Expedient Synthesis of Alphitolic Acid and Its Naturally Occurring 2- O-Ester Derivatives

Article abstract of DOI:10.1021/acs.jnatprod.8b00986

The expedient synthesis of alphitolic acid (1) as well as its natural C-3-epimer and 2-O-ester derivatives was accomplished in a few steps from the readily commercially available betulin (9). A Rubottom oxidation delivered an α-hydroxy group in a stereo- and chemoselective manner. The diastereoselective reduction of the α-hydroxy ketone was key to accessing the 1,2-diol moiety of this class of natural products. Our concise and stereoselective synthetic protocol allowed the gram-scale synthesis of these natural products, which will facilitate future biological evaluations.

Full text of DOI:10.1021/acs.jnatprod.8b00986

Article
pubs.acs.org/jnp
Cite This: J. Nat. Prod. XXXX, XXX, XXX−XXX
Expedient Synthesis of Alphitolic Acid and Its Naturally Occurring
2‑O‑Ester Derivatives
Somin Park, Jihee Cho, Hongjun Jeon,* Sang Hyun Sung, Seunghee Lee, and Sanghee Kim*
College of Pharmacy, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea
S
* Supporting Information
ABSTRACT: The expedient synthesis of alphitolic acid (1)
as well as its natural C-3-epimer and 2-O-ester derivatives was
accomplished in a few steps from the readily commercially
available betulin (9). A Rubottom oxidation delivered an α-
hydroxy group in a stereo- and chemoselective manner. The
diastereoselective reduction of the α-hydroxy ketone was key
to accessing the 1,2-diol moiety of this class of natural
products. Our concise and stereoselective synthetic protocol
allowed the gram-scale synthesis of these natural products,
which will facilitate future biological evaluations.
broad spectrum of interesting biological effects, including anti-
inflammatory,2 antiparasitic,3 antimicrobial,4 and cytotoxic
activities,5 there has been a growing interest in this compound
as a lead compound. Recently, a number of its 2-O-ester
derivatives including 2−7 have been isolated by our colleague,
the late Prof. S. H. Sung6 and others.7 These compounds
showed interesting biological activities including antiplasmo-
dial,8 anti-inflammatory,7b,9 and cytotoxic activities.6,7 To
facilitate further biological investigation10 and the evaluation
of preliminary structure−activity relationships, we developed
an efficient and scalable synthetic route to alphitolic acid and
its natural O-ester derivatives as well as their structural
analogues. In this article, we report our synthetic studies on
these triterpenoids.
lphitolic acid (1, Figure 1) is a pentacyclic triterpenoid
Awith a lupane skeleton. This triterpene has been found in
various plant species and was first isolated from Alphitonia
whitei Braid (Rhamnaceae).1 Since alphitolic acid displays a
RESULTS AND DISCUSSION
■
Structurally, alphitolic acid (1) is a 2-hydroxylated derivative of
betulinic acid (8), which is a natural triterpenoid generally
found in the bark of the white birch tree and several other
plants.4,11 On the one hand, however, betulinic acid (8) is
isolated in low yield12 and is thus rather expensive. On the
other hand, its reduced congener betulin (9) is inexpensive and
commercially available in large quantities. Therefore, betulin
was considered as a suitable starting material for the synthesis
of alphitolic acid (1) and its derivatives. In fact, alphitolic acid
has been previously synthesized from betulin, but the process
was quite lengthy at 13 steps.13 Since the high number of steps
limits the application of this synthetic method to the
preparation of alphitolic acid derivatives, we sought to design
a more concise and efficient method.
The retrosynthetic plan used is depicted in Scheme 1. It was
planned to synthesize alphitolic acid (1) and its O-ester
Figure 1. Structures of alphitolic acid (1), its 2-O-ester natural
derivatives 2−7, and congeners 8 and 9.
Received: November 22, 2018
© XXXX American Chemical Society and
American Society of Pharmacognosy
A
DOI: 10.1021/acs.jnatprod.8b00986
J. Nat. Prod. XXXX, XXX, XXX−XXX

Journal of Natural Products
Article
Scheme 1. Retrosynthetic Route toward Alphitolic Acid (1)
and its 2-O-Ester Derivatives 2−7
Scheme 2. Synthesis of 2-Hydroxy Ketones 10 and 14 as
Substrates for Diastereoselective Reduction
a
derivatives 2−7 from 10 via diastereoselective reduction.
Compound 10 was envisioned to be synthesized from the
triterpene natural product betulonic acid (11) via 2α-
hydroxylation. Previously, the introduction of the 2-hydroxy
group in structurally related triterpenoids was accomplished by
a α-bromination-hydroxylation sequence from the correspond-
ing 3-ketones14 and an oxidative hydroxylation of 3-ketones or
their enol acetate.15 However, the application of these methods
to betulin-type triterpenoids is hindered by the presence of an
oxidatively vulnerable isopropylene group.16 To overcome this
problem, it was decided to utilize a Rubottom-type oxidation
for the 2α-hydroxylation of 11. Betulonic acid (11) was
reported to be readily accessible from betulin (9) through
oxidation.17
a
Reagents and conditions: (a) Jones reagent, acetone, 0 °C, 2 h, 72%;
(b) trimethylsilyl trifluoromethanesulfonate, Et3N, CH2Cl2, −78 °C, 1
h; (c) (i) m-chloroperoxybenzoic acid, NaHCO3, CH2Cl2, 0 °C, 3 h;
(ii) oxalic acid, MeOH, room temperature (rt), 10 min, 62% (3
steps); (d) tert-butyldimethylsilyl trifluoromethanesulfonate, Et3N,
CH2Cl2, −78 °C, 1.5 h, 89%; (e) m-chloroperoxybenzoic acid,
Na2HPO4, CH2Cl2, rt, 5 h, 83%; (f) tetrabutylammonium fluoride,
THF, reflux, 3 h, 83%.
The synthesis began with Jones oxidation of betulin (9) to
give betulonic acid (11) in high yield (Scheme 2).17b For the
formation of electron-rich silyl enol ethers, 11 was treated with
2 equiv of trimethylsilyl trifluoromethanesulfonate at low
temperature. A Rubottom oxidation18,19 of crude silyl enol
ether 12 with m-chloroperoxybenzoic acid at 0 °C followed by
removal of the trimethylsilyl group using oxalic acid yielded α-
hydroxyketone 10 in 62% overall yield (three steps). Only the
desired α-isomer was formed possibly through epoxidation
from the less hindered α-face. In this transformation, an
additional step for the protection of the carboxylic acid
functional group was not necessary.
To obtain alphitolic acid (1) by conversion of the ketone
group of 10 to an equatorial alcohol, compound 10 was
reduced using various reducing agents. Under the most
reductive conditions examined, regardless of the bulkiness of
the hydride donor, the desired equatorial alcohol anti-diol 1
was obtained as the major isomer but in only a slight excess
(Table 1). For example, the small hydride donor NaBH4 led to
a mixture of the diastereomers in only a 2.5:1 ratio (entry 1).20
It was reasoned that the low diastereoselectivity in the NaBH4
reduction of 10 might result from boron chelation to the α-
hydroxy ketone moiety, which hinders the axial approach of
NaBH4 to some extent. This assumption was strengthened by
the results of the reduction with Zn(BH4)2, which is well-
known to reduce α-hydroxy ketones stereoselectively through a
chelation-controlled reduction.21 The reduction of 10 with
Zn(BH4)2 resulted in a mixture of diastereomers in a ratio
(2.6:1) similar to that achieved with NaBH4 (entry 2). The
bulky hydride donor LiAlH(Ot-Bu)3 also provided the
equatorial alcohol 1 as the major isomer with similarly low
Table 1. Diastereoselective Reduction of α-Hydroxy
Ketones 10 and 14
a
b
c
entry SM
reducing agent
NaBH4
Zn(BH4)2
LiAlH(Ot-Bu)3
NaBH(OAc)3
NaBH4
solvent
yield
dr (anti/syn)
1
2
3
4
5
6
10
THF/EtOH
THF/EtOH
THF
THF/AcOH
THF/EtOH
THF
88
93
71
76
73
84
2.5:1
2.6:1
2.3:1
1:10
10:1
10:1
d
10
10
10
14
14
LiAlH(Ot-Bu)3
a
b
Starting material. Combined yield of the anti and syn products.
c
d
The diastereomeric ratio was determined by 1H NMR analysis. Zinc
borohydride was freshly prepared from ZnCl2 and NaBH4 in ether.
selectivity, even though bulky hydride donors generally prefer
equatorial attack for steric reasons (entry 3). The low
selectivity was attributed to the C-4-gem-dimethyl group
adjacent to the ketone, which adds additional steric bulk to
the upper face of the ring.
