O
O
(isomer A)
OH
H
Me
H
O
H
N
N
O
2
3
0.3
O
H
O
Ph
(isomer B)
N
H
3b
OH
H
Me
H
H
H
O
H
N
N
O
0.05
O
O
N
H
H
10
Ph
O
O
Cl
R1
Ph
Me
OH
11
a, b
H
N
O
O
c
O
H
N
O
4
5
26 2.5
N
H
N
O
H
O
H
O
Ph
4
N
H
5
OH R2
NH
O
d
N3
Ph
N3
Ph
OH
H
N
O
H
N
O
2
7
0.3
0.7
6
7
O
H
O
O
Ph
(isomer A)
N
H
14a
OH R2
N
R1
N3
Ph
OH
H
H
O
H
O
H
N
N
O
N
O
6
O
H
O
O
H
N
O
8 (R1 = Me)
O
O
H
N
H
H
14b
Ph
O
e
O
(isomer B)
O
9
OH R2
N
O
O
H
N
Me
H
OH
H
O
H
H
H
H
H
O
H
N
N
O
7
8
102 4.9
130 12.5
42 3.2
O
H
O
O
N
H
O
O
N
H
Ph
H
15a
(isomer A)
Ph
O
3 R2 = iBu
10 R2 = (CH2)2CHMe2
11 R2 = CH2Ph
O
O
O
O
OH
H
O
H
N
N
O
OH R2
O
H
N
R1
H
N
O
N
O
N
H
H
15b
(isomer B)
Boc
Ph
Boc
N
O
R3
12
Ph
Ph
13
OH
H
N
f
O
H
N
O
9
O
H
O
OH R2
N
N
N
16a
(isomer A)
R1
Ph
H
N
H
O
H
14-15 (R3 = Me)
16 (R3 = H)
O
H
O
N
O
OH
H
Ph
R3
O
O
H
N
N
O
10
60
8
O
H
Scheme 1. Reagents and condition: (a) R1MgBr, THF, 0 °C; (b)
SOCl2, TEA, CH2Cl2; (c) R2NH2, i-PrOH; (d) CH3CN, TEA; (e) 9,
H2, Pd/C, THF; (f) i—TFA, CH2Cl2; ii—9, TEA, CH2Cl2.
O
16b
Ph
(isomer B)

A. K. Ghosh et al. / Bioorg. Med. Chem. Lett. 16 (2006) 1869–1873
1871
carbonate 9 in THF in the presence of triethylamine
afforded optically pure inhibitors 3a and 3b as well as
diastereomeric mixture of 10 and 11 in good yields
(60–75%). Preparation of inhibitors 14–16 was carried
out with commercially available Boc-protected epoxide
12 as starting material. Epoxide opening followed by
reaction with chloride 5 provided the corresponding
Boc derivatives 13. Diastereomers were separated by
silica gel chromatography using 25% ethyl acetate in
hexane as the eluent. Removal of the Boc group by
exposure to TFA followed by the reaction of the result-
ing amine with mixed carbonate 912 in the presence of
triethylamine in CH2Cl2 furnished the final inhibitors
14–16 in good yield (47–65%). Thus, the corresponding
oxyindole diastereomers for 14–16 were prepared in an
optically active form. Stereochemical identity of the
oxyindole ring chiral center was not determined for
our preliminary studies.
unsuccessful. Diastereomeric mixtures of N-isoamyl (10)
and N-benzyl (11) derivatives also showed good activity,
with the larger benzyl analog being less potent (26 vs
7 nM). Introduction of an allyl group at the oxyindole
C-3 center (14a and 14b) resulted in a separable mixture
of diastereomers which showed comparable potency (2
and 7 nM, respectively) with that of the methyl analogs
(3a and 3b). In an effort to interact with the residues in
the active site, we have incorporated a 5-methoxy substi-
tuent on the oxyindole aromatic ring. Thus, 5-methoxyi-
satin was converted to inhibitors 15a and 15b as a
1
diastereomeric mixture (1:1 ratio by H NMR analysis)
Table 2. Inhibitory activity of spirocyclic derivatives
Entry
Compound
Ki (nM)
OH
H
N
H
O
H
N
1
5
0.5
The inhibitory potencies of various oxyindole-derived
inhibitors are shown in Table 1. The assay protocol of
Toth and Marshall13 was utilized and the values denote
mean values from two determinations. As can be seen,
both oxyindole diastereomers of the N-isobutyl analogs
3a and 3b have shown potencies of 6 and 3 nM, respec-
tively. It appears that S20 -enzyme active site has only a
slight preference for one diastereomer over the other.
