Bismuth(III) triflate-based catalytic direct opening of oleanolic hydroxy-γ-lactones to afford 12-oxo-28-carboxylic acids

Article abstract of DOI:10.1002/adsc.201100155

The bismuth(III) triflate-based catalytic direct opening of oleanolic hydroxy-γ-lactones affords the corresponding 12-oxo-28-carboxylic acid derivatives, in both acetonitrile and dichloromethane, at reflux, in high yields. Participation of an in situ generated Bronsted acid species from bismuth(III) triflate is most likely involved in the reaction mechanism. Full structural elucidation of the products obtained has been performed by 1D and 2D NMR techniques. Copyright

Full text of DOI:10.1002/adsc.201100155

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DOI: 10.1002/adsc.201100155
Bismuth(III) Triflate-Based Catalytic Direct Opening of Oleanolic
G
Hydroxy-g-lactones to Afford 12-Oxo-28-carboxylic Acids
Jorge A. R. Salvador,a,b,* Vꢀnia M. Moreira,a,c Rui M. A. Pinto,a Ana S. Leal,a
and Christophe Le Rouxd
a
Grupo de Quꢁmica FarmacÞutica, Faculdade de Farmꢂcia da Universidade de Coimbra, Pꢃlo das CiÞncias da Safflde,
Azinhaga de Santa Comba, 3000-548 Coimbra, Portugal
Fax : (+351)-239-488-503; phone: (+351)-239-488-400; e-mail: salvador@ci.uc.pt
Centro de NeurociÞncias e Biologia Celular, Universidade de Coimbra, 3004-517 Coimbra, Portugal
Centro de Quꢁmica de Coimbra, Faculdade de CiÞncias e Tecnologia da Universidade de Coimbra, Rua Larga, 3004-535
b
c
Coimbra, Portugal
d
Laboratoire HØtØrochimie Fondamentale et AppliquØe, UniversitØ Paul Sabatier, 118, route de Narbonne, 31062 Toulouse
Cedex 9, France
Received: March 5, 2011; Revised: June 15, 2011; Published online: October 10, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adcs.201100155.
Abstract: The bismuth
G
clude oleanolic acid 1 (Scheme 1), maslinic, morolic
and moronic acids among others. Oleanolic acid (OA)
1 has attracted a growing interest in the last couple of
decades due to its pharmacological effects, combined
with its low toxicity.[4] In addition, OA 1 and its deriv-
atives have demonstrated antibacterial,[5,6] antiparasit-
ic,[7] antiosteoporotic,[8] antifertillity,[9] antihyperten-
sive and antihyperlipidemic,[10] diuretic,[11] antidiabet-
ic,[12,13] immunomodulatory,[14] anti-inflammatory,[15]
antinociceptive,[16] gastroprotective,[17] hepatoprotec-
tive,[18] and anti-HIV activities,[19] as well as having the
ability to inhibit the complement pathway.[20]
direct opening of oleanolic hydroxy-g-lactones af-
fords the corresponding 12-oxo-28-carboxylic acid
derivatives, in both acetonitrile and dichlorome-
thane, at reflux, in high yields. Participation of an in
situ generated Brønsted acid species from bis
ACHTUNGTRENNUNG
mechanism. Full structural elucidation of the prod-
ucts obtained has been performed by 1D and 2D
NMR techniques.
Keywords: bismuthACHTNUGRTENUNG(III) triflate; hydroxylactones;
oleanolic compounds; 12-oxo-28-carboxylic acids;
pentacyclic triterpenoids
In the past few years, bismuthACTHGNUTERNNUG
(III) salts[21,22] have
emerged as convenient reagents for the development
of new chemical processes[23] under more “ecofriend-
ly” reaction conditions, which avoid the use of large
amounts of toxic and corrosive materials. Indeed, it is
becoming more and more common to find bismuth-
Natural products have been indispensable tools for
ACHTUNGTREN(NGNU III) salts-based processes for the synthesis of com-
understanding the logic of biosynthesis and as plat- pounds of pharmaceutical interest.[24] As part of our
forms for developing front-line drugs.[1] It is no doubt ongoing work on exploiting bismuth-based processes
noteworthy that some of the most interesting natural in natural products chemistry,[25] we herein report the
products currently available come from readily acces- bismuthACTHNUTRGNEUNG(III) triflate-based catalytic direct opening of
sible natural sources such as marine organisms and oleanolic hydroxy-g-lactones to afford the corre-
plants. As the largest class of natural products, terpe- sponding 12-oxo-28-carboxylic acid derivatives, in
noids have a variety of roles in mediating antagonist high yields. This novel reaction greatly simplifies the
and beneficial interactions among organisms.[2] Triter- preparation of oleanolic 12-oxo-28-carboxylic acid de-
penoids have been studied for their anti-inflamma- rivatives which will allow further modifications of the
ACHTUNGTRENNUNGtory, hepatoprotective, analgesic, antimicrobial, anti- oleanoic acid core.
