A new synthesis of the cytotoxic alkaloid Luotonine A

Article abstract of DOI:10.1016/S0040-4039(02)00140-5

A convenient synthesis of the cytotoxic alkaloid Luotonine A has been achieved using cheap and readily accessible reagents. The key intermediate in the synthesis is the tricyclic compound 2,3-dihydro[1H]-pyrrolo[3,4-b]quinolin-3-one (5).

Full text of DOI:10.1016/S0040-4039(02)00140-5

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 1835–1837
A new synthesis of the cytotoxic alkaloid Luotonine A
Sabrina Dallavalle* and Lucio Merlini
Dipartimento di Scienze Molecolari Agroalimentari, Universita` di Milano, via Celoria 2, 20133 Milano, Italy
Received 27 September 2001; revised 11 October 2001; accepted 21 January 2002
Abstract—A convenient synthesis of the cytotoxic alkaloid Luotonine A has been achieved using cheap and readily accessible
reagents. The key intermediate in the synthesis is the tricyclic compound 2,3-dihydro[1H]-pyrrolo[3,4-b]quinolin-3-one (5). © 2002
Elsevier Science Ltd. All rights reserved.
Luotonine A (1) is a cytotoxic alkaloid of natural origin
isolated for the first time in 1997 from the aerial parts
of Peganum nigellastrum Bunge, a plant distributed over
Asia which was employed in traditional medicine for
rheumatism, abscess and inflammation.1 It was found
to be active in vitro against murine leukemia P-388 cells
at a concentration of 1.8 mg/mL.1
Our particular interest was in the possibility of synthe-
sizing camptothecine-like analogues in order to obtain
cytotoxic compounds and to clarify the structure–activ-
ity relationships connected with these molecules. The
absence of labile functions such as the lactone moiety
of camptothecine made Luotonine A appealing from a
synthetic point of view, so we focused our attention on
an alternative approach to the synthesis of 1. In this
note we describe a new straightforward synthetic
method to obtain Luotonine A and analogues which
uses only simple reagents.
O
N
N
N
1
We envisaged the synthesis of 1 via a D/E ring-closing
acylation/reduction reaction and a Friedla¨nder conden-
sation as key steps. Thus, the tricyclic compound 2,3-
dihydro[1H]-pyrrolo[3,4-b]quinolin-3-one 5 (Scheme 1)
can be seen as the precursor of Luotonine A; it can in
turn be obtained by condensation of o-aminobenzalde-
hyde with pyrrolidine-2,3-dione.
The structure of Luotonine is strikingly reminiscent of
the cytotoxic alkaloid camptothecine, whose derivatives
are clinically useful anticancer agents.
This similarity has stimulated much activity directed
toward the synthesis of the alkaloid and four multistep
syntheses of such a compound have been reported. The
first two syntheses started by building the pyrroloquina-
zoline ring-system and used as a key intermediate vasi-
Being the N-unsubstituted pyrrolidine-2,3-dione of lim-
ited stability,6 we have investigated the synthesis of
derivatives with potentially removable protecting
groups in position 1, easily obtainable by condensation
of ethyl oxalate with N-substituted b-aminopropionic
esters (N-benzyl, N-p-methoxyphenyl, N-a-methoxy-
benzyl, N-allyl).
cinone, prepared by
a A
multistep sequence.2,3
Friedla¨nder condensation with o-aminobenzaldehyde
then gave the desired product. Recently Molina and
co-workers proposed a new oxidation method of des-
oxyvasicinone to vasicinone using selenium dioxide.4
After several attempts the N-allyl derivative was chosen
for the easiness of removal of the protecting group.
On the contrary, the third approach was based on the
synthesis of 3-oxopyrroloquinoline, which was then
coupled with 2-sulfinylaminobenzoylchloride to give 1.5
1-Allylpyrrolidine-2,3-dione (3) was prepared by
hydrolysis and decarboxylation of the corresponding
ester 2, which in turn was synthesized by a slight
modification of the original Sundberg method6 with a
one-step reaction from ethyl acrylate, allylamine and
ethyl oxalate (Scheme 1).7
* Corresponding author. Tel.: +39-02-58356816; fax: +39-58356801;
e-mail: sabrina.dallavalle@unimi.it
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)00140-5

