Design and synthesis of basic ionic liquids for the esterification of triterpenic acids

Article abstract of DOI:10.1007/s00706-016-1889-1

Abstract: We present the design and synthesis of Br?nsted-basic ionic liquids and investigate their application in the microwave-assisted esterification of betulinic acid, aiming towards a benign and pyridine-free manufacturing process of the anti-HIV drug, bevirimat. Graphical abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s00706-016-1889-1

Monatsh Chem
DOI 10.1007/s00706-016-1889-1
ORIGINAL PAPER
Design and synthesis of basic ionic liquids for the esterification
of triterpenic acids
1
1
1
2
1
Anna K. Ressmann
•
Maria Schneider
•
Peter Gaertner
•
Matthias Weil
•
Katharina Bica
Received: 1 October 2016 / Accepted: 3 December 2016
Ó Springer-Verlag Wien 2016
Abstract We present the design and synthesis of Brøns-
ted-basic ionic liquids and investigate their application in
the microwave-assisted esterification of betulinic acid,
aiming towards a benign and pyridine-free manufacturing
process of the anti-HIV drug, bevirimat.
Introduction
Since ancient times, natural products from plant materials
have been an important source for the manufacture of
drugs. This trend continues until today, and natural pro-
duct-derived drugs are still frequently found in top
positions according to the worldwide drug sales. In the
Graphical abstract
2000s, 8 out of 29 drugs owe their orgins to a natural
source [1, 2].
The typical commercial isolation process relies on the
extraction of pharmaceutical active ingredients from plant
materials based on the exhaustive use of volatile organic
compounds (VOCs), including the highly solvent con-
suming maceration method, i.e. the soaking of plant
material in solvents or classical Soxhlet extraction that
requires long hours of refluxing and might degrade thermo-
labile natural products. Similarly, volatile organic solvents
are often an issue in the consecutive semi-synthetic trans-
formation of a natural-product derived precursor towards
the final drug, making the pharmaceutical industries a
prime contributor for the generation of hazardous waste.
Consequently, efforts to reduce the exhaustive use of
undesirable solvents as reaction media have been made in
the past years. Recent trends focus on the use of ionic
liquids (ILs, salts melting below 100 °C) for the manu-
facturing of pharmaceutically valuable compounds, both as
solvent replacement in the semi-synthetic preparation of a
final drug or as leaching media for pharmaceutically active
plant components [3–6].
Keywords Ionic liquids Á Natural products Á
Green chemistry Á Microwave-assisted synthesis Á
Crystal structure
Electronic supplementary material The online version of this
article (doi:10.1007/s00706-016-1889-1) contains supplementary
material, which is available to authorized users.
&
Katharina Bica
katharina.schroeder@tuwien.ac.at
Pharmaceutically active triterpenes such as the lupane
derivative betulin have known medical applications and
offer a number of therapeutic uses. Betulin and its deri-
vates, e.g. betulinic acid ((3b)-3-hydroxy-lup-20(29)-en-
28-oic acid, 1) or bevirimat (3-O-(3 ,3 -dimethylsuc-
cinyl)betulinic acid, 2) show antitumor, anti-HIV, antiviral,
1
2
Institute of Applied Synthetic Chemistry, TU Wien,
Getreidemarkt 9/163, 1060 Vienna, Austria
0
0
Institute for Chemical Technologies and Analytics, TU Wien,
Getreidemarkt 9/164-SC, 1060 Vienna, Austria
123

A. K. Ressmann et al.
antibacterial, anti-inflammatory, and antimalarial activities
7]. The derivative bevirimat (2) that is probably one of the
specific ionic liquids (TSILs)’’ are however not limited to
Brønsted-acidic species. Basic ionic liquids are for exam-
ple obtained via selective monoalkylation or protonation of
diamine precursors [13, 14] and have found application in
acid catalysed reactions such as acetalizations, condensa-
tions or esterifications [15, 16] in palladium-catalyzed
cross coupling [17, 18] but also in gas separations. It is
[
most promising derivatives of betulin was found to be
active against Human Immunodeficiency Virus (HIV)
infection. Due to the fact that bevirimat (2) is well absor-
bed and tolerated, it succeeded in phase I and IIa clinical
trials in 2009 [8]. Later on, problems with reduced effi-
ciency for patients carrying resistant viruses due to
naturally occurring polymorphism in the SP1 region of
HIV-1 Gag were found that that restricted further clinical
trials [9]. Recent research focuses on the inclusion of
privileged fragments, e.g. caffeic acid derivatives into
bevirimat to improve activity against the most prevalent
bevirimat-resistant systems [10].
particularly in the development of efficient CO capture
2
and storage processes that benefit from the presence of
basic groups, e.g. amines in functionalized ionic liquids via
chemical fixation of CO [19, 20].