mycotic, virostatic, immunomodulatory, and tonic ef-
The synthesis of the oleanolic hydroxy-g-lactones
fects. They are used in the prevention and treatment 2,[13] 4,[26] 6,[6] 8 and 10 has been accomplished using
of hepatitis and above all for their cytostatic effects.[3] m-chloroperoxybenzoic acid (MCPBA)[27] (Scheme 1).
Oleanane triterpenoids comprise a large group of
pentacyclic triterpenoids present in nature that in- 1 mol% of BiAHCTUNTGRENNUNG
The reaction of the hydroxy-g-lactone 2 with
(III) triflate, in dichloromethane, at
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Jorge A. R. Salvador et al.
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Scheme 1. Synthesis of the oleanolic hydroxy-g-lactones 2, 4, 6, 8 and 10.
reflux, was found to afford the corresponding 12-oxo- tion. Almost no reaction was found to occur using
28-carboxylic acid 11, in 91% crude yield, after 4 h 1 mol% of bismuth bromide (Table 2, entry 1) or
(Table 1, Scheme 2). Screening assays for solvent 5 mol% of bismuth chloride, in dichloromethane
choice using compound 2 as substrate are depicted in (entry 5). A loading amount of 10 mol% of these cat-
Table 1. Thus, the reaction was also found to occur in alysts was needed to obtain full conversion in di-
acetonitrile in the same time, at reflux, quantitatively chloromethane (entries 3 and 6).
(Table 1, entry 2). However, no reaction was found to
Among the other metal triflates screened for this
occur in ethyl acetate and only 18% product was reaction, we found that 10 mol% of lanthanum triflate
identified in the reaction crude after 4 h, at 408C, could complete the reaction in refluxing acetonitrile
using 1,4-dioxane (Table 1, entries 3 and 4).
(entry 11), whereas ytterbium triflate completed the
reaction at 5 mol%, in 5 h, in refluxing dichlorome-
thane (entry 13) and at 10 mol% in refluxing acetoni-
trile (entry 15). Copper triflate afforded 71% of reac-
tion product after 2 h in refluxing dichloromethane, at
1 mol% (entry 16). The reaction was found to be
complete in both solvents tested using 5 mol% of this
catalyst (entries 17 and 18). Despite the good results
Table 1. Opening of the oleanolic hydroxy-g-lactone 2 with
Bi
G
vents.[b]
Entry
Solvent
Temp.
Time [h]
Yield [%][c]
1
2
3
4
CH2Cl2
CH3CN
ethyl acetate
1,4-dioxane
reflux
reflux
408C
408C
4
4
4
4
91
87
–
obtained with copper triflate, we chose bismuthACHTUNGTRENNUNG(III)
triflate as the optimal catalyst for this study based not
only on the short reaction times and high substrate
conversions but also on its properties as an environ-
mentally friendly, inexpensive and easily available re-
agent.
The protons which result from the hydrolysis of
bismuthACTHNUTRGENUG(N III) salts have been known to be the true cat-
alytic species involved in some of the reactions in
which these reagents participate.[28,29] We set out to
determine the nature of the catalysis promoted by
18[d]
[a]
Calculations based on anhydrous salt.
[b]
Reaction conditions: 0.11 mmol of 2, 1 mol% of
Bi(OTf)3·xH2O, 1.5 mL of solvent, reflux.
ACHTUNGTRENNUNG
[c]
[d]
Isolated yield.
The yield was determined by integration of the 13b-H
1
signal in the H NMR spectrum of the crude mixture.
We then moved on to establish whether other bis- bismuth
ACHTUNGTREN(NUNG III) triflate in the obtention of the oleanolic
muth catalysts could perform the same transforma- 12-oxo-28-carboxylic acid 11 from the hydroxy-g-lac-
Scheme 2. Opening of the hydroxy-g-lactone 2 with BiACTHNUGTRENUNG(OTf)3·xH2O to afford the 12-oxo-28-carboxylic acid 11.
