Oxyfunctionalization products of terpenoids with dimethyldioxirane and their biological activity

Article abstract of DOI:10.1248/cpb.55.247

Oxyfunctionalization of the bioactive terpenoids, ursolic acid acetate (1), oleanolic acid acetate (5), lupeol acetate (12), and kaurenic acid (17), with dimethyldioxirane (DMDO) was investigated. Treatment of the terpenoids with DMDO under mild conditions afforded a variety of oxidation and oxydegradation products to yield naturally occurring and/or novel compounds in one step. After chromatographic separation, the structures of the individual isolated products were determined using spectroscopic methods including several homonuclear ( ¹H-¹H) and heteronuclear (¹H-¹³C) shift-correlated 2D-NMR techniques. The inhibitory activity of the terpenoid derivatives against α-glucosidase was investigated and compounds 1, 3, 7, and 9 were found to exhibit potent activity.

Full text of DOI:10.1248/cpb.55.247

February 2007
Chem. Pharm. Bull. 55(2) 247—250 (2007)
247
Oxyfunctionalization Products of Terpenoids with Dimethyldioxirane and
Their Biological Activity
Shoujiro OGAWA,a Keiji HOSOI,a Noriaki IKEDA,a Mitsuko MAKINO,b Yasuo FUJIMOTO,b and
Takashi IIDA*,a
b
a Department of Chemistry and Department of General Studies, College of Humanities and Sciences, Nihon University;
Sakurajosui, Setagaya-ku, Tokyo 156–8550, Japan.
Received July 20, 2006; accepted November 7, 2006; published online November 9, 2006
Oxyfunctionalization of the bioactive terpenoids, ursolic acid acetate (1), oleanolic acid acetate (5), lupeol ac-
etate (12), and kaurenic acid (17), with dimethyldioxirane (DMDO) was investigated. Treatment of the terpenoids
with DMDO under mild conditions afforded a variety of oxidation and oxydegradation products to yield natu-
rally occurring and/or novel compounds in one step. After chromatographic separation, the structures of the in-
dividual isolated products were determined using spectroscopic methods including several homonuclear (1H–1H)
and heteronuclear (1H–13C) shift-correlated 2D-NMR techniques. The inhibitory activity of the terpenoid deriva-
tives against a-glucosidase was investigated and compounds 1, 3, 7, and 9 were found to exhibit potent activity.
Key words dimethyldioxirane; oxyfunctionalization; triterpenoid; diterpenoid; a-glucosidase
In analogy with enzyme-controlled reactions in vivo, such DMDO to obtain more biologically active derivatives as well
as cytochrome P-450 oxidase-dependent systems,1) regio- as to clarify the factors governing its reactivity and O-inser-
and stereoselective oxyfunctionalization in substrates is a key tion position in oxidative transformations of the substrates.
transformation in the efficient and convenient synthesis of bi- The biological activity of the oxidized products was also ex-
ologically active compounds, starting from abundantly avail- amined.
able and less active terpenoids, steroids, and/or alkaloids
occurring in nature. Hence various oxidants mimicking the Results and Discussion
P-450 enzymatic system have been developed by many
The substrates examined in this study are major bioactive
triterpenoids (1, 5, 12) and a diterpenoid (17) occurring in a
groups.2—4)
Recently, the successful use of dioxiranes, a new class of variety of plant species. To prevent the simultaneous oxida-
powerful and versatile cyclic peroxides with a three-mem- tion of hydroxyl groups in the substrates to carbonyls, these
bered ring, has been demonstrated to insert an oxygen atom compounds were protected as their acetyl derivatives prior to
into a variety of substrates.5—7) Applications include the oxy- DMDO reaction. DMDO oxidation was carried out for 24—
functionalization of primary alcohols and aldehydes to the 36 h at room temperature. All of the substrates were found to
corresponding carboxylic acids, secondary alcohols to ke- react readily with DMDO to form a variety of novel oxy-
tones, alkenes to epoxides, amines to nitro compounds, sul- genated products in one step. The reactivity and selectivity of
fides to sulfones or sulfoxides, and phosphines to phosphine the oxidation reactions depended significantly on the struc-
oxides. Of further interest is that dioxiranes display high effi- tures of the substrates.