2
Based on this strategy, we aimed to expand our research
in this area to develop a novel strategy for the manufac-
turing of bevirimat (2), a maturation inhibitor for the
treatment of HIV-1 infection (Scheme 1). In this paper, we
present the design and synthesis of Brønsted-basic ionic
liquids and investigate their application in the esterification
of betulinic acid, aiming towards a benign and pyridine-
free manufacturing process of 2.
However, current strategies for its manufacturing have
significant drawbacks and are hardly applicable on an
industrial scale. The manufacturing process for bevirimat
typically relies on a two-step process: (a) the initial
extraction and purification of betulinic acid (1) from vari-
ous plant sources and (b) the consecutive esterification with
dimethylsuccinic anhydride.
Both steps provide considerable problems: Extraction
yields of betulinic acid are very low and vary between 2.4
and 3.3 wt% [11]. Moreover, the successive esterification
requires harsh reaction conditions; in fact the synthesis of
bevirimat is typically performed via base-catalyzed ester-
ification of betulinic acid using dimethylsuccinic anhydride
under addition of DMAP (4-(dimethylamino)pyridine) in
highly toxic pyridine at reflux for several hours to days.
In our previous work on the synthesis of drug precursors
in alternative reaction media, we demonstrated that the role
of ionic liquids is not limited to a mere solvent replace-
ment; moreover, it is possible to take advantage from their
catalytic properties and directly convert a plant extract to a
desired drug or its precursor. This concept of catalytically
active ionic liquids could be realized using different
Brønsted-acidic ionic liquids as solvent and reaction media
Results and discussion
As the preparation of bevirimat from betulinic acid is typically
performed in toxic and volatile pyridine, a set of basic ionic
liquids was chosen as potential replacement. They should play a
dual role and serve as reaction media and base for the esterifi-
cation of betulinic acid (1). Ionic liquids based on the diamines
1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), 1,4-diazabicy-
clo[2.2.2]octane (DABCO), or 4-dimethylaminopyridine
(DMAP) as structural motif were synthesized. To allow a
comparison with the conventional solvent pyridine, pyri-
dinium-based ionic liquids were also included, although they
would not qualify as Brønsted-basic ionic liquid.
In a first step N-heterocyclic bases were reacted with
alkyl halides to obtain the chloride salt. In a second step the
chloride anion was further optionally exchanged to the
bistriflimide anion via metathesis (Scheme 2).
towards the isolation of ketal intermediates for the syn-
TM
thesis of the anti-influenza drug Tamiflu
via an in situ
Basic precursors 1,8-diazabicyclo[5.4.0]undec-7-ene
(DBU), 1,4-diazabicyclo[2.2.2]octane (DABCO), pyridine,
and 4-dimethylaminopyridine (DMAP) were initially
esterification and ketalization process [12]. Functionalized
ionic liquids, that are also known under the term ‘‘task-
Scheme 1
H
H
H
H
DMAP, pyridine
OH
O
reflux, 24 h
OH
O
O
O
O
O
H
H
H
H
HO
HOOC
O
betulinic acid (1)
bevirimat (2
)
1
23

Design and synthesis of basic ionic liquids for the esterification of triterpenic acids
Scheme 2
Scheme 3
reacted with n-butylchloride to yield their corresponding
hydrophilic chloride-based ionic liquids in moderate to
good yields. The following anion exchange gave the
hydrophobic bistriflimide ionic liquids in excellent yields.
Additionally, a basic ionic liquid functionalized with a
pyridine-subunit was synthesized (Scheme 3).
Once a set of basic ionic liquids was available, the
esterification of betulinic acid (1) was initially investigated
using the conventional solvent pyridine. We developed an
HPLC-based screening procedure that allowed the rapid
quantification of formed bevirimat (2) in the presence of
ionic liquids or conventional solvents. An additional peak
besides bevirimat was observed after conversion of betu-
linic acid under standard conditions that was identified as
regioisomer iso-2, as two possible nucleophilic attacks on
the anhydride can occur (Scheme 4). However, the attack
of the less hindered carbonyl group (green) is more likely
than reaction of the sterically more demanding carbonyl
group (red), resulting in the preferential formation of
bevirimat (2).
For the synthesis of [C PyrImd]Cl (12), 2-(chlor-
1
omethyl)pyridine hydrochloride was neutralized using
sodium hydrogen carbonate. After isolation via simple
extraction and evaporation the obtained unstable product
11 was directly converted with 1-methylimidazole in
methanol to the chloride-based ionic liquid [C PyrImd]Cl.
1
Anion exchange yielded the hydrophobic ionic liquid 13 in
excellent yield.
123

A. K. Ressmann et al.
Scheme 4
˚
To confirm this assumption, HPLC–MS was performed,
O5…O2 = 2.6630(14) A, O5-H5S…O2 = 171(2)°, and
˚
showing that the mass of the two product peaks was the
?
[M ? H ]
for iso-2: O1…O6 = 2.650(3) A, O1-H1S…O6 = 171(4)°;
O5…O2 = 2.700(3) A, O5-H5S…O2 = 178(4)°]. These
hydrogen bonding interactions generate a three-dimensional
network structure in the case of (2) whereas a layered
arrangement of molecules parallel to the ab plane is realized
for iso-2.