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BismuthACHTUNGTRENNUNG(III) Triflate-Based Catalytic Direct Opening of Oleanolic Hydroxy-g-lactones
Table 2. Catalyst screening for the opening of the oleanolic
hydroxy-g-lactone 2.[a]
reaction was found to occur in the presence of
10 mol% of BiPh3 [0.11 mmol of 2, 1 mol%
BiACTHUNTGRENUNG(OTf)3·xH2O, 1.5 mL CH2Cl2, reflux, 4 h], another
Entry
Catalyst
(mol%)
Solvent
Time
[h]
Yield
[%][b]
ꢀ
proton scavenger due to the high lability of the Bi Ph
bond under acidic conditions.[22] In sharp contrast, full
conversion was achieved in the presence of 10 mol%
TfOH (0.11 mmol of 2, 1.5 mL CH2Cl2, reflux, 4 h),
ACHTUNGTRENNUNG
1
2
3
4
5
6
7
8
BiBr3 (1)
BiBr3 (5)
BiBr3 (10)
BiBr3 (10)
BiCl3 (5)
BiCl3 (10)
BiCl3 (10)
CH2Cl2
CH2Cl2
CH2Cl2
CH3CN
CH2Cl2
CH2Cl2
CH3CN
CH2Cl2
CH2Cl2
CH2Cl2
CH3CN
CH2Cl2
CH2Cl2
CH2Cl2
CH3CN
CH2Cl2
CH2Cl2
CH3CN
CH3CN
CH2Cl2
5
5
4
8
5
5
22
5
5
5
2
5
5
4
2
2
4
2
4
4
–
29[c]
98
indicating that bismuthACTHNUTRGNE(UNG III) triflate is hydrolyzed
50[c]
20[c]
96
under these reaction conditions to afford a Brønsted
acid species in situ which acts as the true catalyst.
This species could promote ring opening of the
13b,28-olide group by creating a tertiary carbocation
at C-13 which induces stereoselective 1,2-migration of
the 12b-H to the 13b-position. In fact, as pointed out
66[c]
traces
30[c]
38[c]
92
La
La
La
La
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
G
9
10
11
12
13
14
15
16
17
18
19
20
ˇ
´
by Kocovsky et al., non-concerted Wagner–Meer-
wein-type rearrangements with development of carbo-
cation centres, result in 1,2-migration that occurs on
the same plane (sp2 alignment factor).[30] Thus, a plau-
sible reaction mechanism is formation of a carbocat-
ion centred at C-12 due to the 12b-H!13b-H shift
with rearrangement of the 12a-hydroxy group to
afford the corresponding 12-oxo-28-carboxylic acid 11
(Scheme 3).
Yb
Yb
Yb
Yb
N
62[c]
97
95
94
Cu
Cu
Cu
U
71[c]
94
92
96
93
Bi
Bi
U
We extended this new bismuthACTHNUGTRNEUNG(III) triflate-based
[a]
Reaction conditions: 0.16 mmol of 2, 2 mL of solvent,
reflux.
Isolated yield.
process to other oleanolic hydroxy-g-lactones under
optimized reaction conditions (Scheme 4). Acetoni-
trile was chosen over dichloromethane to avoid ma-
nipulation of halogenated solvents. Thus, the oleanolic
hydroxy-g-lactones 4, 6, 8, and 10 were fully convert-
ed to the respective 12-oxo-28-carboxylic acids 12–15,
in very high yields.
[b]
[c]
The yield was determined by integration of the 13b-H
1
signal on the H NMR spectrum of the crude mixture.
[d]
Calculations based on anhydrous salt.
tone 2 (Scheme 2). When the reaction was performed
The acetoxy and trifluoroacetoxy groups of com-
in the presence of a well-known proton scavenger, pounds 6 and 8 remained unchanged using the opti-
2,6-di-tert-butylpyridine, which binds to protons only mized reaction conditions. Structural elucidation of
and is unable to coordinate to metal ions due to the the oleanolic 12-oxo-28-carboxylic acids 11–15 ob-
bulky tert-butyl groups[29] [0.11 mmol of 2, 1 mol% tained was performed by 1D and 2D NMR tech-
BiACHTUNGTRENNUNG(OTf)3·xH2O, 10 mol% scavenger, 1.5 mL CH2Cl2, niques. The proton signal for 13-H was assigned based
reflux, 4 h], no reaction was found to occur. Again no on the HMBC correlation that it displayed with the
Scheme 3. Suggested mechanism for the opening of the hydroxy-g-lactone 2.