Surprisingly, when the reduction was performed with
NaBH(OAc)3 in the presence of AcOH (entry 4), a switch
in the diastereoselectivity was observed, and syn-diol 1′ was
generated as the major isomer with high selectivity (1:10). Syn-
B
DOI: 10.1021/acs.jnatprod.8b00986
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Journal of Natural Products
Article
a
diol 1′, C-3-epi-alphitolic acid, is also a natural triterpenoid and
has been synthesized once by Sun and co-workers from betulin
(9) in a nine-step route.13 In general, NaBH(OAc)3-mediated
reductions of cyclohexanones possessing axial α-hydroxy
groups provide anti-diols via hydroxy-directing effects.22,23
However, there are few reports on the NaBH(OAc)3
reductions of ketones with an equatorial α-hydroxy group.24
Although the exact mechanism in the case of equatorial α-
hydroxy cyclohexanones remains to be elucidated, it appears
that the equatorial 2-hydroxy group first reacted with
NaBH(OAc)3 and delivered the hydride equatorially in an
intramolecular manner to give syn-diol 1′.
Table 2. NMR Spectroscopic Data of Natural and
Synthetic Alphitolic Acid (1) in Pyridine-d5
b
δH, mult (J in Hz)
δC
natural synthetic
position
natural
synthetic
It was envisioned that the protection of the 2-hydroxy group
would enable the diastereoselective reduction of the ketone
group to equatorial alcohol by eliminating the chelation effect
and/or increasing the torsional strain that develops during the
formation of the axial alcohol. Instead of protecting the 2-
hydroxy group in 10 with a bulky silyl group, the 2-hydroxy-
protected ketone 14 was prepared directly from 11 in two
steps (Scheme 1). Treatment of 11 with tert-butyldimethylsilyl
trifluoromethanesulfonate led to the formation of silyl enol
ether 13 after basic workup. A Rubottom oxidation with m-
CPBA gave 14 in good overall yield. With a small hydride
donor NaBH4, ketone 14 furnished predominantly the
equatorial alcohol 15 via an axial attack (10:1, Table 1, entry
5). Bulky LiAlH(Ot-Bu)3 also provided 15 with the same level
of diastereoselectivity in higher yield (entry 6). TBS
deprotection with TBAF afforded alphitolic acid (1) in good
1a
1b
2
3
4
5
6a
6b
7a
7b
8
2.34, dd (12.5, 4.5)
1.29, m
4.14, m
2.29, dd (12.4, 4.4)
1.35, m
4.08, td (10.3, 4.0)
3.38, d (9.2)
48.6
48.2
69.2
84.1
40.3
56.4
19.2
68.8
83.8
39.9
56
3.42, d (9.4)
1.01, m
1.56, m
1.43, m
1.47, m
1.39, m
0.96, m
1.54, m
1.46, m
1.46, m
1.35, m
18.8
35.1
34.7
41.5
50.1
37.9
21.7
41.1
50.9
37.6
21.3
9
1.50, m
1.46, m
10
11a
11b
12a
12b
13
14
15a
15b
16a
16b
17
18
19
20
21a
21b
22a
22b
23
24
25
26
27
28
29a
29b
30
1.52, m
1.22, m
1.95, m
1.18, m
1.54, m
1.15, m
1.85, m
1.15, m
1
yield. The H and 13C NMR spectra of synthetic 1 were in
26.4
26
good agreement with those of natural 1 (Table 2).6
2.74, dt (12.1, 3.4)
2.70, td (11.6, 3.2)
39
43.2
30.6
38.7
42.8
30.2
After the development of a concise and diastereoselective
synthetic pathway to alphitolic acid (1) and its C-3-epimer 1′,
the route was extended to furnish natural 2-O-ester derivatives
2−7. Hydroxy ketone 10 was an excellent precursor for
preparing 2−7. Most natural 2-O-ester alphitolic acid
derivatives possess one or two phenolic hydroxy groups in
their ester moieties. These phenolic hydroxy groups needed to
be protected for successful esterification with 10. The phenol
groups of commercial acids 16a−e were protected with a TBS
group to give 17a−e, and the acid groups in 17a−e were
activated as the N-succinimidyl esters to give 18a−e (Scheme
3). Chemoselective coupling of 10 with 18a−e in the presence
of LiHMDS at low temperature yielded 19a−e (Scheme 4).
The diastereoselective reduction with LiAlH(Ot-Bu)3 followed
by silyl deprotection afforded desired 2-O-ester derivatives 3−
7 with high diastereoselectivity (10:1). For the synthesis of
natural 2-O-benzoate alphitolic acid (2), 10 was reacted with
benzoic anhydride in the presence of 4-(dimethylamino)-
pyridine to give benzoate 20. The subsequent reduction of 20
with LiAlH(t-BuO)3 afforded desired natural product 2 with
high diastereoselectivity (10:1). The spectral data of the
obtained natural derivatives of alphitolic acid were all in good
agreement with those of the natural products,6,7 and this is the
first synthesis of natural 2-O-ester derivatives 2−7.
1.88, m
1.25, m
2.65, m
1.56, m
1.85, m
1.15, m
2.60, dt (12.4, 2.8)
1.54, m
33.2
32.8
56.9
50.1
48.6
151.6
31.5
56.6
49.7
47.7
151.2
31.1
1.75, dd (11.3, 11.3) 1.72, dd (11.3, 11.3)
3.55, dt (4.8, 11.2)
3.50, td (10.3, 5.3)
2.24, m
1.54, m
2.27, m
1.59, m
1.28, s
1.08, s
0.92, s
1.06, s
1.06, s
2.20, m
1.46, m
2.20, m
1.54, m
1.23, s
38.9
38.5
29.6
17.8
18
16.8
15.2
179.2
110.4
29.2
17.4
17.7
16.4
14.9
178.8
110
1.033, s
0.88, s
1.026, s
c
c
1.020, s
4.95, s
4.79, s
1.80, s
4.91, d (2.3)
4.75, m
1.76, s
19.8
19.4
a
Measured at 600 MHz (1H NMR) and 150 MHz (13C NMR).
b
Measured at 400 MHz (1H NMR) and 125 MHz (13C NMR).
In conclusion, a concise and stereoselective synthesis of
alphitolic acid (1) and its natural 2-O-ester derivatives 2−7
was accomplished in five−seven steps starting from the
commercially available and inexpensive betulin (9). In
addition, natural C-3-epi-alphitolic acid (1′) was synthesized
stereoselectively starting from the same material in five steps. A
key step in the synthesis was the Rubottom oxidation of a silyl
enol ether in the presence of an oxidizable isopropylene group,
which allowed facile access to 2α-hydroxy betulonic acid.
c
Interchangeable assignments.
Substrate-controlled diastereoselective reduction provided a
viable synthetic route to both C-3-epimers of alphitolic acid.
This concise and efficient synthesis allowed the gram-scale
synthesis of alphitolic acid (1), its natural C-3-epimer (1′), and
its 2-O-ester derivatives 2−7 for further evaluation in various
biological tests, including in vivo assays.
C
DOI: 10.1021/acs.jnatprod.8b00986
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Journal of Natural Products
Article
a
(Agilent). NMR spectra were obtained on a JNM-ECZ400S/L1
(JEOL), a Bruker AVANCE 500 (Bruker), a JEOL JNM-ECA600
(JEOL), or an 800-MHz Bruker Avance III HD spectrometer with a 5
mm triple resonance inverse (TCI) CryoProbe (Bruker BioSpin).
High-resolution fast-atom bombardment mass spectrometry
(HRFABMS) spectra were obtained on a JEOL JMS-700
spectrometer (JEOL).
Scheme 3. Preparation of N-Succinimidyl Esters 18a−e
2α-Hydroxybetulonic Acid (10). To a solution of betulonic acid
(11, 1.0 equiv, 25.0 g, 55.0 mmol) in CH2Cl2 (500 mL) were added
triethylamine (5.0 equiv, 38.3 mL, 275 mmol) and trimethylsilyl
trifluoromethanesulfonate (2.0 equiv, 19.9 mL, 110 mmol) at −78 °C.