Either R or S absolute configuration of the oxyindole
chiral center in 3 seems to bind into HIV protease S02-
active site effectively. Nevertheless, our attempts to
assign stereochemical identity of the oxyindole ring were
O
H
O
O
N
H
Ph
22
O
O
OH
H
N
H
H
O
H
N
2
3
4
126 0.5
O
O
O
H
N
H
Ph
26
OH
H
N
O
H
N
>1000
O
O
O
H
N
H
Ph
28
O
Cl
(
)
n
OH
OH
H
N
H
N
H
H
NH
O
H
N
O
Boc
47 1.1
+
N
H
O
O
O
Ph
a
H
N
H
Ph
17 n = 1
18 n = 2
23
19
O
OH
OH
H
N
OH
b
H
H
H
(
)
O
H
N
(
)
n
n
O
H
N
N
N
N
Boc
5
6
>1000
>1000
O
H
O
O
H
O
O
O
O
O
N
H
Ph
Ph
N
H
N
H
Ph
27a
(isomer A)
O
20 n = 1
21 n = 2
22 n = 1
23 n = 2
c
OH
H
H
O
H
N
N
O
H
O
O
OH
b
OH
O
H
H
N
H
H
Ph
(
)
(
)
n
27b
(isomer B)
O
H
N
N
n
N
N
Boc
O
H
O
Ph
O
O
N
H
Ph
N
O
H
OH
H
24 n = 1
25 n = 2
H
H
26 n = 1
27 n = 2
O
H
N
N
d, b
7
8
>1000
>1000
O
O
O
H
N
H
Ph
29a
O
OH
(isomer A)
H
H
O
N
N
(
)
n
H
OH
H
O
H
O
Ph
O
28 n = 1
29 n = 2
O
H
N
N
N
H
O
O
H
O
O
O
N
H
Ph
(isomer B)
Scheme 2. Reagents and conditions: (a) CH3CN, TEA, reflux; (b) i—
TFA, CH2Cl2; ii—9, TEA, CH2Cl2; (c) Grubbs’ 1st generation
catalyst, CH2Cl2, 42 °C; (d) H2, 10% Pd/C, MeOH.
29b

1872
A. K. Ghosh et al. / Bioorg. Med. Chem. Lett. 16 (2006) 1869–1873
and the mixture was separated. However, these inhibi-
tors have shown significantly lower inhibitory activity
compared to unsubstituted inhibitors 3. The N-methyl
oxyindole derivative was also synthesized and the indi-
vidual diastereomers (16a and 16b) displayed Ki values
of 42 and 60 nM, respectively. The fact that the potency
displayed an approximately 10-fold decrease (as
compared to compounds 3a and 3b) suggests that the
oxyindole N–H may be participating in hydrogen
bonding with the enzyme active site.
The spirocyclic oxyindole derivatives were assayed and
their potencies are displayed in Table 2. Acyclic com-
pounds 22 and 23 showed good activity (5 and 47 nM,
respectively) and were generally consistent with those
observed for similar compounds shown in Table 1.
Interestingly, there is a significant reduction in inhibito-
ry potency for the corresponding six and seven-mem-
bered unsaturated and saturated spirocyclic inhibitors.