˚
same
of
585
corresponding
to
(
MW = 584 g/mol). The mixture of isomers was separated
via preparative HPLC and further characterized via NMR
spectroscopy and structure determination using X-ray
diffraction.
Heteronuclear Multiple Bond Correlation experiments
Initial experiments on the esterification of betulinic
acid in conventional solvents showed that extremely
harsh reaction conditions are required to reach complete
conversion of betulinic acid (Table 1). Long reaction
times of 3 days were required even at reflux temperatures
to reach complete conversion. Moreover, a large excess
of DMAP and anhydride is necessary: Although DMAP
is recovered during the reaction there are several car-
boxylic acid groups in the starting material, product and
anhydride after ring-opening that can bind DMPA via
protonation. Consequently, a reduction to one equivalent
of DMAP also decreased the yield dramatically (Table 1,
entry 3).
(
HMBC NMR, Fig. 1) showed a strong correlation of the
carboxy carbon (A) to the two methyl groups (3), which is
not the case for the ester carbon (B). Furthermore, B shows
a correlation to the proton 1. Worth to mention is also the
different solubility in NMR solvents: while bevirimat (2)
was soluble in chloroform, its regioisomer iso-2 was not. In
contrast, signals A and B are shifted in the 2D-NMR for the
regioisomer iso-2, probably due to a solvent effect, since
the regioisomer iso-2 was only soluble in DMSO. Proton 1
proofs the shifting, as it has to correlate to B. The ester B
also shows a correlation to the two methyl groups 2, which
is not the case for carboxylic acid A, thereby confirming
the structure of iso-2.
Since it is well known that reactions may proceed faster
under microwave irradiation, the microwave-assisted
esterification of betulinic acid (1) was considered a
promising strategy. When applying microwave irradiation
heating using pyridine as solvent at 125 °C for 1 h, only
low conversion was observed and mostly unreacted starting
material remained. However, an increase of the reaction
temperature to 160 °C could improve conversion, allowing
a yield of 62% after 1 h only. The yield could be further
increased if performed with higher betulinic acid concen-
trations, allowing the formation of 81% bevirimat in short
times.
Eventually, we were able to grow single crystals of
bevirimat (2) and its regioisomer iso-2 after slow evapo-
ration from a methanolic solution. The crystal structures of
the two compounds could be determined for the first time,
confirming the assumption that the main regioisomer is
bevirimat (2).
The five-membered ring system (C1–C22) is very similar
in the molecular structures of the two compounds (2) and iso-
2
(Fig. 2). As expected, the substituents (C32–C36) at the
ester function show a different orientation for the two iso-
mers, with torsion angles O3–C31–C32–C33 of 58.28(15)°
for (2) and of -41.9(4)° for iso-2. Other main differences
pertain to the orientations of the prop-1-en-2-yl group (C20,
C29, C30) and the carboxyl group (C28, O1, O2) at the
cyclopentane ring, with torsion angles of C18–C19–C20–
C29 = -12.44(17)° for (2) and 149.8(3)° for iso-2, and of
C18–C17–C28–O1 = -172.52(10)° for (2) and 120.1(3)
for iso-2, respectively. In the crystal structure of both com-
pounds, mutual and almost linear hydrogen bonding
interactions of medium strengths between the two acidic
functions of neighbouring molecules are realized [for (2):
In order to avoid the excessive use of pyridine as sol-
vent, we aimed for its successive replacement with
Brønsted basic ionic liquids or their corresponding chlo-
ride-based molten salts. A screening using hydrophilic
basic chloride salts and hydrophobic basic bistriflimide
ionic liquids was performed. 15 mg Betulinic acid (1) were
irradiated under microwave conditions for 1 h at 160 °C in
0.1 g ionic liquid with the addition of 2 eq. DMAP and
3 eq. anhydride. These results were compared with the
results obtained with pyridine under otherwise identical
conditions (Fig. 3).
O1…O6 = 2.6558(143) A˚ ,
O1-H1S…O6 = 170(3)°;
1
23

Design and synthesis of basic ionic liquids for the esterification of triterpenic acids
Fig. 1 2D-NMR of bevirimat
(
2)
When comparing the performance of basic ionic liquids
obtained—particularly in case of bistriflimide-based ionic
with the conventional solvent pyridine, results remained
below our expectations, as only low conversion was
liquids. With the exception of [C DBU]Cl, all chloride-
4
based salts could outperform the bistriflimide derivatives,
123

A. K. Ressmann et al.
Fig. 2 The molecular structures
of bevirimat (2) (top) and its
regioisomer iso-2 (bottom).
