An enantioselective enzymatic desymmetrization route to hexahydro-4H-furopyranol, a high-affinity ligand for HIV-1 protease inhibitors

Article abstract of DOI:10.1016/j.tetlet.2017.07.010

An enantioselective synthesis of (3aS,4S,7aR)-hexahydro-4H-furo[2,3-b]pyran-4-ol, a high-affinity nonpeptide ligand for a variety of potent HIV-1 protease inhibitors is described. The key steps involved a highly enantioselective enzymatic desymmetrization

Full text of DOI:10.1016/j.tetlet.2017.07.010

Tetrahedron Letters xxx (2017) xxx–xxx
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
An enantioselective enzymatic desymmetrization route to hexahydro-
4H-furopyranol, a high-affinity ligand for HIV-1 protease inhibitors
⇑
Arun K. Ghosh , Anindya Sarkar
Department of Chemistry and Department of Medicinal Chemistry, Purdue University, 560 Oval Drive, West Lafayette, IN 47907, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
An enantioselective synthesis of (3aS,4S,7aR)-hexahydro-4H-furo[2,3-b]pyran-4-ol, a high-affinity non-
peptide ligand for a variety of potent HIV-1 protease inhibitors is described. The key steps involved a
highly enantioselective enzymatic desymmetrization of meso-diacetate, an efficient transacetalization,
and a highly diastereoselective reduction of a ketone. This route is amenable to large-scale synthesis
using readily available starting materials.
Received 11 May 2017
Revised 17 June 2017
Accepted 1 July 2017
Available online xxxx
Ó 2017 Elsevier Ltd. All rights reserved.
Keywords:
Enzymatic hydrolysis
Enantioselective
Desymmetrization
HIV-1 protease inhibitors
Ligand
The development of combined antiretroviral therapy (cART)
with HIV-1 protease inhibitors dramatically transformed HIV/AIDS
from a terminal disease to a chronic ailment.1,2 This treatment reg-
imen improved life expectancies of those patients with access to
cART.3,4 Despite this remarkable progress, there are numerous
drug-related complications. Perhaps, the most concerning is the
rapid emergence of multidrug-resistant HIV-1 variants which
severely limit the current and future management of HIV/AIDS.5–
In an effort to further optimize the bis-THF structural template
of darunavir, we explored modifications that would enhance the
‘backbone binding’ as well as improve hydrophobic interactions
in the protease active site.10,17 Interestingly, the ring-fusion, stere-
ochemistry, and position of urethane are critical to inhibitor
potency. Inhibitor 3, with a (3R,3aS,7aR)-hexahydro-4H-furo[2,3-
b]pyran-3-ol, is significantly less potent than inhibitors 1 and
7
In our continuing efforts in developing novel protease inhibitors
(PIs) with broad-spectrum activity against multidrug-resistant
HIV-1 variants, we reported inhibitors containing fused bis-
tetrahydrofuran and tetrahydropyranyl tetrahydrofuran repre-
sented in PIs 1–4 (Fig. 1).8–10 Among these PIs, darunavir (1) is
an FDA approved drug and it is used as a front-line therapy for
HIV/AIDS patients.11,12 Darunavir and its methoxy derivative (1
and 2) contain (3R,3aS,6aR)-bis-tetrahydrofuranyl (bis-THF)
urethane as the P2 ligand.8,9 Darunavir’s exceptional antiretroviral
activity against multidrug-resistant clinical isolates and high
genetic barrier to resistance is attributed to the presence of the
bis-THF ligand.13–15 Darunavir exhibited dual mechanism of action
by blocking HIV-1 protease enzymatic activity as well as by
inhibiting dimerization of protease monomers.16 Structurally, dar-
unavir was designed to make extensive interactions in the active
site of HIV-1 protease, particularly with the backbone atoms from
S2 and S20 subsites.9,17
X
OH
H
H
N
O
N
O
H
S
O
O
O
O
O
H
H
Ph
1
(Darunavir, X = NH2)
2 (TMC-126, X = OMe)
O
OMe
OH
H
N
H
H
N
O
O
H
S
O
Ph
3
O
OMe
OH
H
N
N
O
H
S
O
O
O
O
H
Ph
4
O
⇑
Corresponding author.
