One-pot synthesis of luotonin A and its analogues

Article abstract of DOI:10.1021/ol1029707

Starting with inexpensive reagents, a self-directed chemical process with the aid of a single metal triflate was readily achieved to concomitantly construct quinazoline and pyrroloquinoline cores to afford the synthesis of luotonin A and its analogues. Am

Full text of DOI:10.1021/ol1029707

ORGANIC
LETTERS
2011
Vol. 13, No. 5
920–923
One-Pot Synthesis of Luotonin A and Its
Analogues
Ming-Chung Tseng,†,‡ Yu-Wan Chu,† Hsiang-Ping Tsai,§ Chun-Mao Lin,§ Jaulang
Hwang,‡ and Yen-Ho Chu*,†
Department of Chemistry and Biochemistry, National Chung Cheng University,
Minhsiung, Chiayi 62102, Taiwan, ROC, Institute of Molecular Biology,
Academia Sinica, Nankang, Taipei 11529, Taiwan, ROC, and Department of
Biochemistry, School of Medicine, Taipei Medical University, Taipei 110, Taiwan, ROC
cheyhc@ccu.edu.tw
Received December 8, 2010
ABSTRACT
Starting with inexpensive reagents, a self-directed chemical process with the aid of a single metal triflate was readily achieved to concomitantly
construct quinazoline and pyrroloquinoline cores to afford the synthesis of luotonin A and its analogues. Among all compounds prepared, 2c, 2d,
and 3b exhibit more potent inhibitory activity than luotonin A against human topoisomerase I.
This paper reports the synthesis of luotonin A and its
analogues in one pot. Many naturally occurring products
have quinoline and/or quinazoline rings embedded in their
structures, and luotonin A (1) carries both structural
motifs (Figure 1).1 Our group recently reported the appli-
cation of metal triflates for use in the synthesis of natural
and unnatural quinazoline containing alkaloids such as
asperlicin C, circumdatin F, and sclerotigenin.2 We were
interested in further showcasing this Lewis acid catalyzed
cyclization reaction for other natural product syntheses,
particularly in an intramolecular fashion, and, for exactly
this reason, specifically targeted luotonin A. Here, we
report a synthetic strategy for the simultaneous construc-
tion of quinazoline and pyrroloquinoline cores (BCD
rings) applied to the total synthesis of 1. This concomitant
formation of three rings leading to luotonin A preparation
in one pot, to our knowledge, has not been reported in the
literature and may potentially be useful for the synthesis of
other complex natural products.
Luotonin A is a cytotoxic alkaloid first isolated in 1997
from the plant Peganum nigellastrum.3 This plant has a
history of use in Chinese medicine for the treatment of
rheumatism and inflammation. Being experimentally de-
monstrated as a human DNA topoisomerase I (hTopI)
poison to stabilize a DNA/enzyme binary complex using
the same mechanism of action as camptothecin (CPT),4 1
displays useful cytotoxicity in vitro against the murine
† National Chung Cheng University.
‡ Academia Sinica.
§ Taipei Medical University.
(1) For recent reviews, see: (a) Michael, J. P. Nat. Prod. Rep. 2008, 25,
166. (b) Michael, J. P. Nat. Prod. Rep. 2007, 24, 223. (c) Mhaske, S. B.;
Argade, N. P. Tetrahedron 2006, 62, 9787. (d) Connolly, D. J.; Cusack,
D.; O’Sullivan, T. P.; Guiry, P. J. Tetrahedron 2005, 61, 10153.
(2) (a) Tseng, M.-C.; Yang, H.-Y.; Chu, Y.-H. Org. Biomol. Chem.
2010, 8, 419. (b) Tseng, M.-C.; Lai, C.-Y.; Chu, Y.-W.; Chu, Y.-H.
Chem. Commun. 2009, 445. (c) Tseng, M.-C.; Chu, Y.-H. Tetrahedron
2008, 64, 9515.
(3) Ma, Z.-Z.; Hano, Y.; Nomura, T.; Chen, Y.-J. Heterocycles 1997,
46, 541.
r
10.1021/ol1029707
Published on Web 01/26/2011
2011 American Chemical Society

