Facile synthesis of pyrrolo[2,1-c][1,4]benzodiazepine-5,11-dione dimer

Article abstract of DOI:10.1002/jhet.5570430441

Efficient synthesis of biological important pyrrolo[2,1-c][1,4] benzodiazepine-5,11-dione dimer linked through the C-2 positions by fumarate group is described.

Full text of DOI:10.1002/jhet.5570430441

Jul-Aug 2006
1091
Facile Synthesis of Pyrrolo[2,1-c][1,4]benzodiazepine-5,11-dione Dimer
Naim H. Al-Said
Department of Applied Chemical Sciences, Jordan University of Science and Technology
P.O. Box 3030, Irbid 22110, JORDAN, Fax: 962-2-7095014. e-mail: naim@just.edu.jo
Received September 8, 2005
NO2
H3CO
H3CO
CO2Et
H3CO
H3CO
NO2
2 steps
5 steps
N
COOH
OH
O
H
O
H
N
OCH3
OCH3
O
H
N
H3CO
H3CO
O
H
H
N
N
N
N
H
O
O
O
Efficient synthesis of biological important pyrrolo[2,1-c][1,4]benzodiazepine-5,11-dione dimer linked
through the C-2 positions by fumarate group is described.
J. Heterocyclic Chem., 43, 1091 (2006).
H
N
H
N
Introduction.
OH
OCH3
H
O
CH3
HO
H
Understanding of the interactions at the molecular level
between DNA and some small molecules, which are vital
in antitumor, antibiotic, and antiviral chemotherapy, is
crucial in facilitating the design of new drugs and probes
that can recognize specific DNA sequences [1-4]. The
pyrrolo[2,1-c][1,4]benzodiazepines (PBDs) are a family
of naturally occurring antitumor antibiotics. DC-8 (1),
tomamycin (2), and chicamycin (3) are examples of the
PBDs family [5-7]. These small molecules recognize and
bond to specific DNA sequences [8-9]. The biological
activity of PBDs is associated with their ability to form
covalent binding to DNA via nucleophilic attack of the
C2-NH2 of a guanine base at the electrophilic C11
position within the minor groove of DNA [10-11].
Dilactams of PBD such as 4, lacking the imine moiety (or
carbinolamine equivalent) at N10-C11, were shown to
bind in a non-covalent fashion to DNA [12]. Recently, C-
8 linked bifunctional alkylating dimers, based on PBD
skeleton posses an efficient inter-strand cross-linking
ability and high affinity for DNA, were described [13-16].
Since the imine moiety in such dimer has strong covalent
binding ability to DNA it retards the NMR spectroscopic
study of the non-covalent binding motifs.
N
N
H3CO
OH
O
O
OH
Chicamycin (3)
Dilactam (4)
The current publication describes the first synthesis of PBD
dilactam dimer linked through the C-2 positions. This dimer
lacks the problematic unstable imine moiety design to
recognize and bind to the minor groove of DNA. The synthetic
pathway described here is short, amenable to large-scale
preparation and generally extendable to synthesis of wide
range of dimers with different linkers and functionalities.
Results and Discussion.
The synthesis of PBD dilactam dimer 12 started with
the synthesis of a specifically functionalized dilactam
unit. In the first step, commercially available trans-
hydroxy proline ethyl ester hydrochloride (5) and 6-
nitroveratric acid (6) were coupled. Following the
conversion of 6 to its acid chloride using thionyl chloride,
the resulting acid chloride was added to a solution of 5 in
dichloromethane at 0 °C to afford the precursor 7 in
1
excellent yield (Scheme 1). The H-NMR (CDCl3) of this
pure intermediate indicated the presence of two related
species in a ratio of 7:3 probably due to the restricted
rotation of the amide bond. The methoxy protons were
displayed as two singlets (ꢀ 3.93 and 3.90) and the methyl
protons as two triplets (ꢀ 1.28 and 1.03).
H
H
N
N
HO
HO
N
N
H3CO
H3CO
O
O
Reduction of the nitro group in 7 in methanol over
Pd/C was very slow and required addition of
DC-8 (1)
Tomamycin (2)

