Synthesis of hybrids between the alkaloids rutaecarpine and luotonins A, B

Article abstract of DOI:10.1016/j.tetlet.2008.05.141

The synthesis of 7,12-dihydroindolo[2′,3′:3,4]pyrrolo[2,1-b]quinazolin-5-one, a hybrid compound containing common structural features of the natural alkaloids rutaecarpine (Evodia rutaecarpa) and luotonin A (Peganum nigellastrum), was performed by active methylene group transformations of deoxyvasicinone. The synthesis of 7-hydroxy-8-norrutaecarpine was accomplished via the first total synthesis of bouchardatine (Bouchardatia neurococca) and its acid-catalyzed ring closure. The synthesized alkaloid analogues are the first representatives of a new heterocyclic ring system. Preliminary testing of the synthesized compounds showed cytotoxic activities against HeLa cells and apoptosis inducing effects at a concentration comparable to that of the control drug, etoposide.

Full text of DOI:10.1016/j.tetlet.2008.05.141

Tetrahedron Letters 49 (2008) 4937–4940
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Tetrahedron Letters
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Synthesis of hybrids between the alkaloids rutaecarpine and luotonins A, B
b
a
a
b
a
a
Máté Bubenyák a, , Melinda Pálfi , Mária Takács , Szabolcs Béni , Éva Szöko , Béla Noszál , József Kökösi
*
}
a
}
Department of Pharmaceutical Chemistry, Semmelweis University, Hogyes E. u. 9., Budapest H-1092, Hungary
b Department of Pharmacodynamics, Semmelweis University, Nagyvárad tér 4., Budapest H-1089, Hungary
a r t i c l e i n f o
a b s t r a c t
The synthesis of 7,12-dihydroindolo[20,30:3,4]pyrrolo[2,1-b]quinazolin-5-one, a hybrid compound con-
taining common structural features of the natural alkaloids rutaecarpine (Evodia rutaecarpa) and luotonin
A (Peganum nigellastrum), was performed by active methylene group transformations of deoxyvasicinone.
The synthesis of 7-hydroxy-8-norrutaecarpine was accomplished via the first total synthesis of bouchard-
atine (Bouchardatia neurococca) and its acid-catalyzed ring closure. The synthesized alkaloid analogues
are the first representatives of a new heterocyclic ring system. Preliminary testing of the synthesized
compounds showed cytotoxic activities against HeLa cells and apoptosis inducing effects at a concentra-
tion comparable to that of the control drug, etoposide.
Article history:
Received 23 April 2008
Revised 19 May 2008
Accepted 29 May 2008
Available online 4 June 2008
Keywords:
Hybrid alkaloid
Rutaecarpine
Luotonin
Ó 2008 Elsevier Ltd. All rights reserved.