The reaction mixture was stirred at the same temperature. After 2.5 h,
the mixture was warmed to 0 °C, and water was added. The mixture
was washed with saturated aqueous NaHCO3, dried over anhydrous
MgSO4, filtered, and concentrated in vacuo to give crude silyl enol
ether 12. The crude product was dissolved in CH2Cl2 (500 mL), and
the solution was cooled to 0 °C. Solid NaHCO3 (10 equiv, 46.2 g,
550 mmol) was added, and then m-chloroperbenzoic acid (≤77%, 1.6
equiv, 19.7 g, 88.0 mmol) dissolved in CH2Cl2 (120 mL) was added
in three portions at 0 °C. After 2 h, the reaction was quenched with
10% aqueous Na2SO3 solution (100 mL) at 0 °C, washed with water
and brine, dried over MgSO4, filtered, and concentrated in vacuo. The
crude mixture was dissolved in MeOH (180 mL), and oxalic acid (0.4
equiv, 2.0 g, 22.0 mmol) was added. This solution was stirred for 15
min at room temperature, and then water was added to quench the
reaction. The resulting mixture was extracted with CH2Cl2. The
combined organic layers were washed with saturated aqueous
NaHCO3, filtered, and concentrated in vacuo. The residue was
purified by column chromatography on silica gel (hexane−EtOAc−
AcOH, 82:17:1) to give 2α-hydroxybetulonic acid (10, 16.1 g, 34.1
mmol, 62% for three steps) as a white solid: mp 173 °C; [α]D20 + 5.7
a
Reagents and conditions: (a) tert-butyldimethylsilyl chloride,
imidazole, dimethylformamide (DMF), rt, 0.5 h, 53−98%; (b) N-
hydroxysuccinimide, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide
hydrochloride, CH2Cl2, rt, 16 h, 36−56%.
EXPERIMENTAL SECTION
■
General Experimental Procedures. All the chemicals used
herein were of reagent grade and were used as received. Betulin (9)
was purchased from Jilin Tely Imp. & Exp.Co., Ltd. (Jilin). The
reactions were monitored by thin-layer chromatography (TLC)
performed on silica gel 60 F254 TLC plates. The synthesized products
were purified by flash column chromatography on silica gel 60 (40−
63 μm, 230−400 mesh, Merck). Melting points were measured on a
1
(c 0.50, MeOH); IR (neat) υmax 2944, 1701, 1457, 1375 cm−1; H
NMR (CDCl3, 400 MHz) δ 4.73 (1H, d, J = 1.8 Hz), 4.60 (1H, s),
4.51 (1H, dd, J = 12.6, 6.6 Hz), 2.98 (1H, td, J = 10.6, 4.4 Hz), 2.45
(1H, dd, J = 12.6, 6.6 Hz), 2.27 (1H, m), 2.20 (1H, m), 1.97 (2H, m),
1.71 (1H, d, J = 15.6 Hz), 1.66 (3H, s), 1.44 (12H, m), 1.18 (1H, m),
1.15 (3H, s), 1.12 (3H, s), 1.08 (1H, m), 1.07 (3H, s), 1.02 (2H, m),
0.97 (3H, s), 0.93 (3H, s); 13C NMR (CDCl3, 125 MHz) δ 216.7,
181.5, 150.1, 109.9, 69.4, 57.9, 56.3, 50.1, 49.2, 47.7, 46.9, 42.6, 40.8,
38.3, 38.0, 37.0, 34.1, 32.1, 30.5, 29.6, 25.2, 24.6, 21.3, 21.1, 19.3,
Buchi B-545 apparatus (Buchi Labortechnik AG). Optical rotations
̈ ̈
were measured on a JASCO P-2000 polarimeter (Hachioji) using a 10
cm cell, and IR spectra were acquired using an Agilent 5500a FTIR
a
Scheme 4. Synthesis of 2-O-Ester Derivatives of Alphitolic Acid 2−7
a
Reagents and conditions: (a) lithium bis(trimethylsilyl)amide, 18a−e, THF, −40 °C, 24 h, 63−76%; (b) LiAlH(Ot-Bu)3, THF, 0 °C, 3 h, 50−
79%; (c) tetrabutylammonium fluoride trihydrate, acetic acid, THF, 0 °C, 10 min, 70−100%; (d) benzoic anhydride, 4-(dimethylamino)pyridine,
pyridine, rt, 2 h, 53%.
D
DOI: 10.1021/acs.jnatprod.8b00986
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Journal of Natural Products
Article
19.1, 16.7, 16.2, 14.6; HRFABMS m/z 471.3478 [M + H]+ (calcd for
C30H47O4, 471.3474).
(hexane−EtOAc−AcOH, 87:12:1) to give compound 14 (3.3 g,
5.08 mmol, 83%) as a white solid: decomposed and melted at 218 °C;
20
General Procedure of the Diastereoselective Reduction of
10. To a solution of compound 10 (1 equiv) in solvent (0.1 M) was
added reducing agent (2 equiv) at 0 °C. The reaction was allowed to
stir at 0 °C for 1 h. The reaction mixture was quenched with 1 N HCl,
poured into water, and extracted with EtOAc. The organic layer was
dried over MgSO4, filtered, and concentrated in vacuo. The residue
was purified by column chromatography on silica gel (CH2Cl2−
MeOH−AcOH, 94:5:1) to give alphitolic acid (1) and C-3-epi-
alphitolic acid (1′).
[α]D + 5.5 (c 0.10, MeOH); IR (neat) υmax 2927, 1721, 1694, 837,
1
779 cm−1; H NMR (CDCl3, 400 MHz) δ 4.72 (1H, d, J = 1.4 Hz),
4.59 (1H, s), 4.53 (1H, dd, J = 12.0, 6.4 Hz), 2.99 (1H, td, J = 10.9,
4.4 Hz), 2.21 (3H, m), 1.97 (2H, m), 1.73 (1H, d, J = 13.3 Hz), 1.67
(3H, s), 1.58 (1H, dd, J = 11.5, 11.5 Hz), 1.38 (14H, m), 1.16 (1H,
app d, J = 13.3 Hz), 1.11 (3H, s), 1.08 (3H, s), 1.02 (3H, s), 0.96
(3H, s), 0.91 (3H, s), 0.88 (9H, s), 0.10 (3H, s), −0.01 (3H, s); 13C
NMR (CDCl3, 125 MHz) δ 214.2, 181.7, 150.2, 109.9, 71.3, 56.7,
56.3, 50.4, 50.2, 49.2, 48.3, 46.9, 42.5, 40.8, 38.4, 38.0, 37.1, 34.1,
32.1, 30.5, 29.6, 25.8, 25.6, 25.1, 21.6, 21.1, 19.2, 19.1, 18.5, 16.8,
16.1, 14.6, −4.6, −5.6; HRFABMS m/z 585.4343 [M + H]+ (calcd for
C36H61O4Si, 585.4339).
Alphitolic Acid (1). A white solid: decomposed and melted at 292
20
°C; [α]D + 29.0 (c 0.10, MeOH); IR (neat) υmax 2942, 2869, 1723,
1
1687, 1048 cm−1; H NMR (pyridine-d5, 400 MHz) δ 4.91 (1H, d, J
= 2.3 Hz), 4.75 (1H, m), 4.08 (1H, td, J = 10.3, 4.0 Hz), 3.50 (1H, td,
J = 10.3, 5.3 Hz), 3.38 (1H, d, J = 9.2 Hz), 2.70 (1H, td, J = 11.6, 3.2
Hz), 2.60 (1H, dt, J = 12.4, 2.8 Hz), 2.29 (1H, dd, J = 12.4, 4.4 Hz),
2.20 (2H, m), 1.85 (2H, m), 1.76 (3H, s), 1.72 (1H, dd, J = 11.3, 11.3
Hz), 1.54 (4H, m), 1.46 (4H, m), 1.35 (2H, m), 1.23 (3H, s), 1.15
(3H, m), 1.03 (3H, s), 1.026 (3H, s), 1.020 (3H, s), 0.96 (1H, m),
0.88 (3H, s); 13C NMR (pyridine-d5, 125 MHz) δ 178.8, 151.2, 110.0,
83.7, 68.8, 56.6, 56.0, 50.9, 49.7, 48.2, 47.7, 42.8, 41.1, 39.9, 38.7,
38.5, 37.6, 34.7, 32.8, 31.2, 30.2, 29.2, 26.0, 21.3, 19.4, 18.8, 17.6,
17.4, 16.4, 14.9; HRFABMS m/z 473.3642 [M + H]+ (calcd for
C30H49O4, 473.3631).
General Procedure of the Diastereoselective Reduction of
14. To a solution of compound 14 (1 equiv) in solvent (0.1 M) was
added reducing agent (5 equiv) at 0 °C. The reaction was allowed to
stir at room temperature for 24−36 h. The reaction mixture was
quenched with 1 N HCl, poured into water, and extracted with
EtOAc. The organic layer was dried over MgSO4, filtered, and
concentrated in vacuo. The residue was purified by column
chromatography on silica gel (hexane−EtOAc−AcOH, 90:9:1) to
give compound 15 and compound 15′.
