Reagent optimization for allylic oxidation of 3-O-acetyl-β-boswellic acid into 3-O-acetyl-11-oxo-β-boswellic acid

Article abstract of DOI:10.1007/s10600-013-0451-1

Introduction of the oxo group at the allylic position of 3-acetoxy-urs-12-en-24β-oic acid (ABA) in the presence of the sensitive 3α-acetyl group, thereby converting it into 3-acetoxy-urs-12-en-11-oxo- 24β-oic acid (AKBA), has been tried with various catal

Full text of DOI:10.1007/s10600-013-0451-1

Chemistry of Natural Compounds, Vol. 48, No. 6, January, 2013 [Russian original No. 6, November–December, 2012]
REAGENT OPTIMIZATION FOR ALLYLIC OXIDATION
OF 3-O-ACETYL-ꢀ-BOSWELLIC ACID INTO
3-O-ACETYL-11-OXO-ꢀ-BOSWELLIC ACID
Suvarna Shenvi and G. Chandrasekara Reddy*
UDC 547.918
Introduction of the oxo group at the allylic position of 3-acetoxy-urs-12-en-24ꢀ-oic acid (ABA) in the presence
of the sensitive 3ꢁ-acetyl group, thereby converting it into 3-acetoxy-urs-12-en-11-oxo-24ꢀ-oic acid (AKBA),
has been tried with various catalytic oxidizing agents. Among them sodium chlorite with N-hydroxyphthalimide
and manganese(III) acetate with t-butyl hydroperoxide were found to be good reagents showing excellent
conversion with functional group compatibility.
Keywords: allylic oxidation, t-butyl hydroperoxide, NaClO , Mn(OAc) , NHPI, BHT, NF-kB, apoptosis.
2
3
Allylic oxidation reactions are industrially important due to their wide variety of applications in the synthesis of
pharmaceuticals and fine chemicals [1].
As the interest in natural compounds with biological activity is re-emerging, we have focused our attention on adding
value to the existing natural product isolate. From Boswellia serrata gum resin, four compounds [2] have been isolated, viz.,
ꢀ-boswellic acid (BA), 3-O-acetyl-ꢀ-boswellic acid (ABA), 11-oxo-ꢀ-boswellic acid (KBA), and 3-O-acetyl-11-oxo-ꢀ-boswellic
acid (AKBA), which are responsible for their 5-lipoxygenase inhibitory activity (5-LOXIN) [3]. Interest stems from the fact
that conversion of all four components into one of the most active ingredients would be an ideal thing since AKBA shows the
most pronounced inhibitory activity among all ꢀ-boswellic acids against 5-LOX, MAP-kinase [4], experimental colitis [5],
apoptosis [6] by suppressing NF-kB gene expression [7], and other pharmacological activities [8, 9], and they are found to
belong to a unique class of dual inhibitors of human topoisomerases I and II ꢁ [10]. We have stumbled upon the idea of
converting the mixture of ꢀ-boswellic acids into their acetyl derivatives followed by allylic oxidation to finally get exclusively
AKBA and hence tried different oxidizing agents with a view to identify and optimize the best ones.
Generally, chromium(Vl) based oxidizing reagents such as CrO -pyridine complex [11, 12], CrO 3,5-dimethyl pyrazole
3
3
[13], pyridinium chlorochromate (PCC) [14], PDC-t-butyl hydroperoxide [15], pyridinium dichromate (PDC) [16], sodium
chromate, sodium dichromate [17], sodium dichromate in acetic acid [18], pyridinium flurochloromate (V1) [19],
3,5-dimethylpyrazolium flurochromate (VI) [20], and a combination of N-hydroxydicarboxylic acid imides with a chromium-
containing oxidant [21] were used to perform allylic oxidations.
On the other hand, some stoichiometric oxidative reagents like MnO , KMnO , SeO , Os(VI), N-methylmorpholine-
2
4
2
N-oxide, iron (III), etc. are also used for the oxidation process [22]. Jauch and Bergmann [23] reported that NBS with CaCO
3
works well with longer reaction times.