Bouchardatine
Apoptosis
Norrutaecarpine
In search of new polyheterocyclic systems of potential pharma-
cological value, we prepared new pentacyclic compounds by fusing
pyrroloquinazolone and indole rings. Compounds 1 (Fig. 1), de-
scribed in this Letter, are structurally close to natural alkaloids
such as rutaecarpine 2 and the luotonins 3 which have shown
antitumour activities.1–3
Rutaecarpine-type alkaloids constitute an important class of
indolopyridoquinazolinone heterocycles, which belong to the sub-
group of quinazoline-type alkaloids isolated from both the heart-
wood and the fruit of numerous plants and trees of the Rutaceae
family.3 Their extracts have long been used as important remedies
in Chinese traditional medicine for the treatment of various dis-
eases.4 The Chinese herbal drug, Wu-Chu-Yu, the dried unripe fruit
of Evodia rutaecarpa, showed remarkable activities against head-
ache, cholera, dysentery, worm infestations and post partum
disturbances.5 Rutaecarpine 2 is a major quinazolinocarboline
alkaloid isolated from E. rutaecarpa and shows a variety of pharma-
cological activities including antithrombotic, vasorelaxant and
cyclooxygenase (COX-2) inhibitory effects.6 In 2004, rutaecarpine
was described as an antihypertensive drug which activates the
vanilloid receptor (VR-1).7 Various total syntheses of 2 have been
reported8 including our original synthetic approaches.9
Luotonins A and B, 3a,b, are recently isolated members of a new
alkaloid family extracted from the aerial parts of Peganum nigella-
strum which contain a pyrroloquinazolinoquinoline skeleton.10 The
plant extracts are used in traditional Chinese medicine and are
reported to exhibit activity against a range of ailments including
rheumatism, inflammation, influenza, hepatitis and leukaemia.11
Luotonin A seems to be very promising as a campthotecin-
like antitumour agent selectively inhibiting human DNA topo-
isomerase I.12 In the past decade, luotonin A has become an impor-
tant synthetic target and a variety of preparative methods have
been published.13 The structural similarity between rutaecarpine
2 and luotonins 3a,b is remarkable and both alkaloids exhibit
X
7
O
N
4
N
H
N
1a X=H
1b X=OH
1
X
8
1
14
13
7
O
O
A
C
N
N
B
N
D
4
N
N
H
N
6
E
3a X=H
3b X=OH
2
1
Figure 1. Structure of 8-norrutaecarpine 1a, 7-hydroxy-8-norrutaecarpine 1b,
rutaecarpine 2, luotonin A 3a and luotonin B 3b.
* Corresponding author. Tel.: +36 1 476 3600x3073; fax: +36 1 217 0891.
E-mail address: bmate@gyok.sote.hu (M. Bubenyák).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.05.141

4938
M. Bubenyák et al. / Tetrahedron Letters 49 (2008) 4937–4940
topoisomerase inhibitory activity. An obvious difference between
the two compounds is the flexibility of the six-membered C-ring
of rutaecarpine compared to the rigidity of the five-membered C-
ring of luotonin, which was postulated to be essential for biological
activity.2
Deoxyvasicinone 4, a natural alkaloid of Peganum harmala,
is readily available by cyclocondensation of 2-methoxy-3,4-
dihydro-2H-pyrrole and anthranilic acid.14 Compound 4 is weakly
active towards electrophilic reagents at the C3 carbon atom. Bro-
mination of 4 was carried out with bromine in the presence of
sodium acetate in 75% acetic acid solution at 60 °C, producing
the 3,3-dibromo derivative 5 in 82% yield (Scheme 1). Substitution
of 5 with excess of 25% aqueous hydrazine hydrate solution in eth-
anol (60 °C, 2 h) led to the 3-oxo derivative 6 in 79% yield. A small-
scale method was described to produce 6 directly from 4 via SeO2
oxidation in inert solution in moderate yield.15 Nucleophilic substi-
Br
Br
N
3
N
N
N
O
ii
i
N
N
N
1
O
O
iv
5
4
vi
7
iii
N
H
N
O
N
N
H
N
O
N
N
v
vii
1a
N
N
N
O
O
6
8E
8Z
Scheme 1. Reagents and conditions: (i) 2 equiv Br2, 75% AcOH, CH3COONa, 60 °C (82% yield); (ii) dimethylformamide, 2 equiv POCl3, rt, 1 h, 60 °C 3 h (94% yield); (iii) 25%
NH2NH2ꢀH2O, 60 °C, 2 h (79% yield); (iv) Ph-NHNH2, EtOH, 4 h reflux (89% yield); (v) Ph-NHNH2, EtOH, 6 h reflux (95% yield); (vi) Ph-N þClꢁ, AcOH/H2O, 0 °C 3 h (96% yield);
2
(vii) polyphosphoric acid, 180 °C, 20 min (67% yield).
