Structure-based design and synthesis of HIV-1 protease inhibitors employing beta-D-mannopyranoside scaffolds.

Article abstract of DOI:10.1016/S0960-894X(02)00220-2

A preliminary account on the structure-based design, synthesis and evaluation of peptidomimetic inhibitors of HIV-1 protease containing beta-D-mannopyranoside scaffolds is given. The compounds prepared had IC(50) values in the micromolar range. The results provide a platform for the development of more potent carbohydrate-based inhibitors of HIV-1 and other aspartic proteases.

Full text of DOI:10.1016/S0960-894X(02)00220-2

Bioorganic & Medicinal Chemistry Letters 12 (2002) 1763–1766
Structure-Based Design and Synthesis of HIV-1 Protease
Inhibitors Employing ꢀ-D-Mannopyranoside Scaffolds
Paul V. Murphy,a,* Julie L. O’Brien,a Lorraine J. Gorey-Feretb and
Amos B. Smith, IIIc,*
aChemistry Department, Centre for Synthesis and Chemical Biology, Conway Institute of Biomolecular and Biomedical Research,
University College Dublin, Belfield, Dublin 4, Ireland
bBristol-Myers Squibb Pharmaceutical Company, Wilmington, DE, USA
cDepartment of Chemistry, University of Pennsylvania, Philadelphia, PA 19104, USA
Received 12 February 2002; revised 26 March 2002; accepted 4 April 2002
Abstract—A preliminary account on the structure-based design, synthesis and evaluation of peptidomimetic inhibitors of HIV-1
protease containing b-d-mannopyranoside scaffolds is given. The compounds prepared had IC50 values in the micromolar range.
The results provide a platform for the development of more potent carbohydrate-based inhibitors of HIV-1 and other aspartic
proteases. # 2002 Published by Elsevier Science Ltd.
Several aspartic acid proteases are of interest as ther-
apeutic targets. Examples include renin, HIV-1 protease
and memapsin 2 (b-secretase) which are respectively
involved in hypertension,1 viral infection and Alzhei-
mer’s disease.2 Only in the case of the HIV-1 protease
have inhibitors of this class of enzymes attained clinical
importance.3 This is due mostly to the lack of bioavail-
ability of the potent inhibitors that have been devel-
oped. Also, despite the successes of using protease
inhibitors for treatment of HIV, there remain a number
of problems with current therapies such as overlapping
resistance patterns and long-term side effects.4 Tradi-
tional approaches for development of inhibitors have
involved natural product screening or replacing the
scissile bond of a peptide substrate with a non-cleavable
isostere. More recently, peptidomimetic research, which
has the ultimate goal of developing inhibitors with
improved pharmacokinetic properties, has gained
importance.5
knowledge, the concept was not explored further. Dur-
ing the early 1990s, a series of publications from the
University of Pennsylvania introduced the use of b-d-
glucose (e.g., 1), its enantiomers and a diastereomer (b-
d-mannose) as scaffolds for the design and synthesis of
ligands for the somatostatin (SRIF), the substance P
(SP), and the b2-adrenergic receptors.
Concurrent with the early Penn studies, Olson and co-
workers at Hoffmann LaRoche reported a conceptually
similar approach with the synthesis of a thyrotropin-
releasing hormone mimic, which employed a cyclohex-
ane scaffold.8 Although initially the affinities of the glu-
cose mimics were modest (15 mM range), more potent
ligands have been designed, synthesized and reported
subsequently.9 The Penn group recently obtained a glu-
coside with IC50 of 60 nM for the hSST4 receptor, and a
congener revealed an IC50 of 8 nM at the NK-1 recep-
tor.10 Validation of this concept led other groups to
utilize sugar scaffolds in peptidomimetic research.11
Herein we describe the design and synthesis of first
generation peptidomimetic inhibitors of HIV-1 protease
that incorporate a b-d-mannopyranose scaffold.
In 1980, Farmer proposed the attachment of side chains
to a cyclohexane ring,6 but no experiments were carried
out to assess the validity of this concept. Six years later,
Belanger and DuFresne7 successfully explored the bicy-
clooctane scaffold to generate an opiate ligand; to our
*Corresponding authors. Tel.: +353-1-716-2504; fax: +353-1-716-
2127; e-mail: paul.v.murphey@ucd.ie
0960-894X/02/$ - see front matter # 2002 Published by Elsevier Science Ltd.
PII: S0960-894X(02)00220-2