Due to environmental concerns in recent years, t-butyl hydroperoxide (TBHP) became the oxidant of choice for the
allylic oxidation of steroidal and natural compounds [24]. It has been reported that TBHP must be catalyzed or promoted by
anyone of the reagents such as sodium chlorite [25], chromium (VI) [26], manganese (III) acetate [27], bismuth (III) salts [28],
copper iodide [29], cobalt acetate [30], ruthenium (II) chloride [31], diruthenium caprolactamate [32], RuCl (PPh ) [33], and
2
3 3
cuprous salts [34] to obtain its oxidizing activity. But none of these eco-friendly oxidizing reagents has been tried for allylic
oxidation in ꢀ-boswellic acids, hence the interest to try out some of these reagents.
rd
th
Vittal Mallya Scientific Research Foundation, # 94/3 & 94/5, 23 Cross, 29 Main, BTM II Stage, Bangalore-560 076,
India, fax: +91 80 26687170, e-mail: gcreddy@vmsrf.org. Published in Khimiya Prirodnykh Soedinenii, No. 6, November–December,
2012, pp. 893–896. Original article submitted August 1, 2011.
1008
0009-3130/13/4806-1008 02013 Springer Science+Business Media New York

TABLE 1. Allylic Oxidation of Methyl Ester of 3-O-Acetyl-ꢀ-boswellic Acid (MABA) (1) with Different Reagents
Reaction
conversion%
Entry
Reagents
Solvent ratio
Time, h
T, ꢂC
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
NBS + CaCO3
NBS + MnO2(active)
Mn(OAc)3·2H2O + TBHP + 3 Å molsieves
Mn(OAc)3·2H2O + TBHP + 3 Å molsieves
Mn(OAc)3·2H2O + TBHP + 3 Å molsieves
Mn(OAc)3·2H2O + NaClO2 + 3 Å molsieves
NaClO2 + NHPI
Dioxane + H2O
Dioxane
25–30
25–30
25–30
2–5
50
25–30
50
25–30
50
50
50
50
50–60
50–60
50–60
50–60
25–30
25–30
25–30
25–30
45–50
12–14
12–14
11–12
24
7–8
12
5–6
6–7
5–6
3–4
100
100
100
90
100
90
100
100
100
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc + H2O (3:1)
DCM + H2O (3:1)
Dioxane + H2O (3:1)
CH3CN + H2O (2:1)
THF
NaClO2 + NHPI
NaClO2 + NHPI
NaClO2 + NHPI
NaClO2 + NHPI
100
8–9
60 (impurity)
95
10–15
70–80
No reaction
70
No reaction
40–45
90 (impurity)
50
NaClO2 + BHT
Iodobenzene
Iodobenzene + NaClO2
TBHP
EtOAc + H2O
Dioxane
10–11
12–13
12–13
12–13
12–13
12–13
12–13
10–11
24–25
5–6
EtOAc
EtOAc
EtOAc
TBHP + NaClO2
NMO + OsO4 + TEAA
BiCl3 + TBHP + K-10
CrO3 + TBHP
Acetone + H2O
CH3CN
CH2Cl2
MnO2(active)
SeO2+H2O2
Dioxane
-BuOH + H O
No reaction
t
2
TABLE 2. Allylic Oxidation of 3-O-Acetyl-ꢀ-boswellic Acid with Different Reagents
Reaction
conversion, %
Entry
Reagents
Solvent ratio
Time, h
Yield, %
T, ꢂC
1
2
3
4
Mn(OAc)3·2H2O + TBHP + 3 Å molsieves
NaClO2 + NHPI
EtOAc
50
50
50
7–8
3–4
10–11
12–14
100
100
95
96
91
84
88
CH3CN + H2O (2:1)
EtOAc + H2O (3:1)
Dioxane
NaClO2 + BHT
NBS + MnO2(active)
25–30
100
H
X
H
H
AcO
ROOC
1 - 4
1: R = CH , X = H,H; 2: R = CH , X = O
3
3
3: R = H, X = H,H; 4: R = H, X = O
We have first tried oxidation with different reagents (Table 1) under different experimental conditions with the methyl
ester of 3-O-acetyl-ꢀ-boswellic acid (1) to get the methyl ester of 3-O-acetyl-11-oxo-ꢀ-boswellic acid (2). Among them,
sodium chlorite (NaClO ) with N-hydroxyphthalimide (NHPI) and manganese(III) acetate (Mn(OAc) ) with t-butyl
2
3
hydroperoxide (TBHP) were found to be good reagents showing excellent conversion (100%) at room temperature. The effect
of solvent medium on reaction times has also been studied. For example, NaClO + NHPI reagent works well in solvent media
2
of CH CN–H O (2:1) and CH Cl –H O (3:1) with shorter reaction times and at ambient temperature. Further, these oxidizing
3
2
2
2
2
rd
agents show very good compatibility with the acetyl group present at the 3 position of the parent pentacyclic triterpenoid and
no formation of any side products.