2-O-tert-Butyldimethylsilylalphitolic Acid (15). A white solid:
decomposed and melted at 269 °C; [α]D20 − 15.0 (c 0.10, MeOH); IR
(neat) υmax 2947, 1696, 1456, 1085, 837 cm−1; 1H NMR (CDCl3, 400
MHz) δ 4.72 (1H, d, J = 1.8 Hz), 4.60 (1H, s), 3.66 (1H, td, J = 10.3,
4.0 Hz), 2.98 (2H, m), 2.26 (1H, app d, J = 12.8 Hz), 2.17 (1H, td, J
= 12.1, 3.0 Hz), 1.97 (2H, m), 1.85 (1H, dd, J = 12.6, 4.8 Hz), 1.70
(1H, m), 1.67 (3H, s), 1.57 (1H, dd, J = 11.4, 11.4 Hz), 1.27 (11H,
m), 1.18 (1H, m), 1.00 (3H, s), 0.95 (3H, s), 0.90 (3H, s), 0.87 (9H,
s), 0.85 (3H, s), 0.81 (2H, m), 0.77 (3H, s), 0.72 (1H, m), 0.08 (3H,
s), −0.01 (3H, s); 13C NMR (CDCl3, 125 MHz) δ 181.9, 150.4,
109.7, 83.1, 71.0, 56.4 55.4, 50.3, 49.2, 47.3, 46.9, 42.5, 40.8, 38.75,
38.66, 38.3, 37.1, 34.2, 32.1, 30.6, 29.6, 28.6, 25.9, 25.4, 20.9, 19.3,
18.1, 18.0, 17.3, 16.6, 16.1, 14.6, −3.9, −4.6; HRFABMS m/z
587.4488 [M + H]+ (calcd for C36H63O4Si, 587.4496).
C-3-epi-Alphitolic Acid (1′). A white solid: decomposed and
melted at 280 °C; [α]D20 − 3.1 (c 0.10, MeOH); IR (neat) υmax 2941,
1
2871, 1725, 1696, 1274 cm−1; H NMR (pyridine-d5, 400 MHz) δ
4.90 (1H, d, J = 2.3 Hz), 4.73 (1H, s), 4.28 (1H, dt, J = 11.5, 3.5 Hz),
3.74 (1H, d, J = 2.4 Hz), 3.49 (1H, td, J = 11.8, 5.1 Hz), 2.67 (1H, td,
J = 11.9, 3.1 Hz), 2.58 (1H, app d, J = 12.8 Hz), 2.19 (2H, m), 1.96
(1H, dd, J = 11.9, 4.1 Hz), 1.84 (2H, m), 1.74 (3H, s), 1.68 (2H, td, J
= 11.5, 4.0 Hz), 1.49 (8H, m), 1.33 (2H, m), 1.23 (3H, s), 1.13 (3H,
m), 1.02 (3H, s), 0.91 (3H, s), 0.85 (6H, s); 13C NMR (pyridine-d5,
125 MHz) δ 178.8, 151.3, 109.9, 79.4, 66.3, 56.6, 50.8, 49.7, 48.7,
47.8, 43.2, 42.9, 41.3, 38.9, 38.8, 38.5, 37.5, 34.7, 32.8, 31.2, 30.2,
29.4, 26.0, 22.1, 21.2, 19.4, 18.4, 17.4, 16.4, 14.8; HRFABMS m/z
473.3628 [M + H]+ (calcd for C30H49O4, 473.3631).
2-O-tert-Butyldimethylsilyl-C-3-epi-alphitolic Acid (15′). A white
20
Silyl Enol Ether 13. To a solution of betulonic acid (11, 1.0 equiv,
5.0 g, 11.0 mmol) in CH2Cl2 (110 mL) were added tert-
butyldimethylsilyl trifluoromethanesulfonate (5 equiv, 12.6 mL, 55.0
mmol) and triethylamine (10 equiv, 15.3 mL, 110 mmol) at −78 °C.
After 1.5 h, 1 N NaOH (70 mL) was added, and the reaction mixture
was allowed to stir at room temperature for 12 h. The resulting
solution was quenched with 3 N HCl, diluted with diethyl ether, and
washed with water. The organic layer was dried over anhydrous
MgSO4, filtered, and concentrated in vacuo. The residue was purified
by column chromatography on silica gel (hexane−EtOAc−AcOH,
93:6:1) to give compound 13 (5.6 g, 9.8 mmol, 89%) as a white solid:
decomposed and melted at 245 °C; [α]D20 + 38.7 (c 0.10, MeOH); IR
(neat) υmax 2924, 2855, 1685, 833, 774 cm−1; 1H NMR (CDCl3, 400
MHz) δ 4.73 (1H, d, J = 1.4 Hz), 4.59 (1H, s), 4.50 (1H, d, J = 5.0
Hz), 3.01 (1H, td, J = 10.7, 4.6 Hz), 2.26 (1H, app d, J = 12.8 Hz),
2.17 (1H, td, J = 12.0, 3.2 Hz), 1.96 (3H, m), 1.68 (3H, s), 1.35
(15H, m), 1.06 (2H, m), 1.00 (3H, s), 0.96 (3H, s), 0.94 (3H, s), 0.91
(10H, s), 0.86 (3H, s), 0.84 (3H, s), 0.13 (3H, s), 0.09 (3H, s); 13C
NMR (CDCl3, 125 MHz) δ 182.5, 155.9, 150.5, 109.6, 98.5, 56.5,
53.0, 49.3, 49.1, 47.0, 42.4, 40.5, 40.3, 38.7, 38.2, 37.0, 36.2, 33.5,
32.1, 30.6, 29.8, 28.2, 25.9, 25.6, 21.3, 19.7, 19.5, 19.4, 18.3, 16.1,
15.8, 14.7, −3.9, −4.8; HRFABMS m/z 568.4317 [M]+ (calcd for
C33H60O3Si, 568.4312).
solid: decomposed and melted at 255 °C; [α]D + 204.7 (c 0.10,
MeOH); IR (neat) υmax 2947, 1696, 1456, 1085, 837 cm−1; 1H NMR
(CDCl3, 400 MHz) δ 4.72 (1H, d, J = 1.8 Hz), 4.58 (1H, s), 3.97
(1H, dt, J = 10.7, 3.8 Hz), 3.25 (1H, d, J = 2.8 Hz), 2.98 (1H, td, J =
10.8, 4.6 Hz), 2.24 (1H, app d, J = 11.9 Hz), 2.17 (1H, td, J = 12.2,
3.3 Hz), 1.95 (2H, m), 1.71 (1H, m), 1.66 (3H, s), 1.58 (1H, dd, J =
11.3, 11.3 Hz), 1.31 (15H, m), 0.99 (3H, s), 0.94 (3H, s), 0.90 (3H,
s), 0.86 (9H, s), 0.82 (3H, s), 0.79 (3H, s), 0.04 (6H, s); 13C NMR
(CDCl3, 200 MHz) δ 179.8, 150.4, 109.7, 79.1, 68.0, 56.3, 50.1, 49.2,
47.8, 46.9, 42.5, 42.2, 40.9, 38.4, 38.3, 37.8, 37.0, 34.0, 32.1, 30.5,
29.6, 28.4, 25.8, 25.4, 21.8, 20.8, 19.3, 18.0, 17.9, 17.2, 16.0, 14.7,
−4.5, −4.8; HRFABMS m/z 587.4488 [M + H]+ (calcd for
C36H63O4Si, 587.4496).
Experimental Procedure for the TBS-Deprotection of 15. To
a solution of compound 15 (1 equiv, 2.0 g, 3.4 mmol) in
tetrahydrofuran (THF) was added 1 M tetrabutylammonium fluoride
in THF (2.5 equiv, 8.5 mL, 8.5 mmol) at room temperature. The
reaction was refluxed for 3 h, cooled to room temperature, quenched
with saturated aqueous NH4Cl, and poured into water. The mixture
was extracted with EtOAc, and the organic layer was washed with
brine, dried over MgSO4, filtered, and concentrated in vacuo. The
residue was purified by column chromatography on silica gel
(CH2Cl2−MeOH−AcOH, 94:5:1) to give alphitolic acid (1, 1.67 g,
2.9 mmol, 83%).
2α-Silyloxybetulonic Acid 14. To a solution of compound 13 (1
equiv, 4.0 g, 7.0 mmol) in CH2Cl2 (70 mL) was added sodium
phosphate dibasic (20 equiv, 20.0 g, 141 mmol) at 0 °C. The reaction
mixture was stirred for 0.5 h, and m-chloroperbenzoic acid (≤77%, 1.1
equiv, 1.7 g, 7.7 mmol) dissolved in CH2Cl2 (30 mL) was added in
three portions. The reaction was allowed to stir at room temperature
for 5 h. The solution was washed with water, and the organic layer
was dried over MgSO4, filtered, and concentrated in vacuo. The
residue was purified by column chromatography on silica gel
Experimental Procedure for the Esterification of 10 with 18.