As shown, inhibitor with a cyclohexene ring has shown
a Ki value of 126 nM. However, saturation of the double
bond provided compound 28 with very little activity (Ki
value > 1 lM). Also, all spirocyclic derivatives with a
seven-membered ring have displayed no significant en-
zyme inhibitory activity. A closer inspection of the pre-
liminary model structure reveals that the oxyindole
carbonyls of the spirocyclic derivatives do not overlap
with the sulfone oxygen of 2 that effectively interacts
with the tight-bound water molecule in the active site.
Further structural modifications of the oxyindole deriv-
atives are necessary for effective binding in the active
site.
We have also examined the feasibility of spirocyclic
oxyindole derivatives as the P02-ligand. It has been
shown by us and others that constrained rings in the
HIV protease active site significantly improved enzyme
inhibitory activity.14,15 Our preliminary molecular
modeling suggested that such spirocycles can make
effective interaction in the active site. Scheme 2 shows
the synthesis of six- to seven-membered spirocyclic
oxyindole-derived inhibitors. Opening epoxide 12 with
allylamine in i-PrOH provided quantitative yield of sec-
ondary amine 19. The olefinic chlorooxyindoles (17,
48%) and (18, 52%) were prepared following the same
2-step sequence as described in Scheme 1. Reactions of
these chlorooxyindoles with amine 19 afforded tertiary
amines 20 and 21 as 1:1 mixtures of diastereomers (by
1H NMR) in 68–82% yield. These diastereomers could
be separated and the mixture was utilized in subsequent
reactions. The dienes were then subjected to ring closing
metathesis using Grubbs’ first generation16 catalyst in
refluxing CH2Cl2 to provide six and seven-membered
spirocycles 24 and 25, respectively, in excellent yield
(80–85%). The seven-membered ring diastereomers were
separated at this point by silica gel chromatography,
while the six-membered ring was used as a 1:1 mixture
of diastereomers. The unsaturated rings were converted
into urethane derivatives 26 and 27 containing P2-bis-
tetrahydrofuranyl ligand following the standard
protocol described in Scheme 1. Saturated inhibitors
28 and 29 were prepared by removal of Boc from 24
and 25 and reaction of the resulting amines with carbon-
ate 9 in the presence of triethylamine in CH2Cl2 fol-
lowed by catalytic hydrogenation of the resulting
olefins (60–65% yield).
We have determined the antiviral activity of 3a and 3b
against HIV-1IIIb in MT-2 cells. The results are summa-
rized in Table 3. The IC50 values shown were determined
based on the inhibition of HIV-induced cytopathogenic-
ity in MT-2 cells. All assays were conducted in duplicate,
and the values with standard deviation denote mean val-
ues from two or three. As can be seen, the antiviral
activity of these compounds was substantially limited
compared to saquinavir.17 To improve antiviral poten-
cy, further modifications of functionalities are in pro-
gress. To gain molecular insight, an energy minimized
model of 3(R0)-configuration of oxyindole derivative 3
was created (Fig. 2). The structure was built based on
our published crystal structure of 2-complexed with
Table 3. Antiviral activity of 3a and 3b
Inhibitor
IC50 (lM)
CC50 (lM)
3a
3b
0.30 0.071
0.48 0.38
0.005 0.002
>10
>10
>10
Saquinavir
Figure 2. (30R)-Oxyindole isomer of compound 3 modeled into the active site of HIV-1 protease. The inhibitor (green), superimposed upon the
crystal structure of TMC-114 (magenta).

A. K. Ghosh et al. / Bioorg. Med. Chem. Lett. 16 (2006) 1869–1873
1873
HIV-1 protease.6 The conformation of 3 was optimized
6. Tie, Y.; Boross, P. I.; Wang, Y.-F.; Gaddis, L.; Hussain,
A. K.; Leshchenko, S.; Ghosh, A. K.; Louis, J. M.;
Harrison, R. W.; Weber, I. T. J. Mol. Biol. 2004, 338, 341.
7. Swain, A. L.; Miller, M. M.; Green, J.; Rich, D. H.;
Schneider, J.; Kent, S. B. H.; Wlodawer, A. Proc. Natl.
Acad. Sci. U.S.A. 1990, 87, 8805.