ciency for the O-insertion reaction into “unactivated” tertiary
The oxidation of ursolic acid acetate (1) with DMDO gave
and secondary C–H bonds of hydrocarbons.8—10) Despite re- 11-oxo-ursolic acid acetate (2, 15% yield) and 11,12-dehy-
ports of such versatile reactivity and usefulness in organic droursolic lactone (3, 11% yield) and its 20b-hydroxy deriva-
synthesis, the further detailed, potential ability of dioxiranes tive (4, 10% yield) (Fig. 1). In spite of the structural similar-
remains to be explored.
ity of ursolic acid and oleanolic acid (positional isomer of a
The smallest (C3) cyclic peroxide, dimethyldioxirane methyl group at C-19 and C-20), the DMDO oxidation of 3-
(DMDO), which is generated readily by the reaction of ace- O-acetyloleanolic acid (5) unexpectedly afforded 3-O-acetyl-
tone with Oxone® (2KHSO5·KHSO4·K2SO4),11,12) is a class 12a-chloro-oleanolic lactone (6, 50% yield) as a major prod-
of environmentally friendly oxidant that does not contain a uct together with 3-O-acetyl-9,11-dehydro-12a-hydroxy-
toxic, hazardous heavy metal. DMDO was reported to dis- oleanolic lactone (7, 20% yield) and its 12b-hydroxy deriva-
play high efficiency in the oxyfunctionalization of various tive (8, 10% yield) and 3-O-acetyl-12-oxo-oleanolic lactone
substrates.5—7) DMDO also allowed the reaction to be per- (9, 5% yield). As shown in Fig. 2, the chloride (6) may be
formed under extremely mild, strictly neutral nonhydrolytic produced by the action of hypochloric acid formed from
conditions; particularly noteworthy is the ease of handling CHCl3 and DMDO during storage in a refrigerator. In the ox-
and simplicity of workup procedures in experimental DMDO idation of 1, it is considered that the double bond at C-12,13
oxidation.
is sterically hindered by the methyl group at C-19. Therefore
As a part of our continuing efforts directed toward the hypochloric acid was not accessible to the double bond, and
convenient, efficient synthesis of biologically active com- allylic oxidation occurred preferentially to give an allyl alco-
pounds, we became interested in the efficient utilization and hol (1ꢀ), which in turn was converted to the enone (2) and
development of DMDO oxidation. We report here the oxy- lactone (3) by lactonization and dehydration. On the other
functionalization of ursolic acid acetate (1), oleanolic acid hand, treatment of 5 with DMDO generated in situ in CH2Cl2
acetate (5), lupeol acetate (12), and kaurenic acid (17) by yielded only 2% of the chloride (6) together with 11-oxo-
∗ To whom correspondence should be addressed. e-mail: takaiida@chs.nihon-u.ac.jp
© 2007 Pharmaceutical Society of Japan

248
Vol. 55, No. 2
Fig. 1. Oxidation of Ursolic Acid Acetate with DMDO
Fig. 2. Oxidation of Oleanolic Acid Acetate with DMDO
oleanolic acid acetate (10, 26% yield) and 3-O-acetyl-12a- yield). The major products 18, 19, and 20 and the minor
hydroxyoleanolic lactone (11, 18% yield). These results sug- product 22 were produced via the initially formed epoxide
gest that CH2Cl2 was more stable against DMDO than 17ꢀ, as shown in Fig. 4. The chlorohydrin (21) must be pro-
CHCl3, and that 10 and 11 may be produced through allylic duced by the action of hypochloric acid produced during
oxidation and ring opening of the initially formed a-epoxide the storage of DMDO/CHCl3. Each of the reaction products
(5ꢀ), respectively (Fig. 2). Upon oxidation of lupeol acetate was separated on open column chromatography, followed
(12) with DMDO, 30-nor-20-oxo-lupeol acetate (13, 41% by medium-pressure column chromatography (MPLC) or
yield) and 30-oxo-lupeol acetate (14, 30% yield) were ob- HPLC. The structures of the individually isolated products,
tained as major products, along with two minor products (15, particularly of the novel compounds 4, 6, 16, and 21, were
1
5% yield, and 16, 4% yield). The mechanism of the forma- determined on the basis of 2D homonuclear H–1H and het-
1
tion of major products from the initially formed epoxide eronuclear H–13C shift-correlated 2D-NMR spectroscopy
(12ꢀ) of 12 can be rationalized as follows. Acid-catalyzed and MS spectral data.
