A total synthesis of subarine, a marine alkaloid related to the pyridoacridine family

Article abstract of DOI:10.1002/ejoc.200400043

The synthesis of the marine alkaloid subarine has been accomplished in four steps in 70% overall yield starting from 4-bromo-1,10-phenanthroline. The natural compound and three intermediates in the synthesis were tested at six concentrations on six differ

Full text of DOI:10.1002/ejoc.200400043

SHORT COMMUNICATION
A Total Synthesis of Subarine, a Marine Alkaloid Related to the
Pyridoacridine Family
Laurent Bijeire,[a] Laurent Legentil,[a] Jean Bastide,[a] Francis Darro,[b] Christelle Rochart,[b]
and Evelyne Delfourne*[a,c]
Keywords: Alkaloids / Natural products / Antitumor agents
The synthesis of the marine alkaloid subarine has been ac-
ing various histopathological types. None of these com-
complished in four steps in 70% overall yield starting from pounds exhibits significant antitumor activity.
4-bromo-1,10-phenanthroline. The natural compound and
three intermediates in the synthesis were tested at six con-
centrations on six different human cancer cell lines, includ-
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2004)
Introduction
Since the isolation of the marine alkaloid amphimedine
(1) by Schmitz in 1983,[1] there has been significant interest
in compounds of the 11H-pyrido[4,3,2-mn]acridine fam-
ily,[2] due to the range of potentially useful biological activi-
ties they exhibit, which include antifungal, antiviral and
antitumor properties. Their role as pharmaceutical lead
compounds, along with their limited availability from natu-
ral sources, makes them prime candidates for the develop-
ment of total synthetic routes for alkaloids and their ana-
logues. Among these compounds, ascididemin (2), isolated
in 1988 by Kobayashi[3] from the ascidian Didemnum sp.,
has been the subject of various investigations both to define
new anticancer drugs[4] and to understand the mechanisms
involved in tumor cytotoxicity.[5] Recently, subarine (3), a
new alkaloid, has been isolated from a Singapore as-
cidian.[6] This compound has been suggested to be a signifi-
cant biosynthetic link in the biosynthesis of ascididemin
and other related pentacyclic pyridoacridines. We report
herein the first total synthesis of subarine, which employs
4-bromo-1,10-phenanthroline 7 as the starting material.
Results and Discussion
Synthesis
In our first approach, we used the retrosynthetic analysis
shown in Scheme 1, involving the monoester-monoacid 4,
the key step of the synthesis being the formation of an
arylϪaryl bond, in order to construct the last ring.
[a]
´
Centre de Phytopharmacie, FRE-CNRS 2605, Universite de
Perpignan,
´
52 avenue Paul Alduy, 66860 Perpignan Cedex, France
[b]
[c]
Unibiosreen SA,
40 avenue Joseph Wybran, 1070 Bruxelles, Belgium
´
Laboratoire SPCMIB, Universite Paul Sabatier, UMR-CNRS
5068
118 route de Narbonne, 31062 Toulouse Cedex 4, France
Fax: (internat.) ϩ 33-561-556011
E-mail: delfourn@chimie.ups-tlse.fr
Scheme 1
Eur. J. Org. Chem. 2004, 1891Ϫ1893
DOI: 10.1002/ejoc.200400043
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1891

L. Bijeire, L. Legentil, J. Bastide, F. Darro, C. Rochart, E. Delfourne
SHORT COMMUNICATION
Compound 4 was prepared in two steps from 1,10-phen- data of 3 (1H NMR and 13C NMR) were identical to the
anthroline according to procedures previously described, values reported for the natural product.[6] The overall yield
namely, oxidation with a mixture of HNO3 and H2SO4,[7] of this synthesis was 70%.
and reaction of the resultant 1,10-phenanthroline-5,6-dione
with m-chloroperbenzoic acid.[8] Compound 4 was then
transformed into the key amide intermediate 5 in 62% yield
by the action of dicyclohexylcarbodiimide (DCC) and ani-
line. Unfortunately, attempts to cyclize this derivative 5, by,
amongst other methods, thermolysis, or the action of phe-
nyliodine() bis(trifluoroacetate) (PIFA) in CH2Cl2 with
BF3·Et2O as an activating agent,[9] failed.
Biological results
The drug-induced inhibition of human cancer cell line
growths was assayed for compounds 9, 10, 11 and subarine
(3).[4] Six concentrations (ranging from 10Ϫ5 to 10Ϫ9 ) of
each of these compounds were tested on six different cell
lines of different histopathological types including glioblas-
tomas (Hs683 and U-373MG), breast (MCF-7), colon
(HCT-15 and LoVo) and lung (A549). None of these com-
pounds presents cytotoxic activity (IC50 Ͼ 10Ϫ5 ).
