A short and efficient general synthesis of luotonin A, B and E

Article abstract of DOI:10.1016/j.tet.2004.08.025

Highly convergent synthesis of three luotonins (A, B, and E) has been achieved from readily available starting materials. The key step in the synthesis is formation of quinazolinone skeleton by the condensation of 3-(1,3-dioxolan-2-yl-quinoline-2-carbaldehyde and anthranilamide. Graphical Abstract

Full text of DOI:10.1016/j.tet.2004.08.025

Tetrahedron 60 (2004) 9931–9935
A short and efficient general synthesis of luotonin A, B and E
*
Subhash P. Chavan and Rasapalli Sivappa
Division of Organic Chemistry (Technology) National Chemical Laboratory, Pune 411008, India
Received 19 October 2003; revised 22 July 2004; accepted 13 August 2004
Available online 11 September 2004
Abstract—Highly convergent synthesis of three luotonins (A, B, and E) has been achieved from readily available starting materials. The key
step in the synthesis is formation of quinazolinone skeleton by the condensation of 3-(1,3-dioxolan-2-yl-quinoline-2-carbaldehyde and
anthranilamide.
q 2004 Elsevier Ltd. All rights reserved.
1. Introduction
activity in cell assays relative to parent CPT. The pyridone D-
ring also has been considered to be an absolute requirement for
the activity.6 Surprisingly luotonin, which can be regarded as a
DE-ring modified analog of camptothecin, is cytotoxic1a and
very recently, Hecht et al.7 have demonstrated that luotonin A
stabilizes the human DNA topoisomerase I-DNA covalent
binary complex and mediates topoisomerase I-dependent
cytotoxicity in intact cells (IC50 5.7–12.6 m/mL) in a similar
way to camptothecin. These findings has resulted in increased
quest for obtaining luotonin based CPT-like analogues for
structure-activity relations and hence considerable attention
has been focused on their synthesis.8 Luotonin B has been
obtained from luotonin A by CAN mediated oxidation,9
whereas luotonin E has been synthesized by etherification of
luotonin B with methanol in the presence of BF3–OEt2. So far,
there is no general synthetic approach available to access all
three pyrroloquinazolinone luotonins. These considerations,
coupled with our interest in quinoline10 and quinazoline11
alkaloids, led us to develop a general synthetic route which will
enable access to all the three luotonins.
Luotonins A, B, and E, new heteroaromatic pyrroloquina-
zolinoquinoline alkaloids,1 were isolated from the aerial
parts of Peganum nigellastrum Bunge, a Chinese medicinal
plant used for the treatment of rheumatism, inflammation,
abscesses and other maladies. Luotonins A (1), B (2), and E
(3) possess a unique skeleton comprising of pharmacologi-
cally important quinoline2 and quinazoline3 framework.
Amongst these, luotonin A showed remarkable cytotoxic
activity against mouse leukemia-P388 cells even at low
concentrations (IC50 1.8 mg/mL). Luotonin A is reminiscent
of camptothecin (CPT) 4,4 an inhibitor of topoisomerase I,
derivatives of which are clinically useful anti-cancer agents, in
its pentacyclic structure as well as cytotoxic activity. Hydroxy
lactone E-ring, of CPT considered to be an absolute
requirement for high topoisomerase I mediated cytotoxycity5
as many E-ring modifications like CPT-lactol, CPT-lactam, a
ring opened hydroxy amide, an a-halo lactone, an a-
azidolactone, an a-aminolactone and an a-exo-methylenelac-
tone were either inactive or showed significantly decreased
2. Results and discussion
We were intrigued by the possibility of obtaining pyrrolo-
quinazolinone luotonins A, B and E by delaying the C-ring
construction, where they differ, as shown in our retro
synthetic analysis (Scheme 1).
We envisioned condensation of suitable quinoline aldehyde
with anthranilamide to construct the key intermediate.
Meth-Cohn’s quinoline synthesis12 is ideally suited for the
construction of the AB ring unit with an aldehyde
functionality at 2-position for quinazoline formation while
keeping the other formyl group masked at the 3-position
which could later be unmasked and utilized for the C-ring
Keywords: Luotonin; Synthesis; Quinazolinone; Mitsunobu; Camptothecin.
* Corresponding author. Tel.: C91-2058933002289; fax: C91-
205893614; e-mail: spchavan@dalton.ncl.res.in
0040–4020/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2004.08.025