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S. Dalla6alle, L. Merlini / Tetrahedron Letters 43 (2002) 1835–1837
C2H5OOC
(a)
(b)
NH2
N
CH2-CH=CH2
N
CH2-CH=CH2
O
O
O
O
3
2
O
N
(c)
N
O
CH2-CH=CH2 (d)
(e)
NH
O
N
N
N
N
1
4
5
Scheme 1. Synthesis of 1. Reagents and conditions: (a) CH2ꢀCH–COOEt, EtOH, rt 48 h; then NaOC2H5, H5C2OOC–COOC2H5,
reflux 15 h, 70%; (b) HCl 10%, reflux 2 h, 61%; (c) 2-aminobenzaldehyde, toluene, pTsOH, reflux, 50%; (d) i. PPh3, PdCl2,
DMF/H2O 8/2, reflux 4 h, 100%; ii. HCl 6N reflux 2 h, 98%; (e) NaH, THF, 60°C 1 h; then 2-nitrobenzoylchloride 50°C 1 h; then
Fe, AcOH/EtOH 1/1, reflux 2 h, 38%.
Condensation of 1-allylpyrrolidine-2,3-dione with o-
aminobenzaldehyde was carried out under Friedla¨nder
conditions to obtain the pyrroloquinoline compound
4.8 The allylic protective group was then easily isomer-
ized with PdCl2 to an enamide function and then
of allylamine was dropped. The mixture was allowed to
stand at room temperature for 48 h. In a separate flask
an ethanolic sodium ethoxide solution was prepared by
dissolving 4 g (0.175 mol) of metallic sodium in 100 mL
of absolute ethanol. To this solution 23.7 mL (0.175 mol)
of ethyl oxalate were added, followed by the previously
prepared solution of ethyl b-allylaminopropionate. The
mixture was refluxed for 1.5 h and the solvent was then
removed by distillation. The residual slurry was added to
300 mL of warm water and acidified with HCl 6N. The
mixture was then cooled and allowed to stand for some
time to complete the precipitation. The product was
filtered out and recrystallized from ethanol to obtain 26 g
of 2 in the form of white needles. Yield 70%, mp 82°C,
1H NMR (CDCl3) l: 1.33 (t, J=7.3 Hz, 3H), 3.95 (s,
2H), 4.10 (d, J=6.2 Hz, 2H), 4.31 (q, J=7.3 Hz, 2H),
5.14–5.28 (m, 2H), 5.70–5.90 (m, 1H), 8.83 (brs, 1H). 10
g (0.047 mol) of 2 were refluxed 2 h in 530 mL of HCl
10%. The aqueous solution was then extracted with
CH2Cl2 (300 mL×3) and with ethyl acetate (300 mL×3).
The combined organic layers were dried and evaporated
to give 4 g of 1-allyl-pyrrolidine-2,3-dione (3) as a yellow
quantitatively removed through acidic hydrolysis.9
A
one-pot sequence of acylation of 5 with 2-nitroben-
zoylchloride, reduction of the nitro group and subse-
quent ring-closure led to Luotonine A (1).10
In summary we have devised an improved and new
synthesis of Luotonine A. The friendly reaction condi-
tions, the cheap and readily available reagents make
this method suitable for large-scale production; more-
over it offers the possibility of obtaining camp-
tothecine-like analogues in which the lactone moiety
can be easily replaced by a variously substituted ben-
zene ring. The synthesis and testing of such analogues
for antitumor activity is in progress.
Acknowledgements
1
oil. Yield 61%. H NMR (CDCl3) l: 2.72 (t, J=5.9 Hz,
2H), 3.63 (t, J=5.9 Hz, 2H), 4.13 (d, J=6.6 Hz, 2H),
5.29–5.33 (m, 2H), 5.77 (m, 1H).
This work was financially supported by MURST
(COFIN 99).
8. Synthesis of 4. 517 mg (4.28 mmol) of 2-aminobenzalde-
hyde was dissolved in 25 mL of toluene. To this solution
594 mg (4.28 mmol) of 3 was added together with a
catalytic amount of p-toluenesulfonic acid (0.4 mmol).
The reaction flask was fitted with a Dean–Stark appara-
tus for the distillation of water and the reaction was
refluxed until no more water was collected. The largest
part of the solvent was removed under reduced pressure.
The residue was triturated with ether and filtered to give
4 as a brownish solid. Yield 50%, mp 153–155°C, 1H
NMR (CDCl3) l: 4.40 (dd, J=1.1 and 6.2 Hz, 2H), 4.55
(s, 2H), 5.26–5.40 (m, 2H), 5.83–6.03 (m, 1H), 7.65–7.90
(m, 2H), 7.93 (d, J=9.2 Hz, 1H), 8.26 (s, 1H), 8.42 (d,
J=9.2 Hz, 1H).
9. Synthesis of 5: 200 mg (0.89 mmol) of 4 were dissolved in
2.3 mL of DMF/H2O 8/2 (v/v). After adding 38 mg (0.18
mmol) of triphenylphosphine and 8 mg (0.045 mmol) of
PdCl2, the mixture was refluxed for 4 h and 165 mg of a
pale-gray solid were obtained by filtration. The resulting
solution was diluted with 20 mL of water and extracted
References
1. Ma, Z. Z.; Hano, Y.; Nomura, T.; Chen, Y. J. Heterocy-
cles 1997, 46, 541–546.
2. Ma, Z. Z.; Hano, Y.; Nomura, T.; Chen, Y. J. Heterocy-
cles 1999, 51, 1593–1596.
3. Ross Kelly, T.; Chamberland, S.; Silva, R. A. Tetra-
hedron Lett. 1999, 40, 2723–2724.
4. Molina, P.; Tarraga, A.; Gonzales-Tejero, A. Synthesis
2000, 1523–1525.
5. Wang, H.; Ganesan, A. Tetrahedron Lett. 1998, 39, 9097–
9098.
6. Sundberg, R.; Richard, J.; Bradley, C.; Laurino, J. P. J.
Heterocycl. Chem. 1986, 23, 537–539.
7. Experimental procedure for the synthesis of 3. To a
solution of 15.5 g (0.175 mol) of freshly distilled ethyl
acrylate in 100 mL of absolute ethanol 10 g (0.175 mol)