8. (a) Gunasekara, N. S.; Spencer, C. M.; Keating, G. M.
Drugs 2002, 62, 1217; (b) Gallagher, G., Jr.; Lavanchy, P.
G.; Wilson, J. W.; Hieble, J. P.; DeMarinis, R. M. J. Med.
Chem. 1985, 28, 1533; (c) Haynes, J.; Obiako, B.; Babal,
P.; Stevens, T. Am. J. Physiol. Heart Circ. Physiol. 1999,
276, H1877; (d) Liu, Y.; Liu, D.; Printzenhoff, D.;
Coghlan, M. J.; Harris, R.; Kraft, D. S. Eur. J. Pharmacol.
2002, 435, 153.
9. (a) Alvarez, R. G.; Hunter, I. S.; Suckling, C. J.; Thomas,
M.; Vitinius, U. Tetrahedron 2001, 57, 8581; (b) Sharma,
V. M.; Prasanna, P.; Adi Seshu, K. V.; Renuka, B.;
Laxman Rao, C. V.; Sunil Kumar, G.; Prasad Narasimh-
ulu, P.; Aravind Babu, P.; Puranik, R. C.; Subramanyam,
D. Bioorg. Med. Chem. Lett. 2002, 12, 2303.
using the MMFF94x force field.18
In summary, a series of novel HIV protease inhibitors
incorporating oxyindole-derived P02-ligand has been
designed, synthesized, and evaluated. The oxyindole
derivatives can be readily prepared from isatin. The oxy-
indole derivatives incorporate a basic amine functional-
ity. Various 3-alkyl substitutents on the oxyindole rings
resulted in inhibitors with low nanomolar potency. In
general, acyclic inhibitors are considerably more potent
than their cyclic counterparts. Preliminary structure–ac-
tivity studies have shown that the lactam N–H is critical
to enhanced potency. We have also investigated the fea-
sibility of spiro oxyindoles as the P20-ligands. However,
spirocyclic inhibitors have shown significantly reduced
potencies compared to their acyclic counterparts. Fur-
ther design and optimization of these inhibitors are cur-
rently underway.
10. Powers, J. C. J. Org. Chem. 1966, 31, 2627.
11. Ghosh, A. K.; Thompson, W. J.; Holloway, M. K.;
McKee, S. P.; Duong, T. T.; Lee, H. Y.; Munson, P. M.;
Smith, A. M.; Wai, J. M.; Darke, P. L.; Zugay, J. A.;
Emini, E. A.; Schleif, W. A.; Huff, J. R.; Anderson, P. S.
J. Med. Chem. 1993, 36, 2300.
12. (a) Ghosh, A. K.; Thompson, W. J.; Fitzgerald, P. M. D.;
Culberson, J. C.; Axel, M. G.; McKee, S. P.; Huff, J. R.;
Anderson, P. S. J. Med. Chem. 1994, 37, 2506; (b) Ghosh,
A. K.; Leshchenko, S.; Noetzel, M. J. Org. Chem. 2004,
69, 7822; (c) Ghosh, A. K.; Chen, Y. Tetrahedron Lett.
1995, 36, 505.
Acknowledgment
Financial support by the National Institutes of Health
(GM 53386) is gratefully acknowledged.
References and notes
1. United Nations. 2004 Report on the Global HIV/
AIDS Epidemic: 4th Global Report. New York, USA,
2004.
2. (a) Flexner, C. N. Engl. J. Med. 1998, 338, 1281; (b)
Cihlar, T.; Bischofberger, N. Annu. Rep. Med. Chem.
2000, 35, 177.
13. Toth, M. V.; Marshall, G. R. Int. J. Pept. Protein Res.
1990, 36, 544.
14. (a) Ghosh, A. K.; Swanson, L.; Liu, C.; Cho, H.; Hussain,
A.; Walters, D. E.; Holland, L.; Buthod, J. Bioorg. Med.
Chem. Lett. 2002, 12, 1993; (b) Ghosh, A. K.; Swanson, L.