To a solution of compound 10 (1 equiv, 6.4 mmol) in anhydrous
THF (32 mL) was added 1 M lithium bis(trimethylsilyl)amide in
THF (2 equiv, 12.8 mmol) at −40 °C. After 10 min, a solution of 18
(1.1 equiv, 7.0 mmol) in anhydrous THF (20 mL) was added to the
reaction mixture. The reaction was stirred for 20 h at −40 °C,
quenched with aqueous NH4Cl, poured into water, and extracted with
CH2Cl2. The combined organic layer was washed with brine, dried
E
DOI: 10.1021/acs.jnatprod.8b00986
J. Nat. Prod. XXXX, XXX, XXX−XXX

Journal of Natural Products
Article
over MgSO4, filtered, and concentrated in vacuo. The residue was
purified by column chromatography on silica gel (hexane−EtOAc−
AcOH, 90:9:1) to give compound 19.
= 12.2, 5.8 Hz) 2.27 (1H, app d, J = 12.8 Hz), 2.20 (1H, td, J = 12.2,
3.2 Hz), 1.97 (2H, m), 1.77 (1H, m), 1.67 (3H, s), 1.47 (13H, m),
1.22 (3H, s), 1.18 (2H, m), 1.14 (3H, s), 1.10 (3H, s), 0.99 (3H, s),
0.97 (9H, s), 0.96 (9H, s), 0.95 (3H, s), 0.20 (6H, s), 0.19 (6H, s);
13C NMR (CDCl3, 150 MHz) δ 209.5, 181.0, 166.4, 150.2, 149.5,
147.1, 145.4, 128.0, 122.4, 121.1, 120.4, 115.1, 109.9, 71.8, 57.3, 56.2,
50.3, 49.2, 48.7, 46.9, 46.2, 42.6, 40.8, 38.31, 38.27, 37.0, 34.1, 32.1,
30.5, 29.6, 25.90, 25.86, 25.3, 24.7, 21.2, 21.1, 19.3, 19.0, 18.5, 18.4,
16.6, 16.2, 14.6, −4.07, −4.08, −4.10; HRFABMS m/z 860.5457
[M]+ (calcd for C51H80O7Si2, 860.5443).
Synthesis of Natural 2-O-Ester Derivatives 3−7 from 19. To
a solution of compound 19 (1 equiv, 3.0 mmol) in THF (30 mL) was
added lithium tri-tert-butoxyaluminum hydride (1 M in THF, 2 equiv,
6.0 mmol) at 0 °C. The reaction was stirred at room temperature for
3 h, quenched with 1 N HCl, and diluted with water. The mixture was
extracted with CH2Cl2, and the organic layer was washed with brine,
dried over MgSO4, filtered, and concentrated in vacuo. The residue
was purified by column chromatography on silica gel (hexane−
acetone−AcOH, 83:16:1) to give the corresponding alcohol. To a
solution of the alcohol (1 equiv, 1.8 mmol) in THF (9 mL) were
added acetic acid (1.1 equiv, 2.0 mmol) and tetrabutylammonium
fluoride trihydrate (1.1 equiv, 2.0 mmol) at 0 °C. After 10 min, the
reaction mixture was poured into water and extracted with CH2Cl2.
The combined organic layer was washed with saturated aqueous
NH4Cl, dried over MgSO4, filtered, and concentrated in vacuo. The
residue was purified by column chromatography on silica gel (hexane/
EtOAc/AcOH, 66:33:1) to give natural derivatives 3−7.
2-O-p-Hydroxybenzoylalphitolic Acid (3). A white solid (77% for
two steps): decomposed and melted at 221 °C; [α]D20 −10.7 (c 0.10,
MeOH); IR (neat) υmax 2952, 1699, 1276 cm−1; 1H NMR (pyridine-
d5, 400 MHz) δ 8.30 (2H, m), 7.16 (2H, m), 5.70 (1H, td, J = 10.6,
4.3 Hz), 4.93 (1H, d, J = 2.3 Hz), 4.78 (1H, s), 3.68 (1H, d, J = 10.1
Hz), 3.52 (1H, td, J = 11.4, 5.5 Hz), 2.70 (1H, m), 2.62 (1H, app d, J
= 12.8 Hz), 2.32 (1H, dd, J = 12.4, 4.6 Hz), 2.18 (2H, m), 1.85 (2H,
m), 1.79 (3H, s), 1.74 (1H, dd, J = 11.3, 11.3 Hz), 1.57 (4H, m), 1.46
(1H, m), 1.37 (3H, m), 1.28 (3H, s), 1.18 (5H, m), 1.09 (3H, s), 1.06
(3H, s), 1.03 (1H, m), 1.00 (3H, s), 0.98 (3H, s); 13C NMR
(pyridine-d5, 150 MHz) δ 178.8, 166.7, 163.4, 151.4, 132.4, 122.6,
116.0, 110.0, 79.8, 74.1, 56.6, 55.7, 50.8, 49.8, 47.8, 45.0, 42.9, 41.1,
40.6, 38.6, 37.6, 34.7, 32.9, 31.1, 30.2, 29.1, 26.0, 21.3, 19.4, 18.8,
17.42, 17.40, 16.3, 14.9; HRFABMS m/z 593.3850 [M + H]+ (calcd
for C37H53O6, 593.3842).
2α-Hydroxy-O-p-(tert-butyldimethylsilyloxy)benzoylbetulonic
20
Acid (19a). A white solid (63%): mp 188 °C; [α]D + 17.2 (c 0.50,
MeOH); IR (neat) υmax 2946, 2860, 1716, 1266, 910 cm−1; 1H NMR
(CDCl3, 400 MHz) δ 7.94 (2H, m), 6.82 (2H, m), 5.80 (1H, dd, J =
13.2, 6.4 Hz), 4.72 (1H, s), 4.59 (1H, s), 2.99 (1H, td, J = 10.7, 4.7
Hz), 2.39 (1H, m), 2.27 (1H, m), 2.21 (1H, td, J = 12.3, 3.2 Hz), 1.98
(2H, m), 1.73 (1H, app d, J = 13.3 Hz), 1.67 (3H, s), 1.49 (14H, m),
1.23 (3H, s), 1.19 (1H, m), 1.16 (3H, s), 1.11 (3H, s), 1.06 (1H, m),
1.00 (3H, s), 0.96 (12H, s), 0.20 (6H, s); 13C NMR (CDCl3, 125
MHz) δ 209.3, 181.3, 165.6, 160.2, 150.2, 131.8, 122.9, 119.8, 109.9,
72.1, 57.3, 56.3, 50.3, 49.2, 48.8, 46.9, 46.2, 42.6, 40.9, 38.31, 38.29,
37.0, 34.1, 32.1, 30.5, 29.6, 25.6, 25.3, 24.7, 21.2, 21.1, 19.3, 19.0,
18.2, 16.6, 16.2, 14.6, −4.4; HRFABMS m/z 705.4549 [M + H]+
(calcd for C43H65O6Si, 705.4550).
2α-Hydroxy-O-m,p-bis(tert-butyldimethylsilyloxy)-
20
benzoylbetulonic Acid (19b). A white solid (64%): mp 164 °C;[α]D
+ 10.0 (c 0.50, MeOH); IR (neat) υmax 2950, 2860, 1714, 1510, 1301,
842 cm−1; 1H NMR (CDCl3, 400 MHz) δ 7.54 (2H, m), 6.81 (1H, d,
J = 8.7 Hz), 5.78 (1H, dd, J = 13.0, 6.2 Hz), 4.72 (1H, s), 4.59 (1H,
s), 2.99 (1H, td, J = 10.7, 4.6 Hz), 2.38 (1H, dd, J = 12.4, 6.4 Hz),
2.28 (1H, app d, J = 12.8 Hz), 2.20 (1H, td, J = 12.3, 3.4 Hz), 1.98
(2H, m), 1.73 (1H, m), 1.67 (3H, s), 1.47 (14H, m), 1.23(4H, s),
1.18 (2H, m), 1.15 (3H, s), 1.10 (3H, s), 0.965 (11H, s), 0.960 (10H,
s), 0.20 (6H, s), 0.19 (6H, s); 13C NMR (CDCl3, 125 MHz) δ 209.3,
181.9, 165.6, 151.8, 150.2, 146.7, 123.8, 123.0, 122.5, 120.5, 109.9,
72.1, 57.3, 56.3, 50.3, 49.2, 48.8, 46.9, 46.1, 42.6, 40.9, 38.3, 37.0,
34.0, 32.1, 30.5, 29.6, 25.91, 25.86, 25.3, 24.7, 21.2, 21.1, 19.3, 19.0,
18.5, 18.4, 16.6, 16.2, 14.6, −4.08, −4.11, −4.13; HRFABMS m/z
835.5356 [M + H]+ (calcd for C49H79O7Si2, 835.5364).
