Design and synthesis of a dual linker for solid phase synthesis of oleanolic acid derivatives

Article abstract of DOI:10.3390/molecules16064748

A hydrophilic amino-terminated poly(ethylene glycol)-type dual linker for solid phase synthesis of oleanolic acid derivatives using trityl chloride resin was designed and synthesized for the first time. Model reactions in both liquid and solid phase were performed to show the feasibility of its selective cleavage at two different sites. The biological assay results indicated that the long and flexible alkyl ether functionality in the linker is less likely to be critical for the binding event. Following the successful solid-phase synthesis of model compounds, the potential of this dual linker in reaction monitoring and target identification is deemed worthy of further study.

Full text of DOI:10.3390/molecules16064748

Molecules 2011, 16, 4748-4763; doi:10.3390/molecules16064748
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Design and Synthesis of a Dual Linker for Solid Phase Synthesis
of Oleanolic Acid Derivatives
Shaorong Wang and Weishuo Fang *
State Key Laboratory of Bioactive Substances and Functions of Natural Medicines, Institute of Materia
Medica, Chinese Academy of Medical Sciences and Peking Union Medical College, 1 Xian Nong Tan
Street, Beijing 100050, China; E-Mail: purplewinds@imm.ac.cn
* Author to whom correspondence should be addressed; E-Mail: wfang@imm.ac.cn;
Tel.: +86-10-63165229; Fax: +86-10-63165229.
Received: 23 May 2011; in revised form: 27 May 2011 / Accepted: 30 May 2011 /
Published: 8 June 2011
Abstract: A hydrophilic amino-terminated poly(ethylene glycol)-type dual linker for solid
phase synthesis of oleanolic acid derivatives using trityl chloride resin was designed and
synthesized for the first time. Model reactions in both liquid and solid phase were
performed to show the feasibility of its selective cleavage at two different sites. The
biological assay results indicated that the long and flexible alkyl ether functionality in the
linker is less likely to be critical for the binding event. Following the successful solid-phase
synthesis of model compounds, the potential of this dual linker in reaction monitoring and
target identification is deemed worthy of further study.
Keywords: oleanolic acid; dual linker; solid phase synthesis; analytical construct
1. Introduction
While working on the modification of the A ring of oleanolic acid (OA, 1a, Figure 1) to find
novel antitumor compounds, we found that the C-28 carboxyl group must be protected during the
acylation of C-3 OH, otherwise it would interfere the reaction. Organic synthesis on polymeric
supports offers several advantages over solution-based techniques, including easy isolation of
products by simple filtration from excess reagents after reaction completion, and it has been
extensively used in the development of new hits based on natural products [1]. Although there is

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only few reports related to solid parallel synthesis of triterpenoids by attaching a resin to the C-28
position [2–4], we considered this methodology would not only serve as a means to protect the
carboxyl group, but also could simplify the entire synthesis procedure.
Figure 1. Structures of OA (1a) and 3β-acetoxyl-OA (1b).
Since the OA scaffold was found to be highly unstable under harsh acidic conditions, trityl chloride
resin, well known for its super sensitivity to acids, was chosen to facilitate the reaction. In addition,
considering the steric hindrance around the C-28 position, a flexible anchor would be helpful for
connecting the resin and the carboxyl group in order to improve the yield of the loading reaction. On
the other hand, for the prospective target identification of an active OA analog, a reasonably designed
linker pre-equipped from the start of the synthesis could help immobilizing the hit compounds onto
solid matrix or an affinity tag rapidly without the need for laborious structure-activity relationship
(SAR) studies [5], which would often be encountered in forward chemical genetics [6–8]. Based on
these considerations, a novel dual linker between the triterpene scaffold and a trityl chloride
polystyrene solid support was introduced, as far as we know, for the first time.
2. Results and Discussion
Our designed dual linker also has an amino end, but our approach differs from that of a previous
report [5], by directly attaching to the resin through an acid sensitive linker (linker 2, Figure 2).
Cleavage at site B with a weak protonic acid will release the products bearing linker 1 and a
hydrophilic spacer. This spacer could not only improve the resin’s swelling properties and the loading
capacity as well as the overall yield of the solid phase reaction [9], but also exhibit more desirable
physical properties, provide a friendly environment for target interaction and reduce the binding to
nonspecific proteins in affinity chromatography for target identification [10]. In this initial study, we
employed diethylene glycol to simply demonstrate the feasibility of the method. On the other hand, an
alkoxymethyl ester fragment was chosen as linker 1 to protect the carboxyl group of the triterpene. It
was supposed that the alkoxymethylene group would be sensitive to a Lewis acid like a 2-methoxy-
ethoxymethyl ether (MEM), a commonly used alcohol protective group, so that facile cleavage at site
A will yield the unlabeled product.

Molecules 2011, 16
Figure 2. Diagram of the dual linker for OA solid phase synthesis.
4750
The dual linker was synthesized as illustrated in Scheme 1. Monosulfonation of diethyleneglycol (2)
with tosyl chloride (TsCl) in CH2Cl2 in the presence of pyridine and 4-dimethylaminopyridine (DMAP)
showed poor selectivity towards the symmetrical diol, and yielded only a 48.9% of the monotosylate
3a, along with 17.8% of the ditosylate 3b, respectively. In contrast, use of a stoichiometric amount of
TsCl in the presence of Ag2O and a catalytic amount of KI under neutral condition [11] improved the
yield of the desired product 3a to 90.6%. Then, 3a was treated with NaN3 under reflux conditions in
MeCN, giving the corresponding azide 4 in almost quantitative yield. A portion of DMF should be
added to the system to accelerate the reaction process. Because of the high yield, compound 4 was
hydrogenated in MeOH at atmospheric pressure (catalyzed by Pd/C) to give 2-(2-aminoethoxy)-ethan-
1-ol (5) in 98.3% yield without further purification.
Scheme 1. Synthesis of chloromethyl ether 9.
We initially chose benzyloxycarbonyl chloride (Cbz) as the amino protective group before
chloromethylation of the hydroxyl, but the yield is not satisfactory due to the poor selectivity of Cbz
for the two nucleophiles, OH and NH2. Finally, we found that phthalimide group was the best choice to
meet our needs, giving the protected compound 7 in a yield of 92.3%. Chloromethylation of 7 with
trioxane and dry HCl (dissolved in dry MeOH) did not afford the wanted product but rather gave the
dimer of the molecule itself by connecting with a methylene. Thus an O,S-acetal 8 was prepared in
83.7% yield by reacting of the alcohol 7 with DMSO in the presence of AcOH and Ac2O [12]. Then
chloromethyl ether 9 was obtained by cleavage of 8 with a stoichiometric amount of SOCl2. It should
be mentioned that 9 may be unstable and decompose on heating or during storage, so that it should be
used immediately without any purification after evaporation of the reaction solvent at low temperature.