O
i
H2N
H2N
O
O
+
N
CONH2
N
H
H
HN
11
OH
9
10
ii
NH
Br
N
H
N
N
N
O
iii
iv
N
H
NH
NH
N
O
O
v
12
13
14
OH
N
O
OH
O
H
N
H
O
N
O
N
H
N
H
viii
vi
vii
N
1a
H
N
N
N
17
15
1b
Scheme 2. Reagents and conditions: (i) DCCI, THF, 20 °C, 12 h (78% yield); (ii) Na, ethylene glycol, 180 °C, 30 min (93% yield); (iii) 3 equiv PhNHNH2, EtOH, 80 °C, 3 h (92%
yield); (iv) polyphosphoric acid, 180 °C, 20 min (84% yield); (v) DMF, POCl3, 0 °C, 24 h (89% yield); (vi) NaBH4, MeOH, 20 °C, 2 h (95% yield); (vii) 30% H2SO4, EtOH, reflux, 3 h
(67% yield); (viii) 30% H2SO4, EtOH, reflux, 1 h (72% yield).

M. Bubenyák et al. / Tetrahedron Letters 49 (2008) 4937–4940
4939
tution of 5 or condensation of 3-ketone 6 with excess phenyl
hydrazine in ethanol afforded 3-phenylhydrazone 8 in excellent
yield (89% and 95%, respectively).
improved solubility (which will then allow their biological evalua-
tion) is in progress and will be described later.
Vilsmeier–Haack formylation of 4 using 2 equiv of phosphoryl
chloride in dimethylformamide at 60 °C for 3 h gave the 3-dimeth-
ylaminomethylene derivative 7 in 94% yield. Japp-Klingemann
reaction of 7 with phenyldiazonium chloride16 in acetic acid solu-
tion (0 °C, 3 h) resulted in phenylhydrazone derivative 8 in 96%
yield. Compound 8 exhibits solvent dependent E–Z geometric
isomerism, as indicated by the NMR spectra. In DMSO-d6 the steri-
cally more favoured E form is dominant, due to an intermolecular
hydrogen bond between the amino group and the solvent mole-
cules. In DMSO-d6 the E–Z ratio is 55:45 at rt.17 These facts indicate
a low activation energy for isomerization of the exocyclic C(3)@N
double bond.18
Fischer indolization of compound 8 in polyphosphoric acid gave
8-norrutaecarpine 1a. The crude product was purified by flash
chromatography19 on silica gel to give 1a in good yield. The result-
ing compound, 7,12-dihydroindolo[20,30:3,4]pyrrolo[2,1-b]quinaz-
olin-5-one 1a, which can also be named as 8-norrutaecarpine or
14-norluotonin A constitutes a new pentacyclic ring system, which
contains common structural features of the two bioactive natural
alkaloids.
An alternative synthetic approach was elaborated for the prep-
aration of the pentacyclic ring system starting from anthranyl-
amide 10 (Scheme 2). Condensation of 10 with indole-2-
carboxylic acid 9 using DCCI in tetrahydrofuran solution and sub-
sequent base-catalyzed cyclocondensation provided the indolyl-
quinazolone derivative 12. The same product 12 was also
available by reaction of 2-bromoethylquinazolone 13 with 3 equiv
of phenylhydrazine and indolization of the obtained phenylhydraz-
one 14 in polyphosphoric acid at 180 °C. Vilsmeier–Haack formyla-
Acknowledgement
This work was supported by the Hungarian Scientific Research
Fund (OTKA T048554, OTKA K73804).
References and notes
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Molecules 2008, 13, 272.
4. Tang, W.; Eisenbrand, G. In Chinese drugs Drugs of plant originPlant Origin; Tang,
W., Eisenbrand, G., Eds.; Springer: Berlin, 1992; pp 509–514.
5. Li, M.-T.; Huang, H.-I. Yao Hsueh Hsuesh Pao 1966, 13, 265; Chem. Abstr. 1966,
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6. Hibino, S.; Choshi, T. Nat. Prod. Rep. 2001, 18, 66.