E-mail address: akghosh@purdue.edu (A.K. Ghosh).
Fig. 1. Structures of protease inhibitors 1–4.
http://dx.doi.org/10.1016/j.tetlet.2017.07.010
0040-4039/Ó 2017 Elsevier Ltd. All rights reserved.
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2
A.K. Ghosh, A. Sarkar / Tetrahedron Letters xxx (2017) xxx–xxx
2.8,11 However, incorporation of isomeric (3aS,4S,7aR)hexahydro-
4H-furo[2,3-b]pyran-4-ol (Tp-THF) resulted in inhibitor 4 which
showed excellent protease inhibitory activity (Ki = 2.7 pM) and
improved antiviral activity in MT cells (IC50 = 0.5 nM) compared
to inhibitor 2 (Ki = 14 pM, IC50 = 2.3 nM).18 Furthermore, inhibitor
4 showed retention of antiviral activity against a panel of mul-
tidrug-resistant clinical HIV-1 strains with IC50 values in the
nanomolar range and is superior to other approved PIs and compa-
rable to darunavir.15,18 The X-ray structural analysis revealed that
the hydrogen bonding interactions of both P2 ligand oxygens are
significantly stronger than the bis-THF ligand in darunavir or inhi-
bitor 2 (Fig. 2).19,20 This may be due to the fact that the Tp-THF
ligand of inhibitor 4 has higher affinity for the active site of HIV-
1 protease. For the synthesis of Tp-THF ligand, we previously con-
verted optically active bicyclic lactone 5 to (3aS,4S,7aR)-hexahy-
dro-4H-furo[2,3-b]pyran-4-ol 6.18 Lactone 5 was obtained from
cyclopentene diacetate as the key starting material.21 To expand
the use of the Tp-THF ligand, we have now explored an alternative
synthetic route using readily available and inexpensive starting
materials. Herein, we report a convenient enantioselective synthe-
sis of (3aS,4S,7aR,)-hexahydro-4H-furo[2,3-b]pyran-4-ol using an
efficient enzymatic desymmetrization of meso-diacetate as the
key reaction.
The synthetic strategy for optically active hexahydro-4H-
furopyranol 6 is shown in Scheme 1. The 6,5-fused ring structure
would be obtained through a transacetalization reaction on diol
7. Optically active diol 7 can be derived from the oxidative cleavage
of cyclohexene derivative 8, which could be obtained from
optically active alcohol 9. Alcohol 9 can be prepared from commer-
cially available meso-diacetate 10 by an enzymatic desymmetriza-
tion process.
The synthesis and enzymatic desymmetrization are shown in
Scheme 2. Cis-meso-diacetate 10 was prepared in multigram quan-
tity from commercially available, inexpensive, 1,2,3,6-tetrahy-
drophthalic anhydride 11 by LAH reduction followed by
acetylation as reported in the literature.22 Initially, we carried
out enzymatic desymmetrization of meso-diacetate 10 by treat-
ment with Porcine Pancreatic Lipase (PPL, 5% w/w, Sigma, type II,
crude)23 in 0.1 M phosphate buffer (pH 7) over 24 h to afford opti-
cally active monoacetate 9 in >95% ee. During this enzymatic reac-
tion, aqueous 1 N NaOH was added dropwise to neutralize the
acetic acid formed during the reaction.22,24 However, when the
reaction was performed on a gram-scale, inconsistent yields and
varying degree of optical purity were observed. This is presumably
due to non-enzymatic hydrolysis of the monoacetate 9 and diac-
etate 10 promoted by aqueous 1 N NaOH in the reaction. In a mod-
ified protocol, we subsequently used aqueous 1 N NaHCO3 instead
OH
O
O
Ref.18
O
5
6
O
present work
H
HO
RO
H
H
OAc
OAc
OR
OR
OH
OR
8
7
H
H
OAc
OR
9
10
R = H
R = Ac
Scheme 1. An enzymatic desymmetrization strategy for hexahydro-4H-
furopyranol.