A second set of experiments (Table 2) was carried out with the naturally occurring 3-O-acetyl-ꢀ-boswellic acid (3) to
convert it into AKBA (4) using a few of the above-mentioned most promising oxidizing reagents to see how these reagents
1009

behave in the presence of a naturally occurring free acid group. Here again the results are identical to what we have observed
with the methyl ester of ABA, thereby proving that these oxidizing agents are eco-friendly, comparatively less expensive, and
industrially feasible.
EXPERIMENTAL
Chemicals and Instruments. TBHP-(t-butyl hydroperoxide 5.0–6.0 M in decane). NaClO 80%. BHT-butylated
2
hydroxy toluene 99%. NHPI-N-hydroxypthalimide 97%. All the reagents and solvents were of analytical grade.
Melting points were recorded on an Acro melting point apparatus and uncorrected. Optical rotations were measured
in CHCl at room temperature (~ 25ꢂC). Thin-layer chromatography (TLC) was performed on TLC silica gel 60 F (Merck).
3
254
Chromatograms were developed using hexane–EtOAc (8:2, v/v), and compounds were detected by H SO solution (10%)
2
4
o
with subsequent heating at 100–120 C for 4–5 min. IR spectra were recorded on a Thermo-Nicolet instrument in KBr discs.
Mass spectra were recorded using GCMS-QP2010S (direct probe) and on a QTOF micro . AMPS MAX 10/6A system.
TM
PMR spectra were recorded in CDCl with TMS as internal standard on a Bruker AG spectrometer at 200 MHz, and chemical
3
shifts are recorded in ꢃ units.
Isolation of ꢀ-Boswellic Acids. We extracted the Boswellia serrata gum resin (1 kg) with methanol (2 ꢄ 2 L) in a
percolator. The combined extracts were evaporated under reduced pressure at 45ꢂC to obtain a thick brown residue (450 g).
The residue was stirred with 3% NaOH (5 L) to a uniform emulsion. The aqueous layer was extracted with CH Cl to remove
2
2
the nonacidic part. The aqueous layer was acidified with 1 N HCl to precipitate the total organic acids. The filtered acids were
washed with water several times till neutral. The crude total acids were redissolved in 3% NaOH solution, and the process was
repeated to give off a white powder. The product was dried in vacuum oven below 50ꢂC to get crude powder of ꢀ-boswellic
acids (250 g).
Acetylation. A mixture of the above acids (100 g) was dissolved in pyridine (400 mL), acetic anhydride (200 mL)
was added at rt, and the solution was stirred overnight. The mass was quenched in crushed ice (1 kg), extracted with EtOAc
(3 ꢄ 300 mL), and dried over Na SO . The ethyl acetate layer was evaporated to get a mixture of ABA and AKBA (108 g).
2
4
Separation. The mixture of ꢀ-boswellic acids (50 g) obtained after acetylation was charged on a silica gel column
(60–120 mesh, 1 kg), and elution was carried out using hexane–ethyl acetate (95:5 to 85:15) to obtain pure ABA (8.9 g) and
AKBA (3.9 g).
Methylation. Pure ABA was dissolved (8 g) in DMF (32 mL) and K CO (3.6 g) added. It was heated to 80ꢂC and
2
3
dimethyl sulfate (2.2 g) added. The whole was maintained at this temperature for 6 h. The reaction mass was cooled to room
temperature and quenched with ice, acidified to pH 3–4, and extracted with ethyl acetate (2 ꢄ 75 mL). The combined extract
was dried over Na SO and vacuum evaporated to get the methyl ester of 3-O-acetyl-ꢀ-boswellic acid (1).
2
4
Preparation. a) For entry 1, Table 1. To a solution of 3-O-acetyl-ꢀ-boswellic acid methyl ester (1) (550 mg, 1.07 mmol)
in aq. dioxane (6.5 mL H 0 in 50 mL), NBS (450 mg, 2.5 mmol) and CaCO (500 mg, 5 mmol) were added, and the whole
2
3
stirred for 12 h at 50ꢂC. The mass was concentrated, then poured into water, extracted with CHCl , and evaporated to dryness
3
to get product 2. Isolated yield 500 mg, 88%.
b) For entry 2, Table 1. To a solution of 3-O-acetyl-ꢀ-boswellic acid methyl ester (1) (125 mg, 0.25 mmol) in dioxane
(3 mL), NBS (100 mg, 0.55 mmol) and MnO
(100 mg, 1.15 mmol) were added and stirred for 14 h at rt. The solution
2(active)
was filtered through a celite pad and the mass was evaporated to dryness to get product 2. Isolated yield 118 mg, 89.6%.