Adv. Synth. Catal. 2011, 353, 2637 – 2642
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Jorge A. R. Salvador et al.
COMMUNICATIONS
Scheme 4. Opening of the hydroxy-g-lactones 4, 6, 8 and 10 with Bi
15.
ACHTUNGTRENN(NUG OTf)3·xH2O to afford the 12-oxo-28-carboxylic acids 12–
signal at 211.7 ppm belonging to the carbonyl group Experimental Section
at C-12, for compound 11. The correlation between
the 18b-H signal at 2.75 ppm and the 13-H signal at General Procedure for the Conversion of Oleanolic
2.67 ppm on the COSY spectrum together with the Hydroxy-g-lactones into the Corresponding 12-Oxo-
existence of a NOESY correlation between these two 28-carboxylic Acid Derivatives
signals is consistent with a 13b-H stereochemistry.
3b-Hydroxy-12-oxoolean-28-oic acid (11): Compound
The same pattern was found for compounds 12–15 on
2
(75 mg, 0.16 mmol) and bismuthACTHNUGRTNEUNG(III) triflate (Sigma–Al-
the respective 2D NMR spectra.
In conclusion, we have found that catalytic amounts
of bismuthACHTUNGTRENNUNG(III) triflate catalyze the direct opening of
oleanolic hydroxy-g-lactones to afford the corre-
sponding 12-oxo-28-carboxylic acid derivatives in ace-
tonitrile or dichloromethane, at reflux, in high yields.
This novel reaction is most likely to proceed via an in
situ generated Brønsted acid species from bismuth-
drich, Co) (1.05 mg, 0.0016 mmol) were placed in acetoni-
trile (2 mL), under magnetic stirring, at reflux, for 4 h. The
mixture was then concentrated under reduced pressure.
Ethyl acetate (20 mL) and water (10 mL) were added and
the aqueous phase was further extracted with ethyl acetate
(2”30 mL). The organic phase was washed with 10% aque-
ous NaHCO3 solution (2”20 mL), water (20 mL) and brine
(20 mL), dried with anhydrous Na2SO4, filtered, and concen-
trated under reduced pressure to give compound 11[31] as a
white solid; yield: 72 mg (96%); mp 302–3048C. IR: n=
ACHTUNGTRENNUNG(III) triflate which works as the true catalyst, and fur-
ther illustrates the use of the environmentally friend-
ly, inexpensive, and easily obtainable bismuth salts in
organic chemistry. To the best of our knowledge, com-
pounds 7, 8, 10, 14, and 15 are new.
3500, 1693 cmꢀ1 1H NMR (CDCl3, 400 MHz): d=0.77 (s,
;
3H), 0.85 (s, 3H), 0.90 (s, 3H), 0.94 (s, 3H), 0.97 (s, 3H),
0.98 (s, 6H), 2.67 (d, 1H, J=4.1 Hz, 13b-H), 2.75 (m, 1H,
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Adv. Synth. Catal. 2011, 353, 2637 – 2642

BismuthACHTUNGTRENNUNG(III) Triflate-Based Catalytic Direct Opening of Oleanolic Hydroxy-g-lactones
18b-H), 3.20 (m, 1H, 3a-H); 13C NMR (CDCl3, 100 MHz):
d=15.2, 15.3, 16.4, 18.2, 20.5, 22.5, 23.1, 27, 27.5, 27.9, 30.6,
31.8, 31.8, 33, 33.3, 34.4, 36.1, 36.9, 37.9, 38.5, 38.8, 41.2, 41.9,
47.2, 49.7, 51.8, 55, 78.6 (C-3), 183.9 (C-28), 211.7 (C-12);
EI-MS: m/z (%)=473 (2) M+1; 411 (59); 263 (47); 217
(100); 206 (82); 189 (71); 177 (61); 104 (41).
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[13] M. S. Ali, M. Jahangir, S. S. ul Hussan, M. I. Choudh-
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nancial support. Vꢀnia M. Moreira and Ana S. Leal thank
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(SFRH/BPD/45037/2008 and SFRH/BD/41566/2007).
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