An unprecedented biogenetic-type chemical synthesis of 1(15→11) abeotaxanes from normal taxanes

Article abstract of DOI:10.1021/np900722j

A one-pot chemical process using BF₃.Et₂O for the synthesis of a new class of 1(15→11) abeotaxanes from normal taxanes has been developed. The chemical structures of rearranged 1(15→11) abeotaxane were established by extensive 2D NMR

Full text of DOI:10.1021/np900722j

J. Nat. Prod. 2010, 73, 747–750
747
An Unprecedented Biogenetic-Type Chemical Synthesis of 1(15f11) Abeotaxanes from Normal
Taxanes
T. Narender,*,† K. Papi Reddy,† V. K. Singh,† K. Rajendar,† and J. Sarkar‡
Medicinal and Process Chemistry DiVision and Drug Target DiscoVery and DeVelopment DiVision, Central Drug Research Institute,
Lucknow-226 001, U.P., India
ReceiVed NoVember 8, 2009
A one-pot chemical process using BF3 ·Et2O for the synthesis of a new class of 1(15f11) abeotaxanes from normal
taxanes has been developed. The chemical structures of rearranged 1(15f11) abeotaxane were established by extensive
2D NMR spectroscopic data.
Taxol (1), a diterpeno pseudoalkaloid, was first isolated from
the bark of the Pacific yew, Taxus breVifolia, by Wall and Wani’s
group in collaboration with the National Cancer Institute (NCI) and
is a significant new lead for the treatment of cancer (Figure 1).1 It
has been approved by the FDA for the treatment of advanced
ovarian and metastatic breast cancer and is currently in clinical
trials for other cancers.2 The outstanding cytotoxic activity of taxol
(1) is believed to arise from its unique propensity to hinder cell
replication by preventing microtubule depolymerization.3 The only
major source of 1 is from the needles (0.033%) and bark (0.01%)
of the Pacific yew tree, T. breVifolia, which cannot be considered
a renewable source because of its slow growth, and therefore, the
supply of 1 is quite limited.4
On the other hand, the supply of the key terpenoid fragments
baccatin III (2) (0.2%) and 10-deacetylbaccatin III (3) (0.1%) is
readily obtained from the needles, a rapidly renewable resource.
The semisynthetic route to the production of Taxol involves the
Figure 1. Paclitaxel (1, Taxol: BMS-181339); baccatin III (2); 10-
attachment of the N-benzoyl-(2R,3S)-phenylisoserine (4) side chain
deacetylbaccatin III (3); and phenylisoserine (4, Taxol C-13 side
to the main ring of baccatin III (2) or 10-deacetylbaccatin III (3)
chain).
(Figure 1).5 The scarcity of 1 from natural sources and the economic
challenges associated with the total synthesis of Taxol6 have led
to the search for an alternative compound.
Extensive chemical studies on other Taxus species resulted in
the isolation of a large number of taxoids with basic skeletons I-VI
(Figure 2).7 A normal taxane contains an eight-membered ring
sandwiched between two six-membered rings as in I. Cyclotaxanes
(II) are tetracyclic systems with an additional bond between C-3
and C-11 (6/5/5/6 ring system). The rearranged taxanes possessing
ring systems III-V are called 11(15f1) abeotaxane, 2(3f20)
abeotaxane, and 11(15f1) and 11(10f9) bisabeotaxane, respec-
tively. Compounds with skeleton VI (6/12 ring system) are referred
to as pretaxanes since the bond between C-3 and C-8 is not yet
formed. It is noteworthy to mention that rearranged taxoids (II-V)
retained the microtubule and multi-drug-resistance (MDR) reversing
activities.8 Thus far, 1(15f11) abeotaxanes (VII) with the 5/7/6
ring system have not yet been reported as natural products; therefore
their anticancer activity, tubulin-binding activity, and MDR-
reversing activities have not yet been studied (Figure 2). Herein
we describe an unprecedented chemical synthesis of 1(15f11)
abeotaxanes using BF3 ·OEt2.