2α-Hydroxy-O-p-tert-butyldimethylsilyloxy-m-methoxy-benzoyl-
betulonic Acid (19c). A white solid (69%): mp 163 °C;[α]D20 + 10.9
(c 0.5, MeOH); IR (neat) υmax 2949, 2868, 1714, 1288, 884 cm−1; 1H
NMR (CDCl3, 400 MHz) δ 7.58 (1H, dd, J = 8.3, 1.7 Hz), 7.53 (1H,
d, J = 1.8 Hz), 6.83 (1H, d, J = 8.2 Hz), 5.79 (1H, dd, J = 13.2, 6.4
Hz), 4.72 (1H, s), 4.59 (1H, s), 3.82 (3H, s), 2.99 (1H, td, J = 10.6,
4.7 Hz), 2.40 (1H, dd, J = 12.4, 6.0 Hz), 2.28 (1H, m), 2.21 (1H, td, J
= 12.2, 3.0 Hz), 1.99 (2H, m), 1.73 (1H, m), 1.66 (3H, s), 1.47 (14H,
m), 1.23 (3H, s), 1.20 (1H, m), 1.16 (3H, s), 1.12 (3H, s), 1.06 (1H,
m), 1.01 (3H, s), 0.97 (9H, s), 0.96 (3H, s), 0.15 (6H, s); 13C NMR
(CDCl3, 125 MHz) δ 209.3, 181.8, 165.7, 150.6, 150.2, 149.8, 123.6,
123.2, 120.5, 113.1, 109.9, 72.2, 57.3, 56.3, 55.4, 50.3, 49.2, 48.8, 46.9,
46.2, 42.6, 40.9, 38.3, 37.0, 34.1, 32.1, 30.5, 29.6, 25.6, 25.3, 24.7,
21.2, 21.1, 19.3, 19.0, 18.5, 16.6, 16.2, 14.6, −4.6; HRFABMS m/z
735.4660 [M + H]+ (calcd for C44H67O7Si, 735.4656).
2-O-Protocatechuoylalphitolic Acid (4). A white solid (71% for
20
two steps): decomposed and melted at 246 °C;[α]D − 8.7 (c 0.10,
MeOH); IR (neat) υmax 3340, 2943, 1693, 1448, 1291, 1218 cm−1; 1H
NMR (pyridine-d5, 400 MHz) δ 8.24 (1H, d, J = 1.8 Hz), 7.93 (1H,
dd, J = 8.0, 2.1 Hz), 7.25 (1H, d, J = 8.3 Hz), 6.56 (1H, s), 5.68 (1H,
td, J = 10.6, 4.1 Hz), 4.93 (1H, d, J = 1.8 Hz), 4.79 (1H, s), 3.65 (1H,
d, J = 9.7 Hz), 3.52 (1H, m), 2.69 (1H, m), 2.61 (1H, app d, J = 12.4
Hz), 2.30 (1H, dd, J = 12.4, 4.6 Hz), 2.20 (2H, m), 1.88 (1H, m),
1.86 (1H, m), 1.79 (3H, s), 1.74 (1H, dd, J = 11.3, 11.3 Hz), 1.47
(18H, m), 1.27 (3H, s), 1.17 (5H, m), 1.08 (3H, s), 1.05 (3H, s), 0.99
(3H, s), 0.97 (3H, s); 13C NMR (pyridine-d5, 200 MHz) δ 178.8,
167.0, 152.3, 151.4, 147.0, 123.1, 123.0, 118.0, 116.0, 110.0, 79.8,
74.0, 56.5, 55.7, 50.8, 49.8, 47.7, 45.0, 42.9, 41.1, 40.5, 38.7, 38.6,
37.6, 34.6, 32.9, 31.1, 30.2, 29.1, 26.0, 21.2, 19.4, 18.8, 17.41, 17.37,
16.3, 14.8; HRFABMS m/z 609.3795 [M + H]+ (calcd for C37H53O7,
609.3791).
2α-Hydroxy-O-p-tert-butyldimethylsilyloxycinnamoylbetulonic
20
Acid (19d). A white solid (70%): mp 191 °C;[α]D + 22.0 (c 0.10,
1
MeOH); IR (neat) υmax 2944, 1702, 1457 cm−1; H NMR (CDCl3,
500 MHz) δ 7.65 (1H, d, J = 15.9 Hz), 7.40 (2H, d, J = 8.5 Hz), 6.81
(2H, d, J = 8.5 Hz), 6.35 (1H, d, J = 15.9 Hz), 5.72 (1H, dd, J = 13.1,
6.1 Hz), 4.72 (1H, s), 4.59 (1H, s), 2.99 (1H, td, J = 10.5, 4.6 Hz),
2.34 (1H, dd, J = 12.3, 6.1 Hz), 2.28 (1H, app d, J = 12.8 Hz), 2.21
(1H, app t, J = 10.6 Hz), 1.96 (2H, m), 1.72 (2H, m), 1.67 (3H, s),
1.58 (3H, m), 1.48 (3H, m), 1.41 (7H, m), 1.22 (3H, s), 1.17 (1H,
m), 1.14 (3H, s), 1.10 (3H, s), 1.05 (1H, m), 0.99 (3H, s), 0.963 (9H,
s), 0.956 (3H, s), 0.20 (6H, s); 13C NMR (CDCl3, 125 MHz) δ
209.5, 182.0, 166.4, 157.9, 150.2, 145.2, 129.8, 127.7, 120.4, 115.2,
109.9, 71.8, 57.3, 56.3, 50.3, 49.2, 48.7, 46.9, 46.2, 42.6, 40.8, 38.33,
38.27, 37.0, 34.0, 32.1, 30.5, 29.6, 25.6, 25.3, 24.7, 21.2, 21.1, 19.3,
19.0, 18.2, 16.6, 16.2, 14.6, −4.4; HRFABMS m/z 730.4637 [M]+
(calcd for C45H66O6Si, 730.4629).
2α-Hydroxy-O-m,p-bis(tert-butyldimethylsilyloxy)-
cinnamoylbetulonic Acid (19e). A white solid (76%): mp 163 °C;
[α]D20 + 10.9 (c 0.50, MeOH); IR (neat) υmax 2952, 2859, 1714, 1509,
1290, 1255, 1162 cm−1; 1H NMR (CDCl3, 400 MHz) δ 7.57 (1H, d, J
= 16.0 Hz), 6.98 (2H, dd, J = 6.6, 2.1 Hz), 6.79 (2H, dd, J = 6.6, 2.5
Hz), 6.28 (1H, d, J = 15.6 Hz), 5.72 (1H, dd, J = 13.2, 6.4 Hz), 4.72
(1H, s), 4.59 (1H, s), 2.99 (1H, td, J = 10.7, 4.9 Hz), 2.35 (1H, dd, J
2-O-Vanilloylalphitolic Acid (5). A white solid (56% for two
20
steps): decomposed and melted at 211 °C; [α]D + 25.1 (c 0.10,
1
MeOH); IR (neat) υmax 2936, 1701, 1282, 1218 cm−1; H NMR
(pyridine-d5, 800 MHz) δ 8.02 (1H, dd, J = 8.2, 1.9 Hz), 7.96 (1H, d,
J = 1.8 Hz), 7.23 (1H, d, J = 8.2 Hz), 6.61 (1H, s), 5.74 (1H, td, J =
10.7, 4.7 Hz), 4.94 (1H, d, J = 1.8 Hz), 4.79 (1H, s), 3.69 (3H, s),
3.68 (1H, m), 3.52 (1H, td, J = 10.8, 4.9 Hz), 2.70 (1H, td, J = 11.9,
3.3 Hz), 2.62 (1H, dt, J = 12.8, 3.2 Hz), 2.33 (1H, dd, J = 12.3, 4.8
Hz), 2.23 (2H, m), 1.88 (1H, m), 1.85 (1H, m), 1.79 (3H, s), 1.74
(1H, dd, J = 11.3, 11.3 Hz), 1.54 (4H, m), 1.40 (4H, m), 1.27 (3H,
s), 1.25 (1H, m), 1.24 (1H, m), 1.23 (1H, m), 1.16 (1H, m), 1.12
(1H, m), 1.10 (3H, s), 1.06 (3H, s), 1.01 (3H, s), 0.99 (3H, s), 0.97
F
DOI: 10.1021/acs.jnatprod.8b00986
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Journal of Natural Products
Article
(1H, m); 13C NMR (pyridine-d5, 200 MHz) δ 178.8, 166.7, 153.0,
151.4, 148.3, 124.7, 122.7, 116.1, 113.5, 110.0, 79.7, 74.1, 56.5, 55.8,
55.7, 50.8, 49.8, 47.7, 45.0, 42.9, 41.1, 40.5, 38.7, 38.6, 37.6, 34.7,
32.9, 31.1, 30.2, 29.1, 26.0, 21.2, 19.4, 18.8, 17.41, 17.39, 16.3, 14.8;
HRFABMS m/z 623.3946 [M + H]+ (calcd for C38H55O7, 623.3948).