Scheme 1. One-Pot Synthesis of Luotonin A (1)
Figure 1. Structure of luotonin A (1).
convergent construction of one or two rings (B, C, or D)
from fragment compounds. Thus, further development of
a practical and efficient, preferably one-pot, synthesis of 1
and its derivatives (2a-f and 3a-e) would be of immense
value for SAR study (Figure 2).
By retrosynthetic analysis of 1, a simultaneous construc-
tion of BCD rings by quinazolinone formation (D ring)
and an intramolecular aza-Diels-Alder (BC rings) reac-
tion can in principle be achieved with the highest efficiency.
Since both reactions were known to be catalyzed by Lewis
acids,2,8 we envisaged that a self-directed sequence of
reactions;(i) imine formation, (ii) quinazolinone forma-
tion, (iii) intramolecular aza-Diels-Alder reaction, and
finally (iv) dehydrogenation and (v) aromatization;acti-
vated and catalyzed by metal triflates could concomitantly
form the desired structure 1.
Scheme 1 illustrates our initial success of luotonin A
synthesis of which the BCD rings were assembled from the
appropriate aniline, a propargyl unit, and the glyoxal. The
proposed reaction sequence did concomitantly occur in a
one-step fashion under our experimental conditions. This
total synthesis of 1 started from the commercial, inexpen-
sive isatoic anhydride 4. Upon reacting first with propar-
gylamine in DMF at ambient temperature for 1 h, 4 was
readily converted to a 2-aminobenzamide 5 in nearly
quantitative yield.9 After removing the solvent and exces-
siveamine in vacuo, reflux of this 2-aminobenzamide 5 in o-
xylene with aniline (10 equiv), Yb(OTf)3 (20 mol %), and
40% aqueous glyoxal (15 equiv) for 12 h furnished the
fluorescent product 1 with 35% isolated yield after column
chromatography. The selection of 20 mol % Yb(OTf)3 was
based upon the screening optimization of 11 metal triflates
available in our laboratory (Table S1, Supporting Infor-
mation). This sequence of reactions was remarkably facile,
and because of the air and water stability of the catalyst,
the reaction required no special precautions. Of the metal
triflates investigated, all of them catalyzed the reaction
under the same experimental conditions; however, the
isolated yields (14-26%) from other catalysts were infer-
ior to that with Yb(OTf)3 (Table S1, Supporting Infor-
mation). Albeit In(OTf)3 wasknown tobe a superior Lewis
acid for the hetero Diels-Alder reactions,10 in our hands it
Figure 2. Structures of luotonin A analogues (2a-f) and C-ring
expanded analogues (3a-e).
leukemia P-388 cell line with an IC50 value of 6.3 μM.
Albeit it is not potent enough for cancer chemotheraphy,
luotonin A is nevertheless a lead compound and has been
the subject of a number of total syntheses and bioactivity
investigations.5,6 Many of the syntheses of 1 however
involved lengthy synthetic procedures, harsh experimental
conditions, long reaction hours, and low overall yields.7
Among the syntheses reported, most completed 1 with the
(4) Cagir, A.; Jones, S. H.; Gao, R.; Eisenhauer, B. M.; Hecht, S. M.
J. Am. Chem. Soc. 2003, 125, 13628.
(5) For a recent review on the synthesis and biological activity of
luotonin A and related derivatives, see: Ma, Z.; Hano, Y.; Nomura, T.
Heterocycles 2005, 65, 2203.
(6) Since its discovery in 1997, there have been 74 publications
(SciFinder on January 7, 2011) on the synthesis and biological applica-
tions of luotonin A.
(7) For recent studies of luotonin A syntheses, see: (a) Ju, Y.; Lu, F.,
F.; Li, C. Org. Lett. 2009, 11, 3582. (b) Liang, Y.; Jiang, X.; Yu, Z.-X.
Org. Lett. 2009, 11, 5302. (c) Sridharan, V.; Ribelles, P.; Ramos, M. T.;
Menendez, J. C. J. Org. Chem. 2009, 74, 5715. (d) Nacro, K.; Zha, C.;
Guzzo, P. R.; Herr, R. J.; Peace, D.; Friedrich, T. D. Bioorg. Med. Chem.
2007, 15, 4237. (e) Zhou, H.-B.; Liu, G.-S.; Yao, Z.-J. J. Org. Chem.
2007, 72, 6270. (f) Twin, H.; Batey, R. A. Org. Lett. 2004, 6, 4913.
(g) Mhaske, S. B.; Argade, N. P. J. Org. Chem. 2004, 69, 4563.
(h) Dallavalle, S.; Merlini, L.; Beretta, G. L.; Tinelli, S.; Zunino, F.
Bioorg. Med. Chem. Lett. 2004, 14, 5757. (i) Cagir, A.; Jones, S. H.;
Eisenhauer, B. M.; Gao, R.; Hecht, S. M. Bioorg. Med. Chem. Lett. 2004,
14, 2051. (j) Cagir, A.; Eisenhauer, B. M.; Gao, R.; Thomas, S. J.; Hecht,
S. M. Bioorg. Med. Chem. Lett. 2004, 12, 6287. (k) Chavan, S. P.;
Sivappa, R. Tetrahedron 2004, 60, 9931. (l) Harayama, T.; Hori, A.;
Serban, G.; Morikami, Y.; Matsumoto, T.; Abe, H.; Takeuchi, Y.
Tetrahedron 2004, 60, 10645. (m) Toyota, M.; Komori, C.; Ihara, M.
ARKIVOC 2003, 15. (n) Lee, E. S.; Park, J.-G.; Jahng, Y. Tetrahedron
Lett. 2003, 44, 1883. (o) Osborne, D.; Stevenson, P. J. Tetrahedron Lett.
2002, 43, 5469. (p) Yadav, J. S.; Reddy, B. V. S. Tetrahedron Lett. 2002,
43, 1905. (q) Dallavalle, S.; Merlini, L. Tetrahedron Lett. 2002, 43, 1835.
(r) Toyota, M.; Komori, C.; Ihara, M. Heterocycles 2002, 56, 101.
(s) Ma, Z.-Z.; Hano, Y.; Nomura, T.; Chen, Y.-J. Heterocycles 1999,
51, 1593. (t) Kelly, T. R.; Chamberland, S.; Silva, R. A. Tetrahedron
Lett. 1999, 40, 2723. (u) Wang, H.; Ganesan, A. Tetrahedron Lett. 1998,
39, 9097.
(8) (a) Kouznetsov, V. V. Tetrahedron 2009, 65, 2721. (b) Buonora,
P.; Olsen, J.-C.; Oh, T. Tetrahedron 2001, 57, 6099.
(9) The 2-aminobenzamide 4 intermediate could be isolated in 97%
yield, when needed.
(10) Ali, T.; Chauhan, K. K.; Frost, C. G. Tetrahedron Lett. 1999, 40,
5621.
Org. Lett., Vol. 13, No. 5, 2011
921