An alternative route to 3 is presented in Scheme 2. This
method was based on two key steps, including KMnO4 oxi-
dation of 4-bromo-1,10-phenanthroline 7 and a Stille
arylϪaryl coupling.
Experimental Section
General Procedures: All commercial chemicals were used without
further purification. Flash chromatography was performed on flash
silica gel 60 (Merck 0.015Ϫ0.040 mm). NMR spectra were re-
1
corded at 400 MHz for H and at 100 MHz for 13C.
Methyl 3Ј-(Anilinocarbonyl)-2,2Ј-bipyridine-3-carboxylate (5): DCC
(309 mg, 1.5 mmol) was added to a solution of 3Ј-(methoxycar-
bonyl)-2,2Ј-bipyridine-3-carboxylic acid[7] (166 mg, 0.64 mmol) and
aniline (60 mg, 0.64 mmol) in CH2Cl2 at 0 °C. The reaction was
stirred for 2 h and filtered. The filtrate was concentrated under vac-
uum and the crude product purified by flash chromatography
(CH2Cl2/MeOH, 95:5) to give 5 as a yellow solid (132 mg, 62%),
1
m.p. 218 °C. H NMR (CDCl3): δ ϭ 3.73 (s, 3 H, COOMe), 7.06
(t, J ϭ 7.7 Hz, C4"-H), 7.25 (t, J ϭ 7.7 Hz, 2 H, C3"-H and C5"-
H), 7.39 (d, J ϭ 7.7 Hz, 2 H, C2"-H and C6"-H), 7.44 (dd, J ϭ
7.7, 4.8 Hz, 1 H, C5Ј-H), 7.48 (dd, J ϭ 7.7, 4.8 Hz, 1 H, C5-H),
8.24 (dd, J ϭ 7.7, 1.4 Hz, 1 H, C4Ј-H), 8.26 (dd, J ϭ 7.7, 1.8 Hz,
1 H, C4-H), 8.69 (dd, J ϭ 4.8, 1.4 Hz, 1 H, C6Ј-H), 8.76 (dd, J ϭ
4.8, 1.8 Hz, 1 H, C6-H), 8.92 (br. s, 1 H, NH) ppm. 13C NMR
(CDCl3): δ ϭ 49.42, 119.66 (2C), 123.39, 123.41, 124.51, 124.55,
129.05 (2C), 132.12, 137.69, 137.91, 138.47, 150.20, 150.88, 156.72,
160.20, 165.16, 167.30 ppm.