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Hypertens. 2004, 22, 1819.
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Mohanta, P. K.; Kim, K. Tetrahedron Lett. 2002, 43, 3993; (c) Lee, E. S.; Park, J. G.;
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Tetrahedron 2004, 60, 3417; (e) Hamid, A.; Elomri, A.; Da¨ıch, A. Tetrahedron Lett.
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Lett. 2008, 49, 481 and references cited therein.
9. (a) Kökösi, J.; Hermecz, I.; Szász, G.; Mészáros, Z. Tetrahedron Lett. 1981, 22,
4861; (b) Kökösi, J.; Szász, G.; Hermecz, I. Tetrahedron Lett. 1992, 33, 2995; (c)
Kökösi, J.; Hermecz, I.; Podányi, B.; Szász, G.; Mészáros, Z. J. Heterocycl. Chem.
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Debreczy, L.; Szász, G.; Mészáros, Z. J. Heterocycl. Chem. 1987, 24, 1045; (e)
Hermecz, I.; Kökösi, J.; Vasvári-Debreczy, L.; Horváth, Á.; Podányi, B.; Szász, G.;
Mészáros, Z. F. E. C. S. In Int. Conf. Chem. Biotechnol. Biol. Act. Nat. Prod., [Proc],
3rd 1985 (Pub. 1987) 5, p 332.; . Chem. Abstr. 1989, 110, 231939; (f) Hermecz, I.;
Ko
¨
ko
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si, J.; Poda
nyi, B.; Liko
J.; Rotter, R.; Hermecz, I. Tetrahedron 2000, 56, 7987.
10. Ma, Z.-Z. ; Hano, Y.; Nomura, T.; Chen, Y.-J. Phytochemistry 2000, 53, 1075.
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nyi, B.; Sza
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sz, G. Heterocycles 1994, 37, 903; (g) Hermecz, I.;
´
Kokosi, J.; Poda
¨
´
´
, Zs. Tetrahedron 1996, 52, 7789; (h) Kiss, A.; Kokosi,
¨
¨
¨
tion of 12 using
a slight excess of phosphoryl chloride in
dimethylformamide under mild conditions (0 °C, 24 h) provided
the 3-formylindole derivative 15 in almost quantitative yield.
The prepared compound 15 is identical with bouchardatine, the
recently isolated natural alkaloid, from the aerial parts of Bou-
chardatia neurococca (Rutaceae).20 This procedure is also the first
total synthesis of the indolylquinazolone alkaloid 15.21 Conversion
of the tetracyclic compound 15 into the pentacyclic system was
performed using an analogous literature method to that applied
in the synthesis of luotonin.22 Reduction of the formyl group in
15 with sodium borohydride in methanol and acid-catalyzed ring
closure of the obtained 3-hydroxymethyl indole derivative 17 with
30% sulfuric acid provided 8-norrutaecarpine 1a. Similarly, acid-
catalyzed ring closure of 15 led to 7-hydroxy-8-norrutaecarpine
1b.
Preliminary apoptotic studies were performed as follows: To
test the effects of 8-norrutaecarpine 1a, bouchardatine 15 and
rutaecarpine 2, HeLa cells were incubated with 10ꢁ6 mol/l of the
compounds for 72 h and nucleosomal DNA fragmentation, a mar-
ker of apoptotic cell death, was analyzed by flow cytometry. The
percent of apoptotic cells corresponding to the sub-G1 phase was
found to be 38.6 3.3 with bouchardatine 15, 24.1 3.8 with
8-norrutaecarpine 1a and 14.5 2.8 with rutaecarpine 2. Cells
were incubated with etoposide as a positive control, and 15.4
1.9 percent were observed to undergo apoptosis.
11. Cagir, A.; Jones, S. H.; Gao, R.; Eisenhauer, B. M.; Hecht, S. M. J. Am. Chem. Soc.
2003, 125, 13628.