Scheme 2. Possible Mechanism for the Synthesis of Luotonin A (1)a
a LA, Lewis acid; IADA, intramolecular aza-Diels-Alder.
catalyzed to produce 1 only with 24% isolated yield. These
results might be ascribed to the just right Lewis acidity of
Yb(OTf)3 for the cascade reactions. More Yb(OTf)3 did
not improve the yield of 1 (Table S1, Supporting Infor-
mation). If this reaction was performed at ambient tem-
perature, no trace of 1 was observed.
Previous studies on luotonin A derivatives were focused
on the introduction of substituents on ring E.12
The results of the activity assay showed that E-ring
substituents contributed to the stabilization of, but were
not essential for targeting, the enzyme-DNA binary
complex.12 In this work, we focused on the synthesis of
the A- and C-ring analogues of 1, using readily available
substituted anilines and but-3-yn-1-amine. Results show
that, regardless of the nature of substituents, target com-
pounds 2a-f and 3a-e were all readily synthesized in one-
pot from their corresponding reagents (Tables S2 and S3,
Supporting Information). In our hands, compounds 2d
and 3b gave the highest (40%) and lowest (17%) isolated
yields, respectively. It is worth noting that compounds 3a
and 3b could be considered as the B ring-expanded analo-
gues of natural products rutaecarpine and hortiacine,
respectively.13 The successful application of our one-pot
protocol to access fused pyrrolo- and pyridoquinazolino-
quinolines demonstrates the utility of this new method.
All compounds synthesized in this work were assayed
for biological activities (Figure 3). In literature, only
limited numbers of luotonin A derivatives with substitu-
ents on ring A and C were reported and showed promising
biological activities.14 Inhibitory activities of compounds
1, 2a-f, and 3a-e to hTopI were determined by assaying
the relaxation of a supercoiled pBlueScript SKþ plasmid
DNA (for experiments, see Supporting Information).15
While insufficient mechanistic data exist at present, a
plausible mechanism for the synthesis of 1 is presented in
Scheme 2. First, propargylamine reacted with isatoic
anhydride 4 to form an isolable intermediate 5 followed
by the Lewis acid mediated formation of the imine 6,
concomitantly leading to a ring-closed product 7. Subse-
quently, a dehydrogenation of 7f8f9 forms the stable D
ring in 9, and later, an aromatization of 10f11f1 could
readily proceed through Lewis acid catalyzed H-abstrac-
tion and deprotonation of the intermediary cyclohexadie-
nyl cation. This aromatization of cyclohexadiene 10 and
related hydrocarbons promoted by Lewis acids is well
documented in literature.11 As one of the key steps in this
proposed mechanism, the process of 9f10 could be ratio-
nalized (elegantly proposed by Yu et al.7b) using a Lewis
acid catalyzed, inverse electron-demand aza-Diels-Alder
[4þ þ 2] cycloaddition reaction in the intramolecular
fashion (IADA) between N-chelated N-phenyliminium
azadiene and the electron-rich alkyne dienophile in 9.
The success of the preparation of 1 using the Yb(OTf)3
catalyst prompted us to further pursue utilizing this one-
pot synthesis for other target compounds with greater
diversity (Tables S2 and S3, Supporting Information).
(13) For a recent review, see: Lee, S. H.; Son, J.-K.; Jeong, B. S.;
Jeong, T.-C.; Chang, H. W.; Lee, E.-S.; Jahng, Y. Molecules 2009, 13,
272.
(11) (a) Ikeshita, K.-I.; Kihara; Sonoda, N. M.; Ogawa, A. Tetra-
hedron Lett. 2007, 48, 3025. (b) Oshita, M.; Chatani, N. Org. Lett. 2004,
6, 4323.
(14) Rahman, A. F. M. M.; Kim, D. H.; Liang, J. L.; Lee, E.-S.; Na,
Y.; Jun, K.-Y.; Kwon, Y.; Jahng, Y. Bull. Korean Chem. Soc. 2008, 29,
1988.
(12) Cagir, A.; Jones, S. H.; Eisenhauer, B. M.; Gao, R.; Hecht, S. M.
Bioorg. Med. Chem. Lett. 2004, 14, 2051.
(15) Chan, A. L.-F.; Chang, W.-S.; Chen, L.-M.; Lee, C.-M.; Chen,
C.-E.; Lin, C.-M.; Hwang, J.-L. Molecules 2009, 14, 1342.
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Org. Lett., Vol. 13, No. 5, 2011