Figure 2. Normal taxoid I, rearranged taxoids (abeotaxoids) II-V,
pretaxane VI, and chemically synthesized taxoid VII.
In continuation of our drug discovery program on anticancer
agents we isolated 2-deacetoxytaxinine J (5; 2-DAT-J) in reasonably
good yield (0.1%) from the Indian T. baccata (ssp. wallichiana).
2-DAT-J (5) was also reported from several other Taxus species.9
It exhibits cytotoxicity against L1210 murine leukemia cells and
KB human epidermoid carcinoma cells and has an effect on Ca2+
-
induced microtubule depolymerization.9 Taxoid 5 exhibits no
cytotoxicity and tubulin affinity and is considered a powerful
inhibitor of P-glycoprotein (P-gp) activity, acting as an efficient
reversing agent in MDR cancer cells.10 Botta and co-workers
synthesized a small library of analogues of 5 to develop a potential
MDR-reversing agent.11 Recently we also reported the in vitro
anticancer activity of 5 and its new derivatives and the in vivo
* Corresponding author. Tel: +91 522 2612422. Fax: +91 522 2623405.
E mail: t_narendra@cdri.res.in.
† Division of Medicinal and Process Chemistry.
‡ Division of Drug Target Discovery and Development
10.1021/np900722j  2010 American Chemical Society and American Society of Pharmacognosy
Published on Web 03/19/2010

748 Journal of Natural Products, 2010, Vol. 73, No. 4
Notes
Scheme 1. Synthesis of 1(15f11) Abeotaxane 6 Using BF3 ·OEt2
1
Table 1. Selected Hf13C HMBC Correlations for 6
anticancer activity of 5 in animal models.12 During this chemical
transformation process we attempted to remove the C-13 acetyl
group from 5 using BF3 ·OEt2. In our previous studies BF3 ·OEt2
selected HMBC
correlationsa
position
δH (J in Hz)
2.92, m
1.71, 2.12, m
2.91, m
δC
41.4
27.0
35.0
regioselectively deacetylated the polyacetoxyacetophenones.13
A
1
2
3
4
5
2, 13
4, 14
2, 20
similar attempt was made on 5, which surprisingly resulted in the
regioselective removal of the acetoxy group followed by an
unprecedented rearrangement to provide 1(15f11) abeotaxane 6
(Scheme 1) in reasonably good yield (70%). Extensive 2D NMR
studies were carried out to elucidate the structure of 6 (Figure 3).
The ESI mass spectrum of 6 showed a molecular ion peak at 613
[590 + Na]+, which clearly gave information on the removal of
HOAc (650 - 590 ) 60) from 5 (Figure 4). One of the significant
differences in the 1H NMR spectra of 5 and 6 is the appearance of
two extra olefinic singlets at 4.59 and 4.83 ppm, an olefinic CH
(5.29 ppm), and six methyls in 6 (0.93, 1.41, 1.58, 1.89, 1.96, and
1.99 ppm) instead of eight methyls in 5 (Table 1).
146.4
74.8
5.44, t (3.6)
3, 7, 20, CO
(cinnamoyl)
4, 8
6
2.10, 2.24, m
5.40, t (3.1)
37.1
7
70.9
3, 5
8
45.7
9
5.34, t (7.2)
5.57, t (7.2)
73.5
3, 11
10
11
12
13
14
15
16
17
18
19
20
72.4
63.7
142.5
126.0
32.3
145.7
112.2
20.9
14.8
13.3
114.2
170.1 (CO)
21.2 (CH3)
169.5 (CO)
21.2 (CH3)
169.8 (CO)
21.3 (CH3)
165.6
8, 11, 12, 15
5.29, m
1.65, 2.22, m
1, 11, 18
11, 12
4.59, 4.83, s
1.58,
1.41, s
0.93, s
4.88, 5.24, s
1.89, s
11, 17
11, 16
11, 13
9, 7
s
3, 5
7-OCOCH3
9- OCOCH3
10-OCOCH3
7b
1.96, s
1.99, s
9b
10b
CO cinnamoyl
1
Figure 3. Selected Hf13C HMBC correlations for 6.