c) For entries 3–6, Table 1. To a solution of the methyl ester of 3-O-acetyl-ꢀ-boswellic acid (1) (550 mg, 1.07 mmol)
ꢅ ꢂ
in EtOAc (10 mL), THBP in decane (125 mg, 1.39 mmol) and 3 ꢅ molecular sieves (100 mg) were added, and the mixture was
stirred for 30 min under nitrogen atmosphere at rt. Manganese (III) acetate dihydrate (7.2 mg, 0.027 mmol) were added, and
the mixture was stirred for 12 h at rt. The solution was filtered through a celite pad and the filtrate evaporated to dryness to get
product 2. Isolated yield 545 mg, 96%.
d) For entries 7–11, Table 1. To a solution of 3-O-acetyl-ꢀ-boswellic acid methyl ester (1) (125 mg, 0.25 mmol) in
CH CN–H O (2:1, v/v, 3 mL), NHPI (4.1 mg, 0.025 mmol) was added, followed by addition of NaClO (42.4 mg, 0.375 mmol).
3
2
2
After stirring at 50ꢂC, the reaction mixture was poured into sodium sulfite solution (10%), then extracted with CHCl . The
3
extract was washed with an aq. solution of NaHCO and water, dried over Na SO , and evaporated to dryness to get product
3
2
4
2. Isolated yield 118 mg, 91%.
1010

e) For entry 12, Table 1. To a solution of 3-O-acetyl-ꢀ-boswellic acid methyl ester (1) (125 mg, 0.25 mmol) in
EtOAc–H O (3:1, v/v, 3 mL), BHT (5 mg, 0.023 mmol) was added, followed by addition of NaClO (42.4 mg, 0.375 mmol)
2
2
under stirring at 50ꢂC. The reaction mixture was poured into sodium sulfite solution (10%), then extracted with CHCl . The
3
extract was washed with an aq. solution of NaHCO and water, dried over Na SO , and evaporated to dryness to get product
3
2
4
2. Yield 108 mg, 84.2%.
f) For entries 13 and 17, Table 1. To a solution of 3-O-acetyl-ꢀ-boswellic acid methyl ester (1) (550 mg, 0.5 mmol) in
dioxane (6 mL), iodobenzene (0.5 mmol, for entry 13) and NMO/OsO /TEAA (0.5 mmol each for entry 17) were added, and
4
the whole stirred for 12 h at 50ꢂC. The reaction mass was concentrated, then poured into water, extracted with CHCl , and
3
evaporated to dryness to get product 2. For entry 17 these was no reaction and the reactant was recovered.
g) For entry 18, Table 1. To a solution of 3-O-acetyl-ꢀ-boswellic acid methyl ester (1) (250 mg, 0.5 mmol) in CH CN
3
(3 mL), BiCl (16 mg, 0.05 mmol) and TBHP (450 mg, 5 mmol, 5.0–6.0 M solution in decane) were added. After 12 h, under
3
stirring at 50ꢂC, the catalyst was removed by filtration and the solution poured into sodium sulfite solution (10%). It was then
extracted with CHCl , washed with an aq. solution of NaHCO and water, dried over Na SO , and evaporated to dryness to get
3
3
2
4
product 2. Isolated yield (90 mg, 35%).
Methyl Ester of 3ꢁ-Acetoxy-urs-12-en-24ꢀ-oic Acid (1). R 0.3, mp 189–190ꢂC, (lit., 189ꢂC [2]); [ꢁ] +71.9ꢂ
f
D
–1
+
(c 0.5, CHCl ). IR spectrum (KBr, ꢆ , cm ): 1735 (CH CO), 1710 (COOCH ). EI-MS (m/z, I , %): 512 [M C H O ] (12),
497 (3), 438 (5), 294 (14), 234 (86), 218 (100), 203 (88), 189 (31), 175 (31). H NMR (CDCl , ꢃ, ppm, J/Hz): 0.81 (6H, br,
3
max
3
3
rel
33 52 4
1
3
2 ꢄ CH ), 0.84 to 1.18 (15H, 5 ꢄ CH ), 1.26 to 2.03 (21H), 2.09 (3H, s, OAc), 2.20 (2H, br), 3.67 (3H, s, COOCH ), 5.15 (1H,
3
3
3
t, J = 3.4), 5.33 (1H, t, J = ~3.0).