2-O-trans-p-Coumaroylalphitolic Acid (6). A white solid (50% for
EtOAc−AcOH, 87:12:1) to give compound 2 (1.0 g, 1.7 mmol, 50%)
20
as a white solid: decomposed and melted at 210 °C; [α]D −16.6 (c
0.10, MeOH); IR (neat) υmax 2944, 1713, 1451, 1275, 1115, 711
1
cm−1; H NMR (pyridine-d5, 400 MHz) δ 8.26 (2H, m), 7.49 (1H,
app t, J = 7.4 Hz), 7.39 (1H, d, J = 7.8 Hz), 7.38 (3H, t, J = 7.4 Hz),
6.64 (1H, s), 5.70 (1H, td, J = 10.6, 4.3 Hz), 4.93 (1H, s), 4.77 (1H,
s), 3.66 (1H, d, J = 9.7 Hz), 3.52 (1H, td, J = 11.0, 4.4 Hz), 2.71 (1H,
m), 2.63 (1H, app d, J = 12.9 Hz), 2.29 (1H, dd, J = 12.4, 4.6 Hz),
2.19 (2H, m), 1.87 (2H, m), 1.78 (3H, s), 1.74 (1H, dd, J = 11.3, 11.3
Hz), 1.55 (4H, m), 1.40 (4H, m), 1.27 (3H, s), 1.22 (2H, m), 1.15
(3H, m), 1.08 (3H, s), 1.07 (3H, s), 1.02 (1H, m), 0.99 (3H, s), 0.98
(3H, s); 13C NMR (pyridine-d5, 125 MHz) δ 178.9, 166.6, 151.4,
133.1, 131.8, 130.0, 128.7, 110.0, 79.7, 74.7, 56.6, 55.7, 50.9, 49.8,
47.8, 44.9, 42.9, 41.1, 40.6, 38.8, 38.6, 37.6, 34.7, 32.9, 31.2, 30.2,
29.1, 26.0, 21.3, 19.4, 18.8, 17.4, 16.3, 14.9; HRFABMS m/z 577.3905
[M + H]+ (calcd for C37H53O5, 577.3893).
20
two steps): decomposed and melted at 252 °C;[α]D + 29.0 (c 0.10,
MeOH); IR (neat) υmax 2947, 1703, 1688, 1605, 1253, 1169, 832
cm−1; 1H NMR (pyridine-d5, 800 MHz) δ 8.01 (1H, d, J = 15.8 Hz),
7.54 (2H, d, J = 11.4 Hz), 7.13 (2H, d, J = 8.5 Hz), 6.61 (1H, d, J =
15.9 Hz), 5.62 (1H, td, J = 10.7, 4.4 Hz), 4.94 (1H, s), 4.78 (1H, s),
3.64 (1H, d, J = 9.9 Hz), 3.52 (1H, td, J = 10.7, 4.9 Hz), 2.71 (1H, td,
J = 12.0, 3.4 Hz), 2.62 (1H, app d, J = 12.9 Hz), 2.31 (1H, dd, J =
12.3, 4.7 Hz), 2.23 (2H, m), 1.90 (1H, m), 1.85 (1H, td, J = 13.5, 3.6
Hz), 1.79 (3H, s), 1.74 (1H, dd, J = 11.3, 11.3 Hz), 1.54 (4H, m),
1.42 (3H, m), 1.36 (2H, m), 1.27 (3H, s), 1.23 (2H, m), 1.16 (1H,
m), 1.12 (1H, m), 1.09 (3H, s), 1.06 (3H, s), 1.01 (3H, s), 0.98 (3H,
s), 0.97 (1H, m); 13C NMR (pyridine-d5, 200 MHz) δ 178.8, 167.5,
161.4, 151.4, 144.7, 130.6, 126.2, 116.8, 116.0, 110.0, 79.8, 73.8, 56.5,
55.7, 50.8, 49.8, 47.7, 45.0, 42.9, 41.1, 40.5, 38.8, 38.6, 37.6, 34.7,
32.9, 31.1, 30.2, 29.1, 26.0, 21.3, 19.4, 18.7, 17.42, 17.39, 16.3, 14.8;
HRFABMS m/z 619.3997 [M + H]+ (calcd for C39H55O6, 619.3999).
2-O-trans-Caffeoylalphitolic Acid (7). A white solid (57% for two
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jnat-
prod.8b00986.
20
steps): decomposed and melted at 246 °C;[α]D − 361.1 (c 0.10,
Experimental procedures and spectroscopic data of
MeOH); IR (neat) υmax 2946, 1696, 1603, 1275, 1179 cm−1; 1H
NMR (CD3OD, 800 MHz) δ 7.54 (1H, d, J = 15.8 Hz), 7.03 (1H, d, J
= 2.0 Hz), 6.93 (1H, dd, J = 8.2, 2.0 Hz), 6.76 (1H, d, J = 8.1 Hz),
6.27 (1H, d, J = 15.8 Hz), 5.04 (1H, td, J = 10.8, 4.4 Hz), 4.69 (1H, d,
J = 1.3 Hz), 4.56 (1H, s), 3.21 (1H, d, J = 10.1 Hz), 3.01 (1H, td, J =
10.9, 4.8 Hz), 2.32 (1H, td, J = 11.8, 2.3 Hz), 2.23 (1H, app d, J =
13.0 Hz), 2.08 (1H, dd, J = 12.3, 4.6 Hz), 1.92 (2H, m), 1.71 (1H,
m), 1.67 (3H, s), 1.60 (1H, dd, J = 11.3, 11.3 Hz), 1.55 (2H, m), 1.48
(2H, m), 1.40 (5H, m), 1.28 (2H, m), 1.17 (1H, m), 1.07 (1H, td, J =
13.2, 4.5 Hz), 1.03 (3H, s), 1.01 (3H, s), 1.00 (3H, s), 0.97 (3H, s),
0.94 (1H, dd, J = 11.9, 11.9 Hz), 0.89 (1H, m), 0.85 (3H, s) 13C
NMR (CD3OD, 200 MHz) δ 181.1, 170.1, 152.8, 150.3, 147.6, 147.4,
128.7, 123.6, 117.3, 116.6, 115.9, 111.0, 81.9, 74.8, 58.4, 57.5, 52.6,
51.2, 49.3, 46.2, 44.4, 42.8, 41.8, 40.5, 40.4, 39.0, 36.2, 34.2, 32.6,
31.6, 29.9, 27.6, 23.0, 20.4, 20.3, 18.5, 18.0, 17.4, 15.9; HRFABMS m/
z 635.3958 [M + H]+ (calcd for C39H55O7, 635.3948).
compounds including 11, 17a−e, and 18a−e; fully
1
assigned H and 13C NMR spectroscopic data of 2−7;
1H and 13C NMR copies of novel compounds and
natural compounds 1−7 and 1′ (PDF)
AUTHOR INFORMATION
Corresponding Authors
*Phone: +82 28802488. E-mail: jeonhj@snu.ac.kr. (H.J.)
*Phone: +82 28802487. E-mail: pennkim@snu.ac.kr. (S.K.)
■
ORCID
Sang Hyun Sung: 0000-0002-0527-4815
Sanghee Kim: 0000-0001-9125-9541
Notes
2α-Hydroxy-O-benzoylbetulonic Acid (20). To a solution of
compound 10 (1 equiv, 3.4 g, 7.2 mmol) in pyridine (36 mL) was
added DMAP (0.05 equiv, 44 mg, 0.36 mmol) and benzoic anhydride
(3 equiv, 5.2 g, 21.6 mmol) at room temperature. The reaction was
stirred at the same temperature for 2 h. The mixture was quenched
with saturated aqueous NH4Cl, poured into water, and extracted with
EtOAc. The organic layer was washed with brine, dried over MgSO4,
filtered, and concentrated in vacuo. The residue was purified by
column chromatography on silica gel (hexane−acetone−AcOH,
94:5:1) to give compound 20 (2.2 g, 3.9 mmol, 53%) as a white
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Research
Foundation of Korea (NRF) grant funded by the Korea
government (MSIT) (No. 2018R1A5A2024425).
DEDICATION
■
This work is dedicated to the late Prof. Sang Hyun Sung, who
passed away in July 2018 at the early age of 50.
20
solid: mp 165 °C; [α]D − 26.6 (c 0.50, MeOH); IR (neat) υmax
1
2946, 1717, 1273, 1119, 712 cm−1; H NMR (CDCl3, 400 MHz) δ
8.04 (2H, m), 7.54 (1H, t, J = 7.6 Hz), 7.41 (2H, t, J = 7.8 Hz), 5.83
(1H, dd, J = 13.2, 6.4 Hz), 4.72 (1H, s), 4.59 (1H, s), 2.98 (1H, td, J
= 12.7, 4.8 Hz), 2.42 (1H, dd, J = 12.4, 6.4 Hz), 2.24 (2H, m), 1.95
(2H, m), 1.74 (1H, m), 1.68 (3H, s), 1.49 (14H, m), 1.25 (3H, s),
1.21 (1H, m), 1.17 (3H, s), 1.13 (3H, s), 1.06 (1H, m), 1.00 (3H, s),
0.97 (3H, s); 13C NMR (CDCl3, 200 MHz) δ 209.1, 179.8, 165.8,
150.1, 133.0, 129.9, 129.8, 128.3, 109.9, 72.4, 57.3, 56.2, 50.3, 49.2,
48.8, 46.8, 46.2, 42.6, 40.9, 38.29, 38.27, 37.0, 34.0, 32.1, 30.4, 29.6,
25.3, 24.7, 21.2, 21.1, 19.3, 19.0, 16.7, 16.2, 14.6; HRFABMS m/z
575.3735 [M + H]+ (calcd for C37H51O5, 575.3737).