M.; Cho, H.; Leshchenko, S.; Hussain, K. A.; Kay, S.;
Walters, D. E.; Koh, Y.; Mitsuya, H. J. Med. Chem. 2005,
48, 3576.
15. (a) Fairlie, D. P.; Tyndall, J. D. A.; Reid, R. C.; Wong, A.
K.; Abbenante, G.; Scanlon, M. J.; March, D. R.;
Bergman, D. A.; Chai, C. L. L.; Burkett, B. A. J. Am.
Chem. Soc. 1996, 118, 8511; (b) Abbenante, G.; Bergman,
D. A.; Brinkworth, R. I.; March, D. R.; Reid, R. C.;
Hunt, P. A.; James, I. W.; Dancer, R. J.; Garnham, B.;
Stoermer, M. L.; Fairlie, D. P. Bioorg. Med. Chem. Lett.
1996, 6, 2531; (c) Reid, R. C.; Pattenden, L. K.; Tyndall, J.
D. A.; Martin, J. L.; Walsh, T.; Fairlie, D. P. J. Med.
Chem. 2004, 47, 1641; (d) Glenn, M. P.; Pattenden, L. K.;
Reid, R. C.; Tyssen, D. P.; Tyndall, J. D. A.; Birch, C. J.;
Fairlie, D. P. J. Med. Chem. 2002, 45, 371.
3. Tamalet, C.; Pasquier, C.; Yahi, N.; Colson, P.; Poizot-
Martin, I.; Lepeu, G. J. Med. Virol. 2000, 61, 181.
4. (a) Ghosh, A. K.; Shin, D. W.; Swanson, L.; Krishnan,
K.; Cho, H.; Hussain, K. A.; Walters, D. E.; Holland,
L.; Buthod, J. Il Farmaco 2001, 56, 29; (b) Ghosh, A.
K.; Kincaid, J. F.; Cho, W.; Walters, D. E.; Krishnan,
K.; Hussain, K. A.; Koo, Y.; Cho, H.; Rudall, C.;
Holland, L.; Buthod, J. Bioorg. Med. Chem. Lett. 1998,
8, 687; (c) Koh, Y.; Nakata, H.; Maeda, K.; Ogata, H.;
Bilcer, G.; Devasamudram, T.; Kincaid, J. F.; Boross,
P.; Wang, Y.-F.; Tie, Y.; Volarath, P.; Gaddis, L.;
Harrison, R. W.Weber, I. T.; Ghosh, A. K.; Mitsuya, H.
Antimicrob. Agents Chemother. 2003, 47, 3123; (d) Yoshim-
ura, K.; Kato, R.; Kavlick, M. F.; Nguyen, A.; Maroun, V.;
Maeda, K.; Hussain, K. A.; Ghosh, A. K.; Gulnik, S. V.;
Erickson, J. W.; Mitsuya, H. J. Virol. 2002, 76, 1349.
5. (a) De Meyer, S.; Azijn, H.; Surleraux, D.; Jochmans, D.;
Tahri, A.; Pauwels, R.; Wigerinck, P.; de Bethune, M.-P.
Antimicrob. Agents Chemother. 2005, 49, 2314; (b) De
Meyers, S.; Peeters, M. Conference on Retroviruses and
opportunistic Infections (11th CROI), February 8–11,
2004, San Francisco, CA. Abstracts 533 and 620.
16. Grubbs, R. H.; Chang, S. Tetrahedron 1998, 54, 4413.
17. Roberts, N. A.; Martin, J. A.; Kinchington, D.; Broad-
hurst, A. V.; Craig, J. C.; Duncan, I. B.; Galpin, S. A.;
Handa, B. K.; Kay, J.; Krohn, A.; Lambert, R. W.;
Merrett, J. H.; Mills, J. S.; Parkes, K. E. B.; Redshaw, S.;
Ritchie, A. J.; Taylor, D. L.; Thomas, G. J.; Machin, P.
Science 1990, 248, 358.
18. Halgren, T. A. J. Comput. Chem. 1999, 20, 720.