ring opening of the epoxide ring, followed by the addition of
DMDO (path a) or by a proton loss from C-30 (path b), leads derivatives synthesized in this study against a-glucosidase
to the ketone (13) or a,b-unsaturated aldehyde (14) (Fig. 3). was evaluated, and the ursolic acid derivatives (1, 3) and
Biological Activity The inhibitory activity of terpenoid
Next, we examined the oxidation of kaurenic acid (17). oleanolic acid derivatives (7, 8, 9) were found to exhibit
Treatment of 17 with DMDO/CHCl3 afforded 15,16-dehy- more potent activity than the original terpenoids (Table 1).
dro-17-hydroxykaurenic acid (18, 32% yield), 16,17-dihy-
Experimental
droxykaurenic acid (19, 18% yield), 15,16a-epoxy-17-hy-
Melting points were determined on an electric micro hot stage and are un-
droxykaurenic acid (20, 16% yield), 17-chloro-16a-hydroxy-
kaurenic acid (21, 3% yield), and dicarboxylic acid (22, 3%
corrected. IR spectra were obtained on a Bio Rad FTS-7 FT-IR spectrometer
(Philadelphia, PA, U.S.A.) for samples in KBr pellets. 1H- and 13C-NMR

February 2007
249
Fig. 3. Oxidation of Lupeol Acetate with DMDO
Fig. 4. Oxidation of Kaurenic Acid with DMDO
JEOL JMS-GCmate mass spectrometer. A Shimadzu GC-2010 gas chromato-
graph equipped with a flame ionization detector was used isothermally at
300 °C fitted with a chemically bonded, fused silica capillary column
(25QC3/BPX5; 25 mꢁ0.32 mm i.d.; film thickness, 0.25 mm; SGE, Mel-
bourne, Australia). The apparatus used for MPLC consisted of a YRD-880
RI-detector (Shimamura Tech. Co. Ltd., Tokyo, Japan) and a uf-3040s chro-
matographic pump using silica gel (230—400 mesh; Nacalai Tesque, Kyoto,
Japan) as the adsorbent and benzene–EtOAc mixture as the eluent. The
preparative normal-phase HPLC apparatus consisted of an Hitachi (Tokyo,
Japan) L-7100 pump equipped with a Shodex RI detector (Tokyo, Japan)
and a Kaseisorb LC 120-5 column (250 mmꢁ10 mm i.d.; Tokyo Kasei
Kogyo Co. Ltd., Tokyo, Japan) using benzene–EtOAc (9 : 1—5 : 5, v/v) mix-
tures as eluent. TLC was performed on precoated silica gel plates (0.25 mm
layer thickness; E. Merck, Darmstadt, Germany) using hexane–EtOAc as the
developing solvent.