Displacement ellipsoids are
displayed at the 40% probability
level
Table 1 Esterification of betulinic acid in conventional solvents
a,b
Entry
Solvent
Conditions
Conversion betulinic acid 1/%
Yield regioisomer iso-2/%
Yield bevirimat 2/%
1
2
3
4
5
6
Pyridine
Pyridine
Pyridine
Pyridine
Pyridine
Pyridine
24 h, reflux
72
100
34
5
6
66
88
41
5
72 h, reflux
12
3
c
24 h, reflux
1 h, MW 125 °C
1 h, MW 160 °C
1 h, MW 160 °C
–
69
92
7
62
81
d
11
MW microwave irradiation
a
3
5 mg betulinic acid (1), 2 eq. DMAP, 3 eq. anhydride, 0.5 cm pyridine
1
b
c
d
Yields were determined via HPLC
5 mg betulinic acid (1), 1 eq. DMAP, 3 eq. anhydride
1
3
.1 cm pyridine used
0
which might be related to the additional basicity introduced
by the anion. However, the DMAP-based chloride salt was
able to compete with pyridine as solvent, and showed high
conversion and yield of bevirimat.
Moreover, a surprising effect on regioselectivity was
found: The bistriflimide ionic liquids preferably yielded the
regioisomer iso-2 instead of bevirimat (2). This effect is
particularly pronounced in case of [C DBU]N(Tf) that
4
2
1
23

Design and synthesis of basic ionic liquids for the esterification of triterpenic acids
of bevirimat and its regioisomer are reported for the first
time.
In future studies, we will address the direct preparation
of bevirimat from plane bark in a reactive dissolution
strategy, aiming to combine ionic liquid-based extraction
with the in situ esterification for a streamlined manufac-
turing process.
Experimental
1
13
H and C NMR spectroscopic data were recorded with a
Bruker AC 200 (200 MHz) or a Bruker Advance Ultra-
Shield 400 (400 MHz) spectrometer. The chemical shifts
(
d) are reported in ppm by using tetramethylsilane as the
internal standard. Infrared spectra were recorded on a
Perkin–Elmer Spectrum 65 FT IR spectrometer that was
equipped with a Specac MK II Golden Gate Single
Reflection ATR (attenuated total reflectance) unit.
Melting points above room temperature were measured
on a Kofler hot-stage microscope or on an automated
melting point system OPTI MELT of Stanford Research
Systems. High-resolution mass spectrometry was per-
formed on a Thermo Scientific LTQ Orbitrap XL hybrid
FTMS (Fourier Transform Mass Spectrometer) equipped
with Thermo Scientific MALDI Interface or Thermo Fis-
cher Exactive Plus Orbitrap (LC-ESI?) and a Shimadzu
IT-TOF Mass Spectrometer.
Fig. 3 Esterification of betulinic acid in basic ionic liquids. Per-
TM
formed on a BIOTAGE Initiator sixty microwave unit with 15 mg
3
betulinic acid (1) in 100 mg ionic liquid or 0.1 cm pyridine at
1
60 °C for 1 h (hold time)
gave twice the yield of the regioisomer iso-2 compared to
bevirimat (2). In contrast, all chloride-based salts gave the
desired isomer bevirimat preferentially. This anion-de-
pendent change in regioselectivity might be caused by a
different coordination behavior of the two anion species. In
contrast to the reactive chloride anion, the bistriflimide
anion is generally considered as a weakly or even non-
coordinating anion, and discrete complexes are only
formed in the absence of more competent anion [21, 22].
Microwave reactions were performed on a BIOTAGE
TM
Initiator
sixty microwave unit. The reported times are
hold times. For this purpose the reaction mixtures were
transferred into a glass pressure microwave tube equipped
with a magnetic stirring bar, sealed with a Teflon septum
and exposed to microwave irradiation at the required
temperature. The absorption mode was set to ‘‘Very high’’.
For the determination of betulinic acid (1), bevirimat
(2), and regioisomer iso-2 a Maisch ReproSil 100 C18
Conclusion
We showed the design and synthesis of a set of Brønsted-
basic ionic liquids based on commercially available
diamines via selective monoalkylation and optional anion
exchange in high yields. The obtained Brønsted-basic ionic
liquids were applied in the esterification of the triterpene
betulinic acid, aiming for a novel and pyridine-free strategy
towards the preparation of the antiviral drug bevirimat.
Here, we found an unexpected regioselectivity effect in the
esterification of betulinic acid with dimethylsuccinic
anhydride: While chloride-based basic salts yielded the
desired bevirimat, all hydrophobic bistriflimide-based ionic
liquids gave the undesired regioisomer preferentially.
Under optimized conditions using microwave irradiation, it
was possible to reduce the reaction time to 60 min only,
while simultaneously substituting toxic pyridine with a
DMAP-derived basic salt. Moreover, the crystal structures
250 9 4.6, 5 lm was used with MeOH:H O (0.1%
2
3
TFA) = 88:15 as solvent and a flow of 1 cm /min; detec-
tion was done at 210 nm, at 30 °C column oven
temperature, 25 °C tray temperature on a Jasco HPLC unit
equipped with a PDA detector. Retention times were
7.1 min for the internal standard, 15.0 min for betulinic
acid (1), 23.4 min for regioisomer iso-2, and 24.3 min for
bevirimat (2).
