First formal synthesis of (+)-nimbidiol. Synthesis, X-ray structure and anticancer activity of a novel ring C aromatic diterpene: Dimethyl (+)-podocarpa-8,11,13-triene-12,13-dicarboxylate

Article abstract of DOI:10.1016/S0040-4039(03)00105-9

A novel ring C aromatic diterpene (4) has been prepared in three steps from natural (+)-manool (1). The structure and anticancer activity data for 4 has been investigated. This key intermediate (4) was easily transformed into 7-deoxo nimbidiol dimethyl ether (8). The present work represents the first formal synthesis of (+)-nimbidiol (10).

Full text of DOI:10.1016/S0040-4039(03)00105-9

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 1859–1862
First formal synthesis of (+)-nimbidiol.
Synthesis, X-ray structure and anticancer activity of a novel
ring C aromatic diterpene: dimethyl
(+)-podocarpa-8,11,13-triene-12,13-dicarboxylate
Jorge L. Zambrano,a,* Viale Rosalesb and Tatsuhiko Nakanob
aDepartamento de Qu´ımica, Universidad Simo´n Bolivar, Valle de Sartenejas, Baruta, Caracas 1080-A, Venezuela
bCentro de Qu´ımica, Instituto Venezolano de Investigaciones Cient´ıficas (I.V.I.C.), Apartado 21827, Caracas 1020-A, Venezuela
Received 2 October 2002; revised 9 January 2003; accepted 10 January 2003
Abstract—A novel ring C aromatic diterpene (4) has been prepared in three steps from natural (+)-manool (1). The structure and
anticancer activity data for 4 has been investigated. This key intermediate (4) was easily transformed into 7-deoxo nimbidiol
dimethyl ether (8). The present work represents the first formal synthesis of (+)-nimbidiol (10). © 2003 Elsevier Science Ltd. All
rights reserved.
Abietane and biosynthetically related polycyclic diter-
penes are a major group of ring C aromatic diterpenes.1
Abietane diterpenes show a wide range of biological
activities, e.g. antibiotic,2 antivirus,3 antioxidant,4
antimalarial5 and cytotoxic6 activity. This enormous
biological data has motivated numerous synthetic
investigations of racemic abietanes via polyene cycliza-
tion.7 While enantioselective syntheses of abietane diter-
penes are known, they are scarce.8
cal) were used sequentially to transform (+)-manool to
the exocyclic diene 3 in 52% overall yield. The utility of
diene 3 is demonstrated for the synthesis of diterpene 4.
We report herein the synthesis, structural and complete
anticancer activity data for diester 4 and its synthetic
application in the formal synthesis of (+)-nimbidiol
(10).
Synthesis of compound 4: Our synthesis started with
diene 3,15b which was readily prepared from (+)-manool
(1) in two steps and 52% overall yield (Scheme 1).12 To
construct the C-ring of abietane we envisioned using a
Diels–Alder reaction between diene 3 and dimethyl
acetylenedicarboxylate (DMAD) with a concomitant
aerobic oxidation.13 In practice, the transformation
worked as planned after significant optimization. The
Commercially available (+)-manool (1)9 has been used
as a key intermediate for the efficient syntheses of
drimane10 and abietane-type11 terpenes. Common to all
these endeavors was the use of naturally occurring
(+)-manool as starting material. In this study, two
cleavage reactions (one oxidative and one photochemi-
Scheme 1. (a) KMnO4, acetone, rt; (b) hw, pentane, −30°C, 52% overall yield; (c) DMAD, 110–220°C, xylene, 3 days, 48%.
Keywords: diterpene; podocarpa; nimbidiol.
* Corresponding author. Tel.: +58(212)9063990; fax: +58(212)9063961; e-mail: jzambrano@usb.ve
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(03)00105-9