1092
N. Al-Said
Vol 43
catalytic amount of hydrochloric acid to accomplish
the conversion of the starting material. Fortunately,
this process was accompanied by a simultaneous
cyclization of the resulting amine to give the
required dilactam 8.
In
summary,
the
first
pyrrolo[2,1-c][1,4]-
benzodiazepine-5,11-dione dimer has been synthesized
and characterized. The approach to this dimer is
extendable to synthesis of a wide range of dimers with
different linkers and functionalities.
Scheme 1
H
N
O
NO2
H3CO
H3CO
CO2Et
H3CO
H3CO
NO2
H3CO
H3CO
H
a, b
c
N
COOH
N
OH
X
O
O
d
6
7
9, X = OMs
8, X = OH
e, f
H
O
H
N
H
N
N
O
OCH3
OCH3
O
H
H
H3CO
H3CO
g
H3CO
H3CO
H
O
H
N
N
N
N
N
H
O
X
O
O
O
12
10, X= N3; 11, X = NH2
Reagents and conditions a) SOCl2, reflux; b) trans-4-Hydroxyproline ethyl ester hydrochloride (5), CH2Cl2, 87%; c) H2/Pd(C), MeOH/HCl(cat)
,
93%; d) MeSO2Cl, Pyr., 87%; e) NaN3, EtOH-DMF, 80° C, 95%; f) H2/Pd/C, MeOH, then Fumaric acid, EDCI, HOBtO, DMF, 63%.
EXPERIMENTAL
Forming a dimer of dilactam 8 employing ester linkages
was abandoned because several attempts to couple the
hydroxyl group of the dilactam 8 with fumaric acid and
succinic acid or their acid chlorides under different
conditions were unsuccessful.
Melting points were determined on an Electrothermal melting
point apparatus and are uncorrected. The spectra were recorded
1
using Nicolet 7199 FT-IR. H-NMR spectra were recorded on
Bruker AM-300 spectrometer. High-resolution mass spectra
(HRMS) and FAB mass spectra were obtained using a Kratos
AEI MS-9 and MS-50 mass spectrometer and reported as m/z.
Therefore, we decided to use the more reactive amine at
C-2 position. The conversion of the hydroxyl group to
amine was smoothly achieved in two steps. First, the
hydroxyl group at C-2 position was converted to mesylate
9 in pyridine at 0 °C. Then, the resulting mesylate was
treated with sodium azide in ethanol-dimethylformamide
solution at 80 °C to afford the azido derivative 10. The
remaining problem in our synthesis consisted formally of
the task of achieving reduction of the azido group to the
corresponding amine 11 and coupling of the resulting
amine with suitable linker. Thus, conversion of azido
group to the required amine 11 was effected by
hydrogenolysis method in methanol. The resulting amine
11 was used in the next step without purification.
Coupling of the amine 11 with fumaric acid was
accomplished by a conventional method used for amide
formation. The fumaric acid was first activated by
N-[2-Nitro-4,5-dimethoxybezoyl]-L-hydroxyproline ethyl ester (7).
A solution of nitroveratric acid (6) (2.27 g, 10 mmol) was
heated with excess thionyl chloride (6 mL) for 2 h. The solution
was then concentrated and the residue was dissolved in
dichloromethane (200 mL). Then, trans-hydroxy-L-proline ethyl
ester hydrochloride (5) (2.93, 15 mmol) and triethylamine (4
mL) were added to the acid chloride at 0 °C. The reaction
mixture was stirred for 3 h, washed with 5% HCl (100 mL), and
the organic layer was concentrated. The residue was purified by
column chromatography on silica gel (5% MeOH in ethyl
acetate) to give amide 7 (3.21 g, 87% yield); [ ]D –72 (c 5.75,
CHC13); FT-IR (CHCI3) 3438, 1740 and 1647 cm-1; H-NMR
1
(CDCl3): ꢀ 7.65 and 6.83 (2s, 1H each, aromatic-H), 4.75 (t, J=8
Hz, 1H, CHOH), 4.20 (q, J=7 Hz, 2H, OCH2), 3.93 (s, 3H,
OCH3), 3.45 (dd, J=11 Hz, J’=4 Hz, 2H, CH2), 1.28 (t, J=8 Hz,
1
3H, CH3) for the major species. H-NMR (CDCl3): ꢀ 7.55 and
6.73 (2s, 1H each, aromatic-H), 4.50 (t, J = 8 Hz, 1H, CHOH),
4.10 (q, J = 7 Hz, 2H, OCH2), 3.90 (s, 3H, OCH3), 1.03 (t, J = 8
Hz, 3H, CH3) for the minor species; HRMS calcd. for
C16H20N2O8 368.1220, found 368.1226.
Anal. Calcd. for C16H20N2O8: C, 52.17; H, 5.47; N, 7.61.
Found: C, 52.20; H, 5.40; N, 7.55
employing
1-(3-dimethylaminopropyl)-3–ethylcarbodi-
imide hydrochloride (EDCI) and 3-hydroxy-1,2,3-benzo-
triazin-4(3H)-one (HOBtO) followed by addition of
amine 11 to furnish the first PBDs dimer 12 in reasonable
overall yield.

