1,2,5,6-Tetra-O-benzyl-D-mannitol derivatives as novel HIV protease inhibitors

Article abstract of DOI:10.1016/S0960-894X(03)00677-2

The synthesis and structure-activity relationships of HIV protease inhibitors derived from carbohydrate alditols are discussed. We disclose a new series of 1,2,5,6-tetra-O-alkyl-D-mannitol exhibiting sub-micromolar activity against HIV-protease. This seri

Full text of DOI:10.1016/S0960-894X(03)00677-2

Bioorganic & Medicinal Chemistry Letters 13 (2003) 3601–3605
1,2,5,6-Tetra-O-benzyl-D-mannitol Derivatives as Novel
HIV Protease Inhibitors
Abderrahim Bouzide, Gilles Sauve,*,y Guy Sevigny and Jocelyn Yelle*
Pharmacor Inc., 535 West, Cartier blvd., Laval, Quebec, Canada H7V 3S8
Received 28 February 2003; accepted 16 May 2003
Abstract—The synthesis and structure–activity relationships of HIV protease inhibitors derived from carbohydrate alditols are
discussed. We disclose a new series of 1,2,5,6-tetra-O-alkyl-d-mannitol exhibiting sub-micromolar activity against HIV-protease.
This series of inhibitors are non-nitrogen containing HIV-protease inhibitors and they are readily prepared in a few chemical steps
from inexpensive commercially available starting materials.
# 2003 Published by Elsevier Ltd.
The human immunodeficiency virus (HIV) has been
identified as the etiologic agent of acquired immuno-
deficiency syndrome (AIDS).1 The pol gene of the
human immunodeficiency virus (HIV) encodes the
aspartic protease which mediates proteolytic processing
of the gag and the gag pol viral gene products, liberating
functional enzymes and structural proteins which are
essential for the formation of the mature, infectious
virus.2 Inactivation of the aspartic protease leads to the
formation of noninfectious virions.3 As a result the HIV
protease has become one of the major targets for thera-
peutic intervention in AIDS and in HIV infection.4
Despite the success of the FDA-approved HIV protease
inhibitors saquinavir,5 ritonavir,6 indinavir,7 nelfinavir,8
amprenavir9 and lopinavir,10 there is an urgent need for
new and improved HIV protease inhibitors due to
increasing viral resistance, a matter that is now of great
concern.11 The high cost of synthesis is today a barrier to
the widespread use of the currently approved protease
inhibitors, notably in the less developed countries. It is thus
important to develop new improved protease inhibitors
accessible at low cost.12 Moreover the challenge today is to
discover orally bioavailable, long half-life compounds with
activity against several protease resistant strains.
appropriate stereochemistry made them a starting
material of choice in various syntheses.13 Herein we
report the synthesis of a new class of HIV-protease inhib-
itors of the general structure 1 (Chart 1), prepared
readily from carbohydrate alditols. These compounds
have been evaluated in an enzyme assay and their struc-
ture–activity relationships (SAR) have been investigated.
The 1,2,5,6-tetra-O-benzyl-d-mannitol 1a was easily pre-
pared from d-mannitol (Scheme 1). Conversion of d-
mannitol to the acetonide 2 was achieved according to
the literature.14 Treatment of 2 with benzyl bromide in
the presence of NaH in DMF afforded the corresponding
fully benzylated intermediate, which under the action of
HCl in MeOH produced 1a. Compounds 1b–1e were
prepared similarly in good yields, ranging from 65 to 80%.
1,2,5,6-Tetra-O-benzyl-l-mannitol 3, 1,2,5,6-tetra-O-
benzyl-dulcitol 4 and 1,2,5,6-tetra-O-benzyl-d-sorbitol 5
(Chart 2) were prepared respectively from l-mannitol,
dulcitol and d-sorbitol following the same steps as in
Scheme 1.
Preparation of diol 8, corresponding to compound 1a
having one of the hydroxyls of the central diol inverted
was achieved in a four-step process. Monoprotection of
the diol 1a using silver oxide15 in the presence of
p-methoxybenzyl chloride (PMBCl) in dichloromethane
furnished 6 in 72% yield. Oxidation of 6 with pyr-
idinium chlorochromate to the corresponding ketone,
followed by reduction with sodium borohydride in
methanol, gave the desired epimer 7, along with 6 in
The carbohydrates have been extensively used in the
synthesis of HIV-protease inhibitors. Their low cost and
*Corresponding authors. Tel.: +1-514-496-2943; fax: +1-450-973-
8404; e-mail: jyelle@procynbiopharm.com
yPresent address: INRS-Institut Armand-Frappier, 531, des Prairies
Blvd, Building 27, Office D-103, Laval, Quebec, Canada H7V1B7.
0960-894X/$ - see front matter # 2003 Published by Elsevier Ltd.
doi:10.1016/S0960-894X(03)00677-2