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Before attempting the solid-phase experiment, preliminary liquid-phase tests were first performed
on the assessment of dual cleavage ability of the linker (Scheme 2). Treatment of 1a with 9 (~2 equiv)
and N,N-diisopropylethylamine (DIPEA) (3 equiv) in CH2Cl2, furnished 10a in 97.2% yield. The
C-3 hydroxyl of 1a is inert under this reaction condition. After hydrolysis of the phthalimide group in
an ethanol solution of methylamine (33 wt. %), the amino product was obtained in 78.2% yield. Trityl
chloride was chosen to simulate the resin, and we first attempted to couple it with 11a through a
carbamate, but were unsuccessful. Alternatively, a direct connection of the amine and trityl chloride in
the presence of DIPEA proved effective, affording 12a in a yield of 85.4%. Then, the cleavage
conditions for the two different sites in 12a were explored. It was found that a solution of 1% (v/v)
trifluoroacetic acid (TFA) in CH2Cl2 could be used for removing the trityl group to afford 11a, albeit
accompanied with the formation of 1a. Little improvement was achieved by reducing the concentration
of TFA to 0.5%. Thus, a mixture of AcOH, 2,2,2-trifluoroethanol (TFE) and CH2Cl2 (1:2:7 v/v/v) was
employed, giving solely the desired product 11a. On the other hand, several Lewis acids, such as
MgBr2, ZnBr2, AlCl3/N,N-dimethylaniline, etc, were utilized for the cleavage of the MEM-like moiety.
Ultimately, anhydrous TiCl4 could smoothly release the free carboxyl group of OA without any side
reactions after carefully controlling the reaction time. We also repeated the same reaction sequence
starting from 3β-acetoxyl-OA 1b. From the results, it was found that the finely tuned cleavage conditions
of our designed dual linker did not affect the integrity of the triterpene skeleton and C-3 acetate.
Scheme 2. Dual cleavage property of the designed linker.
O
MeNH2
DIPEA
RO
O
9
1a or 1b
+
COO
10a
N
O
O
R=H
R=Ac
10b
O
O
N
COO
O
COO
NH2
O
H
12a
12b
R=H
R=Ac
11a
11b
R=H
R=Ac
TiCl4
AcOH/TFE
RO
RO
11a or 11b
COOH
RO
1a
R=H
1b
R=Ac
Compound 11a was then loaded onto trityl chloride resin (Nankai HECHENG, 0.97 mmol/g) by
procedures similar to those in solution described above. As indicated in Scheme 3, 11a was incubated
with pre-swelled resin (~2 equiv) in CH2Cl2 at room temperature in the presence of DIPEA. The
reaction was quenched by MeOH as soon as the substrate disappeared in solution, as monitored by
TLC. The loading of the substrate was calculated as 0.34 mmol/g (substrate/resin) based on the weight
gain of the dried resin. Treatment of 13 with TiCl4 in CH2Cl2 easily recovered the free OA in 94.6%,

Molecules 2011, 16
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while the treatment with AcOH/TFE could release the amino end from the resin in 93.5% smoothly.
To show the viability of the dual linker for solid phase organic synthesis of derivatives of OA, a model
reaction has been performed. The swelled resin 13 was mixed with excess E-(2-thiophene)acrylic acid,
N,N’-diisopropylcarbodiimide (DIC) and DMAP in DMF. After 1 day, the reaction was terminated and
the modified resin was filtered, dried and subjected to cleavage reactions under the conditions
described above. The final products 15 and 16 were obtained in yields of 92.0% and 89.9%,
respectively. The structures of the cleaved compounds 1a, 11a, 15, and 16 were confirmed by MS,
1H- and 13C-NMR analysis.
Scheme 3. Dual cleavage property of the designed linker on the resin.
Next, we measured and compared the cytotoxicity of two compounds, 1a and 10a, on non-small cell
lung cancer cell line A549 (IC50 51 μM and 50 μM, respectively), and their inhibition activity of
human umbilical vein endothelial cells ECV304 (IC50 20 μM and 10.3 μM, respectively). The results
showed that both activity of these two compounds were on the same level, and indicated that the long
and flexible alkyl ether functionality in the linker is less likely to be critical for the binding event, and
the major binding interaction will come from the compound scaffold. This demonstrates its ability for
application in the subsequent target identification program.