Figure 3. Analysis of hTop1 inhibition by luotonin A (1) and its analogues (2a-f, 3a-e). For experimental conditions, see the
Supporting Information.
As shown in Figure 3, our preliminary result demon-
strated that, to our delight, compounds 2c, 2d, and 3b
exhibited more potent inhibitory activities than luotonin A
(1) and the control CPT against hTopI: 48%, 54%, 54%,
18%, and 25% inhibition, respectively, at a concentration of
5μM. It is also noted that compounds 2e and 3e did not show
any inhibitory activity against hTopI. This preliminary data
on the SAR study clearly indicated that compounds 2d and
3b at 5 μM possess a 2.2 times stronger inhibitory activity on
hTop1 compared to CPT (5 μM); the result shown in
Figure 3 likely could serve as a useful starting point for hit
identification in inhibitor discovery. Using the convenient
one-pot method developed in this work, a combinatorial
approach on the design and synthesis of luotonin A tem-
plated libraries is in progress and the results on the identifi-
cation of potent inhibitors will be reported in due course.
In summary, we have developed a one-pot protocol for
the synthesis of luotonin A (1) and its analogues (2a-f and
3a-e) under Lewis acid catalysis. Our method avoids the
need for harsh basic or acidic conditions and allows
concomitant construction of multiple rings (B, C, and
D). This is the shortest synthesis for luotonin A. Compared
with elegant syntheses previously developed for luotonin A
(1) by research groups of Batey7f (10% yield in eight steps)
and Yao7e (47% yield in five steps), our approach presents
the advantages of a one-pot route, moderate but accepta-
ble isolated yield (especially for the key concomitant three-
ring cyclization step), and no need for the isolation and
purification of any intermediates. The chemistry presented
in this work efficiently achieves the synthesis of the luoto-
nin A natural product as well as the preparation of its
analogues and should contribute to the SAR study in this
family of antitumor agents to further identify luotonin A
analogues with more potent anticancer activity.
Acknowledgment. This work was supported by a multi-
year Grant-in-Aid from the ANT Technology (Taipei,
Taiwan) and grants from the National Science Council
(Taiwan, ROC). We thank S.-K. Hsiao (National Chung
Cheng University) for his contribution at the early stage of
this work. We also thank reviewers for valuable and
constructive comments.
Supporting Information Available. Tables S1-S3, ex-
perimental conditions and procedures, 1H and 13C NMR
data and spectra of all compounds 1, 2a-f, and 3a-e.
This material is available free of charge via the Internet at
http://pubs.acs.org.
Org. Lett., Vol. 13, No. 5, 2011
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