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A. Bouzide et al. / Bioorg. Med. Chem. Lett. 13 (2003) 3601–3605
Chart 1. 1,2,5,6-Tetra-O-alkyl-alditol.
Scheme 2. Inversion of the C-3 hydroxyl of 1a.
75% total yield (2:1 ratio in favour of 7). SnCl2 medi-
ated selective deprotection of PMB protecting group16
affording 8 in 85% yield (Scheme 2).
In the synthesis of the monohydroxylated compound
17, diol 1a was reacted with thiocarbonyldiimidazole, to
give the cyclic thiocarbonate 16 in 86% yield. Slow addi-
tion of tributyltin hydride and 2,20-azobis(isobutyro-
nitrile)19 to a solution of 16 in toluene at reflux afforded
17 in 66% yield (Scheme 5).
Compound 1f was prepared from 2 according to
Scheme 3. Selective O-benzylation of the primary
hydroxyls was carried out in the presence of dibutyl tin
oxide at reflux of toluene, with azeotropic removal of
water. Treatment of the resulting dibutyl stannylene
with benzyl bromide in the presence of cesium fluoride
afforded 9 in 82% yield.17 Alkylation of the remaining
secondary hydroxyls was achieved as described pre-
viously in Scheme 1, to produce 10 in 88% yield.
Removal of the isopropylidene group under acidic con-
ditions gave 1f in 95% yield. Compounds 1g–1q, 1u–1ff,
and 1hh–1jj were prepared similarly in yields ranging
from 55 to 70%. Compounds 1r and 1gg were prepared
respectively from 1p and 1ee by reduction in the pre-
sence of LiAlH4. Basic hydrolysis of 1o afforded 1t,
which under the action of LiAlH4 produced 1s.
HIV-Protease Inhibition
The IC50 values of the synthetic compounds targeting
the HIV protease were determined with a fluorometric
assay according to the Matayoshi assay20 and are
reported in Tables 1 and 2.
Although the lead compound 1a has shown a moderate
activity against HIV protease (Table 1), we decided to
pursue this project for the following reasons: (1) 1a is
very stable and non-nitrogen containing HIV-protease
inhibitor,21 (2) 1a is prepared in only two steps from the
commercially available 3,4-O-isopropylidene-d-manni-
tol, and (3) contrary to other HIV protease inhibitors,
1a does not contain any carbonyl or sulfonyl group to
In the synthesis of 13, the diepoxide 11 (Scheme 4),
readily available from d-mannitol in 42% yield,18 was
heated with phenol in DMF at 110 ꢀC in the presence of
potassium carbonate to give diol 12 in 88% yield. Di-O-
benzylation of 12 in the presence of NaH in DMF, fol-
lowed by the hydrolysis of the isopropylidene group in
the presence of 3 N HCl produced 13 in 96% yield.
0
interact with Il50and Il50 .22
The activity of 1a is attributed to the hydrophobic
interactions induced by the four benzylic groups into
the subsites S1, S2, S10, and S20, and to the interaction
of the (3S,4S) central diol with the Asp25 and Asp250.
No activity was observed below a concentration of 50
mM when one OH group of the central diol was epimer-
ized (8) or deoxygenated (17) (Table 1). The (R,R) ste-
reochemistry at C-2 and C-5 was critical for the activity
of 1a, for example, 1,2,5,6-tetra-O-benzyl-d-sorbitol 5
Compound 15 was prepared by ring opening of the
diepoxide 12 with PhMgBr in the presence of
.
CuBr SMe2, to afford diol 14 in 86% yield. Dibenzyla-
tion of 14 and hydrolysis of the isopropylidene group
afforded 15 in 95% yield.
Scheme 1. Synthesis of compound 1a.
Chart 2. Other diastereoisomers of 1a.