Molecules 2011, 16
4753
Since Merrifield and Tam first introduced the concept of “multidetachable” resins as early as
1979 [13,14], dozens of similar studies have been published [15–19]. Among these, a technique, named
“analytical construct”, for solid phase reaction monitoring has been developed by Geysen et al. [20] and
attracted much attention [21–30]. The construct typically comprises two selective-cleavable in-line
linkers, involving a MS sensitizer (usually an amine) which can greatly improve the ionization
properties of the materials and thus allowing easy detection by electrospray mass spectrometry [31]
and a MS splitter which is an isotope label giving rise to a characteristic split-peak pattern in
MS [20,32,33]. Accordingly, the amino-terminated dual linker designed by us may act as an analytical
construct too, whose reaction monitoring ability is worthy of study in the near future.
3. Experimental
3.1. General
1H- and 13C-NMR spectra were measured on a Varian Mercury-400 or Varian Mercury-300
spectrometer. The mass spectra (MS) were measured on Agilent 1100 LC/MSD high performance ion
trap mass spectrometer or LCQ ESI mass spectrometer.
3.2. Materials
Oleanolic acid was a reference compound (purity > 98%) purchased from the Division of Chinese
Materia Medica and Natural Products, National Institute for the Control of Pharmaceutical and
Biological Products (NICPBP), Ministry of Public Health, China. Trityl chloride resin (loading
capacity 0.97 mmol/g) was purchased from Nankai HECHENG S&T Co. (Tianjing, China). All other
reagents were of standard quality and used without further purification. All solvents were dried before
use through standard procedures.
3.3. Tosylation of Diethyleneglycol (2)
To a stirred solution of diethyleneglycol (2, 0.1 mL, 1.0 mmol) in CH2Cl2 (10 mL) was added
fresh Ag2O (350 mg, 1.5 mmol), TsCl (210 mg, 1.1 mmol), and KI (33 mg, 0.2 mmol). The reaction
mixture was stirred at 0 °C for 4 h, then filtered through a small pad of silica gel, and washed with
EtOAc. Evaporation of the solvent in vacuo, followed by column chromatography on silica gel
(hexane-EtOAc = 1:1), gave the desired monotosylate product 3a (as a colorless oil) and a spot of
ditosylate 3b (also a colorless oil) as a by-product.
1
2-(2-{[(4-Methylbenzene)sulfonyl]oxy}ethoxy)ethan-1-ol (3a) (242.5 mg, 90.6%): H-NMR (300 MHz,
CDCl3): δ 7.78 (2H, d, J = 8.1 Hz, o-H of Ph), 7.33 (2H, d, J = 8.1 Hz, m-H of Ph), 4.17 (2H, t, J = 4.5 Hz,
TsOCH2CH2O-), 3.66 (4H, m, -CH2CH2OCH2CH2-), 3.51 (2H, t, J = 4.5 Hz, -OCH2CH2OH),
2.43 (3H, s, -CH3), 2.18 (1H, s, -OH). 13C-NMR (75 MHz, CDCl3): δ 144.89, 132.87, 129.78 (2C),
127.86 (2C), 72.41, 69.12, 68.47, 61.52, 21.55. ESI-MS: m/z [M + H]+ 261.2, [M + Na]+ 283.1.
2-(2-{[(4-Methylbenzene)sulfonyl]oxy}ethoxy)ethyl 4-methylbenzene-1-sulfonate (3b) (18.2 mg, 4.3%):
1H-NMR (300 MHz, CDCl3): δ 7.77 (4H, d, J = 8.1Hz, o-H of Ph), 7.34 (4H, d, J = 7.8 Hz, m-H of Ph),
4.08 (4H, t, J = 4.5 Hz, TsOCH2CH2O-), 3.59 (4H, t, J = 4.5 Hz, TsOCH2CH2O-), 2.44 (6H, s, -CH3).

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13C-NMR (75 MHz, CDCl3): δ 144.91 (2C), 132.74 (2C), 129.83 (4C), 127.86 (4C), 68.96 (2C),
68.64 (2C), 21.56 (2C). ESI-MS: m/z [M + H]+ 415.1, [M + NH4]+ 432.1, [M + Na]+ 437.1, [M + K]+ 453.1.
3.4. Synthesis of 2-(2-azidoethoxy)ethan-1-ol (4)
To a stirred solution of 3a (75.0 mg, 0.29 mmol) in dry CH3CN (2 mL) was added NaN3 (56.2 mg,
0.87 mmol) and DMF (200 μL). The reaction mixture was heated to reflux for 9 h, then another batch
of NaN3 (56.2 mg, 0.87 mmol) was added to the system. After another 20 h, the reaction was quenched
with water (2 mL), and extracted with CH2Cl2. The organic layer was dried over Na2SO4 and
concentrated in vacuo to afford product 4 (36.4 mg, 96.2%) as a colorless oil. This compound was used
1
in the next step without further purification. H-NMR (300 MHz, CDCl3): δ 3.76 (2H, t, J = 4.2 Hz,
-OCH2CH2OH), 3.70 (2H, t, J = 4.8Hz, N3CH2CH2O-), 3.62 (2H, t, J = 4.2Hz, -OCH2CH2OH),
13
3.41 (2H, t, J = 4.8 Hz, N3CH2CH2O-), 2.05 (1H, br s, -OH). C-NMR (75 MHz, CDCl3): δ 72.38,
70.09, 61.78, 50.70. ESI-MS: m/z [M + NH4]+ 149.0.
3.5. Synthesis of 2-(2-aminoethoxy)ethan-1-ol (5)
A suspension of compound 4 (30 mg, 0.23 mmol), 10% Pd/C (10 mg) in MeOH (1 mL) was stirred
under 1 atm of hydrogen pressure for 8 h, and then filtered. The filtrate was concentrated in vacuo to
afford product 5 (23.6 mg, 98.3%) as a colorless oil. This compound was used in the next step without
further purification. 1H-NMR (300 MHz, CDCl3): δ 3.64 (2H, t, J = 4.5Hz, -OCH2CH2OH), 3.49 (2H,
t, J = 4.5 Hz, -OCH2CH2OH), 3.46 (2H, t, J = 5.1 Hz, H2NCH2CH2O-), 2.87 (3H, br s, -NH2 and -OH),
2.81 (2H, t, J = 5.1 Hz, H2NCH2CH2O-). 13C-NMR (75 MHz, CDCl3): δ 72.45 (2C), 61.16, 41.34.
ESI-MS: m/z [M + H]+ 106.1, [M + Na]+ 128.1.
3.6. Synthesis of 2-[2-(2-hydroxyethoxy)ethyl]-2,3-dihydro-1H-isoindole-1,3-dione (7)
Compound 5 (1.05 g, 10 mmol) and phthalic anhydride (1.53 g, 10.1 mmol) were mixed together
and stirred until the system become clear. Then the mixture was heated to 100 °C and maintained for
1.5 h. After cooling to room temperature, CHCl3 (15 mL) and water (6 mL) were added. The organic
layer was dried over Na2SO4 and concentrated in vacuo to afford product 7 (2.17 g, 92.3%) as a white
1
solid. This compound was used in the next step without further purification. H-NMR (300 MHz,
CDCl3): δ 7.81 (2H, m, Ph), 7.69 (2H, m, Ph), 3.88 (2H, t, J = 5.4Hz, -NCH2CH2O-), 3.72 (2H, t,
J = 5.4 Hz, -NCH2CH2O-), 3.66 (2H, t, J = 4.2Hz, -OCH2CH2OH), 3.57 (2H, t, J = 4.2Hz,
-OCH2CH2OH), 2.51 (1H, br s, -OH). 13C-NMR (75 MHz, CDCl3): δ 168.37 (2C), 133.95 (2C),
131.93 (2C), 123.23 (2C), 72.13, 68.27, 61.63, 37.47. ESI-MS: m/z [M + H]+ 236.2, [M + Na]+ 258.1,
[M + K]+ 274.1.