R
6.38, d (15.9)
7.63, d (15.9)
118.4
144.9
1′
ꢀ
CO (cinnamoyl),
2′, 6′
1′
134.4
128.1
128.9
130.3
2′, 6′
3′, 5′
4′
7.47, m
7.33, m
7.33, m
ꢀ, 4′
1′
2′, 6′
a HMBC correlations from proton(s) stated to the indicated carbon.
b Very weak four-bond correlations.
The 13C NMR spectrum showed a characteristic downfield
quaternary carbon at 63.7 ppm. The DEPT-135 and HSQC spectra
confirmed the extra olefinic methylene at 112.2 ppm and olefinic
methyne at 126.0 ppm (Table 1). In 2-DAT-J (5)14 there were three-
bond correlations between H-13 and C-11, which were missing in
the rearranged product 6. In addition to that in 5 H-10 gave three-
bond correlations with C-8 (46.3 ppm) as well as C-15 (39.3 ppm).
H-9 gave three-bond correlations with C-11 (135 ppm) and two-
bond correlations with C-8, whereas in 6 H-10 gave correlations
with C-8 (45.7 ppm), C-12 (142.5 ppm), C-15 (145.7 ppm), and
C-11 (63.7 ppm) and H-9 also gave correlations with C-11 (63.7
ppm). The newly generated olefinic methylene protons (4.59 and
Figure 4. Plausible mechanism for the formation of 1(15f11)
abeotaxanes 6, 10, and 12.

Notes
Journal of Natural Products, 2010, Vol. 73, No. 4 749
Experimental Section
Scheme 2. Synthesis of 1(15f11) Abeotaxane 10
General Experimental Procedures. Melting points were recorded
on a Buchi-530 capillary melting point apparatus and are uncorrected.
1
IR spectra were recorded on a Perkin-Elmer AC-1 spectrometer. H
NMR spectra were run on a Bruker Advance DPX 300 MHz in CDCl3,
and 13C NMR spectra were recorded at 75 and 50 MHz in CDCl3.
COSY, HMBC, and HSQC spectra were recorded at 300 MHz in
CDCl3. Chemical shifts are reported in ppm relative to CHCl3 (7.26)
in CDCl3, and TMS was used as internal standard. ESIMS were
recorded on JEOL SX 102/DA-6000.
Chromatography was done with silica gel (60-120 mesh) using
mixtures of EtOAc and n-hexane as eluents. EtOAc and n-hexane were
dried and purified by distillation prior to use. Reactions that required
the use of anhydrous and inert conditions were carried out under an
N2 atmosphere. 1,4-Dioxane was distilled over Na.
Representative Procedure for the Synthesis of 1(15f11) Abeo-
taxane (6). To a magnetically stirred solution of 2-deacetoxytaxinine-J
(5) (250 mg, 0.38 mmol) in anhydrous 1,4-dioxane (25 mL) was slowly
added BF3 ·OEt2 (0.1 mL, 0.76 mmol) at room temperature. The solution
was stirred for 12 h at room temperature. After dilution with Et2O (100
mL), the solution was washed with H2O (3 × 30 mL) to decompose
the BF3 ·OEt2 complex. The solution was dried over anhydrous Na2SO4
and filtered, and the solvent was evaporated under reduced pressure.
The crude mixture was purified by silica gel column chromatography
using n-hexane-EtOAc (70:30) to afford compound 6 (185 mg, 70%):
mp 121-123 °C; IR (KBr) νmax 2924, 2854, 1739, 1634, 1446, 1370,
1243, 1166, 1033, 760 cm-1; MS (ESI) m/z 613.2 [M + 23]+; 1H NMR
and 13NMR data in Table 1.