12. Nacro, K.; Zha, C.; Guzzo, P. R.; Herr, R. J.; Peace, D.; Friedrich, T. D. Bioorg. Med.
Chem. 2007, 15, 4237.
13. Selected articles: (a) Ma, Z.-Z. ; Hano, Y.; Nomura, T.; Chen, Y.-J. Heterocycles
1999, 51, 1593; (b) Kelly, T. R.; Chamberland, S.; Silva, R. A. Tetrahedron Lett.
1999, 40, 2723; (c) Osborne, D.; Stevenson, P. J. Tetrahedron Lett. 2002, 43,
5469; (d) Dallavalle, S.; Merlini, L. Tetrahedron Lett. 2002, 43, 1835; (e)
Harayama, T.; Morikami, Y.; Shigeta, Y.; Abe, H.; Takeuchi, Y. Synlett 2003, 6,
847; (f) Twin, H.; Batey, R. A. Org. Lett. 2004, 6, 4913; (g) Cheng, K.; Rahier, N. J.;
Eisenhauer, B. M.; Gao, R.; Thomas, S. J.; Hecht, S. M. J. Am. Chem. Soc. 2005, 127,
838; (h) Xiao, X.; Antony, S.; Pommier, Y.; Cushman, M. J. Med. Chem. 2006, 49,
1408; (i) Zhou, H.-B. ; Liu, G.-S. ; Yao, Z.-J. J. Org. Chem. 2007, 72, 6270; (j)
Bowman, W. R.; Elsegood, M. R.; Stein, T.; Weaver, G. W. Org. Biomol. Chem.
2007, 5, 103; (k) Mason, J. J.; Bergman, J. Org. Biomol. Chem. 2007, 5, 2486. and
references cited therein.
14. Chatterjee, A.; Ganguly, M. Phytochemistry 1968, 7, 307–311.
15. Molina, P.; Tárraga, A.; González-Tejero, A. Synthesis 2000, 1523.
16. Vogel, A. I. Practical Organic Chemistry; Longman Group Ltd: Essex, 1978. pp
687–720.
17. 1H NMR (600 MHz, DMSO-d6) d 3.00 (t, J = 7.21 Hz, 2H), 4.19 (t, J = 7.21 Hz, 2H),
6.90 (t, J = 6.63 Hz, 1H), 7.31 (m, 2H, overlap), 7.32 (m, 2H, overlap), 7.48 (t,
J = 7.80 Hz, 1H), 7.75 (d, J = 7.82 Hz, 1H), 7.80 (td, J = 1.24, 7.81 Hz, 1H), 8.14, (d,
J = 7.82 Hz, 1H), (d NH 10.03 and 11.96), E:Z ratio: 55:45.
18. Tóth, G.; Szöllo}sy, Á.; Almásy, A.; Podányi, B.; Hermecz, I.; Breining, T.;
Mészáros, Z. Org. Magn. Reson. 1983, 21, 687–693.
19. Preparation of 8-norrutaecarpine 1a: 3.3 mmol of
8 was added to
polyphosphoric acid (10.0 g, Fluka) at 180 °C and the reaction mixture was
stirred for 30 min. Cold water (100 ml) was added and the aqueous mixture
was stirred for 1 h. The precipitated crystals were filtered off, treated with 10%
NaOH solution and water. The resulting dried dark-green crude solid was
chromatographed on Kieselgel 60 (Merck) (230–400 mesh ASTM) solid phase,
eluting with CH2Cl2/EtOAc (4:1). The middle fractions gave a pale yellow solid
In conclusion, we have synthesized a new pentacyclic indo-
lopyrroloquinazoline ring system. Facile alternative preparative
procedures have been carried out from readily available starting
materials. In all these methods, the synthetic strategy involved
construction of the BC ring and/or to build the connection between
the B and D rings from substituted quinazoline derivatives.