Molecules 2011, 16
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3.7. Synthesis of 2-(2-{2-[(methylsulfanyl)methoxy]ethoxy}ethyl)-2,3-dihydro-1H-isoindole-1,3-dione (8)
To a stirred solution of 7 (2.17 g, 9.2 mmol) in DMSO (27.1 mL) was added AcOH (5.4 mL,
94.4 mmol) and Ac2O (17.9 mL, 191.4 mmol). The reaction mixture was stirred at room temperature
for 20 h, then poured into a cold water (271 mL) solution of Na2CO3 (27.1 g). The mixture was
extracted with CH2Cl2. The organic layer was dried over Na2SO4 and concentrated in vacuo. The
residue was purified by column chromatography on silica gel (hexane-EtOAc = 5:1) to give the
1
product 8 (2.28 g, 83.7%) as a viscous oil. H-NMR (300 MHz, CDCl3): δ 7.82 (2H, m, Ph),
7.69 (2H, m, Ph), 4.58 (2H, s, -SCH2O-), 3.89 (2H, t, J = 6.0 Hz, -NCH2CH2O-), 3.73 (2H, t,
13
J = 6.0 Hz, -NCH2CH2O-), 3.63 (4H, br s, -OCH2CH2O-), 2.07 (3H, s, -SCH3). C-NMR (75 MHz,
CDCl3): δ 168.17 (2C), 133.84 (2C), 132.07 (2C), 123.16 (2C), 75.32, 69.74, 67.80, 66.91, 37.16,
13.64. ESI-MS: m/z [M + H]+ 296.4, [M + Na]+ 318.4.
3.8. Synthesis of {2-[2-(1,3-dione-2,3-dihydro-1H-isoindol-2-yl)ethoxy]ethoxy}methyl olean-12-en-28-
oate (10a)
To a stirred solution of 8 (51 mg, 0.17 mmol) in dry CH2Cl2 (1.0 mL) was added SOCl2 (12.3 μL,
0.17 mmol) at 0 °C. The reaction was warmed to room temperature and stirred for 2 h before it was
evaporated under reduced pressure at 30 °C to remove the excess SOCl2. The crude product 2-{2-[2-
(chloromethoxy)ethoxy]ethyl}-2,3-dihydro-1H-isoindole-1,3-dione (9) could be used in the next step
without further purification. The residue mentioned above dissolved in dry CH2Cl2 (0.5 mL) was
added to a solution of the mixture of oleanolic acid (1a, 21.2 mg, 0.046 mmol) and DIPEA (21.3 μL,
0.13 mmol) in dry CH2Cl2 (0.5 mL). The mixture was stirred at room temperature for 2 h before it was
diluted with CH2Cl2 (10 mL) and washed with a saturated solution of NaHCO3. The organic layer was
dried over Na2SO4 and concentrated in vacuo. The residue was purified by column chromatography on
1
silica gel (hexane-EtOAc = 3:1) to give the product 10a (31.8 mg, 97.2%) as a white solid. H-NMR
(400 MHz, CDCl3): δ 7.84 (2H, m, Ph), 7.72 (2H, m, Ph), 5.28 (1H, s, H-12), 5.24–5.18 (2H, ABq,
J = 6.0 Hz, -COOCH2O-), 3.90 (2H, t, J = 6.0 Hz, -NCH2CH2O-), 3.74 (4H, m, -NCH2-,
-OCH2CH2OCH2O-), 3.65 (2H, m,-OCH2CH2OCH2O-), 3.21 (1H, dd, J = 10.8, 4.4Hz, H-3α),
2.85 (1H, d, J = 10.8 Hz, H-18), 1.12 (3H, s, CH3), 0.98 (3H, s, CH3), 0.91 (3H, s, CH3),
0.89 (6H, s, CH3), 0.78 (3H, s, CH3), 0.72 (3H, s, CH3). 13C-NMR (100 MHz, CDCl3): δ 177.07,
168.20 (2C), 143.61, 133.91 (2C), 132.12 (2C), 123.22 (2C), 122.47, 89.37, 78.98, 69.62, 69.47, 68.00,
55.21, 47.59, 46.85, 45.83, 41.69, 41.17, 39.32, 38.73, 38.43, 37.15, 37.00, 33.81, 33.06, 32.74,
32.27, 30.66, 28.09, 27.58, 27.19, 25.81, 23.59, 23.39, 22.91, 18.31, 16.99, 15.56, 15.30. ESI-MS:
m/z [M + Na]+ 726.4, [M + K]+ 742.4, [M + Cl]− 738.6.
3.9. Synthesis of [2-(2-aminoethoxy)ethoxy]methyl olean-12-en-28oate (11a)
A mixture of 10a (13.5 mg, 0.020 mmol) and a solution of methylamine in ethanol (0.7 mL, 33 wt. %)
was stirred at room temperature for 0.5 h before it was concentrated in vacuo. The residue was purified
by column chromatography on silica gel (CHCl3-MeOH = 10:1) to give the product 11a (8.6 mg,
1
78.2%) as a white solid. H-NMR (400 MHz, CDCl3): δ 5.32–5.26 (3H, m, H-12, -COOCH2O-),
3.78 (2H, t, J = 4.4 Hz, -NCH2CH2O-), 3.62 (2H, t, J = 4.8 Hz, -OCH2CH2OCH2O-), 3.52 (2H, t,