Table 1. Inhibitory Activity of Oxyfunctionalized Compounds (10 mM)
against a-Glucosidase
Inhibition
(%)
Inhibition
(%)
Inhibition
(%)
Compound
Compound
Compound
1
2
3
4
5
6
7
8
83.2
<10
62.7
<10
<10
20.9
53.8
37.3
9
10
11
12
13
14
15
16
46.8
ꢂ10
ꢂ10
11.9
16.7
31.6
ꢂ10
16.3
17
18
19
20
21
22
ꢂ10
ꢂ10
24.5
ꢂ10
ꢂ10
ꢂ10
A concentrated DMDO solution (0.17 mol) in CHCl3 was prepared
spectra were measured on a JEOL JNM-EX 270 FT instrument (Tokyo, according to the previously reported method using Oxone® (caroate;
Japan) at 270 and 67.8 MHz, respectively, in CDCl3 containing 0.1% Me4Si
2KHSO5·KHSO4·K2SO4), NaHCO3, and acetone.11,12) The substrates 1, 5,
as an internal standard. Chemical shifts are expressed as d ppm relative to 12, and 17 used in this study were from our laboratory collection, and the
Me4Si. 13C-NMR signals corresponding to methyl (CH3), methylene (CH2), hydroxyl group at C-3 in 1, 5, and 12 was acetylated in the usual manner.
methine (CH), and quaternary (C) carbons were differentiated by means of
General Oxidation Procedure with DMDO To a solution of a ter-
distortionless enhancement by polarization transfer experiments. 2D-NMR penoid (500 mg) in CH2Cl2 (4.0 ml) was added a previously prepared solu-
spectra were measured on a JEOL GSX-400 spectrometer. Low-resolution tion of DMDO (0.17 mol, 8.0 ml) in CHCl3. The mixture was left at room
electron-impact or fast atom bombardment mass spectra were recorded on a
JEOL JMS-GCmate mass spectrometer at 70 eV. High-resolution electron- under reduced pressure. The above procedure was repeated several times,
impact or fast atom bombardment mass spectra were measured on a until the spot of the substrate disappeared with monitoring by TLC (total re-
temperature for 12 h, and excess DMDO and solvent were evaporated off

250
Vol. 55, No. 2
action times, 24—36 h). After the reaction, the product was purified by pas- 1.56 (3H, s, H-30), 2.04 (3H, s, CH3CO), 2.22 (1H, m, H-19), 4.49 (1H, dd,
sage through an open column on silica gel (70—230 mesh), followed by Jꢅ5.2, 10.9 Hz, H-3), 9.43 (1H, s, H-29). 13C-NMR d: 38.5 (C-1), 23.7 (C-
MPLC on silica gel or by HPLC.
2), 80.9 (C-3), 37.8 (C-4), 55.3 (C-5), 18.2 (C-6), 34.4 (C-7), 41.3 (C-8),
Physicochemical Data for New Compounds 3-O-Acetyl-20b-hy- 50.1 (C-9), 37.0 (C-10), 21.3 (C-11), 27.8 (C-12), 37.0 (C-13), 43.5 (C-14),
droxy-urs-11-en-28,13-olide (4): Colorless amorphous solids crystallized 27.3 (C-15), 35.4 (C-16), 44.5 (C-17), 48.5 (C-18), 46.4 (C-19), 79.7 (C-20),
from aqueous methanol, mp 276—278 °C. FAB-MS m/z: 512.3503 [M]ꢃ 29.2 (C-21), 39.7 (C-22), 28.0 (C-23), 16.5 (C-24), 16.2 (C-25), 16.1 (C-26),
(Calcd for C32H48O5: 512.3501). EI-MS m/z: 512 [M]ꢃ (10), 494
[MꢄH2O]ꢃ (100), 484 [MꢄCOO]ꢃ (4), 468 [MꢄCH3COOH]ꢃ (26), 450
[MꢄCH3COOHꢄH2O]ꢃ (84). IR (KBr) cmꢄ1: 3495 (OH), 1734 (CꢅO).
14.7 (C-27), 19.4 (C-28), 195.7 (C-29), 24.3 (C-30), 171.0 (CH3CꢅO), 21.3
(CH3CO).
