An efficient synthesis of a lycobetaine-tortuosine analogue: A potent topoisomerase inhibitor

Article abstract of DOI:10.1055/s-2006-956482

An efficient gram-scale synthesis that uses a Suzuki cross-coupling reaction to yield 5-methyl-2,9-dimethoxyphenanthridinium chloride, a lycobetaine-tortuosine analogue and potent topoisomerase inhibitor, is presented. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2006-956482

LETTER
3461
An Efficient Synthesis of a Lycobetaine–Tortuosine Analogue: A Potent
Topoisomerase Inhibitor
Synthesis of
a
Lyc
a
obetaine–T
r
ortuosin
l
e
A
nalo
-
gue Heinz Merz,a Thierry Muller,b Sylvia Vanderheiden,b Gerhard Eisenbrand,a Doris Marko,*c Stefan Bräse*b
a
b
c
Department of Chemistry, Division of Food Chemistry & Environmental Toxicology, University of Kaiserslautern,
67663 Kaiserslautern, Germany
University of Karlsruhe (TH), Institute of Organic Chemistry, Fritz-Haber-Weg 6, 76131 Karlsruhe, Germany
Fax +49(721)608858; E-mail: braese@ioc.uka.de
Institute of Applied Biosciences, Section of Food Toxicology, University of Karlsruhe (TH), Kaiserstraße 12, 76131 Karlsruhe, Germany
Received 28 September 2006
Here we report an efficient gram-scale synthesis for the
preparation of 2,9-dimethoxy-5-methyl-phenanthridini-
um chloride (1), a lycobetaine–tortuosine analogue and
potent topoisomerase inhibitor (Figure 2).
Abstract: An efficient gram-scale synthesis that uses a Suzuki
cross-coupling reaction to yield 5-methyl-2,9-dimethoxyphenan-
thridinium chloride, a lycobetaine–tortuosine analogue and potent
topoisomerase inhibitor, is presented.
Key words: cross-coupling, Suzuki reaction, topoisomerase inhib-
itor, phenanthridine alkaloids, heterocycles
OMe
MeO
Cl
The pyrrolophenanthridine alkaloids lycobetaine,1a also
called ungeremine,1b,c as well as tortuosine and
criasbetaine2 are minor constituents of Amaryllidaceae, a
world-wide plant family predominantly found in warmer
regions (Figure 1). Phenanthridine alkaloids and their de-
rivatives have attracted much attention over the past years
due to potent antiviral,3a antimicrobial3b and antitumor3c
properties. Lycobetaine, particularly, has been identified
as a selective topoisomerase IIb poison, potently inhibit-
ing the growth of human tumor cell lines.1a
N
Me
1
Figure 2 2,9-Dimethoxy-5-methylphenanthridinium chloride (1),
an analogue of natural pyrrolophenanthridine alkaloids
Lycobetaine (ungeremine) was discovered in 19655a and
isolated in 1970.5b Lycorine, a more common alkaloid of
Amaryllidaceae, provided the basis for oxidation to lyco-
betaine with selenium dioxide.1c,6 In 1978, Zee-Cheng et
al.7 developed a practical photochemical reaction giving
access to ungeremine and related pyrrolophenanthridines.
Straightforward syntheses of ungeremine were published
by Siddiqui et al.1b in 1990 and by Lauk et al.8 in 1991.
The former is based on a Suzuki cross-coupling reaction
whereas the latter uses a radical cyclization to build the
phenanthridine skeleton. Various natural pyrrolophen-
anthridinium alkaloids have also been prepared via a
Ziegler–Ullmann reaction by Stark et al.2
R
R'
O
O
MeO
MeO
N
N
R = O : lycobetaine
R' = OMe: tortuosine
R' = O : criasbetaine
Figure 1 Naturally occurring pyrrolophenanthridine alkaloids
We have developed an effective large-scale synthesis of
various substituted 5-methylphenanthridinium chlorides
based on a Suzuki cross-coupling reaction and have
applied this method to produce 5-methyl-2,9-dimethoxy-
phenanthridinium chloride (1; Figure 2).
To further investigate these properties, comprehensive
structure–activity studies were performed to identify rele-
vant pharmacophores. Replacement of the 4,5-ethylene
bridge in the pyrrolophenanthridine skeleton by a 5-meth-
yl group and modifications of substituents in positions 2,
8 and 9 led to 2,9-dimethoxy-5-methylphenanthridinium
chloride (1; Figure 2). This compound shows equipotent
growth inhibitory properties compared to its natural ana-
logue lycobetaine (data not shown). Chloride 1 was also
found to be a potent inhibitor of human topoisomerases I
and II.4
The precursors for the Suzuki cross-coupling reaction are
readily obtained. 4-Methoxyphenyl carbamic tert-butyl
ester (3) was prepared by reacting para-anisidine (2) and
di-tert-butyl dicarbonate in tetrahydrofuran9 (Scheme 1).
Ortho-lithiation of 3 with tert-butyllithium,10 reaction
with trimethyl borate, and subsequent hydrolysis yielded
boronic acid 4. 2-Bromo-4-methoxybenzaldehyde (6) was
synthesized in one step from acetal 5 via ortho-lithiation11
using tert-butyllithium followed by addition of
tetrabromomethane12 and subsequent acid hydrolysis
(Scheme 1).
SYNLETT 2006, No. 20, pp 3461–3463
1
8
.1
2
.2
0
0
6
Advanced online publication: 08.12.2006
DOI: 10.1055/s-2006-956482; Art ID: G28806ST
© Georg Thieme Verlag Stuttgart · New York