Scheme 3. Synthesis of 1(15f11) Abeotaxane 12
1(15f11) Abeotaxane (10): mp 130-132 °C; IR (KBr) νmax 3019,
2924, 2855, 1738, 1632, 1445, 1371, 1249, 1035, 759 cm-1; 1H NMR
data δ 5.63 (d, J ) 6.9 Hz, 1H), 5.42 (m, 3H), 5.11 (s, 1H), 4.88 (s,
1H), 4.82 (s, 1H), 4.60 (s, 1H), 4.37 (t, J ) 2.9 Hz, 1H), 3.12 (m, 1H),
3.00 (m, 1H), 2.33-2.14 (m, 4H), 2.06 (s, 3H), 2.02 (s, 3H), 1.96 (s,
3H), 1.84-1.73 (m, 2H), 1.67 (s, 3H), 1.47 (s, 3H), 0.91 (s, 3H); 13C
NMR data δ 170.9, 170.3, 170.2, 151.3, 146.5, 142.4, 126.5, 114.5,
112.1, 73.8, 73.6, 72.9, 71.5, 64.1, 46.4, 41.7, 37.7, 34.7, 34.2, 32.1,
27.7, 21.7, 21.5, 21.1, 15.0, 13.5; MS (ESI) m/z 478.2 [M + 18]+,
483.3 [M + 23]+.
4.83 ppm) gave three-bond correlations with C-11 (63.7 ppm) and
a methyl group at 20.9 ppm (C-17) (Figure 3 and Table 1). In
addition there were correlations between H-9 (H-5.34: C-73.5 ppm)
and H-10 (5.57: 72.4 ppm) in the COSY spectrum to rule out
structure 7 (Scheme 1). Attachment of the exocyclic double bond
(C-15 and C-16) to C-1 as in 6 or to C-11 as in 8 was confirmed
by the HMBC data (Scheme 1). In the HMBC spectrum H-9 (5.34
ppm) and the H-18 methyl (1.41 ppm) gave three-bond correlations
with the carbon at 63.7 ppm, which supports the attachment of the
exocyclic double bond (C-15 and C-16) to C-11 and not C-1. On
the basis of the NMR analysis the structure was assigned as 6
(Figure 3 and Table 1). The relative configuration at C-11 was
determined by NOESY data.15
1
1(15f11) Abeotaxane (12): H NMR data δ 8.00 (d, J ) 8.8 Hz,
2H), 7.44 (d, J ) 8.8 Hz, 2H), 5.69 (d, J ) 6.8 Hz, 1H), δ 5.62 (s,
1H), 5.54 (m, 1H), 5.44 (d, J ) 6.8 Hz, 1H), 5.34 (s, 1H), 5.30 (s,
1H), 4.99 (s, 1H), 4.89 (s, 1H), 4.64 (s,1H), 3.05 (m, 1H), 2.98 (m,
1H), 2.34 (m, 4H), 2.08 (s, 3H), 2.04 (s, 3H), 1.97 (s, 3H), 1.79 (m,
2H), 1.66 (s, 3H), 1.49 (s, 3H), 1.04 (s, 3H); 13C NMR data δ 170.2,
169.8, 1969.4, 164.3, 146.3, 146.1, 142.6, 139.5, 130.9 (2C), 128.9,
128.7 (2C), 125.5, 114.1, 111.7, 75.7, 73.2, 72.4, 70.7, 63.8, 45.9, 41.1,
37.5, 35.0, 32.3, 27.3, 21.24, 21.21, 21.15, 20.7, 14.7, 13.0; MS (ESI)
m/z 621.2 [M + 23]+.
Acknowledgment. We are thankful to the Director, CDRI, Lucknow,
for facilities and constant encouragement for the program on natural
products, K. R. Arya for plant material collection, J. P. Chaturvedi
and Rajesh (BIT, Ranchi) for technical support, SAIF for physical data,
Dr. Shilah A. Bonnett (Portland State University) for reading the
manuscript, G. Madhur in assisting in the manuscript preparation, and
CSIR, New Delhi, for financial support. This is CDRI communication
No. 7846.