Scheme 2. (a) POBr3, PBr3, 110 °C, 6 h. (b) KMnO4, KOH, H2O,
reflux, 2.5 h. (c) DCC, MeOH,
0
°C, 2 h. (d) 2-
(NHBoc)C6H4SnMe3, Pd(PPh3)4, 1,4-dioxane, reflux, N2, over-
night. (e) TFA, CH2Cl2, room temp., overnight
4-Bromo-2,2Ј-bipyridyl-3,3Ј-dicarboxylic Acid (8): A solution of 4-
bromo-1,10-phenanthroline (750 mg, 2.9 mmol), KMnO4 (1.4 g,
8.7 mmol) and NaOH (232 mg, 5.8 mmol) in water (29 mL) was
refluxed for 2.5 h. The reaction mixture was filtered and the filtrate
was extracted with CH2Cl2 (30 mL). The aqueous layer was made
acidic (pH 2) with 1  HCl and concentrated to dryness. Methanol
was added to the residue and the mixture was filtered to give 8
as a brown solid which was used in the next step without further
purification. 1H NMR ([D6]DMSO): δ ϭ 7.55 (dd, J ϭ 4.8, 7.7 Hz,
1 H, C5Ј-H), 7.81 (d, J ϭ 5.3 Hz, 1 H, C5-H), 8.10 (d, J ϭ 7.7 Hz,
1 H, C4Ј-H), 8.41 (d, J ϭ 5.3 Hz, 1 H, C6-H), 8.65 (d, J ϭ 4.8 Hz,
1 H, C6Ј-H) ppm.
Compound 7 was prepared as described previously, by
action of a mixture of phosphorus oxybromide and phos-
phorus bromide on the corresponding hydroxy-derivative
6,[10] itself synthesized in 79% yield from 8-aminoquinoline,
according to the four-step procedure of Snyder and Fre-
ier.[11] The dicarboxylic acid 8 was obtained from 7 by alka-
line permanganate oxidation according to the method re-
ported by Wimmer and Wimmer[12] for the preparation of
binicotinic acid from phenanthroline. It was then trans-
formed into the corresponding diester 9 by action of DCC
and methanol, in 88% yield over the two steps. The pal-
ladium(0)-catalyzed Stille cross-coupling reaction of 9 with
N-(tert-butoxycarbonyl)-2-(trimethylstannyl)aniline[13] gave
the expected adduct 10 (63%) together with the already-
cyclized compound 11 (18%). Reaction of 10 with trifluoro-
acetic acid gave subarine (3) in 98% yield by hydrolysis of
the Boc group and subsequent cyclization. This alkaloid
was also obtained under similar conditions from compound
Dimethyl 4-Bromo-2,2Ј-bipyridinyl-3,3Ј-dicarboxylate (9): DCC
(2.8 g, 13.56 mmol) was added to a solution of the diacid 8 in meth-
anol (47 mL) at 0 °C. The reaction mixture was stirred for 2 h and
filtered through Celite. The filtrate was concentrated and the crude
product was purified by flash chromatography (CH2Cl2/MeOH,
99:1) to give the diester 9 as a white solid (748 mg, 88% from 4-
bromo-1,10-phenanthroline), m.p. 222Ϫ224 °C. IR (CHCl3): ν˜ ϭ
1
1656, 1734 cmϪ1. H NMR (CDCl3): δ ϭ 3.74 (s, 3 H, COOMe),
3.77 (s, 3 H, COOMe), 7.41 (dd, J ϭ 4.8, 7.7 Hz, 1 H, C5Ј-H), 7.59
11 by cleavage of the protecting group. The spectroscopic (d, J ϭ 5.3 Hz, 1 H, C5-H), 8.12 (dd, J ϭ 1.8, 8.1 Hz, 1 H, C4Ј-
1892  2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.eurjoc.org
Eur. J. Org. Chem. 2004, 1891Ϫ1893

A Total Synthesis of Subarine
SHORT COMMUNICATION
H), 8.42 (d, J ϭ 5.3 Hz, 1 H, C6-H), 8.68 (dd, J ϭ 1.8, 4.8 Hz, 1 H,
was added and the organic layer was separated. The aqueous layer
C6Ј-H) ppm. 13C NMR (CDCl3): δ ϭ 52.56, 52.69, 123.45, 127.63, was extracted with CH2Cl2 (2 ϫ 10 mL), the combined organic
128.26, 130.97, 131.82, 137.95, 149.22, 150.37, 155.09, 156.06, layers were dried over MgSO4 and concentrated under vacuum.
166.47, 167.64 ppm. C14H11N2O4Br (351.1543): calcd. C 47.86, H
3.13, N 7.98; found C 48.06, H 3.11, N 8.07.
The crude product was purified by flash chromatography (CH2Cl2/
MeOH, 95:5) to give the alkaloid 3 as a pale-yellow solid (110 mg,
98% from 10, 98% from 11), m.p. Ͼ 260 °C. IR (CHCl3): ν˜ ϭ 1646,
Dimethyl 4-{[(2-tert-Butoxycarbonyl)amino]phenyl}-2,2Ј-bipyridinyl-
3,3Ј-dicarboxylate (10) and tert-Butyl 4-[(3-Methoxycarbonyl)pyri-
din-2-yl]-5-oxo-5HϪbenzo[c][2,7]naphthyridine-6-carboxylate (11):
A mixture of the bromo derivative 9 (220 mg, 0.63 mmol), N-(tert-
butoxycarbonyl)-2-(trimethylstannyl)aniline[9] (404 mg, 1.13 mmol)
and tetrakis(triphenylphosphane)palladium(0) (38 mg, 0.031
mmol) in 1,4-dioxane (14 mL) was refluxed under a nitrogen at-
mosphere overnight. After filtration through Celite, H2O (50 mL)
and EtOAc (100 mL) were added. The organic layer was separated
and the aqueous layer was extracted with EtOAc (2 ϫ 100 mL).
The combined organic layers were dried over MgSO4 and concen-
trated under vacuum. The crude product was purified by flash
chromatography (CH2Cl2/MeOH, 98:2) to give 10 and 11.
1683 cmϪ1 1H NMR (CDCl3): δ ϭ 3.62 (s, 3 H, COOMe), 6.90
.
(d, J ϭ 8.0 Hz, 1 H, C6"-H), 7.33 (dd, J ϭ 8.9, 8.0 Hz, 1 H, C5"-
H), 7.51 (dd, J ϭ 7.9, 7.7 Hz, 1 H, C4"-H), 7.53 (dd, J ϭ 7.1,
5.1 Hz, 1 H, 1 H, C5Ј-H), 8.15 (d, J ϭ 5.5 Hz, 1 H, C5-H), 8.24
(dd, J ϭ 7.7 Hz, C3"-H), 8.48 (dd, J ϭ 1.8, 8.1 Hz, 1 H, C4Ј-H),
8.83 (dd, J ϭ 5.1, 1.8 Hz, 1 H, C6Ј-H), 8.92 (d, J ϭ 5.5 Hz, 1 H,
C6-H), 10.10 (br. s, 1 H) ppm. 13C NMR (CDCl3): δ ϭ 52.31,
115.45, 116.57, 116.80, 118.81, 122.19, 123.07, 123.79, 124.69,
131.47, 137.70, 138.01, 142.48, 150.58, 152.01, 161.66, 161.73,
162.13, 165.55 ppm. C19H13N3O3 (331.3): calcd. C 68.88, H 3.93,
N 12.69; found C 68.38, H 3.88, N 12.61.