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S. P. Chavan, R. Sivappa / Tetrahedron 60 (2004) 9931–9935
under basic medium. Accordingly equimolar mixture of
aldehyde 7 and anthranilamide 8 in ethanol was refluxed for
2 h in presence of 15% NaOH solution to afford the
dihydroquinazolinone 6 in 92% yield.14 Dihydroquinazoli-
none 6 was oxidized to quinazolinone 5 with KMnO4 in
refluxing acetone. Alternatively, dihydroquinazolinone 6
was obtained by heating the reactants in N,N-dimethyl
acetamide (DMA) to 80 8C while quinazolinone 5 was
obtained by heating the reactants to 150 8C in presence of
NaHSO3. Interestingly, the quinazoline 5 could be readily
obtained in a single step when 20% KOH was employed in
the above transformation. Such facile oxidation of dihydro-
quinazolinone to the quinazolinone is precedented.15
Having been successful in the construction of the key
intermediate 5 with AB and DE rings and suitably placed
handle for the further C-ring annulation, all that left was
to identify and realize the synthetic transformations for
the different luotonins which vary in the substitution
at the 7-position, that is, on C-ring. In this direction,
the deprotection of acetal 5 with 10% HCl: THF unveiled
the aldehyde 9, which was subsequently reduced to alcohol
10 with NaBH4 in quantitative yield. Alcohol 10 was
Scheme 1.
annulation on to quinazolinone ring to obtain the title
compounds (Scheme 2).
Thus aldehyde 7 was prepared from acetanilide according to
Meth-Cohn’s procedure.13 The condensation of 7 with
anthranilamide to dihydro quinazolinone 6 was effected
Scheme 2.
Scheme 3.