2-O-Benzoylalphitolic Acid (2). To a solution of compound 20 (1
equiv, 2.0 g, 3.4 mmol) in THF (17 mL) was added lithium tri-tert-
butoxyaluminum hydride (1 M in THF, 2 equiv, 6.8 mL, 6.8 mmol) at
0 °C. The reaction was stirred at room temperature for 24 h,
quenched with 1 N HCl, and diluted with water. The mixture was
extracted with CH2Cl2, and the organic layer was washed with brine,
dried over MgSO4, filtered, and concentrated in vacuo. The residue
was purified by column chromatography on silica gel (hexane−
REFERENCES
■
(1) (a) Cheung, H. T.; Feng, M. C. J. Chem. Soc. C 1968, 9, 1047−
1051. (b) Guise, G. B.; Ritchie, E.; Taylor, W. C. Aust. J. Chem. 1962,
15, 314−321.
(2) Aguirre, M. C.; Delporte, C.; Backhouse, N.; Erazo, S.; Letelier,
M. E.; Cassels, B. K.; Silva, X.; Alegria, S.; Negrete, R. Bioorg. Med.
Chem. 2006, 14, 5673−5677.
(3) Ballin, N. Z.; Traore, M.; Tinto, H.; Sittie, A.; Molgaard, P.;
Olsen, C. E.; Kharazmi, A.; Christensen, S. B. J. Nat. Prod. 2002, 65,
1325−1327.
(4) Schuhly, W.; Heilmann, J.; Calis, I.; Sticher, O. Planta Med.
̈
1999, 65, 740−743.
(5) Kim, K. H.; Choi, S. U.; Lee, K. R. Planta Med. 2012, 78, 86−89.
(6) Kang, K. B.; Kim, J. W.; Oh, W. K.; Kim, J.; Sung, S. H. J. Nat.
Prod. 2016, 79, 2364−2375.
(7) (a) Novakovic, M.; Nikodinovic-Runic, J.; Veselinovic, J.; Ilic-
Tomic, T.; Vidakovic, V.; Tesevic, V.; Milosavljevic, S. J. Nat. Prod.
G
DOI: 10.1021/acs.jnatprod.8b00986
J. Nat. Prod. XXXX, XXX, XXX−XXX

Journal of Natural Products
Article
2017, 80, 1255−1263. (b) Raju, R.; Gunawardena, D.; Ahktar, M. A.;
̈
Low, M.; Reddell, P.; Munch, G. Molecules 2016, 21, 1521−1527.
(8) Suksamrarn, S.; Panseeta, P.; Kunchanawatta, S.; Distaporn, T.;
Ruktasing, S.; Suksamrarn, A. Chem. Pharm. Bull. 2006, 54, 535−537.
(9) Reutrakul, V.; Anantachoke, N.; Pohmakotr, M.; Jaipetch, T.;
Yoosook, C.; Kasisit, J.; Napaswa, C.; Panthong, A.; Santisuk, T.;
Prabpai, S.; Kongsaeree, P.; Tuchinda, P. Planta Med. 2010, 76, 368−
371.
(10) Further investigation of other biological activities will be
reported in due course.
(11) (a) Higa, M.; Noha, N.; Yokaryo, H.; Ogihara, K.; Yogi, S.
Chem. Pharm. Bull. 2002, 50, 590−593. (b) Chang, C.; Wu, T.; Hsieh,
Y.; Kuo, S.; Chao, P. J. Nat. Prod. 1999, 62, 327−328.
́
(12) Soica, C. M.; Dehelean, C. A.; Peev, C.; Aluas, M.; Zupko, I.;
́
Kasa, P., Jr.; Alexa, E. Nat. Prod. Res. 2012, 26, 968−974.
(13) Hao, J.; Zhang, X. L.; Zhang, P.; Liu, J.; Zhang, L.; Sun, H.
Tetrahedron 2009, 65, 7975−7984.
(14) Sommerwerk, S.; Csuk, R. Tetrahedron Lett. 2014, 55, 5156−
5158.
(15) (a) Wen, X.; Zhang, P.; Liu, J.; Zhang, L.; Wu, X.; Ni, P.; Sun,
H. Bioorg. Med. Chem. Lett. 2006, 16, 722−726. (b) Wen, X.; Sun, H.;
Liu, J.; Wu, G.; Zhang, L.; Wu, X.; Ni, P. Bioorg. Med. Chem. Lett.
2005, 15, 4944−4948.
(16) Pramanick, S.; Mukhopadhyay, S.; et al. Synth. Commun. 2005,
35, 2143−2148.
(17) (a) Regueiro-Ren, A.; Swidorski, J. J.; Liu, Z.; Chen, Y.; Sin, N.;
Sit, S. Y.; Chen, J.; Venables, B. L.; Zhu, J.; Nowicka-Sans, B.; Protack,
T.; Lin, Z.; Terry, B.; Samanta, H.; Zhang, S.; Li, Z.; Easter, J.; Beno,
B. R.; Arora, V.; Huang, X. S.; Rahematpura, S.; Parker, D. D.;
Haskell, R.; Santone, K. S.; Cockett, M. I.; Krystal, M.; Meanwell, N.
A.; Jenkins, S.; Hanumegowda, U.; Dicker, I. B. J. Med. Chem. 2018,
61, 7289−7313. (b) Kim, D. S.; Chen, Z.; Nguyen, v. T.; Pezzuto, J.
M.; Qiu, S.; Lu, Z.-Z. Synth. Commun. 1997, 27, 1607−1612.
(18) Rubottom, G. M.; Vazquez, M. A.; Pelegrina, D. R. Tetrahedron
Lett. 1974, 15, 4319−4322.
(19) Yajima, A.; Mori, K. Eur. J. Org. Chem. 2000, 2000, 4079−4091.
(20) Low diastereoselectivities were generally observed when 2-
hydroxy-3-ketones of pentacyclic triterpenoids were reduced with
NaBH4. For representative examples, see: (a) Fan, P.; Li, T.; Ye, Y.;
Luo, Q.; Yuan, H.; Lou, H. Phytochem. Lett. 2016, 18, 99−104.
(b) Sommerwerk, S.; Heller, L.; Serbian, I.; Csuk, R. Tetrahedron
2015, 71, 8528−8534. (c) Wen, X.; Sun, H.; Liu, J.; Cheng, K.;
Zhang, P.; Zhang, L.; Hao, J.; Zhang, L.; Ni, P.; Zographos, S. E.;
Leonidas, D. D.; Alexacou, K.-M.; Gimisis, T.; Hayes, J. M.;
Oikonomakos, N. G. J. Med. Chem. 2008, 51, 3540−3554.
̈
nkelmann, P.; Lodige, M.; Walter,
ller, M.;
̈
(21) Husain, S. M.; Stillger, T.; Du
L.; Breitling, E.; Pohl, M.; Burchner, M.; Krossing, I.; Mu
̈
Romano, D.; Molinari, F. Adv. Synth. Catal. 2011, 353, 2359−2362.
(22) Saksena, A. K.; Mangiaracina, P. Tetrahedron Lett. 1983, 24,
273−276.
(23) For representative examples of NaBH(OAc)3-mediated
reductions of axial α-hydroxy cyclohexanones, see: (a) Nishimura,
E.; Yasuno, Y.; Shinada, T. Tetrahedron 2018, 74, 2664−2668.
(b) Watanabe, T.; Shibata, H.; Ebine, M.; Tsuchikawa, H.;
Matsumori, N.; Murata, M.; Yoshida, M.; Morisawa, M.; Lin, S.;
Yamauchi, K.; Sakai, K.; Oishi, T. J. Nat. Prod. 2018, 81, 985−997.
(c) Hoffmann, H. M. R.; Dunkel, R.; Mentzel, M.; Reuter, H.; Stark,
C. B. W. Chem. - Eur. J. 2001, 7, 4771−4789.
(24) Wittman, M. D.; Kadow, J. F.; Vyas, D. M. Tetrahedron Lett.
2000, 41, 4729−4731.
H
DOI: 10.1021/acs.jnatprod.8b00986
J. Nat. Prod. XXXX, XXX, XXX−XXX