3462
K.-H. Merz et al.
LETTER
NH2
NHBoc
OMe
(Boc)2O, THF
r.t., 86%
(MeO)2SO2
MeO
PhNO2, xylene, r.t., 86%
OMe
OMe
N
2
3
OMe
7
NHBoc
(HO)2B
1) t-BuLi, Et2O, –20 °C
O
S
MeO
2) B(OMe)3, then r.t., H2O/NH4Cl, 63%
O
OMe
N
O
OMe
4
Me
8
OMe
OMe
Scheme 3 Preparation of 2,9-dimethoxy-5-methylphenanthridini-
um methylsulfate (8)
1) t-BuLi, Et2O, –20 °C then –78 °C, CBr4
2) r.t., 10% HCl, 50%
Br
OMe
MeO
OMe
H
O
5
6
O
MeO
O
S
OMe
MeO
Scheme 1 Synthesis of precursors 4 and 6
N
O
Me
OMe
8
Suzuki cross-coupling reaction of 4 and 6 was performed
in presence of tetrakis(triphenylphosphine)palladium and
sodium carbonate in 1,2-dimethoxyethane. Acid hydroly-
sis of the reaction mixture afforded 2,9-dimethoxy-
phenanthridine (7) in 75% yield under optimized
conditions (Scheme 2).
NaCl 16%, H2O
r.t. then 0 °C, 95%
Cl
N
Me
1
Scheme 4 Synthesis of 2,9-dimethoxy-5-methylphenanthridinium
chloride (1)
O
H
NHBoc
Br
(HO)2B
+
e.g. 2,8-dimethoxy-5-methylphenanthridinium chloride,
by simply adapting precursors 4 and 6.
OMe
OMe
6
4
Synthesis of 2,9-Dimethoxyphenanthridine (7)
OMe
Tetrakis(triphenylphosphine)palladium (948 mg, 0.821 mmol, 0.05
equiv) was added to a degassed solution of 2-bromo-4-methoxy-
benzaldehyde (6; 3.53 g, 16.41 mmol, 1 equiv) in anhyd 1,2-
dimethoxyethane (40 mL) and the resulting solution was stirred at
r.t. After 5 min, 2-borono-4-methoxycarbanilate (4; 5.26 g, 19.70
mmol, 1.2 equiv) and an aq solution of Na2CO3 (5.22 g, 49.24
mmol, 3 equiv, 40 mL) were added and the resulting mixture was
heated at 100 °C. After 5 h, a solution of concd HCl (8 mL) was
slowly added to the cold solution and the resulting mixture was
heated at 100 °C. After 30 min, an aq solution of 10 M NaOH was
added until pH 7 was reached. The aqueous phase was extracted
with Et2O (2 × 150 mL) and CHCl3 (2 × 150 mL). The combined
organic layers were dried (MgSO4), filtered and concentrated in
vacuo. The residue was purified by flash column chromatography
(EtOAc–cyclohexane, 7:3) to afford 2,9-dimethoxyphenanthridine
(7; 2.94 g, 75%) as a slightly yellow solid; mp >210 °C (dec.). IR
1) Pd(PPh3)4, Na2CO3,
DME, r.t. then 100 °C
MeO
2) HCl 4 N, 100 °C, 75%
N
7
Scheme 2 Synthesis of 2,9-dimethoxyphenanthridine (7)
Quaternization of 2,9-dimethoxyphenanthridine (7) was
carried out in the presence of dimethylsulfate in a mixture
of nitrobenzene and xylene to yield 2,9-dimethoxy-5-
methylphenanthridinium methylsulfate (8; Scheme 3).
Anion exchange of the methylsulfate salt 8 was achieved
in an aqueous solution of sodium chloride. Target com-
pound 1 was isolated as chloride salt by precipitation at
0 °C in nearly quantitative yield (Scheme 4).
1
(KBr): 1030, 1501, 1516, 1614, 2831, 2935, 2974, 3009 cm–1. H
NMR (400 MHz, CDCl3): d = 4.00 (s, 3 H, OCH3), 4.02 (s, 3 H,
OCH3), 7.28 (dd, J8,7 = 8.9 Hz, J8,10 = 2.4 Hz, 1 H, H-8), 7.36 (dd,
J3,1 = 2.8 Hz, J3,4 = 9.0 Hz, 1 H, H-3), 7.75 (d, J = 2.5 Hz, 1 H), 7.76
In summary, we were able to prepare 2,9-dimethoxy-5-
methylphenanthridinium chloride (1) on a gram scale
using a six-step convergent synthesis with an acceptable
overall yield of 17%. This synthetic route can be used to
obtain other interesting pyrrolophenanthridine analogues,
(d, J = 2.5 Hz, 1 H), 7.92 (d, J7,8 = 8.9 Hz, 1 H, H-7), 8.07 (d, J4,3
=
9.0 Hz, 1 H, H-4), 9.03 (s, 1 H, H-6). 13C NMR (100.6 MHz,
CDCl3): d = 55.5 (OCH3), 55.6 (OCH3), 102.6 (C-1), 103.2 (C-10),
117.7 (C-8), 118.3 (C-3), 121.6 (C-6a), 124.8 (C-10b), 130.4 (C-7),
131.4 (C-4), 133.9 (C-10a), 139.9 (C-4a), 150.4 (C-6), 158.0 (C-2),
161.3 (C-9). MS (ES+): m/z = 239. HRMS (ES+): m/z calcd for
Synlett 2006, No. 20, 3461–3463 © Thieme Stuttgart · New York