Jul-Aug 2006
Pyrrolo[2,1-c][1,4]benzodiazepine-5,11-dione Dimer
1093
(1laS)-2(R)-Hydroxy-7,8-dimethoxy-2,3,5,10,11,1la-hexahydro-
5,11-dioxo-1H-pyrrolo[2,1-c][1,4]benzodiazepine (8).
10% ethyl acetate in hexane to give pure amine 11. The
resulting amine was dissolved in DMF (10 mL) containing
triethylamine (1 mL). The resulting mixture was added to a
DMF (10 mL) solution of fumaric acid (116 mg, 1 mmol), EDCI
(2.2 mmol) and HOBtO (2 mmol). The reaction mixture was
stirred for 24 h. The mixture was concentrated under reduced
pressure. The residue was dissolved in chloroform and the
resulting solution was washed with water, dried and
concentrated. The solid residue was collected and washed with
cold methanol to give pure dimmer 12 (920 mg, 63% yield) as a
white powder; mp. 310 °C (dec); [ꢁ]D +399 (c 3.95, DMSO);
Amide 7 (1.84 g, 5 mmol) in methanol (150 mL) containing
few drops of conc. HCl was hydrogenated (50 psi, 345 KPa) at
40 °C for 18 h over Pd/C. The reaction mixture was filtered and
the filtrate was concentrated. The solid residue was washed with
cold methanol to furnish 8 (1.38 g, 93% yield) as white solid;
m.p 254 °C; FT-IR (CHC13) 3422, 1674, 1649 and 1519 cm-1;
1H-NMR (DMSO-d6): ꢀ 10.30 (s, 1H, NH), 7.25 and 6.68 (2s,
1H each, aromatic-H), 5.13 (d, J = 4 Hz, 1H, CHOH), 4.28 (q, J
= 4 Hz, 1H, NCHH), 4.10 (dd, J = 8 and 6 Hz, 1H, CH), 3.78 (s,
3H, OCH3), 3.60 (dd, J = 12 and 4 Hz, 2H, NCHH), 3.40 (dd, J
= 12 and 5 Hz, 1H, NCHH); HRMS calcd for C14H16N2O5
292.1060, found 292.1061.
1
FT-IR (CH2Cl2) 1677, 1654, 1620, 1606 and 1432 cm-1; H-
NMR (DMSO-d6): ꢀ 10.32 (s, 2H, NH), 8.53 and 8.52 (2s, 1H
each NH), 7.25 and 6.76 (2s, 2H each, aromatic-H), 6.68 (s, 2H,
HC=CH), 4.28 (dd, J = 12 and 5 Hz, 2H), 4.15 (dd, J = 8 and 5
Hz, 2H), 3.98 (dd, J = 12 and 8 Hz, 2H), 3.76 and 3.7 (2s, 3H
each, OCH3), 3.28 (m, 2H), 2.65 (m, 2H), and 2.27 (m, 2H); 13C-
NMR (DMSO-d6) ꢀ 170.0, 164.9, 163.7, 151.8, 145.3, 132.5,
130.8, 117.6, 111.9, 104.4, 55.6, 51.6, 47.4, 30.9; FABMS calcd.
for C32H35N6O10 662.2, found 663.1 (MH+, 5%).
Anal. Calcd. for C14H16N2O5: C, 57.53; H, 5.52; N, 9.51.
Found: C, 57.28; H, 5.47; N, 9.63.
(11aS)-2(R)-Mesy1-7,8-dimethoxy-2,3,5,10,11,1la-hexahydro-
5,11-dioxo-lH-pyrrolol[2,1-c][l,4]benzodiazepine (9).
Methanesulfonyl chloride (0.7 g, 6 mmol) was added to a
solution of 8 (1.17 g, 4 mmol) in dichloromethane (50 mL)
containing pyridine (3 mL). The reaction mixture was stirred at
room temperature for 3 h followed by addition of 10% HCl (100
mL). The organic layer was separated, dried and concentrated.
The residue was purified by flash chromatography (ethyl
acetate) to afford mesylate 9 (1.2 g, 87% yield) as a white solid;
mp. 185° C; [ꢁ]D 299 (c 6.94, MeOH); FT-IR (CHC13)
Anal. Calcd. for C32H34N6O10: C, 58.00; H, 5.17; N, 12.68.
Found: C, 58.32; H, 5.32; N, 12.51.
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