S. P. Chavan, R. Sivappa / Tetrahedron 60 (2004) 9931–9935
9933
subjected to Mitsunobu conditions to obtain luotonin A.
However this operation required the laborious column
chromatography to obtain pure luotonin A. Alternatively,
cyclodehydration of 10 with 60% ethanolic H2SO4 at 80 8C
afforded luotonin A 1. Luotonin B 2, E 3 can be regarded
structurally as a- hydroxy amide and a- alkoxy amide
respectively, which are normally obtained by the treatment
of aldehyde with amide under strong acidic conditions. We
reasoned that the treatment of acetal 5 with strong acid16
would provide luotonin B via intramolecular addition of
amide on to aldehyde, formed by the deprotection of acetal.
Addition of methanol to the above conditions would lead to
the formation of luotonin E (Scheme 3).
1H3HH), 7.28 (dt, JZ1.78, 7.39 Hz, 1H), 7.20 (bs, 1H),
6.89 (dt, JZ1.22, 7.52 Hz, 1H), 6.76 (d, JZ8.3 Hz, 1H),
6.27 (d, JZ1.78 Hz, 1H), 6.07 (s, 1H), 5.83 (bs,1H), 4.03–
4.42 (m, 4H). 13C NMR (50 MHz, CDCl3): d: 165.07 (C),
153.04 (C), 147.62 (C), 147.00 (C), 137.04 (CH), 133.69
(CH), 130.94 (CH), 129.55 (CH), 128.66 (CH), 127.93 (2
CH), 127.55 (C), 127.39 (C), 119.81 (CH), 117.52 (C),
115.75 (CH), 101.80 (CH), 65.70 (CH2), 65.59 (CH2), 65.33
(CH). Mass (m/z): 347 (MC), 304 (40), 288 (70), 273 (51),
251 (39), 231 (60), 204 (20), 158 (36), 132 (55), 119 (40), 77
(70) Anal. Calcd for C20H17N3O3: C, 69.15; H, 4.93; N,
12.10. Found: C, 69. 08; H, 4.91; N, 12.15.
3.1.2. 2-(3-[1,3]Dioxolan-2-yl-quinolin-2-yl)-1H-quina-
zolin-4-one [5]. Method A. To a solution of 6 (3.47 g,
10 mmol) in acetone (100 mL) was added KMnO4 (1.00 g)
and the mixture was refluxed on a steam bath. Slowly the
pink color of KMnO4 disappeared. KMnO4 was added in
portions of 50 mg each time and refluxing was continued
until the pink color persisted. The hot acetone solution was
filtered, excess of acetone was distilled off and excess of
KMnO4 in the filtrate was destroyed with sodium sulphite.
The resulting solution was extracted with CHCl3 (5!
50 mL) and solvent was dried (anhydrous Na2SO4), filtered
and removed under reduced pressure to furnish the pure
quinazolinone 5 (3.20 g, 95% yield) as white solid.
As anticipated, the treatment of the acetal 5 with 60% HCl at
100 8C in THF resulted in the formation of luotonin B.
Treatment of 5 with conc. HCl in methanol at room
temperature furnished luotonin E.
In conclusion we have developed a short, highly practical,
and efficient synthesis of luotonin A, B, and E which is
amenable to the synthesis of substituted analogues as the
substituted starting materials are readily accessible2,17 and
obviates the need for the costly and unstable aminobenzal-
dehyde. We have elaborated the repertoire of Meth-Cohn
aldehyde annulation for synthesis of quinazoline
heterocycles.
Method B. Sodium hydrogen sulfite (1.07 g, 10 mmol) was
added to a solution of anthranilamide (1.37 g, 10 mmol) and
quinoline aldehyde 7 (2.29 g, 10 mmol) in N,N-dimethyl-
acetamide (DMA) (30 mL). The mixture was heated with
stirring at 150 8C for 2 h and poured into ice water
(300 mL). The precipitate was collected, washed with
water, and dried in vacuo to give pure quinazolinone 5
(3.40 g, 98% yield) as white solid; mp 214 8C. IR (KBr)
3310, 3125, 2956, 2883, 1675, 1598, 1509, 1482, 1437,
1215, 950 cmK1. 1H NMR (200 MHz; CDCl3): d: 11.12 (bs,
1H), 8.60 (s, 1H), 8.27 (d, JZ8.15 Hz, 1H), 8.07 (d,
JZ8.15 Hz, 1H), 7.82 (d, JZ8 Hz, 1H), 7.6–7.74 (m, 3H),
7.53 (s, 1H), 7.54 (t, JZ8.46 Hz, 1H), 7.42 (t, JZ8.46 Hz,
1H), 4.06 (m, 4H). 13C NMR (50 MHz, CDCl3): d: 161.51
(C), 149.20 (C), 148.69 (C), 146.07 (C), 145.85 (C), 136.36
(CH), 134.35 (CH), 131.57 (C), 130.90 (CH), 129.66 (CH),
128.55 (CH), 128.47 (CH),128.27 (C), 127.97 (CH), 127.63
(CH),126.5 (CH), 122.35 (C), 99.86 (CH), 65.28 (2CH2).
Mass (m/z): 345 (MC, 5), 301 (20), 285 (10), 273 (100), 245
(30), 119 (50). Anal. Calcd for C20H15N3O3: C, 69.56; H,
4.38; N, 12.17. Found: C, 69.48; H, 4.21; N, 12.23.
3. Experimental
3.1. General
Melting points are uncorrected. IR spectra were recorded
with a Perkin–Elmer FT-IR 16015 spectrometer and 1H and
13C NMR spectra with a Bruker AC-200 or Bruker MSL-
300. Chemical shifts are given on a d (ppm) scale with
tetramethylsilane (TMS) as the internal standard. Mass
Spectra were recorded on Finnigan-Mat 1020C Mass
spectrometer and were obtained at an ionization potential
of 70 eV and mass values are expressed as m/z values.
Column chromatography was carried out on silica gel
(60–120 mesh).
3.1.1. 2-(3-[1,3]Dioxalon-2-yl-quinolin-2-yl)-2,3-dihydro-
1H-quinazoli-4-one [6]. Method A. To a solution of
anthranilamide (3.40 g, 25 mmol) and quinoline alde-
hyde12,13 7 (5.75 g, 25 mmol) in 100 mL of 95% ethanol
was added 1.6 mL of 15% aq. NaOH solution. The mixture
was heated under reflux for 1 h and the precipitated solid
was filtered and air-dried to give dihydroquinazolinone 6
(7 g, 80%) as a white solid.
3.1.3. 2-(3-Hydroxymethyl-quinolin-2-yl)-3H-quinazo-
lin-4-one [10]. A solution of 5 (0.450 g, 1.5 mmol) in a
1:1 mixture of 10% HCl: THF (15 mL) was stirred for 2 h.
After the reaction was completed (TLC), the reaction
mixture was extracted with CH2Cl2 (3!20 mL), dried
(anhydrous Na2SO4), filtered, and concentrated. The
aldehyde 9 obtained thereby was suspended in methanol
(20 mL) to which NaBH4 (114 mg, 3.2 mmol) was added at
0 8C and the resulting solution was stirred for 1 h at room
temperature. Methanol was removed under the reduced
pressure and the residue quenched with dil HCl (2 mL), and
the solution was stirred for 5 min. Water (15 mL) and DCM
(25 mL) were then added. The layers were separated, and
the aqueous layer was extracted with DCM (3!15 mL).
Method B. Anthranilamide (3.40 g, 25 mmol) and quinoline
aldehyde 7 (5.75 g, 25 mmol) were heated in 25 mL of N,N-
dimethylacetamide (DMA) at 80 8C for 3 h and then poured
into ice water (300 mL). The precipitated solid was filtered
and air-dried to give dihydroquinazolinone 6 (7.0 g, 80%) as
a white solid; mp 140 8C. IR (KBr): 3366, 3185, 2911, 1676,
1
1664, 1508, 1487, 1377, 935 cmK1. H NMR (200 MHz;
CDCl3): 8.38 (s, 1H), 8.08 (d, JZ8.3 Hz, 1H), 7.95 (dd,
JZ1.48, 7.52 Hz, 1H), 7.79 (d, JZ8.18 Hz, 1H),7.68
(dt, JZ1.5, 6.95 Hz, 1H), 7.53 (dt, JZ1.22, 8.3 Hz,