To demonstrate the regioselectivity, the C-5 O-cinnamoyl group
of 5 was selectively removed using NH2OH to give 9, which was
subsequently reacted with BF3 ·OEt2 to afford the 1(15f11)
abeotaxane 10 (Scheme 2). To further study the generality of the
reaction, the C-5 p-chlorobenzoate 11 was prepared from 9 using
the DCC and DMAP protocol and subsequently reacted with
BF3 ·Et2O to give the rearranged 1(15f11) abeotaxane 12 (Scheme
3).
The mechanism for the formation of the 1(15f11) abeotaxane
skeleton appears to be the regioselective removal of the C-13
acetoxy group by BF3 ·OEt2 followed by migration of the C-11-C-
12 double bond to C-12-C-13. The resultant acetate ion may
abstract the C-1 proton, which could lead to a cyclopropyl ring
involving C-1, C-11, and C-15. The strain associated with the
cyclopropyl ring may lead to ring-opening between C-15 and C-1
and a subsequent Wagner-Meerwein-type H+ shift from C-16 to
C-1 to furnish the required 1(15f11) abeotaxanes 6, 10, and 12.
Further studies, however, are required to confirm the exact reaction
mechanism (Figure 4).
Supporting Information Available: Spectroscopic data of unusual
1(15f11) abeotaxane 6, 2-DAT-J 5 (2D: HMBC, HMQC, COSY,
NOESY, etc.), and 9-12 (1D data) and their in vitro anticancer activity
are available free of charge via the Internet at http://pubs.acs.org.
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chemical synthesis of 1(15f11) abeotaxanes from normal taxoids
using BF3 ·OEt2. Our simple and convenient method has paved the
way toward the synthesis of a new class of 1(15f11) abeotaxanes.
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1
(14) 2DAT-J (5): H NMR data δ 2.03 (1H, m, H-1), 1.81-1.91 (2H, m,
H-2), 2.74 (1H, dd, H-3), 5.58 (1H, dd, H-5), 1.90-1.99 (2H, m, H-6),
5.69 (1H, dd, H-7, 5.95 (1H, d, H-9), 6.31 (1H, d, H-10), 5.81 (1H,
dd, H-13), 1,01 and 2.81 (2H, m, H-14), 1.10 (3H, s, H-15), 1.63
(3H, s, H-16), 2.35 (3H, s, H-18), 0.97 (3H, s, H-19), 5.03 and 5.40
(2H, s, H-20); O-cinnamoyl: 6.57 (1H, d, H-R), 7.75 (1H, d, H-ꢀ),
7.40 (3H, m, H-3′-5′ and H-4′), 7.51 (2H, m, H-2′,6′), O-acetyl: 1.72
(3H, s), 2.00 (3H,s), 2.05 (3H, s), 2.08 (3H, s); 13C NMR (CDCl3, 50
MHz) δ 40.1 (C1), 27.2 (C-2), 37.4 (C-3), 146.2 (C-4), 74.8 (C-5),
34.5 (C-6), 70.9 (C-7), 46.3 (C-8), 76.7 (C-9), 71.7 (C-10), 135.0 (C-
11), 137.2 (C-12), 70.6 (C-13), 31.8 (C-14), 39.3 (C-15), 27.2 (C-
16), 31.1 (C-17), 15.3 (C-18), 13.1 (C-19), 116.0 (C-20); O-cinnamoyl:
166.1 (CO), 118.3 (C-R), 145.7 (C-ꢀ), 134.0 (C-1′), 129.0 (C-3′,5′),
128.1 (C-2′,6′), 130.6 (C-4′); O-acetyl: CO: 169.3, 169.9, 170.2, 170.3,
CH3: 20.8, 21.0, 21.0, 21.4; MS (ESI) m/z 673 [M + 23]+.
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activity data.
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NP900722J