[1]
F. J. Schmitz, S. K. Agarwal, S. P. Gunasekera, J. Am. Chem.
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10: White-yellow solid (184 mg, 63%), m.p. 228 °C. IR (CHCl3)
ν˜ ϭ 1699, 1728 cmϪ1. 1H NMR (CDCl3): δ ϭ 1.44 (s, 9 H, CMe3),
3.38 (s, 3 H, COOMe), 3.77 (s, 3 H, COOMe), 6.31 (s, 1 H, NH),
7.10 (m, 2 H, C6"-H and C5"-H), 7.31 (d, J ϭ 5.0 Hz, 1 H, C5-H),
7.38 (dd, J ϭ 8.4, 5.5 Hz, 1 H, C4"-H), 7.43 (dd, J ϭ 4.8, 8.1 Hz,
1 H, C5Ј-H), 7.99 (d, J ϭ 8.4 Hz, 1 H, C3"-H), 8.21 (dd, J ϭ 1.6,
8.1 Hz, 1 H, C4Ј-H), 8.70 (dd, J ϭ 1.6, 4.8 Hz, 1 H, C6Ј-H), 8.72
(d, J ϭ 5.0 Hz, 1 H, C6-H) ppm. 13C NMR (CDCl3): δ ϭ 28.25
(3C), 52.14, 52.43, 80.60, 121.81, 123.10, 123.45, 124.64, 127.71,
128.76, 128.92, 129.51, 135.31, 136.93, 138.00, 146.46, 149.51,
150.60, 153.04, 156.19, 156.59, 167.25, 167.41 ppm. C25H25N3O6
(463.5): calcd. C 64.79, H 5.40, N 9.07; found C 64.80, H 5.62, N
8.89.
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11: White-orange solid (49 mg, 18%), m.p. Ͼ 164 °C. IR (CHCl3)
1
ν˜ ϭ 1675, 1727, 1771 cmϪ1. H NMR (CDCl3): δ ϭ 1.56 (s, 9 H,
[5]
CMe3), 3.64 (s, 3 H, COOMe), 7.10 (dd, J ϭ 8.4, 1.2 Hz, 1 H, C6"-
H), 7.37 (ddd, J ϭ 7.8, 8.4, 1.2 Hz, 1 H, C4"-H), 7.45 (dd, J ϭ 8.0,
4.8 Hz, 1 H, C5Ј-H), 7.58 (ddd, J ϭ 7.8, 1.4, 8.4 Hz, 1 H, C5"-H),
8.13 (d, J ϭ 5.5 Hz, 1 H, C5-H), 8.29 (dd, J ϭ 1.4, 8.4 Hz, 1 H,
C3"-H), 8.42 (dd, J ϭ 1.8, 8.0 Hz, C4Ј-H), 8.80 (dd, J ϭ 4.8,
1.8 Hz, 1 H, C6Ј-H), 8.91 (d, J ϭ 5.5 Hz, 1 H, C6-H) ppm. 13C
NMR (CDCl3): δ ϭ 27.79 (3C), 52.49, 87.11, 115.25, 115.88,
117.08, 119.40, 123.19, 124.19, 125.07, 125.13, 132.72, 137.15,
139.12, 142.60, 151.39, 151.76, 152.92, 159.06, 161.75, 162.85,
166.47 ppm. C24H21N3O5 (431.4): calcd. C 66.82, H 4.87, N 9.74;
found C 66.67, H 4.98, N 9.51.
[6]
[7]
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[10]
[11]
Methyl 2-(5-Oxo-5,6-dihydrobenzo[c][2,7]naphthyridin-4-yl)nicotin-
ate (Subarine) (3): TFA (1 mL) was added dropwise to a solution
of compound 10 (156 mg, 0.34 mmol), or compound 11 (147 mg,
0.34 mmol) in CH2Cl2 (10 mL) The solution was stirred at room
temperature overnight. A saturated solution of NaHCO3 (10 mL)
[12]
[13]
Received January 22, 2004
Eur. J. Org. Chem. 2004, 1891Ϫ1893
www.eurjoc.org
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1893