A Thermo Scientific LTQ Orbitrap XL hybrid FTMS
(Fourier Transform Mass Spectrometer) equipped with
Thermo Scientific MALDI Interface or Thermo Fischer
Exactive Plus Orbitrap (LC-ESI?) and a Shimadzu IT-
TOF Mass Spectrometer were used for high resolution
mass spectrometry.
123

A. K. Ressmann et al.
1
13
Complete H and C spectroscopic data of all known
ionic liquids or their precursors are reported in the elec-
tronic supplementary information (Online Resource 1).
1-Butyl-1,4-diazabicyclo[2.2.2]octane-1-ium-chloride
[C DABCO]Cl (4)
1,4-Diazabicylco[2.2.2]octane (10.01 g, 0.089 mol) was
4
3
dried in vacuo for 1 h and then dissolved in 50 cm EtOAc
Single crystal diffraction
under argon atmosphere. Chlorobutane (8.18 g, 0.096 mol,
3
.1 eq.) in 50 cm EtOAc was added dropwise to the
1
Single crystals of 2 and iso-2 of good optical quality were
pre-selected under a polarising microscope, embedded in
perfluorinated polyether and mounted on MiTeGen
solution within 30 min. The solution was refluxed over-
night and NMR showed full conversion of the starting
material. After cooling to r.t. the white solid was removed
Ò
MicroLoops . X-ray diffraction data of (2) and iso-2 were
collected on a Bruker Kappa APEX-II CCD diffractometer
via filtration, washed with cold Et
-
2
O and dried in vacuo
2
(1 9 10
mbar) overnight. A colourless solid was
working with graphite-monochromated Mo–Ka radiation
obtained in 48% (8.80 g) yield. M.p.: 127–129 °C. Ana-
lytical data was in accordance with literature [27].
˚
(
k = 0.71073 A) and using narrow u- and x-scan frames.
Corrections for absorption were applied with the multi-
scan technique using SADABS [23]. The structures were
solved with SHELXS97 (direct methods) [24] and refined
1
-Butylpyridinium chloride [C Pyr]Cl (5)
4
A mixture of pyridine (107.8 g, 1.36 mol) and butylchlo-
ride (151.4 g 1.64 mol, 1.2 eq.) were refluxed for 4 days.
As NMR indicated no full conversion, 0.1 eq. butylchloride
was added and refluxed overnight. A solid precipitated and
was filtered, washed with ethyl acetate and diethylether and
dried in vacuo for 48 h. An off white solid was crystallized
from ethyl acetate and acetonitrile to obtained colorless
crystals in 82% (191 g) yield. M.p.: 129–131 °C Analytical
data was in accordance with literature [28].
2
on F with SHELXL-2014 [25]. All non-hydrogen atoms
were refined with anisotropic displacement parameters. H
atoms were placed in calculated positions and thereafter
treated as riding, except the carboxyl H atoms bonded to
O1 and O5 that were located from difference Fourier maps
and refined freely.
Crystallographic data of 2: C H O , M = 584.80,
r
3
6
56
6
3
colourless fragment, 0.72 9 0.50 9 0.20 mm , orthorhom-
˚
bic, space group P2 2 2 (no. 19), a = 7.8085(7) A,
1 1
1-Butyl-4(dimethylamino)pyridin-1-ium-chloride
1
3
b = 18.8181(17) A, c = 22.1212(19) A, V = 3250.5(5) A ,
3
Z = 4, d = 1.195 g/cm , T = 100 K. 142564 reflections
˚
˚
˚
[C DMAP]Cl (6)
4
4-(Dimethylamino)pyridine (3.01 g, 0.025 mol) were dis-
3
solved in 10 cm anhydrous ACN. Chlorobutane (3.83 g,
x
collected (hmax = 32.7°) and merged to 11932 independent
data with I [ 2r(I) (Rint = 0.027); final R indices (all data)
R = 0.0362, wR = 0.0991, GooF 1.078, 395 parameter.
0.042 mol, 1.6 eq.) was added and the solution was stirred
at 80 °C for 48 h under argon. The white solid was
removed via filtration, washed with CAN, and dried in
-2
1
2
Crystallographic data of iso-2: C H O , M = 584.80,
3
3
colourless plate, 0.45 9 0.35 9 0.05 mm , orthorhombic,
6
56
6
r
vacuo (1 9 10
mbar) overnight. A white solid was
˚
space group P2 2 2 (no. 19), a = 8.0491(10) A,
1 1
obtained in 67% (3.64 g) yield. M.p.: 250 °C (dec).
Analytical data was in accordance with literature [29].