O
H
H
H
H
OAc
OAc
Ref. 22
O
O
10
11
PPL (5% w/w),
pH 7 buffer,
1 N NaHCO3
(84%)
H
H
H
1. Swern Ox.
-78 oC
OAc
O
OAc
OH
2. HC(OMe)3
CSA (0.2 eq)
MgSO4
H
O
9
(99% ee)
12
(84% for 2-steps)
1. O3, Pyridine
2. NaBH4, Ethanol
(80%)
H
H
HO
HO
HO
MeO
OAc
O
CSA
CH2Cl2
OAc
OAc
O
(70%)
O
13
13A
O
14
O
Scheme 2. Synthesis of bicyclic acetal 14.
Me
O
Asp30'
of 1 N aqueous NaOH to neutralize the liberated acetic acid. This
condition afforded monoacetate 9 in 84% yield and high enan-
tiomeric purity (99% ee). The reaction yield and optical purity were
reproducible even on a 60 g scale reaction.25
Asp25'
Asp25
O
N
S
O
O
O
H
H
For the synthesis of the ligand alcohol 6, monoacetate 9 was
oxidized using Swern oxidation to provide the corresponding alde-
hyde in 94% yield. Protection of the aldehyde with ethylene glycol
in presence of a catalytic amount of camphorsulfonic acid (CSA)
gave the desired acetal in poor yield (11%). We screened a number
of other protecting groups, acid catalysts and solvents and the
results are shown in Table 1. The use of bis(trimethylsilyloxy)
ethane (1.3 equiv) in the presence of TMSOTf afforded 30% yield
of the corresponding 1,3-dioxolane derivative (entry 2). Reaction
of aldehyde with trimethyl orthoformate (30 equiv) in methanol
at 23 °C in the presence of a catalytic amount of PPTS afforded
N
H
O
H
O
O
O
H
Gly27
Å
H
3
.
3
Å
.0
3
H
N
N
Asp30
O
Asp29
Fig. 2. The key hydrogen bonding interactions of hexahydro-4H-furopyranol ligand
of inhibitor 4 with Asp29 and Asp30.
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A.K. Ghosh, A. Sarkar / Tetrahedron Letters xxx (2017) xxx–xxx
3
Table 1
Optimization of acetal protection for compound 12.
Entry
Reagent
Acid (equiv)
Temp (°C)
Time (h)
Yield (%)
1a
2b
3c
4b
5b
Ethylene glycol
TMSO(CH2)2OTMS
HC(OMe)3
HC(OMe)3
HC(OMe)3
CSA (0.2)
80
0.5
2
24
72
12
11
30
TMSOTf (13)
PPTS (0.3)
PPTS (0.2)
CSA (0.2)
ꢀ78
23
27
23
23
76d
89d
a
b
c
Solvent: benzene.
Solvent: CH2Cl2.
Solvent MeOH.
d
Anhydrous MgSO4 was used.
dimethyl acetal 12 in 27% yield (entry 3). When the reaction was
carried out in CH2Cl2 with 0.2 equivalent of PPTS, acetal yield
improved to 76% (entry 4). The use of a catalytic amount of CSA
(0.2 equivalent) and in the presence of anhydrous MgSO4 in CH2Cl2
at 23 °C for 12 h provided acetal 12 in 89% yield (entry 5).
Oxidative cleavage of the olefin by ozonolysis in the presence of
pyridine as the organocatalyst26 in CH2Cl2 at ꢀ78 °C, followed by
reduction of the resulting crude dialdehyde with sodium borohy-
dride at 0 °C to 23 °C afforded diol 13. Treatment of diol 13 with
a catalytic amount (30 mol%) of CSA in CH2Cl2 at 23 °C for 2 h fur-
nished bicyclic acetal 14 via intermediate 13A in 56% yield over 3
steps.