Molecules 2011, 16
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J = 5.2 Hz, -OCH2CH2OCH2O-), 3.21 (1H, dd, J = 12.4, 4.0 Hz, H-3α), 2.89-2.85 (3H, m, H-18,
-NCH2CH2O-), 1.14 (3H, s, CH3), 0.98 (3H, s, CH3), 0.92 (3H, s, CH3), 0.90 (6H, s, CH3),
13
0.78 (3H, s, CH3), 0.74 (3H, s, CH3). C-NMR (75 MHz, CDCl3): δ 177.13, 143.49, 122.44, 89.21,
78.74, 72.21, 69.84, 69.29, 55.11, 47.48, 46.81, 45.71, 41.62, 41.22, 41.10, 39.24, 38.64, 38.37, 36.90,
33.71, 32.95, 32.66, 32.23, 30.57, 28.02, 27.50, 27.04, 25.73, 23.50, 23.30, 22.83, 18.22, 16.94, 15.53,
15.23. ESI-MS: m/z [M + H]+ 574.5, [M + Na]+ 596.7.
3.10. Synthesis of (2-{2-[(triphenylmethyl)amino]ethoxy}methyl olean-12-en-28-oate (12a)
To a solution of 11a (63.4 mg, 0.11 mmol) in CH2Cl2 (2.0 mL) was added DIPEA (91.3 μL,
0.55 mmol) and trityl chloride (92.1 mg, 0.33 mmol). The reaction mixture was stirred at room
temperature for 3 h before it was concentrated in vacuo. The residue was purified by column
chromatography on silica gel (hexane-EtOAc = 6:1) to afford product 12a (77.0 mg, 85.4%) as a white
solid. 1H-NMR (400 MHz, CDCl3): δ 7.47 (6H, d, J = 7.6Hz, o-H of Ph), 7.27 (6H, t, J = 7.6 Hz, m-H
of Ph), 7.18 (3H, t, J = 7.2 Hz, p-H of Ph), 5.30-5.22 (3H, m, H-12, -COOCH2O-), 3.73 (2H, t,
J = 4.8 Hz, -NCH2CH2O-), 3.60 (2H, t, J = 5.2 Hz, -OCH2CH2OCH2O-), 3.53 (2H, t, J = 4.8 Hz,
-OCH2CH2OCH2O-), 3.21 (1H, dd, J = 10.8, 4.4 Hz, H-3α), 2.86 (1H, dd, J = 13.6, 4.0 Hz, H-18),
2.36 (2H, br s, -NCH2CH2O-), 1.13 (3H, s, CH3), 0.98 (3H, s, CH3), 0.91 (3H, s, CH3), 0.90 (3H, s, CH3),
0.89 (3H, s, CH3), 0.77 (3H, s, CH3), 0.72 (3H, s, CH3). 13C-NMR (75 MHz, CDCl3): δ 177.04,
145.99 (3C), 143.53, 128.58 (6C), 127.69 (6C), 126.13 (3C), 122.41, 89.33, 78.83, 71.31, 70.51, 69.66,
69.31, 55.11, 47.50, 46.78, 45.74, 42.95, 41.62, 41.10, 39.24, 38.64, 38.34, 36.92, 33.71, 32.98, 32.68,
32.20, 30.57, 28.02, 27.52, 27.09, 25.75, 23.50, 23.30, 22.83, 18.22, 16.94, 15.50, 15.23. ESI-MS:
m/z [M + H]+ 816.7.
3.11. Cleavage at Site A of 12a with TiCl4
To a solution of 12a (10.6 mg, 0.013 mmol) in CH2Cl2 (1.0 mL) was added TiCl4 (5 μL,
0.045 mmol) dropwise. The reaction mixture was stirred at room temperature for 0.5 h before it was
poured into water (1 mL) and extracted with CH2Cl2. The organic layer was washed with a saturated
solution of NaHCO3, dried over Na2SO4 and concentrated in vacuo. The residue was purified by
column chromatography on silica gel (hexane-acetone = 5:1) to afford product 1a (5.3 mg, 89.6%) as a
1
white solid. H-NMR (300 MHz, pyridine-d5): δ 5.49 (1H, br s, H-12), 3.44 (1H, m, H-3α),
3.29 (1H, dd, J = 13.5, 3.6 Hz, H-18), 1.29 (3H, s, CH3), 1.24 (3H, s, CH3), 1.02 (9H, s, CH3),
0.95 (3H, s, CH3), 0.90 (3H, s, CH3). 13C-NMR (75 MHz, pyridine-d5): δ 180.20, 144.88, 122.61,
78.13, 55.88, 48.18, 46.72, 46.55, 42.23, 42.06, 39.82, 39.44, 39.01, 37.44, 34.29, 33.35 (2C), 33.24,
31.03, 28.85, 28.39, 28.16, 26.25, 23.89, 23.85, 23.76, 18.86, 17.49, 16.62, 15.63. ESI-MS: m/z
[M−H]− 455.3.
3.12. Cleavage at Site B of 12a with TFE
A mixture of 12a (25.0 mg, 0.031 mmol) and a solution of AcOH/TFE/CH2Cl2 (1.0 mL,
v/v/v = 1:2:7) was stirred at room temperature for 2 h before it was quenched with a saturated solution
of NaHCO3 (0.4 mL). The water layer was extracted with CH2Cl2, dried over Na2SO4 and concentrated