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P. V. Murphy et al. / Bioorg. Med. Chem. Lett. 12 (2002) 1763–1766
The structure based design work (Fig. 1) used the crys-
tal structure of inhibitor 2 bound to HIV-1 protease;12
this compound contains the hydroxyethylene isostere
common to many aspartic acid protease inhibitors.13
The de novo software Growmol14 was initially used as a
tool to aid in the structure based design. This software
generated modified cyclohexane derivative 3 in the
active site.15 Superimposition of 2 on 3 and structural
modifications using crystal structures of other ligand
HIV-1 protease complexes led us to propose 516 and
related compounds as preliminary targets for synthesis
and evaluation.17
The 2-hydroxyl was protected with the TIPSgroup and
regiospecific cleavage of the methoxybenzylidene group
proceeded smoothly to give 11. Tosylation and sub-
stitution with azide gave 12. Reduction of the azide to
13 was carried out using Lindlars catalyst and hydrogen
in ethanol (Scheme 2). The amine 13 was converted into
carbamate 14 by reaction with benzylchloroformate.
Removal of the TIPSgroup from 14 gave 15. Reaction
of 15 with DDQ gave 5. Amine 13 was also used to
prepare 16 by first of all acetylation and subsequent
21
removal of the TIPSgroup (Scheme 3).
Compounds 5, 8 and 15–18 were evaluated as inhibitors
of HIV protease and data is summarized in Table 1.22
Some of the compounds were moderate inhibitors (IC50,
4.48–9.95 mM) whereas 17 was inactive. The activity
shown by 8 is also encouraging as the synthetic route to
this compound is relatively short and should facilitate
preparation of a range of analogues as will be required
to generate compounds with higher binding affinities.
Molecular modelling using the complex of HIV-1 pro-
tease with 2 did indicate that the methoxyphenyl group
in 15 and 16 and the phenyl group in 8 could have steric
interactions with residues in the S2 subsite, assuming
that the compounds bind as predicted. However, large
conformational changes have been observed in HIV-1
protease in response to large groups on inhibitors and it
has been suggested before that attempts can be made to
probe protein flexibility during inhibitor develop-
ment.13,23 Further modifications to the carbohydrate
scaffold such as preparation of phenyl glycosides (see
Fig. 2) should increase binding. The inhibitory activity
Figure 1. Design of b-d-mannopyranoside-based HIV-protease inhi-
bitors. Inhibitor 2 is shown in its enzyme bound conformation.
From the design work, it was apparent that the b-d-
mannopyranoside ring has good overlap with the pep-
tide-like backbone and hydroxyethylene isostere of the
inhibitor and could be a replacement for at least part of
peptide secondary structures that bind to enzymes. It
was also obvious that monosaccharides could be strate-
gically used to orient binding groups into protease sub-
sites and given that there are considerable similarities in
the way that substrates and inhibitors generally bind to
aspartic acid proteases18 the monosaccharides and rela-
ted structures could ultimately have general application
as inhibitors of these enzymes.
The synthesis of 5 and related compounds 15 and 16
began with commercially available methyl b-d-gluco-
pyranoside 6. This compound was converted to 7 by
introducing a benzylidene group onto the 4- and 6-OH
groups followed by regioselective benzylation using
dibutyltin oxide, benzyl bromide and tetrabutyl-
ammonium iodide in benzene.11x Inversion of configur-
ation at C-2 to give 8 was achieved by oxidation
followed by reduction of the resulting ketone using
sodium borohydride.19 The b-d-mannopyranoside 8
was converted to 9 by removing the benzylidene pro-
tecting group using TFA in dichloromethane and treat-
ing of the crude reaction mixture with excess acetic
anhydride and pyridine to give 9.20 The acetates were
then removed from 9 using catalytic sodium methoxide
in methanol and the methoxybenzylidene derivative 10
obtained by reaction of the appropriate dimethoxyacetal
with camphorsulfonic acid in acetonitrile (Scheme 1).
Scheme 1. Reagents and conditions: (a) PhCH(OMe)2, CSA, MeCN,
80%; (b) Bu2SnO, BnBr, Bu4NI; (c) DMSO, Ac2O (1:2), rt, 40% two
steps; (d) NaBH4, CH2Cl2, silica gel, rt, 90% (mannose/glucose ratio,
7:1); (e) TFA, CH2Cl2, rt; (f) Ac2O, Py, rt; (g) NaOMe, MeOH, rt; (h)
4-MeOC6H4CH(OMe)2, CSA, MeCN.
Scheme 2. Reagents and conditions: (a) TIPSOTf, 2,6-lutidine,
CH2Cl2; (b) DIBAL-H, CH2Cl2 96% for two steps; (c) TsCl, Py, 0 ꢀC;
(d) NaN3, DMF, 80 ꢀC, 40% for two steps; (e) Lindlar catalyst, H2,
EtOH.

P. V. Murphy et al. / Bioorg. Med. Chem. Lett. 12 (2002) 1763–1766
1765
C. Gregory Paris, Novartis Pharmaceuticals, for train-
ing in use of the Growmol software and Ralph Hirsch-
mann for assistance in preparation of the manuscript.
P.M. and J.O.B. thank Enterprise Ireland for Interna-
tional Collaboration Travel Grants.
References and Notes
Scheme 3. Reagents and conditions: (a) BnOCOCl, Et3N, CH2Cl2; (b)
TBAF, THF, 0 ꢀC; (c) Ac2, Py, rt; (d) DDQ, CH2Cl2: H2O (10:1), 0 ꢀC.
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Table 1. HIV-1 protease inhibition data for b-d-mannopyranosidesa
Compd
Inhibition IC50 (mM)b
5
8
15
16
17
18
4.66 (Æ2.21)
4.85 (Æ1.63)
4.48 (Æ0.98)
7.74 (Æ3.19)
Not active
8.95 (Æ1.48)
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manuscript, Regine S. Bohacek and Novartis Pharma-
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1
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