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J. L. Zambrano et al. / Tetrahedron Letters 44 (2003) 1859–1862
Diels–Alder adduct was obtained by heating a mixture
of diene and DMAD (1:3 ratio) in a sealed tube at
110°C for 24 h. To induce aromatization, the tempera-
ture was rapidly increased to 220°C for additional 48 h.
The yield was strongly dependent on the reaction tem-
perature (e.g. >220°C gave decomposition, <220°C
showed incomplete conversion) and stoichiometry (e.g.
threefold excess of alkyne was required to compensate
its lost by thermal polymerization).14 Under these opti-
mized conditions for aromatization15a the desired
product (4) was obtained in 48% isolated yield.15c
X-Ray determination of compound 4:18b While the spec-
tral data (1H and 13C NMR), elemental analyses and
LR/HRMS for compound 4 were completely consistent
with its structure, further confirmation was sought by
means of single crystal X-ray analyses. X-Ray quality
crystals of 4 were grown by slow evaporation of a
hexane–ethyl acetate solution. An ORTEP depiction of
4 is shown in Figure 1. It is interesting to notice that
two independent molecules were found in the asymmet-
ric unit and have different conformations and geomet-
ric parameters. The molecular dimensions for both
structures in the rings A and B are slightly different
compared to the previously reported values for abietane
compounds,16 with mean bond distances being
,
,
C(sp3)ꢀC(sp3) 1.492 A for structure I and 1.491 A for
structure II. The longest CꢀC distance in structure I
,
[1.564 (5) A] was found at the ring A/B junction
according to previous reports.17 However, structure II
showed its longest CꢀC bond in ring A [C31ꢀC36=
Figure 1. ORTEP plot of 4 (structures I and II). Displace-
ment ellipsoids are drawn at 50% probability level and H
atoms are omitted for clarity
,
1.561 (5) A]. In both structures the carbonyl esters are
located anti to each other in order to minimize unfavor-
able dipole–dipole interactions. Neither carbonyl
groups are coplanar with the aromatic ring in structures
Table 1. In vitro selected antitumor activity data for 4
I
and II, according to their dihedral angles
Cell line
Cytotoxicity log GI50 (M)a,b
C17ꢀC16ꢀC18ꢀO1=31.6° and C47ꢀC46ꢀC48ꢀO10=
46.8° respectively. The aromatic ring is slightly twisted
near the decalin system (C17ꢀC10ꢀC11ꢀC14=3.8° for I
and C47ꢀC40ꢀC41ꢀC44=1.2° for II).18a
Leukemia (HL-60 TB)
Non-small cell lung (HOP-92)
Colon (KM12)
−5.06
−5.00
−4.80
−4.80
−5.91
−4.98
−4.99
−4.84
−6.29
−5.19
CNS (SF-268)
Antitumor testing of compound 4: Evaluation of antitu-
mor activity was performed on compound 4 at the
National Cancer Institute (NCI), following the known
in vitro oriented antitumor screening program against a
panel of 62 tumor cell lines derived from nine cancer
types (leukemia, non-small cell lung, colon, CNS,
melanoma, ovarian, renal, prostate and breast) accord-
ing to a standard protocol.19 In each test, dose–
response curves for each cell line were measured with
five different drug concentrations. The concentration
causing 50% cell growth inhibition (GI50) was calcu-
lated. Table 1 shows the biological data for selected cell
lines. Compound 4 was active against all cell lines with
mean log GI50 values ranging from −4.51 (ovarian can-
cer/OVCAR-8) to −6.29 (breast cancer/T-47D). The
cytotoxicity activity against human leukemia cell (HL-
60) of 4 (Table 1, log GI50=−5.06) showed a strong
similarity with the value reported for incanone, a struc-
turally related natural abietane diterpene (−5.22).20 As
previously mentioned, compound 4 was particularly
active against breast cancer cell T-47D. However, lab-
dane diterpenoids isolated from natural sources have
Melanoma (MALME-3M)
Ovarian (IGROV1)
Renal (RXF 393)
Prostate (DU-145)
Breast (T-47D)
Mean value
a Data obtained from NCIs in vitro tumor cells screen.
b Mean value over all 62 cell lines tested.
shown non-specific cytotoxicities against several tumor
cell lines.21
Formal synthesis of (+)-nimbidiol (10):
To further demonstrate the utility of this methodology,
we investigated the use of diester 4 as a chiral building
block for the synthesis of (+)-nimbidiol (10).22 (+)-Nim-
bidiol (10) is a diterpene isolated from the root bark of
Azadirachta indica.23 Thus, we investigated the conver-
sion of the two aromatic ester groups of 4 into the
1,2-bis-phenols of nimbidiol. We initially attempted this