Ent-17-chloro-16-hydroxy-kauran-19-oic Acid (21): An amorphous pow-
1H-NMR d: 0.86 (3H, s, H-23), 0.87 (3H, s, H-27), 0.94 (3H, s, H-25), 1.03 der. FAB-MS m/z: 354.1959 [M]ꢃ (Calcd for C20H31O3Cl: 354.1962). EI-MS
(3H, d, Jꢅ8.6 Hz, H-29), 1.06 (3H, s, H-26), 1.16 (3H, s, H-24), 1.22 (3H, s, m/z: 354 [M]ꢃ (6), 336 [MꢄH2O]ꢃ (21), 323 (55), 305 (100), 300
H-30), 2.06 (3H, s, CH3CO), 4.49 (1H, dd, Jꢅ5.6, 10.6 Hz, H-3), 5.54 (1H,
dd, Jꢅ3.2, 10.5 Hz, H-11), 5.93 (1H, dd, Jꢅ1.5, 10.5 Hz, H-12). 13C-NMR
d: 38.0 (C-1), 23.3 (C-2), 80.6 (C-3), 37.8 (C-4), 54.8 (C-5), 17.6 (C-6),
31.2 (C-7), 41.7 (C-8), 52.9 (C-9), 36.3 (C-10), 128.8 (C-11), 133.4 (C-12),
89.6 (C-13), 42.0 (C-14), 25.5 (C-15), 22.1 (C-16), 44.5 (C-17), 54.5 (C-18),
39.5 (C-19), 71.1 (C-20), 35.8 (C-21), 26.5 (C-22), 27.7 (C-23), 15.9 (C-24),
18.0 (C-25), 18.9 (C-26), 16.0 (C-27), 179.5 (C-28), 12.6 (C-29), 28.5 (C-
30), 171.0 (CH3CꢅO), 21.3 (CH3CO).
[MꢄH2OꢄHCl]ꢃ (58). IR (KBr) cmꢄ1: 3200 (OH), 1700 (COOH). 1H-
NMR d: 0.95 (3H, s, H-20), 1.24 (3H, s, H-18), 3.76, 3.84 (2H, ABq,
Jꢅ11.0 Hz, H-17). 13C-NMR d: 40.6 (C-1), 19.0 (C-2), 37.0 (C-3), 43.7 (C-
4), 56.8 (C-5), 21.9 (C-6), 41.7 (C-7), 45.2 (C-8), 55.7 (C-9), 39.7 (C-10),
18.6 (C-11), 26.0 (C-12), 46.2 (C-13), 37.8 (C-14), 51.8 (C-15), 80.7 (C-16),
53.5 (C-17), 28.9 (C-18), 183.5 (C-19), 15.5 (C-20).
Inhibition of a-Glucosidase13) The inhibitory activity of all samples
against a-glucosidase (Sigma G3651) was measured spectrophotometrically
at pH 6.8 and 37 °C at 20 min using p-nitrophenyl a-D-glucopyranoside
(PNP-G) 0.1 mM as a substrate and 0.017 units/ml of enzyme in 50 mM
sodium phosphate buffer containing 100 mM NaCl. The activity was deter-
mined by measuring the librated p-nitrophenol at 405 nm due to the hydroly-
3-O-Acetyl-12a-chloro-oleanan-28,13-olide (6): Colorless amorphous
solids crystallized from aqueous methanol, mp 243—246 °C. FAB-MS m/z:
533.3396 [M]ꢃ (Calcd for C32H50O4Cl: 533.3397). FAB-MS m/z: 533
[M]ꢃ (4), 487 [MꢄHCOOH]ꢃ (1), 473 [MꢄCH3COOH]ꢃ (8), 437
[MꢄCH3COOHꢄHCl]ꢃ (6) 154 (100), 136 (100). IR (KBr) cmꢄ1: 1724, sis of PNP-G by a-glucosidase as measured on a microplate reader (Model
1729 (CꢅO). 1H-NMR d: 0.85 (3H, s, H-24), 0.88 (3H, s, H-23), 0.90 (3H, 3550 Microplate Reader, Bio-Rad, Tokyo, Japan) at room temperature after
s, H-30), 0.91 (3H, s, H-25), 1.00 (3H, s, H-29), 1.20 (3H, s, H-27), 1.39
the addition of 50 mM sodium bicarbonate (pH 10.0, 50 mM) to the reaction
(3H, s, H-26), 2.05 (3H, s, CH3CO), 4.17 (1H, br s, H-12), 4.52 (1H, dd, mixture.