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in vacuo. The residue was purified by column chromatography on silica gel (CHCl3-MeOH = 10:1) to
afford product 11a (16.5 mg, 94.0%) as a white solid.
3.13. Synthesis of {2-[2-(1,3-dione-2,3-dihydro-1H-isoindol-2-yl)ethoxy]ethoxy}methyl 3β-acetoxylolean-
12-en-28-oate (10b)
To a stirred solution of 8 (59 mg, 0.20 mmol) in dry CH2Cl2 (1.0 mL) was added SOCl2 (14.4 μL,
0.20 mmol) at 0 °C. The reaction was warmed to room temperature and stirred for 2 h before it was
evaporated under reduced pressure at 30 °C to remove the excess SOCl2. The crude product 9 could be
used in the next step without further purification. The crude product dissolved in dry CH2Cl2 (0.5 mL)
was added to a solution of the mixture of 3β-acetoxyl-OA (1b, 22.9 mg, 0.046 mmol) and DIPEA
(21.3 μL, 0.13 mmol) in dry CH2Cl2 (0.5 mL). The mixture was stirred at room temperature for 2 h
before it was diluted with CH2Cl2 (10 mL) and washed with a saturated solution of NaHCO3. The
organic layer was dried over Na2SO4 and concentrated in vacuo. The residue was purified by column
chromatography on silica gel (hexane-EtOAc = 6:1) to give the product 10b (32.7 mg, 95.4%) as a
white solid. 1H-NMR (300 MHz, CDCl3): δ 7.82 (2H, m, Ph), 7.69 (2H, m, Ph), 5.25 (1H, br s, H-12),
5.23–5.15 (2H, ABq, J = 6.0 Hz, -COOCH2O-), 4.46 (1H, t, J = 8.1 Hz, H-3α), 3.88 (2H, t,
J = 5.7 Hz, -NCH2CH2O-), 3.70 (4H, m, -NCH2-,-OCH2CH2OCH2O-), 3.62 (2H, t, J = 4.5 Hz,
-OCH2CH2OCH2O-), 2.82 (1H, m, H-18), 2.02 (3H, s, CH3 of acetyl), 1.09 (3H, s, CH3), 0.89 (6H, s,
13
CH3), 0.87 (3H, s, CH3), 0.84 (3H, s, CH3), 0.83 (3H, s, CH3), 0.70 (3H, s, CH3). C-NMR (75MHz,
CDCl3): δ 176.99, 170.90, 168.12 (2C), 143.56, 133.86 (2C), 132.06 (2C), 123.16 (2C), 122.33, 89.27,
80.80, 69.55, 69.40, 67.93, 55.22, 47.43, 46.76, 45.73, 41.60, 41.10, 39.27, 38.03, 37.59, 37.07, 36.83,
33.74, 33.00, 32.60, 32.19, 30.58, 27.96, 27.49, 25.70, 23.53, 23.44, 23.32, 22.83, 21.23, 18.13, 16.91,
16.62, 15.29. ESI-MS: m/z [M + NH4]+ 763.5, [M + Na]+ 768.5.
3.14. Synthesis of [2-(2-aminoethoxy)ethoxy]methyl 3β-acetoxylolean-12-en-28-oate (11b)
A mixture of 10b (129 mg, 0.17 mmol) and a solution of methylamine in ethanol (6.0 mL, 33 wt. %)
was stirred at room temperature for 0.5 h before it was concentrated in vacuo. The residue was purified
by column chromatography on silica gel (CHCl3-MeOH = 10:1) to give the product 11b (86.9 mg,
82.0%) as a white solid. 1H-NMR (300 MHz, CDCl3): δ 5.28 (3H, m, H-12, -COOCH2O-), 4.48 (1H, t,
J = 7.8 Hz, H-3α), 4.00 (2H, br s, -NH2), 3.77 (2H, m, -NCH2CH2O-), 3.63 (4H, m, -OCH2CH2O-),
3.02 (2H, m, -NCH2CH2O-), 2.86 (1H, m, H-18), 2.04 (3H, s, CH3 of acetyl), 1.13 (3H, s, CH3),
0.92 (6H, s, CH3), 0.90 (3H, s, CH3), 0.86 (3H, s, CH3), 0.85 (3H, s, CH3), 0.73 (3H, s, CH3).
13C-NMR (75 MHz, CDCl3): δ 177.05, 170.87, 143.47, 122.29, 89.15, 80.71, 72.56, 69.80, 69.26,
55.11, 47.34, 46.75, 45.62, 41.54, 41.31, 41.02, 39.18, 37.94, 37.51, 36.75, 33.67, 32.93, 32.54, 32.17,
30.54, 27.90, 27.42, 25.65, 23.47, 23.35, 23.26, 22.77, 21.20, 18.05, 16.88, 16.56, 15.26. ESI-MS: m/z
[M + H]+ 616.4.
3.15. Synthesis of (2-{2-[(triphenylmethyl)amino]ethoxy}methyl 3β-acetoxylolean-12-en-28-oate (12b)
To a solution of 11b (82.8 mg, 0.14 mmol) in CH2Cl2 (2.0 mL) was added DIPEA (155.5 μL,
0.95 mmol) and trityl chloride (188.3 mg, 0.68 mmol). The reaction mixture was stirred at room