Bicyclic acetal 14 was converted to hexahydro-4H-furopyranol
6 as shown in Scheme 3. Removal of the acetate was carried out
by treatment of 14 with potassium carbonate in methanol at
23 °C for 30 min to afford alcohol 15 in 98% yield. Alcohol 15 was
subjected to an elimination reaction using a protocol developed
by Grieco and co-workers.27 Alcohol 15 was treated with o-nitro-
phenylselenonitrile and n-tributylphosphine in THF at 23 °C for
45 min to provide the corresponding selenide derivative. Oxidation
of the resulting selenide with m-CPBA in CH2Cl2 at 23 °C for 10 min
resulted in elimination of the corresponding selenoxide to afford
olefin 16 in 71% yield over 2-steps. We also explored alternative
pathways to convert alcohol 15 to olefin 16. Tosylation of alcohol
15 with tosyl chloride in the presence of triethylamine and a cat-
alytic amount of DMAP at 23 °C for 2 h provided tosylate derivative
17 in 70% yield. However, elimination of the tosylate with sodium
iodide and DBU in 1,2-dimethoxyethane at 90 °C for 3 h furnished
olefin 16 in low yield (31%).28 Further attempted elimination of the
tosylate with potassium tert-butoxide in DMSO provided only trace
amount of elimination product and mostly resulted in the decom-
position of the tosylate. We carried out oxidative cleavage of exo-
cyclic olefin 16 by ozonolysis in CH2Cl2 at ꢀ78 °C to furnish the
corresponding ketone in 88% yield. Reduction of the resulting
Optically active ligand alcohol 6 was converted to HIV-1 pro-
tease inhibitor 20 as shown in Scheme 4. Alcohol 6 was converted
to the activated mixed carbonate 18 by treatment with disuccin-
imidyl carbonate in the presence of Et3N in CH2Cl2 at 23 °C for
18 h.29 Reaction of carbonate 18 with the known (hydroxyethyl)-
sulfonamide isostere30 19 in the presence of diisopropylethy-
lamine at 23 °C for 1 h afforded inhibitor 20 in 85% yield over 2
steps. Inhibitor 20 was evaluated in an enzyme inhibitory assay
following a protocol described by Toth and Marshall.31 As reported
previously, inhibitor 20 showed a Ki value of 10 pM, comparable to
that of Darunavir (16 pM).18
In conclusion, we have carried out an optically active synthesis
of the P2 ligand, (3aS,4S,7aR)-hexahydro-4H-furo[2,3-b]pyran-4-ol.
This nonpeptide ligand is a high affinity P2 ligand for a variety of
exceptionally potent HIV-1 protease inhibitors. The key step
involved the synthesis of optically active ((1R,6S)-6-(hydrox-
ymethyl)cyclohex-3-en-1-yl)methyl acetate 9 by a very efficient
enzymatic desymmetrization of meso-diacetate 10 using commer-
cially available and inexpensive porcine pancreatic lipase. The
enzymatic desymmetrization was carried out in large scale provid-
ing acetate 9 in high optical purity (up to 99% ee). Optically active
alcohol 9 was efficiently converted to (3aS,4S,7aR)-hexahydro-4H-
furo[2,3-b]pyran-4-ol. This ligand alcohol was then converted to
HIV-1 protease inhibitor 20. Further studies, particularly scopes
and application are in progress in our laboratory.
O
H
O
OH
H
O
O
O
O
N
N
O
H
O
+
O
6
Et3N, CH2Cl2
(89%)
ketone with sodium borohydride in ethanol at 23 °C for 15 min
23
provided alcohol 6 ([
a]
ꢀ31.1 (c 1, CHCl3) in 90% yield.
D
O
H
O
O
O
H
N
1. n-Bu3P,
2-NO2PhSeCN
O
O
H
OR
O
O
O
2. m-CPBA, NaHCO3
18
O
O
(71%)
NH2
16
OH
i-Pr2NEt
CH2Cl2
14 R = Ac
15 R = H
K2CO3,
MeOH
N
H2N
Ph
1. O3, Pyridine
S
(96%)
O
O
(79%)
2. NaBH4
Ethanol
TsCl, Et3N
DMAP
19
(70%)
NH2
DBU, NaI
glyme
(31%)
OH
H
H
N
N
O
O
H
S
H
OTs
O
O
O
O
O
H
OH
O
Ph
17
6
20
O
O
Scheme 3. Synthesis of ligand alcohol 6.
Scheme 4. Synthesis of HIV-1 protease inhibitor 20.
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A.K. Ghosh, A. Sarkar / Tetrahedron Letters xxx (2017) xxx–xxx
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Financial support by the National Institutes of Health is grate-
fully acknowledged.
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A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2017.07.
010.
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