Unfortunately, the low solubility of hybrid compounds 1 is a
limitation preventing correct estimation of their biological activity.
The preparation of various substituted derivatives that may exhibit
in 67% yield, mp: 305 °C. UV: in EtOH kmax (loge): 349.2 (4.444), 333.2 (4.538),
319.6 (4.490), 239.2 (4.442), 226 (4.453), 202.8 (4.481) nm. 1H NMR (400 MHz,
DMSO-d6) d 5.12 (s, 2H), 7.19 (ddd, J = 0.80, 7.13, 7.92 Hz, 1H), 7.34 (ddd,
J = 1.07, 7.03, 8.17 Hz, 1H), 7.52 (td, J = 0.72, 8.26 Hz, 2H), 7.74 (d, J = 7.64 Hz,
1H), 7.80 (d, J = 7.99 Hz, 1H), 7.85 (ddd, J = 1.55, 7.15, 8.28 Hz, 1H), 8.24 (dd,
J = 1.31, 7.80 Hz, 1H), 12.41 (s, 1H, NH). 13C NMR (150 MHz, DMSO-d6) d 45.7,
113.3, 119.7, 120.4, 120.5, 121.5, 124.9, 125.0, 125.8, 126.0, 126.7, 133.7, 134.2,
142.4, 149.0, 149.1, 159.6. HRMS (ESI): calcd for (M+H)+ (C17H12N3O) requires
m/z 274.2967, found 274.2959.

4940
M. Bubenyák et al. / Tetrahedron Letters 49 (2008) 4937–4940
20. Wattanapiromsakul, C.; Forster, P. I.; Waterman, P. G. Phytochemistry 2003, 64,
609.
(loge
): 391.5 (4.035), 371.0 (4.179), 262.5 (4.105), 231.5 (4.073) nm. 1H NMR
(600 MHz, DMSO-d6) d 7.35 (ddd, J = 1.10, 7.10, 8.18 Hz, 1H), 7.43 (ddd, J = 1.08,
7.02, 8.18 Hz, 1H), 7.61 (ddd, J = 1.53, 7.11, 8.47 Hz, 1H), 7.69 (d, J = 8.19 Hz,
1H), 7.86 (d, J = 7.65 Hz, 1H), 7.92 (ddd, J = 1.53, 7.18, 8.39 Hz, 1H), 8.22 (dd,
J = 1.24, 7.61 Hz, 1H), 8.27 (d, J = 8.14 Hz, 1H), 10.43 (s, 1H), 13.12 (s, 1H, indole
NH), 13.63 (s, 1H, amide NH). 13C NMR (150 MHz, DMSO-d6) d 113.3, 115.1,
120.2, 121.8, 123.3, 125.4, 125.5, 126.1, 127.4, 127.6, 134.9, 135.8, 135.8, 145.3,
148.3, 161.1, 187.5. HRMS (ESI): calcd for (M+H)+ (C17H12N3O2) requires m/z
290.2961, found 290.2948.
21. Preparation of bouchardatine 15: 1.71 ml (18 mmol) of phosphoryl chloride
(Aldrich) was dissolved in 9 ml of anhydrous dimethyl formamide at 0 °C.
0.522 g (2 mmol) of 2-(2-indolyl)-quinazolone 12 was dissolved in 13 ml of
dimethyl formamide and added dropwise to the first solution. The mixture was
stirred at 0 °C for 24 h, then the solution was added dropwise to 15 ml of
saturated sodium hydrogencarbonate solution. Ten millilitres of 10% sodium
hydroxide solution were poured onto the mixture, and the precipitated solid
was filtered off and washed with water. Recrystallization from ethanol
22. Chavan, S. P.; Sivappa, R. Tetrahedron 2004, 60, 9931.
provided
a beige solid in 89% yield, mp: over 260 °C. UV: in EtOH kmax