Jꢅ5.2, 10.5 Hz, H-3). 13C-NMR d: 38.2 (C-1), 21.3 (C-2), 80.6 (C-3), 37.8
(C-4), 55.3 (C-5), 17.6 (C-6), 34.4 (C-7), 42.4 (C-8), 44.8 (C-9), 36.4 (C-
10), 29.3 (C-11), 64.9 (C-12), 91.6 (C-13), 43.0 (C-14), 27.5 (C-15), 23.5
(C-16), 45.3 (C-17), 51.9 (C-18), 39.8 (C-19), 31.8 (C-20), 33.9 (C-21), 29.0
(C-22), 27.9 (C-23), 16.5 (C-24), 16.8 (C-25), 20.2 (C-26), 18.9 (C-27),
179.0 (C-28), 33.2 (C-29), 23.6 (C-30), 171.0 (CH3CꢅO), 21.3 (CH3CO).
Acknowledgements This work was supported by a Grant-in-Aid (No.
15510182) for Scientific Research from the Ministry of Education, Culture,
Sports, Science, and Technology of Japan and a Nihon University Multidis-
ciplinary Research Grant.
3-O-Acetyl-12a-hydroxy-olean-9-en-28,13-olide (7): Colorless amor- References
phous solids crystallized from aqueous methanol, mp 201—203 °C. EI-MS
m/z: 512.3503 [M]ꢃ (Calcd for C32H48O5: 512.3502). EI-MS m/z: 512 [M]ꢃ
(10), 497 [MꢄCH3]ꢃ (10), 452 [MꢄCH3COOH]ꢃ (14), 437
[MꢄCH3COOHꢄH2O]ꢃ (5), 249 (100). IR (KBr) cmꢄ1: 2928 (OH), 1708,
1776 (CꢅO). 1H-NMR d: 0.88 (6H, s, H-23, H-24), 0.93 (3H, s, H-27), 0.99
(3H, s, H-30), 1.01 (3H, s, H-29), 1.22 (3H, s, H-25), 1.35 (3H, s, H-26),
2.06 (3H, s, CH3CO), 4.21 (1H, d, Jꢅ3.7 Hz, H-12), 4.47 (1H, dd, Jꢅ5.4,
10.5 Hz, H-3), 5.45 (1H, d, Jꢅ3.7 Hz, H-11). 13C-NMR d: 37.5 (C-1), 24.1
(C-2), 80.2 (C-3), 38.1 (C-4), 51.5 (C-5), 17.4 (C-6), 32.9 (C-7), 44.3 (C-8),
157.8 (C-9), 38.9 (C-10), 118.3 (C-11), 65.7 (C-12), 89.5 (C-13), 41.7 (C-
14), 27.1 (C-15), 27.2 (C-16), 43.5 (C-17), 44.1 (C-18), 36.9 (C-19), 31.7
(C-20), 34.2 (C-21), 20.7 (C-22), 28.0 (C-23), 16.5 (C-24), 24.6 (C-25), 28.6
(C-26), 20.9 (C-27), 179.3 (C-28), 33.3 (C-29), 24.1 (C-30), 171.0
(CH3CꢅO), 21.3 (CH3CO).
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