9934
S. P. Chavan, R. Sivappa / Tetrahedron 60 (2004) 9931–9935
The combined organic layers were washed with brine and
then dried (anhydrous Na2SO4). Evaporation and column
chromatography yielded the alcohol 10 (0.370 g, overall
93%) as a white solid; mp 211–212 8C. IR (KBr): 3373,
220–223 8C (lit.1b mp 222–225 8C); Spectral data was in
agreement with the reported.1b
1
1681, 1518, 1461, 1377, 945 cmK1. H NMR (200 MHz;
Acknowledgements
CDCl3): d: 11.49 (br s, 1H), 8.43 (d, JZ7.81 Hz, 1H), 8.31
(s, 1H), 8.20 (d, JZ8.31 Hz, 1H), 7.93–7.78 (m, 4H), 7.72–
7.57 (m, 2H), 6.4 (t, 6.5 Hz, 1H), 5.09 (d, 6.5 Hz, 2H). 13C
NMR (50 MHz, CDCl3): d: 161.02 (C), 150.52(C), 148.13
(C), 147.10 (C), 146.23 (C), 141.21 (CH), 135.06 (CH),
134.53 (C), 131.17 (CH), 130.61 (CH), 129.62 (CH), 129.21
(CH), 128.69 (C), 128.34 (CH), 127.50 (CH), 127.11 (CH),
122.72 (C), 64.5 (CH2). Mass (m/z): 303 (MC, 70), 285 (95),
274 (20), 257 (18), 246 (40), 229 (15), 155 (28), 143 (35),
128 (100), 119 (38), 101 (30), 92 (35). Anal. Calcd for
C18H13N3O2: C, 71.28; H, 4.32; N, 13.85. Found: C, 71.19;
H, 4. 44; N, 13.89.
We thank Prof. A. Ganesan, and Prof. H. Wang, for kindly
providing authentic luotonin A sample and spectra. R. S.
thanks CSIR, New Delhi for financial support. Funding from
CSIR, New Delhi to SPC under YSA scheme is gratefully
acknowledged.
References and notes
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(25 mL) were then added. The layers were separated, and
the aqueous layer was extracted with DCM (3!25 mL).
The combined organic layers were washed with brine and
then dried (Na2SO4). Evaporation and column chromato-
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