1
3
b = 11.5602(13) A, c = 34.314(4) A, V = 3192.9(7) A ,
3
Z = 4, d = 1.217 g/cm , T = 90 K. 35271 reflections
˚
˚
˚
x
2-(Chloromethyl)pyridine (11)
collected (hmax = 25.8°) and merged to 6104 independent
data with I [ 2r(I) (Rint = 0.061); final R indices (all data)
R = 0.0638, wR = 0.0956, GooF 1.039, 396 parameter.
2-Picolylchloride hydrochloride (7.49 g, 45.66 mmol) was
dissolved in water, 5.75 g NaHCO (68.49 mmol, 1.5 eq.)
3
1
2
was added and the solution stirred for 15 min at r.t. The
solution was extracted with DCM three times, the
combined organic layers were washed with brine and dried
over Na SO , filtered, and evaporated to dryness. A red oil
Deposition numbers CCDC 1507241 for (2) and
507242 for iso-2 contain the supplementary crystallo-
1
graphic data for this paper in CIF format. These data can be
obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via http://www.ccdc.cam.ac.uk/data_
request/cif.
2
4
was obtained in 87% (5.07 g) yield. The unstable com-
pound 11 was directly used in a next step [30].
1
-Methyl-3-(pyridin-2-ylmethyl)-1H-imidazol-3-ium
1
-Butyl-1,8-diazabicyclo[5.4.0]undec-7-enium chloride
chloride [C PyrImd]Cl (12)
1
[
C DBU]Cl (3)
4
Freshly prepared 11 (5.07 g, 39.70 mmol, 1.1 eq.) was
3
dissolved in 30 cm anhydrous methanol under argon
1
,8-Diazabicyclo[5.4.0]undec-7-en (6.63 g, 43.55 mmol)
and 4.43 g chlorobutane (47.90 mmol, 1.1 eq.), were
atmosphere. 1-Methylimidazole (2.96 g, 36.09 mmol) was
added dropwise. The solution was stirred at 60 °C over-
night. The dark slurry was crystallized from EtOAc to yield
heated under argon at 50 °C for 24 h. The mixture was
-
2
washed with EtOAc, and dried in vacuo (1 9 10 mbar).
Yield: 9.24 g (87%) of an orange oil. Analytical data was
in accordance with literature [26].
1
2 as light-brown and very hygroscopic crystals in 98%
1
23

Design and synthesis of basic ionic liquids for the esterification of triterpenic acids
(
7.39 g) yield. M.p.: 36–38 °C. Analytical data was in
accordance with literature values [31].
(7.83 mmol) to yield the product as yellowish liquid in
96% (3.14 g). Analytical data was in accordance in with
literature [34].
General procedure for the synthesis of bistriflimide ionic
liquids
1-Methyl-3-(pyridine-2-ylmethyl)imidazolium
To a solution of the chloride ionic liquid in water lithium
bis(trifluoromethane)sulfonimide [C PyrImd]N(Tf)
1
2
bis(trifluoromethane)sulfonimide (LiN(Tf) , 1.1 eq.) was
2
(13, C H F N O S )
12 12 6 4 4 2
added. The reaction mixture was vigorously stirred for
According to the general procedure compound 13 was
prepared from 2.02 g 12 (9.63 mmol) and 3.07 g LiN(Tf)2
(10.56 mmol) to yield the product as brown liquid in 95%
3
0 min at room temperature. The formation of a second
phase was observed. The mixture was extracted with DCM
and the combined organic layers were subsequently washed
with water until no chloride could be detected in the
1
(4.16 g). H NMR (400 MHz, DMSO-d ): d = 9.23 (s,
6
1H), 8.57 (s, 1H), 8.56 (s, 1H), 7.86 (dd, J = 1.74 Hz,
1
organic phase (tested with AgNO solution). The organic
3
J2 = 3.85 Hz), 7.78 (t, J = 1.90 Hz, 1H), 7.72 (t,
J = 1.68 Hz, 1H), 7.49 (d, J = 7.80 Hz; 1H), 7.41 (dd,
J = 4.88 Hz, J = 6.83 Hz, 1H), 5.57 (s, 2H), 3.90 (s,
phase was dried over Na SO , filtered and the solvent was
2
4
evaporated and the residue was dried in vacuo.
-Butyl-1,8-diazabicyclo[5.4.0]undec-7-enium bis(trifluo-
romethane)sulfonimide [C DBU]N(Tf)
1
2
1
3
3
1
H) ppm; C NMR (100 MHz, DMSO-d ): d = 154.1,
50.0, 137.9, 137.8, 124.1, 123.6, 122.9, 119.9 (q,
6
1
4
2
J = 321.9 Hz), 53.5, 36.3 ppm; IR: mꢀ = 3158, 1597,
(
7, C H F N O S )
15 25 6 3 4 2
1
8
5
575, 1479, 1441, 1347, 1329, 1178, 1132, 1051, 998,
According to the general procedure compound 7 was
prepared from 4.41 g 3 (18.01 mmol) and 5.71 g LiN(Tf)2
30, 789, 752, 740, 720, 654, 612, 599, 569, 533, 514,
-1
07 cm
;
HR-ESI-TOF–MS:
m/z = 174.1021
(
19.82 mmol, 1.1 eq.) to yield the product as yellowish oil
?