Molecules 2011, 16
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temperature for 3 h before it was concentrated in vacuo. The residue was purified by column
chromatography on silica gel (hexane-EtOAc = 12:1) to afford product 12b (82.3 mg, 71.3%) as a
white solid. 1H-NMR (300 MHz, CDCl3): δ 7.49 (6H, d, J = 7.5 Hz, o-H of Ph), 7.27 (6H, t, J = 7.2
Hz, m-H of Ph), 7.18 (3H, t, J = 7.2 Hz, p-H of Ph), 5.26 (3H, m, H-12, -COOCH2O-), 4.51 (1H, t,
J = 7.8 Hz, H-3α), 3.74 (2H, t, J = 4.5 Hz, -NCH2CH2O-), 3.61 (2H, t, J = 4.8 Hz, -OCH2CH2OCH2O-),
3.53 (2H, t, J = 4.8 Hz, -OCH2CH2OCH2O-), 2.88 (1H, dd, J = 13.5, 3.3 Hz, H-18), 2.37 (2H, t,
J = 4.8 Hz, -NCH2CH2O-), 2.05 (3H, s, CH3 of acetyl), 1.15 (3H, s, CH3), 0.93 (3H, s, CH3),
0.92 (3H, s, CH3), 0.89 (3H, s, CH3), 0.88 (3H, s, CH3), 0.87 (3H, s, CH3), 0.75 (3H, s, CH3).
13C-NMR (75 MHz, CDCl3): δ 177.05, 170.90, 146.04 (3C), 143.58, 128.61 (6C), 127.72 (6C), 126.18
(3C), 122.38, 89.36, 80.82, 71.35, 70.57, 69.70, 69.34, 55.23, 47.47, 46.82, 45.74, 42.99, 41.65, 41.15,
39.30, 38.06, 37.62, 36.86, 33.76, 33.01, 32.64, 32.25, 30.61, 27.99, 27.53, 25.73, 23.55, 23.46, 23.35,
22.86, 21.23, 18.16, 16.97, 16.63, 15.32. ESI-MS: m/z [M + H]+ 858.5, [M + Na]+ 880.5.
3.16. Cleavage at Site A of 12b with TiCl4
To a solution of 12b (25.0 mg, 0.030 mmol) in CH2Cl2 (1.0 mL) was added TiCl4 (22 μL,
0.20 mmol) dropwise. The reaction mixture was stirred at room temperature for 0.5 h before it was
poured into water (1 mL) and extracted with CH2Cl2. The organic layer was washed with a saturated
solution of NaHCO3, dried over Na2SO4 and concentrated in vacuo. The residue was purified by
column chromatography on silica gel (hexane-EtOAc = 4:1) to afford product 1b (12.0 mg, 82.8%) as
1
a white solid. H-NMR (300 MHz, CDCl3): δ 5.27 (1H, br s, H-12), 4.49 (1H, t, J = 7.8 Hz, H-3α),
2.81 (1H, dd, J = 13.5, 4.2 Hz, H-18), 2.04 (3H, s, CH3 of acetyl), 1.12 (3H, s, CH3), 0.94 (3H, s, CH3),
0.92 (3H, s, CH3), 0.90 (3H, s, CH3), 0.86 (3H, s, CH3), 0.85 (3H, s, CH3), 0.74 (3H, s, CH3).
13C-NMR (75 MHz, CDCl3): δ 184.15, 171.01, 143.58, 122.53, 80.89, 55.27, 47.53, 46.52, 45.80,
41.51, 40.87, 39.25, 38.03, 37.67, 36.96, 33.76, 33.04, 32.49, 32.42, 30.65, 28.02, 27.64, 25.88, 23.56,
23.50, 23.36, 22.85, 21.29, 18.14, 17.15, 16.63, 15.37. ESI-MS: m/z [M−H]− 497.3.
3.17. Cleavage at Site B of 12b with TFE
A mixture of 12b (26.6 mg, 0.031 mmol) with a solution of AcOH/TFE/CH2Cl2 (1.0 mL,
v/v/v = 1:2:7) was stirred at room temperature for 2 h before it was quenched with a saturated solution
of NaHCO3 (0.4 mL). The water layer was extracted with CH2Cl2, dried over Na2SO4 and concentrated
in vacuo. The residue was purified by column chromatography on silica gel (CHCl3-MeOH = 10:1) to
afford product 11b (14.3 mg, 92.7%) as a white solid.
3.18. Loading of 11a onto Trityl Chloride Resin
The trityl chloride resin (332.4 mg, 0.32 mmol) was swelled in CH2Cl2 (1 mL) for 1 h. Then a
solution of 11a (98.8 mg, 0.17 mmol) was added to the system, followed by DIPEA (177.8 μL,
1.21 mmol) dropwise. The reaction mixture was stirred at room temperature for 16 h. The system was
treated with MeOH (1 mL) to cap unloaded resin. The resin was filtered and washed sequentially with
CH2Cl2, DMF, CH2Cl2, diethyl ether and dried in vacuo for 24 h, affording resin 13 (406.2 mg). The

Molecules 2011, 16
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loading of the substrate was calculated as about 0.34 mmol/g (substrate/resin) based on the weight gain
of the resin.
3.19. Acidic Cleavage at Site A of Resin 13 with TiCl4
Resin 13 (36.0 mg, 0.012 mmol) was swelled in CH2Cl2 (1 mL) for 1 h. TiCl4 (8.2 μL, 0.060 mmol)
was added dropwise. The reaction mixture was stirred at room temperature for 0.5 h before the resin
was filtered and washed with MeOH and CH2Cl2. The filtrate was concentrated in vacuo to afford
product 1a (5.3 mg, 94.6%) without any by-products.
3.20. Acidic Cleavage at Site B of Resin 13 with TFE
Resin 13 (39.5 mg, 0.013 mmol) was mixed with a solution of AcOH/TFE/CH2Cl2 (1.0 mL,
v/v/v = 1:2:7) and the mixture was stirred at room temperature for 2 h before the resin was filtered and
washed with MeOH and CH2Cl2. The filtrate was concentrated in vacuo, and the residue was purified
by column chromatography on silica gel (CHCl3-MeOH = 10:1) to afford product 11a (7.2 mg, 93.5%).
3.21. Coupling Reaction of (E)-(2-thiophene)acrylic Acid with Resin 13
Resin 13 (88.8 mg, 0.030 mmol) was swelled in DMF (1 mL) for 3 h. Then, (E)-(2-thiophene)acrylic
acid (268.7 mg, 1.74 mmol), DMAP (21.3 mg, 0.17 mmol) and DIC (381.9 μL, 2.44 mmol) were
added to the system. The reaction mixture was stirred at room temperature for 24 h before the resin
was filtered, washed sequentially with DMF, MeOH, CH2Cl2, diethyl ether and dried in vacuo for 24 h,
affording resin 14 (92.6 mg). The conversion of the substrate was calculated as 93.3% based on the
weight gain of the resin.
3.22. Acidic Cleavage at Site A of Resin 14 with TiCl4
Resin 14 (42.3 mg, 0.0137 mmol) was swelled in CH2Cl2 (1 mL) for 1 h. TiCl4 (9.6 μL, 0.085 mmol)
was added dropwise. The reaction mixture was stirred at room temperature for 0.5 h before the resin
was filtered and washed with MeOH and CH2Cl2. The filtrate was concentrated in vacuo, and the
residue was purified by column chromatography on silica gel (hexane-acetone = 10:1) to afford
product 15 (6.4 mg, 92.0%) and unreacted substrate 1a (0.5 mg).
1
3β-[(E)-(2-Thiophene)acryloxyl]-12-en-28-oate (15): H-NMR (400 MHz, CDCl3): δ 7.76 (1H, d,
J = 15.6 Hz, -CH = CHCO-), 7.36 (1H, d, J = 4.8 Hz, H-3 of thiophene), 7.25 (1H, d, J = 3.2 Hz,
H-5 of thiophene), 7.05 (1H, t, J = 4.0 Hz, H-4 of thiophene), 6.23 (1H, d, J = 16.0 Hz, -CH = CHCO-),
5.29 (1H, br s, H-12), 4.63 (1H, t, J = 8.0 Hz, H-3α), 2.83 (1H, dd, J = 10.4, 3.2 Hz, H-18),
1.14 (3H, s, CH3), 0.97 (3H, s, CH3), 0.93 (6H, s, CH3), 0.91 (6H, s, CH3), 0.77 (3H, s, CH3).
13C-NMR (100 MHz, CDCl3): δ 183.78, 166.67, 143.59, 139.68, 136.75, 130.70, 128.23, 128.01,
122.57, 117.60, 80.98, 55.33, 47.55, 46.53, 45.83, 41.56, 40.92, 39.28, 38.09, 37.94, 37.00, 33.78,
33.05, 32.53, 32.43, 30.66, 28.09, 27.66, 25.91, 23.63, 23.57, 23.40, 22.88, 18.19, 17.17, 16.82, 15.40.
ESI-MS: m/z [M + Cl]− 591.4.