J. L. Zambrano et al. / Tetrahedron Letters 44 (2003) 1859–1862
1861
Scheme 2.
Scheme 3. (a) MeLi (8 equiv.), CeCl3, THF, −78°C; (b) 30% H2O2, p-TsOH cat., THF, 0°C to rt; (c) CH2N2, ether, 45% overall
yield.
Acknowledgements
transformation by means of a Baeyer–Villiger reaction
on the corresponding dimethyl ketone 5 with the hope
of selectively forming 6 (Scheme 2). Unfortunately the
formation of dimethyl ketone 5 using MeLi/Me3SiCl24
or MeMgCl25 gave a complex crude mixture from
which the desired dimethyl ketone could not be iso-
lated.26 Next we investigated the selective oxidation of
the two carbon–carbon bonds of the bis-tertiary alcohol
7 by a hydroperoxide rearrangement.27 Exhaustive
methylation of diester 4 with excess MeLi (8 equiv.)28
gave the diol 7, which due to its instability to silica gel
chromatography was used as is (Scheme 3). Exposure
of the crude diol 7 to a solution of hydrogen peroxide
in acidic THF29 yielded 7-deoxo nimbidiol 630 which
was immediately dimethylated to give the known 7-
deoxo nimbidiol dimethyl ether 8 in 45% overall yield
(Scheme 3).31 The two steps conversion of 8 to nimbid-
iol 10 has previously been reported by Majetich and
co-workers.32
We thank the FONACIT (Venezuela) for research
grants partly supporting this work. The authors also
thank the National Cancer Institute (NCI), Develop-
mental Therapeutics Program (Bethesda, MD) for test-
ing 4 in their in vitro cytotoxicity screen. We thank
Professor George O’Doherty (University of Minnesota)
for his comments on the preparation of the manuscript.
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MHz) l 4.78 (dd, J=2.29, 2.75 Hz, 1H), 4.74 (d, J=1.83
Hz, 1H), 4.63 (dd, J=2.29, 2.75 Hz, 1H), 4.52 (d, J=1.83
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(tdt, J=2.29, 5.5, 13.29 Hz, 1H), 1.74–1.69 (m, 1H),
1.65–1.57 (m, 2H), 1.55–1.39 (m, 4H), 1.18 (td, J=4.13,
9.17 Hz, 1H), 1.11 (dd, J=2.75, 12.83 Hz, 1H), 0.94 (s,
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35.9, 33.8, 33.4, 22.7, 22.0, 20.7, 19.2. HRMS (EI) m/z
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heated in a sealed tube under vacuo at 110°C for 24 h
and then at 220°C for an additional 48 h. After cooling,
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1
1228, 1070 cm−1. H NMR (CDCl3, 500 MHz) l 6.80 (s,
1H), 6.50 (s, 1H), 3.85 (s, 3H), 3.81 (s, 3H), 2.78–2.88 (m,
2H), 2.18–2.45 (m, 2H), 1.19–1.95 (m, 7H), 1.18 (s, 3H),
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2920, 2842, 1725, 1609, 1438, 1308, 1135, 979 cm−1 1H
.
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2.81 (m, 1H), 2.31–2.28 (m, 1H), 1.91–1.87 (m, 1H),
1.78–1.58 (m, 3H), 1.49–1.46 (m, 1H), 1.37 (dt, J=3.67,
13.29 Hz, 1H), 1.26 (dd, J=2.29, 12.83 Hz, 1H), 1.21 (dd,
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