-
(C H N ), 279.9178 (C F NO S ).
1
10 12 3 2 6 4 2
in 96% (8.46 g).
d = 3.62–3.64 (m, 2H), 3.43–3.46 (m, 6H), 2.79–2.82
m, 2H), 2.11 (quint, J = 5.95 Hz, 2H), 1.73–1.81 (m, 6H),
H
NMR (400 MHz, CDCl3):
Esterification of betulinic acid (1)
Conventional heating: to 15 mg (± 0.5 mg) betulinic acid
(1) 3 eq. dimethylsuccinic acid anhydride and 2 eq. DMAP
(
1
.60 (quint, J = 5.97 Hz, 2H), 1.35 (sext, J = 7.1 Hz,
1
3
3
were added in a screw-cap vial. 0.5 cm of pyridine were
2
H,), 0.96 (t, J = 7.27 Hz, 3H,) ppm;
C NMR
3
added and the mixture was refluxed overnight. 20 mm of
(
100 MHz, CDCl ): d = 166.5, 119.9 (q, J = 321.4 Hz),
3
3
the solution were taken and mixed with 80 mm 2 N HCl,
5
1
1
7
5.3, 53.9, 49.1, 47.1, 30.6, 28.5, 28.3, 25.9, 23.1, 19.9,
9.7, 13.6 ppm; IR: mꢀ = 2938, 2876, 1651, 1619, 1529,
449, 1349, 1328, 1226, 1175, 1134, 1053, 984, 915, 845,
3
3
900 mm methanol, and 200 mm internal standard, fil-
tered, and analyzed via HPLC.
-
1
88, 762, 739, 653, 614, 600, 569, 538, 512, 502 cm
;
3
Microwave irradiation: in a 5 cm microwave vial 15 mg
?
HR-ESI-TOF-MS:
m/z = 209.2005
(C H N ),
13 25 2
(
± 0.5 mg) betulinic acid (1) 3 eq. dimethylsuccinic acid
anhydride, and 2 eq. DMAP were added and dissolved in
-
2
79.9179 (C F NO S ).
2 6 4 2
3
0.5 cm of pyridine or in 0.1 cm pyridine or ionic liquids
3
1
-Butyl-1,4-diazabicyclo[2]octanium bis(trifluoromethane)-
3
sulfonimide [C DABCO]N(Tf) (8)
4
and heated to 160 °C for 1 h. To the solution 200 mm 2 N
HCl were added and the solution was diluted to 10 cm with
2
3
According to the general procedure compound 8 was
prepared from 2.00 g 4 (9.77 mmol) and 3.10 g LiN(Tf)2
3
3
methanol. An aliquot of 1 cm was taken, 200 mm internal
standard were added, filtered, and analyzed via HPLC.
(
10.75 mmol, 1.1 eq.) to yield the product as white solid in
9% (4.43 g). M.p.: 87–89 °C. Analytical data was in
accordance in with literature [32].
9
0
0
Synthesis of reference compounds: 3-O-(3 ,3 -dimethyl-
succinyl)betulinic acid, bevirimat (2)
1
-Butylpyridinium bis(trifluoromethane)sulfonimide
For the separation and isolation of bevirimat several
charges of esterifications were combined and purified via
[
C Pyr]N(Tf) (9)
4
2
According to the general procedure compound 9 was
prepared from 3.01 g 5 (17.53 mmol) and 5.54 g LiN(Tf)2
preparative HPLC. (MeOH:H O (0.1% TFA) = 83:17,
2
3
210 nm, 20 cm /min. The regioisomer iso-2 was synthe-
(
19.29 mol, 1.1 eq.) to yield the product as colorless liquid
sized and isolated using [C DBU]N(Tf) as solvent and
4
2
in 94% (6.86 g). Analytical data was in accordance in with
literature [33].
purified applying the same method. The separated regioi-
somers (2) and iso-2 were then crystallized from methanol.
1
Characterization of bevirimat (2): H NMR (400 MHz,
1
-Butyl-4-(dimethylamino)pyridinium bis(trifluoromethane)-
sulfonimide [C DMAP]N(Tf) (10)
4
CDCl ): d = 4.65 (s, 1H), 4.53 (s, 1H), 4.45–4.96 (m, 1H),
2
3
According to the general procedure compound 10 was
prepared from 1.55 g 6 (7.12 mmol) and 2.29 g LiN(Tf)2
2.93–2.99 (m, 1H), 2.79 (d, J = 15.20 Hz, 1H), 2.34 (d,
J = 15.20 Hz, 1H), 2.21–2.24 (m, 2H), 2.10 (s, 1H),
123