LETTER
Synthesis of a Lycobetaine–Tortuosine Analogue
3463
C15H13NO2: 239.0946; found: 239.0944. Anal. Calcd for
C15H13NO2: C, 75.30; H, 5.48; N, 5.85. Found: C, 74.38; H, 5.39; N,
5.79.
References
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Synthesis of 2,9-Dimethoxy-5-methylphenanthridinium
Methylsulfate (8)
Dimethyl sulfate (8 mL) was added to a solution of compound 7
(2.6 g, 10.87 mmol, 1 equiv) in nitrobenzene (40 mL) and xylene
(20 mL). The resulting mixture was stirred at r.t. After 1 h, the pre-
cipitate was filtered off, washed with cold Et2O (2 × 50 mL) and
dried under vacuum to afford 2,9-dimethoxy-5-methylphenanthri-
dinium (8; 3.49, 88%) as a white solid. IR (KBr): 1005, 1221, 1253,
1614, 3008 cm–1. 1H NMR (400 MHz, DMSO-d6): d = 3.63 (s, 3 H,
SOCH3), 3.81 (s, 3 H, OCH3 at C-9), 3.90 (s, 3 H, OCH3 at C-2),
4.23 (s, 3 H, NCH3), 7.11–7.45 (m, 4 H, H-1, H-3, H-8, H-10), 7.81–
7.97 (m, 2 H, H-4, H-7), 9.02 (s, 1 H, H-6). 13C NMR (100.6 MHz,
DMSO-d6): d = 46.2 (NCH3), 55.4 (SOCH3), 57.7 (2 × OCH3),
105.0 (C-1), 106.2 (C-10), 119.1 (C-6a), 121.9 (C-3, C-8), 122.8
(C-7), 127.3 (C-10b), 129.4 (C-10a), 135.6 (C-4), 137.4 (C-4a),
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2,9-Dimethoxy-5-methylphenanthridinium Chloride (1)
Compound 8 (3.093 g, 8.46 mmol, 1 equiv) was added to an aq so-
lution of 16% NaCl (80 mL) and the resulting mixture was stirred
for 1 h at r.t. and for 30 min at 0 °C. The precipitate was then filtered
off, washed with cold H2O (30 mL) and dried under vacuum to af-
ford 2,9-dimethoxy-5-methylphenanthridinium chloride (1; 2.34 g,
95%) as a slightly yellow solid; mp 203–204 °C. IR (KBr): 1025,
1
1221, 1617, 2838, 3006 cm–1. H NMR (400 MHz, DMSO-d6):
d =3.79 (s, 3 H, OCH3 at C-9), 3.83 (s, 3 H, OCH3 at C-2), 4.17
(s, 3 H, NCH3), 7.24 (dd, J3,1 = 2.2 Hz, J3,4 = 8.9 Hz, 1 H, H-3), 7.31
(d, J1,3 = 2.1 Hz, 1 H, H-1), 7.38 (m, 2 H, H-8, H-10), 7.87 (d, J7,8
=
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H-6). 13C NMR (100.6, MHz, DMSO-d6): d = 46.1 (NCH3), 57.8
(2 × OCH3), 105.1 (C-1), 106.3 (C-10), 119.0 (C-6a), 122.0 (C-3,
C-8), 122.7 (C-7), 127.2 (C-10b), 129.5 (C-10a), 135.5 (C-4), 137.3
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Synlett 2006, No. 20, 3461–3463 © Thieme Stuttgart · New York