Antitumor agents. 274. A new synthetic strategy for E-ring SAR study of antofine and cryptopleurine analogues

Article abstract of DOI:10.1021/ol902819j

Chemical equation presented A new versatile synthetic methodology for the synthesis of enantiomerically pure natural phenanthroindolizidines and phenanthroquinolizidines has been established and described. Natural products R-antofine and R-cryptopleurine, as well as a novel E-ring expanded analogue 13c (E7), 12-oxo-S-antofine (17), and 12N-methyl-12-aza-S-antofine (18) were synthesized with the new method. This strategy will greatly facilitate future SAR studies on the natural alkaloids with E-ring variations.

Full text of DOI:10.1021/ol902819j

ORGANIC
LETTERS
2010
Vol. 12, No. 7
1416-1419
Antitumor Agents. 274. A New Synthetic
Strategy for E-Ring SAR Study of
Antofine and Cryptopleurine Analogues
Xiaoming Yang,† Qian Shi,*,† Kenneth F. Bastow,‡ and Kuo-Hsiung Lee*,†
Natural Products Research Laboratories and DiVision of Medicinal Chemistry and
Natural Products, Eshelman School of Pharmacy, UniVersity of North Carolina,
Chapel Hill, North Carolina 27599-7568
qshi1@email.unc.edu; khlee@unc.edu
Received December 7, 2009
ABSTRACT
A new versatile synthetic methodology for the synthesis of enantiomerically pure natural phenanthroindolizidines and phenanthroquinolizidines
has been established and described. Natural products R-antofine and R-cryptopleurine, as well as a novel E-ring expanded analogue 13c (E7),
12-oxo-S-antofine (17), and 12N-methyl-12-aza-S-antofine (18) were synthesized with the new method. This strategy will greatly facilitate future
SAR studies on the natural alkaloids with E-ring variations.
Phenanthroindolizidines and phenanthroquinolizidines are
two series of natural alkaloids primarily found in the
Asclepiadaceae and Moracea plant families. The leaves of
these plants have been used since ancient times to treat
asthma, bronchitis, rheumatism, etc.1 To date, over 60
compounds have been isolated and characterized, such as
R-tylophorine, R-antofine, and R-cryptopleurine (Figure 1),
which are well-known representatives reported to have potent
antitumor activity. Due to low natural abundance and
interesting anticancer activity, total synthesis has attracted
much attention to assist the development of both the natural
compounds and new analogues.
Figure 1. Representative structures of phenanthroindolizidines and
phenanthroquinolizidines.
electrophilic addition.3-7 Other strategies include a chiral
auxiliary approach,8 chiral allylic alcohol,9 enantioselec-
tive catalysis for intramolecular alkene carboamination,10
and enantioselective phase-transfer alkylation;11 some of
To date, numerous synthetic strategies have been reported
in the literature.2 One representative strategy, first reported
by Rapoport and co-worker, used building blocks such as
proline, glutamic acid, aminoadipate, and pyroglutamate as
the sources of the chiral center followed by intramolecular
(3) Buckley, T. F.; Rapoport, H. J. Org. Chem. 1983, 48, 4222–4232
.
(4) Nordlander, J. E.; Njoroge, F. G. J. Org. Chem. 1987, 52, 1627–
1630
.
† Natural Products Research Laboratories.
(5) Furstner, A.; Kennedy, J. W. J. Chem.sEur. J. 2006, 12, 7398–
‡ Division of Medicinal Chemistry and Natural Products.
(1) Gellert, E. J. Nat. Prod. 1982, 45, 50–73.
7410
.
(6) Jin, Z.; Li, S. P.; Wang, Q. M.; Huang, R. Q. Chin. Chem. Lett.
(2) For reviews, see: (a) Li, Z.; Jin, Z.; Huang, R. Synthesis 2001, 16,
2365–2378. (b) Chemler, S. R. Curr. Bioact. Compd. 2009, 5, 2.
2004, 15, 1164
.
(7) Faber, L.; Wiegrebe, W. HelV. Chim. Acta 1976, 59, 2201–2212
.
10.1021/ol902819j  2010 American Chemical Society
Published on Web 03/02/2010