A. K. Ressmann et al.
2
2
0
.02–2.10 (m, 1H), 1.84–2.02 (m, 2H), 1.16–1.61 (m,
9H), 0.93 (s, 3H), 0.89 (s, 3H), 0.82 (s, 3H), 0.77 (s, 3H),
9. Fun A, Maarseveen NM, Pokorna J, Maas REM, Schipper PJ,
Konvalinka J, Nijhuis M (2011) Retrovirology 8:70
0. Zhao Y, Gu Q, Morris-Natschke SL, Chen CH, Lee KH (2016) J
Med Chem 59:9262
1
1
3
.73 (s, 3H) ppm;
C NMR (100 MHz, CDCl3):
d = 182.4, 182.3, 169.4, 149.4, 108.6, 80.1, 55.8, 53.9,
11. J a¨ ger S, Trojan H, Kopp T, Laszczyk MN, Scheffler A (2009)
Molecules 14:2016
4
3
2
8.3, 48.2, 45.9, 44.7, 41.2, 39.7, 39.4, 37.4, 36.8, 36.6,
6.2, 36.1, 32.7, 31.2, 29.5, 28.9, 27.6, 26.3, 24.1, 22.7,
2.6, 19.9, 18.4, 17.4, 16.8, 16.0, 15.6, 13.4 ppm; MS: m/
1
1
2. Ressmann AK, Gaertner P, Bica K (2011) Green Chem 13:1442
3. Chiappe C, Melai B, Sanzone A, Giulia Valentini G (2009) Pure
Appl Chem 81:2035
z = 585.6 (positive), 583.5 (negative).
1
Characterization of the undesired iso-2: H NMR
14. Yang ZZ, He LN, Miao CX, Chanfreau S (2010) Adv Synth Catal
352:2233
1
1
1
5. Xing G (2013) Monatsh Chem 144:1369
(
400 MHz, DMSO-d ): d = 12.1 (s, 2H), 4.69 (s, 1H), 4.57
6
6. Khaligh NG (2014) Monatsh Chem 145:1643
7. Ye C, Xiao JC, Twamley B, LaLonde AD, Norton MG, Shreeve
JM (2007) Eur J Org Chem 30:5095
(
s, 1H), 4.31–4.35 (m, 1H), 2.92–2.99 (m, 1H), 2.49 (m,
2
2
3
0
H), 2.19–2.25 (m, 1H), 2.10–2.13 (m, 1H), 1.79–1.81 (m,
H), 1.65 (s, 3H), 1.30–1.61 (m, 28H), 1.17 (s, 3H), 1.16 (s,
H), 0.95 (s, 3H), 0.88 (s, 3H), 0.81 (s, 3H), 0.80 (s, 3H),
18. Forsyth AS, Fr o¨ hlich U, Goodrich P, Gunaratne HQN, Hardacre
C, McKeown A, Seddon KR (2010) New J Chem 34:723
1
9. Zhang X, Zhang X, Dong H, Zhao Z, Zhang S, Huang Y (2012)
Energy Environ Sci 5:6668
1
3
.79 (s, 3H) ppm; C NMR (100 MHz, DMSO-d6):
d = 177.7, 176.0, 172.8, 150.8, 110.1, 80.1, 55.8, 55.1,
20. Ramdin M, Loos TW, Vlugt TJH (2012) Ind Eng Chem Res
51:8149
5
3
2
0.1, 49.0, 47.1, 43.9, 42.4, 40.7, 40.6, 40.5, 38.1, 37.9,
7.1, 36.8, 34.2, 32.2, 30.6, 29.7, 28.1, 25.7, 25.6, 25.1,
3.4, 20.9, 19.4, 18.2, 16.9, 16.3, 16.2, 14.88 ppm; MS: m/
2
1. Klingshirn MA, Broker GA, Holbrey JD, Shaughnessy KH,
Rogers RD (2002) Chem Commun 13:1394
2
2. Bridget Williams D, Stoll ME, Scott BL, Costa DA, Oldham WJ
Jr (2005) Chem Commun 11:1438
z = 585.4 (positive), 583.4 (negative). Analytical data was
in accordance with literature [35].
23. Krause L, Herbst-Irmer R, Sheldrick GM, Stalke D (2015) J Appl
Cryst 48:3
2
2
2
4. Sheldrick GM (2005) Acta Cryst A 64:112
5. Sheldrick GM (2015) Acta Cryst C 71:3
6. Gutierrez BC, Casal BD, Gonzalez AMD (2006) J Mol Catal A
Chem 253:203
Acknowledgements We are grateful to Botanical Garden Vienna for
the collections and supply of bark from platanus acerifola for the
preparation of reference material. The X-ray centre of TU Wien is
acknowledged for providing access to the single crystal
diffractometer.
2
2
2
7. Yang Z-Z, He L-N, Peng S-Y, Liu A-H (2010) Green Chem
1
2:1850
8. Xu W, Wang L-M, Nieman RA, Angell CA (2003) J Phys Chem
B 107:11749
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