A. Bouzide et al. / Bioorg. Med. Chem. Lett. 13 (2003) 3601–3605
3603
Scheme 3. Synthesis of compound 1f.
Scheme 4. Synthesis of compounds 13 and 15.
Scheme 5. Monodeoxygenation of diol 1a.
Table 1. IC50 of tetra-O-benzyl-alditols
substitution of the benzyl groups at positions 2 and 5
with para-fluoro-benzyl group (1f) showed an encoura-
ging sub-micromolar activity. Replacement of the para-
fluoro with para-chloro (1g) or para-bromo (1h) gave
less active inhibitors, and no gain in activity was noticed
when the fluoro group was switched from para position
to meta (1i) or ortho position (1j). On the other hand,
substitution of the benzyl groups at positions 1 and 6
with ortho-fluoro-benzyl groups (1w) resulted in a good
enhancement in activity (3-fold more active than 1a)
Furthermore, the inhibition is increased when both
ortho positions are occupied by the fluoro group as in
the case of 1x which is 8-fold more active than 1a. As we
anticipated, the combination of para-fluoro-benzyl at
positions 2, 5 and ortho-fluoro-benzyl at positions 1,6
leading to inhibitor 1jj permitted the best IC50.
No.
IC50, mM
1a
3
4
5
8
13
15
17
2.4
>50
>50
>50
>50
>50
>50
>50
where the R stereochemistry at C-2 was inverted to S did
not show any activity at a concentration below 50 mM.
The compounds 1,2,5,6-tetra-O-benzyl-l-mannitol 3
and 1,2,5,6-tetra-O-benzyl-dulcitol
4 were inactive
(Table 1).
In order to minimize the hydrophobic character of the
inhibitors and improve their bioavailability, hydrophilic
groups such as esters (1p and 1ee) carboxylic acids (1q
and 1ff), carboxamides (1s), alcohols (1r and 1gg) and
amines (1t) were added on the aromatic groups. These
derivatives present variable activity.
The benzyloxy groups are essential for the activity of 1a
as it was substantiated by the low activity of compound
13 lacking the methylene groups and the low activity of
compound 15 lacking CH2O groups. Consequently, one
can conclude that shortening these groups may prevent
the aromatic group to reach the appropriate subsite.
A new series of HIV-protease inhibitors were readily
prepared in few steps from inexpensive alditols.
Although these inhibitors do not contain a carbonyl or
sulfonyl group to interact with Il50and Il50 0 via a water
Substitution of all the benzyl groups at the para-posi-
tions with CH3, CF3, F and CN groups (Table 2, 1b–
1e) did not show any improvement.23 Interestingly,

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A. Bouzide et al. / Bioorg. Med. Chem. Lett. 13 (2003) 3601–3605
Table 2. IC50 of tetra-O-benzyl-d-mannitol derivatives
Heimbach, J. C.; Dixon, R. A. F.; Scolnick, E. M.; Siga, I. S.
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1d
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4-CF3
4-F
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7.0
6.0
1.6
3.1
0.7
2.0
2.1
1.8
3.7
6.4
2.6
10.0
3.7
5.4
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26
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1g
1h
1i
H
H
H
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4-Br
3-F
1j
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1k
1l
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1o
1p
1q
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27
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1s
1t
1u
1v
1w
1x
1y
1z
1aa
1bb
1cc
1dd
1ee
1ff
1gg
1hh
1ii
1jj
2.5
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0.33
4.2
6.0
5.2
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3.0
4.3
2.0
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IC50’s. The mode of interaction between these inhibitors
and the enzyme is still unknown in the absence of an
X-ray structure. Further improvement will be published
in due course.
Acknowledgements
The authors would like to thank Mr. Patrick Soucy for
the biological evaluation of the molecules, and Dr.
Gervais Berube for revising the manuscript.
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improvement as well.