them have also been used in the synthesis of seco
structures.
altered via ring-closing metathesis and various functional
groups can be readily introduced during E-ring formation
and subsequent reactions. The resulting new analogues would
provide useful SAR information about the alkaloid E-ring.
In contrast to Rapoport’s lactam moiety, another novelty in
our strategy is the use of an oxazolidinone moiety, which
not only allows efficient formation of the D-ring but also is
easily cleaved to provide the useful synthetic precursor 7.
Figure 2. Ring and position numbering of phenanthroindozlidines
and phenanthroquinolizidines.
Scheme 1. Synthesis of the Key Intermediate 7
It has been reported that a major side effect prohibiting
the therapeutic use of these natural alkaloids is their CNS
toxicity, such as disorientation and ataxia.12 Analogues with
higher polarity may be desirable to ameliorate such side
effects by preventing the compounds from crossing the
blood-brain barrier. However, only a few polar antofine
analogues with a C-14 OH group have been synthesized.13
No E-ring substituted analogues (at C11-C14) have been
reported to date, despite the large pool of approaches
presently available. As a result, current SAR study is still in
a premature stage, and additional phenanthroindolizidine and
phenanthroquinolizidine analogues with diverse structural
features, especially polar functionalities on the E-ring, are
urgently needed for a more extensive study of the biological
properties.
In this paper, we report the design and synthesis of a key
intermediate 7 that should prove to be a versatile precursor
to a series of interesting E-ring modified analogues (Figure
2). Possible modifications include incorporation of heteroa-
toms, introduction of polar groups such as hydroxy and
amino groups, and addition of multiple substitutions, all of
which are not readily accessible by reported synthetic
methods. Two natural products, R-antofine and R-cryptopleu-
rine, were synthesized to verify the feasibility of this new
strategy, and three new compounds, 7-membered analogue
E7 (13c), 12-oxo-S-antofine (17), as well as 12N-methyl-
12-aza-S-antofine (18) were synthesized for the first time to
further corroborate the versatility of this method.
As shown in Scheme 1, compound 1 was obtained via
three steps as reported in the literature in 45% yield.14
Subsequent reduction with LiAlH4 was followed by oxidiza-
tion with Py·SO3/DMSO to yield an aldehyde, which was
reductively aminated by using D-serine methyl ester hydro-
chloride to give 2 in an overall yield of 59% (three steps).
Construction of the oxazolidinone ring system was ac-
complished by reaction of 2 with Im2CO in CH2Cl2 to afford
3 in 76% yield. The D-ring was formed by acylation of acid
4 to yield 5 in 79% yield. Compound 6 was obtained from
5 by reduction of ketone to methylene in two steps. Refluxing
6 with 6 N NaOH (aq) in MeOH successfully cleaved the
oxazolidinone to give the key intermediate 7 in 95% yield,
from which a series of interesting modifications could be
achieved.
In our innovative approach, the E-ring is constructed after
the D-ring has been conjugated. This synthetic strategy has
the following merits: the size of the E-ring can be easily
(8) (a) Comins, D. L.; Chen, X.; Morgan, L. A. J. Org. Chem. 1997,
62, 7435–7438. (b) Ihara, M.; Takino, Y.; Tomotake, M.; Fukumoto, K.
J. Chem. Soc., Perkin Trans. 1 1990, 2287–2292. (c) Suzuki, H.; Aoyagi,
S.; Kibayashi, C. J. Org. Chem. 1995, 60, 6114–6122.
E7 (13c) was first synthesized along with two natural
products, R-antofine (13a) and R-cryptopleurine (13b), from
7 as described below. The amino group was protected with
a Boc group to furnish 8 in 96% yield (Scheme 2). The
hydroxy group was then oxidized by Py·SO3 to give an
aldehyde, which was converted to an alkene 9 by Wittig
(9) Kim, S.; Lee, T.; Lee, E.; Lee, J.; Fan, G. J.; Lee, S. K.; Lee, D. J.
Org. Chem. 2004, 69, 3144–3149.
(10) Zeng, W.; Chemler, S. R. J. Org. Chem. 2008, 73, 6045–6047.
(11) Kim, S. H.; Lee, J.; Lee, T.; Park, H. G.; Kim, D. Org. Lett. 2003,
5, 2703–2706.
(12) Suffness, M.; Douros, J. Anticancer Agents Based on Natural
Product Models; Academic Press: New York, 1980; pp 465-487.
(13) (a) Gao, W.; Busson, S.; Grill, S. P.; Gullen, E. A.; Hu, Y. C.;
Huang, X.; Zhong, S.; kaczmarek, C.; Gutierrez, J.; Francis, S.; Baker, D. C.;
Yu, S.; Cheng, Y. C. Bioorg. Med. Chem. Lett. 2007, 17, 4338–4342. (b)
Gao, W.; Lam, W.; Zhong, S.; kaczmarek, C.; Baker, D. C.; Cheng, Y. C.
Cancer Res. 2004, 64, 678–688.
(14) Su, C. R.; Damu, A. G.; Chiang, P. C.; Bastow, K. F.; Morris-
Natschke, S. L.; Lee, K. H.; Wu, T. S. Bioorg. Med. Chem. 2008, 16, 6233–
6241.
Org. Lett., Vol. 12, No. 7, 2010
1417