Molecules 2011, 16
4760
3.23. Acidic Cleavage at Site B of Resin 14 with TFE
Resin 14 (42.6 mg, 0.0138 mmol) was mixed with a solution of AcOH/TFE/CH2Cl2 (1.0 mL,
v/v/v = 1:2:7). And the mixture was stirred at room temperature for 2 h before the resin was filtered
and washed with MeOH and CH2Cl2. The filtrate was concentrated in vacuo, and the residue was
purified by column chromatography on silica gel (CHCl3-MeOH = 10:1) to afford product 16
(7.7 mg, 89.1%) and unreacted substrate 11a (0.8 mg).
1
[2-(2-Aminoethoxy)ethoxy]methyl 3β-[(E)-(2-thiophene)acryloxyl]-12-en-28-oate (16): H-NMR (300
MHz, CDCl3): δ 7.75 (1H, d, J = 15.6Hz, -CH = CHCO-), 7.35 (1H, d, J = 4.8 Hz, H-3 of thiophene),
7.23 (1H, d, J = 2.7 Hz, H-5 of thiophene), 7.03 (1H, t, J = 3.9 Hz, H-4 of thiophene), 6.23 (1H, d,
J = 15.9 Hz, -CH = CHCO-), 5.28 (3H, m, H-12, -COOCH2O-), 4.61 (1H, t, J = 7.8 Hz, H-3α), 4.32
(2H, br s, -NH2), 3.78 (2H, br s, -NCH2CH2O-), 3.63 (4H, br s, -OCH2CH2O-), 2.99 (2H, br s,
-NCH2CH2O-), 2.86 (1H, d, J = 11.1 Hz, H-18), 1.14 (3H, s, CH3), 0.94 (3H, s, CH3), 0.92 (6H, s,
13
CH3), 0.90 (6H, s, CH3), 0.74 (3H, s, CH3). C-NMR (75 MHz, CDCl3): δ 177.21, 166.63, 143.58,
139.64, 136.73, 130.68, 128.22, 128.00, 122.44, 117.57, 89.24, 80.91, 71.03, 69.95, 69.35, 55.30, 47.48,
46.89, 45.76, 41.70, 41.18, 39.33, 38.11, 37.90, 36.90, 33.79, 33.03, 32.68, 32.31, 30.65 (2C), 28.07,
27.58, 25.76, 23.58 (2C), 23.39, 22.91, 18.21, 17.03, 16.83, 15.38. ESI-MS: m/z [M + H]+ 710.4.
3.24. Cytotoxicity Assay
Cells grown in 96-well plates were treated with gradient concentrations of each compound for 48 h.
Cells were fixed with 50% trichloroacetic acid and stained with 0.4% sulforhodamine B dissolved
in 1% acetic acid. Cells were then washed with 1% acetic acid to remove unbound dye. The
protein-bound dye was extracted with 10 mM Tris base to determine the optical density at 564 nm
wavelength using a SPECTRAmax PLUS 384 microplate spectrophotometer (Molecular Devices,
Sunnyvale, CA, USA). The percentage of cell survival as a function of drug concentration was plotted
to determine the IC50 value, which stands for the drug concentration needed to kill cells by 50%.
3.25. Cell Migration Assay
Human umbilical vein endothelial cells grown in 24-well plates as confluent monolayers were
mechanically scratched using a 20 µL pipette tip to create the wound. Cells were washed with
phosphate-buffered saline to remove the debris, and complete culture media were then added to allow
wound healing. Phase contrast images of the wound were taken at three random locations first
immediately after wounding and then at the same location after 24 h to examine wound closure by
migrating cells. Cells migrated into the wound area were then counted, and the extent of wound
closure was determined.
4. Conclusions
In summary, a hydrophilic amino-terminated poly(ethylene glycol)-type dual linker for the solid
phase synthesis of oleanolic acid derivatives using trityl chloride resin was designed and synthesized
for the first time. Although the sensitivity of the linker to acids limits its applications, careful planning

Molecules 2011, 16
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of the synthetic approach may overcome this problem. Following the successful solid-phase synthesis
of model compounds 15 and 16, the preparation of an OA analog library and the potential of this dual
linker in reaction monitoring and target identification are under investigation in our lab.
Acknowledgments
This work was supported by the Beijing Natural Science Foundation (Grant No. 7102114) and the
Foundation subsidized by the Ministry of Science and Technology of the People’s Republic of China
(Grant No. 2009ZX09311-004, 2009ZX09301-003-4-1). The authors thank Jun Zhou (Nankai
University, China) for working on the bioactivity measurement.
Conflict of Interest
The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds are available from the authors.
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