Methyl Ester of 3ꢁ-Acetoxy-urs-12-en-11-oxo-24ꢀ-oic Acid (2). R 0.47, mp 189ꢂC, (lit., 188–190ꢂC [2]);
f
–1
[ꢁ] +86ꢂ (c 0.5, CHCl ). IR spectrum (KBr, ꢆ , cm ): 1735 (CH CO), 1710 (COOCH ), 1655 and 1610 (ꢁ, ꢀ unsaturated
C=O). EI-MS (m/z, I , %): 526 [M , C H O ] (26), 511 (3), 467 (6), 407 (5), 273 (100), 232 (60), 189 (5), 135 (50),
161 (35). H NMR (CDCl , ꢃ, ppm, J/Hz): 0.79 to 0.82 (6H, m, 2 ꢄ CH ), 0.89 to 1.25 (15H, 5 ꢄ CH ), 1.35 to 2.06 (18H), 2.09 (3H,
D
3
max
3
3
+
rel
33 50 5
1
3
3
3
s, OAc), 2.17 (1H, br), 2.41 (1H, s), 2.53 (1H, br), 3.68 (3H, s, COOCH ), 5.33 (1H, t, J = ~3.0), 5.55 (1H, s).
3
3ꢁ-Acetoxy-urs-12-en-24ꢀ-oic Acid (3). R 0.41, mp 252–253ꢂC, (lit., 251–253ꢂC [2, 23]); [ꢁ] + 66.2ꢂ (c 0.5,
f
D
–1
CHCl ). IR spectrum (KBr, ꢆ , cm ): 3350 and 2965 (COOH), 1727 (CH CO), 1702 (COOH) and 1275 (C-O-C). EI-MS
3
max
3
+
1
(m/z, I , %): 498 [M C H O ] (7), 483 (5), 470 (3), 438 (7), 377 (2), 218 (100), 203 (25), 189 (5), 175 (17). H NMR
rel
32 50 4
(CDCl , ꢃ, ppm, J/Hz): 0.81 to 0.87 (6H, m, 2 ꢄ CH ), 0.91 to 1.24 (15H, 5 ꢄ CH ), 1.34 to 2.03 (20H), 2.10 (3H, s, OAc), 2.15
3
3
3
(2H, br), 2.23 (1H, br), 5.15 (1H, t, J = 3.4), 5.31 (1H, br.t, J = ~3.0).
3ꢁ-Acetoxy-urs-12-en-11-oxo-24ꢀ-oic Acid (4). R 0.54, mp 272–273ꢂC, (lit., 271–274ꢂC [2, 23]); [ꢁ] +94ꢂ
f
D
–1
(c 0.5, CHCl ). IR spectrum (KBr, ꢆ , cm ): 3350, 1728 (CH CO), 1706 (COOH), 1658 (ꢁ, ꢀ unsaturated C=O), and 1274
3
max
+
3
(C-O-C). EI-MS (m/z, I , %): 512 [M , C H O ] (8), 497 (3), 452 (15), 434 (5), 408 (4), 273 (100), 232 (75), 189 (12), 175
(30), 161 (15). H NMR spectrum (CDCl , ꢃ, ppm, J/Hz): 0.78 to 0.83 (6H, m, 2 ꢄ CH ), 0.94 to 1.35 (15H, 5 ꢄ CH ), 1.43 to
rel
32 48 5
1
3
3
3
2.04 (18H), 2.09 (3H, s, OAc), 2.28 (1H, br), 2.41 (1H, s), 2.55 (1H, br), 5.30 (1H, t, J = ~3.0), 5.56 (1H, s).
Conclusion. Among various oxidizing agents used to convert naturally occurring 3-O-acetyl-ꢀ-boswellic acid (3)
into AKBA (4), it was found that sodium chlorite (NaClO ) with N-hydroxyphthalimide (NHPI) and manganese (III) acetate
2
(Mn(OAc) ) with t-butyl hydroperoxide (TBHP) showed excellent conversion (100%). The effect of solvent medium on
3
reaction times is clearly shown. Thus, it can be concluded that these oxidizing agents are eco-friendly, comparatively less
expensive, and industrially feasible.
ACKNOWLEDGMENT
We express our sincere gratitude to Dr. Anil Kush, CEO, Vittal Mallya Scientific Research Foundation, for his keen
interest and encouragement and also wish to thank Mr. A. C. Karunakara and Ms. Aparna Bhat for analytical help.
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