S. Dalla6alle, L. Merlini / Tetrahedron Letters 43 (2002) 1835–1837
1837
with dichloromethane (×3). The organic layer was washed
with water, dried and evaporated to afford further 35 mg
10. Synthesis of Luotonine A. To a suspension of 40 mg
(0.22 mmol) of 5 in 2 mL of THF 15 mg (0.62 mmol),
NaH (60% in mineral oil, previously washed with hexane)
was added with cooling in one portion. The mixture was
heated at 60°C until the evolution of gas ceased (ca. 1 h).
It was then ice-cooled and 32 ml (0.24 mmol) of 2-
nitrobenzoylchloride was added. After heating at 50°C
for an additional hour the solvent was evaporated. The
resulting solid was suspended in 5 mL of a mixture of
acetic acid:absolute ethanol 1:1 and 73 mg (1.3 mmol) of
Fe was added. After refluxing for 2 h the iron was
removed by filtration and the filter was carefully washed
with ethanol, ethyl acetate and chloroform. The
combined organic layers were washed with a saturated
solution of sodium bicarbonate and then with water.
After drying and removal of the solvent the product
was purified by flash chromatography (from ethyl
acetate:hexane 9:1 to ethyl acetate). Yield 38%. Syn-
thetic 1 matched the spectroscopic data reported in litera-
ture.1
(overall
yield
100%)
of
2-propenyl-1,2-dihy-
dro[1H]pyrrolo[3,4-b]quinolin-3-one, mp 167–168°C, 1H
NMR (CDCl3) l: 1.86 (dd, J=7.0 and 1.8 Hz, 3H), 4.69
(s, 2H), 5.38 (m, 1H), 7.34 (d, J=14.3 Hz, 1H), 7.67
(ddd, J=8.5, 8.5 and 1.5 Hz, 1H), 7.81 (ddd, J=8.5, 8.5
and 1.5 Hz, 1H), 7.92 (dd, J=8.5 and 1.5 Hz, 1H), 8.27
(s, 1H), 8.40 (dd, J=8.5 and 1.5 Hz, 1H). 2-Propenyl-1,2-
dihydro[1H]pyrrolo[3,4-b]quinolin-3-one (160 mg, 0.72
mmol) was refluxed in 5.6 mL of HCl 6N for 2 h. The
solution was then neutralized with K2CO3 and extracted
fivefold with ethyl acetate. The combined organic layers
were dried and evaporated. Purification by flash chro-
matography (ethyl acetate:methanol 7:3) afforded 130 mg
of 5 as a white solid, Yield 98%, mp>270°C (dec.), 1H
NMR (DMSO-d6) l: 4.53 (s, 2H), 7.69 (ddd, J=8.5, 8.5
and 1.47 Hz, 1H), 7.82 (ddd, J=8.5, 8.5 and 1.47 Hz,
1H), 8.09 (dd, J=8.5 and 1.5 Hz, 1H), 8.16 (dd, J=8.5
and 1.5 Hz, 1H), 8.55 (s, 1H), 9.21 (brs, 1H).