Scheme 2
.
Synthesis of Antofine, Cryptopleurine, and E7
Scheme 3. Synthesis of 12-Oxo-S-antofine
analogues with different functional groups on the E ring for
further derivatization (unpublished data). Moreover, this
novel strategy is also applicable for other natural products
in this family, such as tylophorine, which has different
substitution patterns on the phenanthrene scaffold.
reagent in 68% yield over two steps. After the Boc group
was removed, an appropriate unsaturated acid (e.g., 10a,
acrylic acid; 10b, 3-butenoic acid; 10c, 4-pentenoic acid)
was introduced to afford 10 in an average yield of 80% using
EDC or DEPC (for 10b, double-bond isomerization occurred
on the N-amide side chain using EDC/HOBt). Cycloalkene
analogues (11a-c) were obtained by cyclization with
Grubb’s second-generation (G2) catalyst and then underwent
hydrogenation with H2/Pd/C in MeOH to afford 12a-c in
high yields. The desired target alkaloids R-antofine (13a) and
R-cryptopleurine (13b) as well as an E-ring expanded
analogue (13c) were obtained by reduction of amide to amine
in an average yield of 70%. The analytical information
obtained from our synthesized 13a and 13b agreed with the
data reported in the literature.4,9
The versatility and usefulness of this strategy were further
demonstrated by the synthesis of two new analogues: 12-
oxo-S-antofine (17, Scheme 3) and 12N-methyl-12-aza-S-
antofine (18, Scheme 4). Compounds 17 and 18 contain a
ketone and an additional nitrogen, respectively, in the E-ring.
Compound 14 was obtained by reacting 9 with 2-meth-
oxyacrylic acid after removal of the Boc group in 83% yield.
The ring closure was accomplished using G2 catalyst to give
compound 15 in 88% yield. Subsequent reduction by LiAlH4
in THF and hydrolysis with HCl afforded 17 in 50% yield
over two steps to generate a carbonyl functionality at C12
position. For the synthesis of compound 18, the alcohol 8
was oxidized to the aldehyde, which underwent reductive
amination to give an intermediate amine. After removal of
the Boc group, both nitrogens were connected by HCHO to
produce 18 in 45% yield over four steps. By virtue of similar
strategies, we are able to synthesize a series of cryptopleurine
Scheme 4. Synthesis of 12N-Methyl-12-aza-S-antofine
The new compound E7 was screened in Vitro against a
panel of human tumor cell lines including A549 (lung), KB
(nasopharyngeal), DU-145 (prostate), and HCT-8 (colon),
using R-antofine and R-cryptopleurine as a comparison. The
results (GI50) are listed in Table 1. Interestingly, compound
E7 showed significant cytotoxic activity and was as potent
as R-antofine against A549 cell growth but with improved
selectivity relative to KB and DU145 tumor cell lines. This
result suggests that the E-ring size may affect the interaction
between the molecule and the binding site in the target, and
Table 1. Anticancer Activities (GI50) of R-Antofine,
R-Cryptopleurine, E7, 17, and 18
A549
(nM)
DU145
(nM)
KB
(nM)
HCT-8
(nM)
compd
R-antofine
R-cryptopleurine
22
1.38
25
290
660
25
1.59
179
710
2030
36
1.51
102
480
1720
ND
1.09
10
420
1000
E7
17
18
1418
Org. Lett., Vol. 12, No. 7, 2010

the expansion of the E-ring may play a role in the anticancer
selectivity. Although moderate to significant reduction in
cytotoxicity was observed for compounds 17 and 18, further
modifications on the ketone and substituents on the imida-
zolidine nitrogen are of great interest to potentially improve
the anticancer activity.
template are being studied in our laboratories, aiming to
discover interesting molecules with strong anticancer activity
and reduced CNS toxicity. The detailed synthetic pathways
to other analogues, their biological activity, and mechanistic
study results will be reported in due time.
Acknowledgment. This study was supported by Grant No.
CA17625-32 from the National Cancer Institute awarded to
K.H.L.
In conclusion, we have established a novel strategy to
synthesize new phenanthroindolizidine and phenanthroquino-
lizidine analogues, which provides a useful tool for managing
CNS toxicity and exploring the SAR profile, especially on
the E-ring, for which no analogues have been reported so
far, despite substantial progress in the development of their
syntheses. Three new E-ring modified analogues E7, 17, and
18 were first reported through our novel strategy detailed
above to verify our method’s feasibility and apparent
versatility. Other modifications using R-cryptopleurine as
Supporting Information Available: General experimental
information, cytotoxicity assay, detailed experimental pro-
1
cedures, and copies of H NMR and 13C NMR spectra of
key intermediates 3, 5-7, 9, 11, 12, 15 and 16 and final
products 13, 17, and 18. This material is available free of
charge via the Internet at http://pubs.acs.org.
OL902819J
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